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Intermediate Frequency
Hello,
I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Does the IF stage conditions the incoming signal? What are the advantages of the IF stage? Just confused. Can anyone throw some light on this? Thanks. |
Intermediate Frequency
hello!
radio_rookie wrote: ... IF was used in Superhet transceivers. well, IF __is__ used in superhet receivers... :) My question is why doesn't anyone use zero IF now a days. .... In fact, you can very well use zero IF in a receiver: what you get, doing so, is a "direct conversion", or "synchrodyne", receiver, which is simple, easy to design and realize, cheap, and yet can offer very high level performances - as is well known to radio amateurs ;) But... yes, there's a but, and a price to pay: in a DCR, you put all the required gain (a hundred of dBs, give or take a few) in the base band (or audio frequency) chain, and this is bad for noise! Most electronic devices are sensitive to pressure, and a sound is a pressure wave... you may end up with something which can be quite a decent microphone! :) Moreover, in a DCR it may be rather difficult to implement an effective AGC, so switching from weak to loud signals may be annoying, and even dangerous for hearing! Also, it's not easy to obtain a reasonable S-meter... Last, but not least, a simple DCR is inherently a DSB receiver, lest you implement it with rather complex mixers - which add much to the circuit total complexity. Adding all of this up, a superhet may be a good choice... -- 73 es 51 de i3hev, op. mario Il vero Radioamatore si riconosce... dal call in firma! - Campagna 2005 "Sono un Radioamatore e me ne vanto" it.hobby.radioamatori.moderato http://digilander.libero.it/hamweb http://digilander.libero.it/esperantovenezia |
Intermediate Frequency
"radio_rookie" wrote in message
ps.com... doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Does the IF stage conditions the incoming signal? What are the advantages of the IF stage? Actually, it is fairly common -- it is called direct conversion and it is very popular for simple rigs. It is not without issues. Converting down to audio means that you cannot eliminate the "other sideband". Also, since the RF amp needs to be broadband, you can only get limited gain, so you end up needing a lot of gain in the audio stage. This is achievable, but it is a little tricky to manage oscillations, ringing and the like. In addition, getting narrow bandwidth at audio is also a little dicey, and you can't have it at RF unless you tune the RF ... also tricky. Which is why most receivers these days are still superhets. You can choose an IF that allows you to manage the bandwidth, and even so, multiple conversion is pretty popular. Many commercial rigs are triple conversion, typically with IF's around 60 MHz, 10 MHz and 455 kHz, with a very liberal interpretation of "around" g ... |
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In article ,
i3hev, mario held wrote: Last, but not least, a simple DCR is inherently a DSB receiver, lest you implement it with rather complex mixers - which add much to the circuit total complexity. As I understand it, in order to do SSB via direct conversion, you must use a modern version of the old IQ phasing technique. This requires doing the zero-IF mixdown twice, on two different versions of the signal (with a uniform 90-degree phase shift being applied to one copy of the RF or LO signal). The resulting two baseband outputs are then processed (with a further 90-degree phase shift being applied to one of them) and carefully mixed. This results in reinforcement of the desired sideband and suppression of the other. The circuitry needed to apply the necessary phase shifts is not trivial (if you want enough accuracy to deliver acceptable opposite-sideband rejection), and is not necessarily simpler or less expensive than the filtering and extra stage of mixing done in a traditional IF-based SSB receiver or transmitter. These days, of course, you can apply the phase shift by converting the two baseband signals to digital format, and implementing the final 90-degree phase shift via a digital FIR all-pass filter. This of course requires your design to have a pair of high-linearity ADCs, a DSP, and a DAC to reproduce the final (mixed) signal. The "holy Grail" these days seems to be a direct-from-RF system, in which the RF signal is _directly_ sampled (at a ferociously-high sampling rate), and all of the phase shifts and downconversion and mixing are done digitally. RF-grade ADCs with the necessary linearity and speed aren't particularly inexpensive, especially if you need your system to deliver a very high dynamic range which can work properly even in the presence of strong in-band or near-band interferers. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Intermediate Frequency
See URL:
http://www.fas.org/man/dod-101/navy/...perhet.htm#ads and http://www.eie.polyu.edu.hk/~ensurya...1/Chapter1.htm CL "radio_rookie" wrote in message ps.com... Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Does the IF stage conditions the incoming signal? What are the advantages of the IF stage? Just confused. Can anyone throw some light on this? Thanks. |
Intermediate Frequency
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Intermediate Frequency
From: radio_rookie on Thurs, Sep 7 2006 1:17 pm
I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Does the IF stage conditions the incoming signal? What are the advantages of the IF stage? Just confused. Can anyone throw some light on this? This can be a HUGE subject, but, since this is "homebrew" we can 'distill' it to a few things: :-) 1. Ever-present random NOISE in the front end. Can't escape it. Since the amount of noise voltage reaching the demodulator can be reduced by the square-root of relative bandwidth, IF bandpass filtering can cut down that random noise, yield a constant selectivity regardless of RF input. 2. Direct conversion to baseband is subject to dynamic range limitations v. the amount of RF input power and RF input selectivity. i.e., a very strong signal well out of the desired RF input range might mess with the sampler causing intermodulation distortion. 3. Lowest RF input level (which determines the "sensitivity" specification) requires a very low- noise sampler to equate to a full superhet with an IF chain. Samplers are not noise-free. Samplers must compete on the tenths of microvolts (or less) noise with conventional active mixers of now to meet high-sensitivity specifications of today. 4 Software (as in an SDR architecture) is NOT simple to implement, even in a very fast processor. While it is easy to change demodulation modes, one needs to understand the math behind the demodulation process. If you have the TIME and the smarts, go for it; if not, it may be months before your project works and then it may not work very well. 5. Not all RF input signals are AM or derivatives of that (on-off keying, SSB on HF). For FM or combination AM-PM as in the "modem" fashion, it might be much easier to implement via a separate IF plus separate demodulator per mode. 6. In the beginning (1918 and Ed Armstrong in Paris right after WW1), vacuum tubes were NOT what one could call the best, noise-free, or even with much gain. The superhet form allowed the same selectivity (via the IF bandpass) at any desired RF input frequency; that did not exist before the superhet. Since that was a quantum-level improvement at the time, it had a mystique about it that caused nearly all designers to follow the IF chain idea with its diode or tube "detector" (really a rectifier-mixer). The math of modulation had been published in 1915 (John R. Carson of AT&T) but had yet to spread. It was not intuitive to the non-mathematical and so few designers got "into" possible new ways to mix and demodulate. With better tubes that came after, the IF and 2nd IF and even 3rd IF as discretes was easier to design and make. That lasted until roughly 1980 or about 6 decades, all superhets having IF chains in a familiar arrangement. It was "comfortable." More importantly, it worked. 7. If you want selectable bandpass filtering at all frequencies, the IF with its input bandpass filters at most any bandwidth you want is the easiest to design-in and build. That way you lop off the signals on either side as close to the antenna as you can get. 8. Heterodyning (mixing) down to one frequency, the IF, makes it easier to work and debug with a semi-direct-conversion system. Especially so if the desired RF inputs have many bands. 9. On the other hand, if portability, light weight, and low power drain is a requirement (as in military field receivers), plus all sorts of demodulation modes, the SDR or Software Designed Radio is the thing to do, using samplers, A-D conversion and demodulation in a processor subsystem. Note: You combine the front end of a conventional IF with the processor sub-system replacing the IF back end and 'detectors' to get the best of both. There isn't any one simple answer. It is all a trade-off between what is desired and what you can design and make and how much you have to build plus your budget. Its all wonderfully complex to decide and I love it. :-) |
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radio_rookie wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Thanks. Andy writes: This is the eighth response to your question. All of the others are accurate. HOWEVER, there is one technical problem with a homodyne that outweighs the previous resonses... The L.O, has to be EXACTLY the same frequency as the incoming signal to provide proper demodulation...... With a superhet, depending on the modulation, you can be many Khz in error ( for AM) or a few dozen off ( for voice SSB). With homodyne, you may have to process a psuedo-baseband signal which has frequency components you don't want..... that is a bitch.... In order to achieve the LO to be EXACTLY the same frequency (phase doesn't matter since you can use I and Q), it is necessary to achieve frequency lock. That requires a much higher S/N than a simple superhet with a detector... So you lose sensitivity. In some systems , you can lock to a remote carrier, but you aren't really talking about those methods, I don't. think.... So, the major technical problem is not SIMPLICITY or BANDWIDTH or NUMBER OF STAGES...,.. it is how to obtain an LO of the correct frequency.... All the other problems are simple compared to this.... for most systems... Andy W4OAH PS I welcome dissent and would like very much to learn if anything I have said is in error...... |
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AndyS wrote:
Andy writes: This is the eighth response to your question. All of the others are accurate. HOWEVER, there is one technical problem with a homodyne that outweighs the previous resonses... The L.O, has to be EXACTLY the same frequency as the incoming signal to provide proper demodulation...... With a superhet, depending on the modulation, you can be many Khz in error ( for AM) or a few dozen off ( for voice SSB). With homodyne, you may have to process a psuedo-baseband signal which has frequency components you don't want..... that is a bitch.... In order to achieve the LO to be EXACTLY the same frequency (phase doesn't matter since you can use I and Q), it is necessary to achieve frequency lock. That requires a much higher S/N than a simple superhet with a detector... So you lose sensitivity. In some systems , you can lock to a remote carrier, but you aren't really talking about those methods, I don't. think.... So, the major technical problem is not SIMPLICITY or BANDWIDTH or NUMBER OF STAGES...,.. it is how to obtain an LO of the correct frequency.... All the other problems are simple compared to this.... for most systems... Andy W4OAH PS I welcome dissent and would like very much to learn if anything I have said is in error...... I believe that what you said is true, but only for AM. What other kind of modulation requires better frequency accuracy from a direct conversion receiver LO than from a superhet LO? I believe that only a small fraction of today's amateurs are interested in AM reception, but of course it's the bread and butter of the SWL and BCL. I've build direct conversion receivers for many years. Their simplicity is particularly evident when you compare a transceiver having a direct conversion receiver with one having a superhet receiver -- to be honest, be sure to include all the extra filtering necessary with the superhet. The single biggest disadvantage to DC receivers, in my opinion, is the difficulty of making a good AGC, particularly in conjunction with narrow audio bandwidth. And they do have their own set of potential problems, such as unintended AM demodulation, the effects of LO leakage and radiation, and the difficulties in making a clean, stable, high gain audio amplifier. But all can be overcome once one understands the causes of the problems. All but perhaps the last one will be present in a digital version, too, so a casually designed and/or built one is likely to be a poor performer. Roy Lewallen, W7EL |
Intermediate Frequency
Dave Platt wrote:
As I understand it, in order to do SSB via direct conversion, you must use a modern version of the old IQ phasing technique. ... that's correct :) the problem here is that you need a phasing filter offering a constant (and precise) 90 degrees phase shift all over the receiver band, and that can be tricky to do... These days, of course, you can apply the phase shift by converting the two baseband signals to digital format... well, you're opening doors to digital radio :) But, AFAIK, the actually attainable dynamics does not seem to incite enthusiastic greetings... ;) -- 73 es 51 de i3hev, op. mario Il vero Radioamatore si riconosce... dal call in firma! - Campagna 2005 "Sono un Radioamatore e me ne vanto" it.hobby.radioamatori.moderato http://digilander.libero.it/hamweb http://digilander.libero.it/esperantovenezia |
Intermediate Frequency
On Fri, 08 Sep 2006 19:05:49 +0200, "i3hev, mario held"
wrote: Dave Platt wrote: As I understand it, in order to do SSB via direct conversion, you must use a modern version of the old IQ phasing technique. ... that's correct :) the problem here is that you need a phasing filter offering a constant (and precise) 90 degrees phase shift all over the receiver band, and that can be tricky to do... This is not that hard and there are tricks to do this over very wide ranges. The easiest is create a fixed 90 degree reference and down convert it. Another is use one of the I&Q output DDS chips. These days, of course, you can apply the phase shift by converting the two baseband signals to digital format... well, you're opening doors to digital radio :) But, AFAIK, the actually attainable dynamics does not seem to incite enthusiastic greetings... ;) The problem is finding fast A/D with enough bits (more than 16) and using 32bit processors to grind the data. Those two things are not easy or cheap.. yet. But we are getting there. Allison |
Intermediate Frequency
Roy Lewallen wrote: I believe that what you said is true, but only for AM. What other kind of modulation requires better frequency accuracy from a direct conversion receiver LO than from a superhet LO? I believe that only a small fraction of today's amateurs are interested in AM reception, but of course it's the bread and butter of the SWL and BCL. Roy Lewallen, W7EL Andy responds I don't disagree with anything you've said. However , i've found it a lot easier to make the final frequency adjustments in a superhet since it can be done at a lower frequency. Trying to sync up at 915 is more challenging than at 455 khz, obviously. And for systems like FSK and SSB, you gotta do it somewhere..... And with FM, since the beat formed by the carrier depends on the modulation index of the received signal, getting rid of it can be squirrelly..... Like youself, I've built both types , both as home projects, and as commercial products, for a long long time. In the DC versions, if I use I/Q and combine them to form the audio, the AGC is the same as with superhet SSB. And I generally use PIN diodes before the front end for the first AGC stage... But, that's just my own preference..... Andy W4OAH |
Intermediate Frequency
radio_rookie wrote:
Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Does the IF stage conditions the incoming signal? What are the advantages of the IF stage? Just confused. Can anyone throw some light on this? Thanks. Hey radio rookie, Thanks for posting this excellent question. The answers you received have taught me more than a few textbooks have. Some very good minds have weighed in on your query. Keep up the good work and welcome to ham radio. To the respondents, I say "Thanks!" for approaching ideal NG activity! Makes one proud to be a ham. John AB8O |
Intermediate Frequency
On Fri, 08 Sep 2006 22:04:06 +0200, "i3hev, mario held"
wrote: wrote: This is not that hard and there are tricks to do this over very wide ranges. The easiest is create a fixed 90 degree reference and down convert it. that's true but, imho, there's no meaning in removing the (moderate) complexities of the typical superhet receiver just to put (trickier) complexities in quadrature downconverting... apart from part count, controlling the reciprocal phase in two "identical" converters is in itself not an easy task, since just a single degree of dephasing is enough to degrade unwanted sideband rejection. I disagree with trickier vs moderate. For a given level of performance the two are comparable. The only context is one may be familiar vs different as in new. The phase of two converters is pretty easy to maintain, having done it. However, creative part was down converting the LO rather than the more critical small signal paths. Since the LO would only at most require low pass (VFO at 70-124mhz with fixed IQ generation at 70mhz would give a simple dc-54mhz LO) to prevent the 70mhz and higher components. If passive DBMs are used phase shifts from gain and added LCs can be avoided. Also it's possible to put inline measurement of the resulting LOs and feed back any phase error (found unneeded) for correction. To build a superhet (ignore selectivity and overload performance for the moment) to cover dc-54mhz you need upconversion and then down conversion to IF and lots of filters along the way with switching. Then you have the problem of various LOs and carrier osc (SSB/CW) getting back into previous stages causing undesired tones. The more conversions the greater the problem and the better shielding must be to avoid it. Having built both each has challenges and to do it as a high performing reciever (or TX) requires a very similar effort even if the areas where the effort need be applied differ. The one big difference is that the image reject system is less hardware between antenna and the baseband and that hardware is more frequency agile. The best available analog answer is the KK7B MiniR2 (or R2-PRO), base design uses SBL-1 mixers good from 1-500mhz making generation of an 90degree LO the only real work. As you go up in frequency a quadrature splitter will have both phase and level bandwidth that increases making a tuneable VHF or UHF IF practical. (FYI: accepable (under 1degree and under 1db level variation) bandwidth at 14mhz is 1mhz.) Of course the image reject detector system could exist at any fixed frequency such as 66 mhz with a fixed LO. Then the upconverter only needs a conventional LO of 66 to 120mhz still yeilds an RX that tunes from DC to 54mhz and has few if any adjustments. This would yeild a Single conversion RX with a VHF IF that is remarkably simple and needs few filters. If you take that to the next step using DSP at baseband where it's cheap (using PS and soundcard) you have a very straightforward high performing multimode radio. This is the current SDR approach. When you consider a mainboard with ram, sound and other needed bits running at 1+ ghz is dirt cheap (found free sometimes) and small this is achieveable. Especially considering the software is already out there. Also unused CPU cycles could also do the "glass front pannel" and manage things like tuning the DDS or PLL. ... Those two things are not easy or cheap.. yet. But we are getting there. I'm sure we will get there soon enough, and it may well be that just a few years will suffice; but, as for now... they are not easy or cheap! :) Actually it's progressing faster than you'd notice. The number of Ham radios with IF DSP as the detection is considerable. Most of the upper models of Tentec, Yaesu, Kenwood and others are already doing it. Also the vendors of the DSP processors do have boards for proto work to venture into this realm. With more people doing it and plenty of alreay published work it may be more assembly of components (be they hardware or software) than construction. While this wanders far from the IF question posted it's an example fo how you can have IFs from baseband through UHF or higher. Allison |
Intermediate Frequency
On 7 Sep 2006 13:17:19 -0700, "radio_rookie"
wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison |
Intermediate Frequency
I don't disagree with anything you've said. However , i've
found it a lot easier to make the final frequency adjustments in a superhet since it can be done at a lower frequency. Trying to sync up at 915 is more challenging than at 455 khz, obviously. And for systems like FSK and SSB, you gotta do it somewhere..... And with FM, since the beat formed by the carrier depends on the modulation index of the received signal, getting rid of it can be squirrelly..... You mentioned FM. I have a general question about the Tayloe mixer. Is it possible to receive NBFM or FSK from VHF with it? I'm a little lost in the question how to use the for outgoing phases or I/Q to demodulate FM. All I found on the Net was doing shortwave SSB demodulation. Are there other analog switches going higher than the mentioned 70MHz for FST3253 or 74HC4066? Perhaps video switches? What is better: Tayloe 4-phases or a switched mixer with one output only? In the latter case I would need a conventional IF filter after the switching mixer? If I need a DSP at the baseband doing math with the phases or I/Q I would think of an www.wavefrontsemi.com DSP AL3101/2CG DSP-1K will suffice? Wolud it suffice? It is a little simple DSP mainly for doing FIR - but FAST and with 24-bits including audio AD converters ip to 50KHz. The DSP runs with 50MHz up to 1000 instructions long until it repeats the prog. The nice think is a very low pin-count package and cheap too. There is maybe a middle way with a device like the www.cypress.com PSoC family of mixed-mode Microcontroller with programmable analog cells. There even exists a PSoC app note describing a heterodyne FSK receiver for 130KHz. Maybe a www.microchip.com dsPIC is better? Would it better having the IF not at zero but at a usually higher IF, say 25KHz (remember the NFBM!)? regards - Henry |
Intermediate Frequency
"john" wrote in message
... radio_rookie wrote: Some very good minds have weighed in on your query. I hate to sound "college professor-y", but you really need to be familiar with the literature. One of the challenges with anything on the Internet is that anyone can say anything. That is one of the great advantages, too! The problem is, especially in a technical forum like this, you will get responses from folks who don't have a clue but want to sound knowledgable. What is perhaps surprising, is that sometimes, especially on this NG, some of the truly giant minds in the field will also weigh in. Unless you are familiar with who is who, you will have a tough time assessing the responses. But if you review the books and magazine articles on the subject, and browse the archives of various other forums, you will see that some of the leading experts have posted here. Recognize that they might not sound too different than someone who is simply guessing, and be critical in your analysis of the responses you have received. ... |
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xpyttl wrote:
"john" wrote in message ... radio_rookie wrote: Some very good minds have weighed in on your query. I hate to sound "college professor-y", but you really need to be familiar with the literature. One of the challenges with anything on the Internet is that anyone can say anything. That is one of the great advantages, too! The problem is, especially in a technical forum like this, you will get responses from folks who don't have a clue but want to sound knowledgable. What is perhaps surprising, is that sometimes, especially on this NG, some of the truly giant minds in the field will also weigh in. Unless you are familiar with who is who, you will have a tough time assessing the responses. But if you review the books and magazine articles on the subject, and browse the archives of various other forums, you will see that some of the leading experts have posted here. Recognize that they might not sound too different than someone who is simply guessing, and be critical in your analysis of the responses you have received. .. Your advice is taken. But, look thru the thread: there was reasonable consensus and I recognized some "big hitters". I didn't mean to suggest that one learns best on the NG. Rather, my intent was to support and give kudos to those who bring the NG up to a reasonably high standard: no flames, insults, diversions...you know, the usual stuff. Here are some of my "favorite" NG remarks "Get help" "You have serious issues" "What's YOUR mensa number?" "I spent xx years at xx and xx years building xx and have xx degrees, so I know what I'm talking about" As one monitors these NG's, one begins to "smell" the diff between the knowledgeable and the blow-hards. The only problem comes when they are one and the same. Luckily, this is uncommon. I appreciate your advice, "Professor" :) John AB8O |
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"jawod" wrote in message ...
Your advice is taken. But, look thru the thread: there was reasonable consensus and I recognized some "big hitters". Yes, this particular thread seemed to gather surprisingly few space cadets ... |
Intermediate Frequency
Hamateur wrote:
wrote: On 7 Sep 2006 13:17:19 -0700, "radio_rookie" wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison I can easily agree that an IF amp's job is to cleanly and efficiently amplify a specific, modulated, carrier frequency and to allow for gain control feedback. But I don't see how "selectivity" should be considered a function of an IF amp (other than they're not amplifying what they shouldn't amplify). It seems easier to think of "selectivity" as a property of a tuner or several tuner stages. It's hard for me to think of IF "stages" as improving tuner selectivity when my homebrew 40m DC recvr seems to be selective enough so that when I listen to CW the pitch will not change audibly. The frequency may fluctuate a little, but certainly not enough to loose a signal, and it does not drift monotonically enough to worry about (except perhaps as a matter of pride). this is not selectivity, this is stability. selectivity is filtering nearby strong signals, which direct conversion has more trouble with, especially as they get closer. the IF allows cheap, narrow, lower freq filters, which will have great side skirts to remove the nearby strong signals. The superhet's conversion mixers/filters/amps seem to be considered sub-steps of "IF stages", but I find it easier to think of the mixer/filter steps as "stages of tuners interlaced with IF amplifier stages". ok, then. Since the final conversion step may represent a detection, the idea of "selectivity" as being interlaced with IF amps has a more tidy representation in my mind. Comments, criticisms, corrections, caveats - are always welcome. |
Intermediate Frequency
On Sat, 09 Sep 2006 15:13:15 -0000, Hamateur
wrote: wrote: On 7 Sep 2006 13:17:19 -0700, "radio_rookie" wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison I can easily agree that an IF amp's job is to cleanly and efficiently amplify a specific, modulated, carrier frequency and to allow for gain control feedback. But I don't see how "selectivity" should be considered a function of an IF amp (other than they're not amplifying what they shouldn't amplify). First that last is the central description of what an IF should do. And the word that defines what should or should not be amplified is selectivity (or bandwidth). That come from the former use of distributed selectivity in IF stages, AKA those old IF cans. Since stages were coupled with tuned circuits it was possible to add both gain and selectivity. However in modern designs the IF is preceeded by a crystal filter giving lumped selectivity. In the end the when people talk about an IF, gain, gain control and selectivity are central parameters of that circuit block. It seems easier to think of "selectivity" as a property of a tuner or several tuner stages. Usually image rejection is perfomed there. Selectivity as in 3khz bandwidth would be difficult to do at 50mhz! It's hard for me to think of IF "stages" as improving tuner selectivity when my homebrew 40m DC recvr seems to be selective enough so that when I listen to CW the pitch will not change audibly. The frequency may fluctuate a little, but certainly not enough to loose a signal, and it does not drift monotonically enough to worry about (except perhaps as a matter of pride). DC gets its slectivity at baseband using bandpass or peaking filters. Also if it's not a image reject design it sees images making it's selectivity effectively twice the bandpass filters width. Example of DC at 7.1mhz... if the desired signal is 7.1 and offending signals at 7.101 and 7.099 what do you hear? That is where selectivity is important. Drift is a seperate issue and with care very managable. The superhet's conversion mixers/filters/amps seem to be considered sub-steps of "IF stages", but I find it easier to think of the mixer/filter steps as "stages of tuners interlaced with IF amplifier stages". I'd prefer to not hear that. It muddies the functional description of what the stage does. It is better to think of RF, Mixer, IF and detector as distinct systems with functional goals even though the raw parts used could be very similar. You use "tuners" in ways that are better described with different and more specific terms. For example a tuned circuits at 14.000mhz even with very good Q will be broad compared to the desired signal. In fact it's barely narrow enough if the IF is 455khz to suppress the images (lo at 13.545 and image at 13.090). However, at an IF of 455khz with four tuned circuits of decent Q will give enough selectivity for an AM signal but marginal for close spaced SSB signals. Since the final conversion step may represent a detection, the idea of "selectivity" as being interlaced with IF amps has a more tidy representation in my mind. Valid and very traditional designs were exactly that. However consider lumped gain used with crystal filters. Same effect very different looking. Lumped vs distributed selctivity and the same for gain. Old tube designs would have multiple IF stages at moderate gain with with multiple tuned circuits for selectivity. Current solid state would use a ceramic or crystal filter with lumped gain in the form of an IC or two following. Both could be designed to provide the exact same gain and slectivity profiles yet their topology is different. In the we can use the same terms to talk about both as black boxes but differing terms when discussing the content. Allison |
Intermediate Frequency
Hamateur wrote:
wrote: On 7 Sep 2006 13:17:19 -0700, "radio_rookie" wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison I can easily agree that an IF amp's job is to cleanly and efficiently amplify a specific, modulated, carrier frequency and to allow for gain control feedback. But I don't see how "selectivity" should be considered a function of an IF amp (other than they're not amplifying what they shouldn't amplify). It seems easier to think of "selectivity" as a property of a tuner or several tuner stages. It's hard for me to think of IF "stages" as improving tuner selectivity when my homebrew 40m DC recvr seems to be selective enough so that when I listen to CW the pitch will not change audibly. The frequency may fluctuate a little, but certainly not enough to loose a signal, and it does not drift monotonically enough to worry about (except perhaps as a matter of pride). this is not selectivity, this is stability. selectivity is filtering nearby strong signals, which direct conversion has more trouble with, especially as they get closer. the IF allows cheap, narrow, lower freq filters, which will have great side skirts to remove the nearby strong signals. I tend to think of "stability" more in terms of random fluctuations. Instability may or may not effect selectivity. As long as my desired selection remains decipherable, I would say that selectivity has been accomplished regardless of whether there's any kind of instabilty. Filtering strong nearby signals seems more about "exclusivity" than "selectivity". I would rather say that IF stages maintain selectivity while they are excluding undesired mixing products and other signals. As long as a recvr includes my desired frequency and it does not drift out of my receiver's bandwidth requiring me to retune, then I would say that the recvr is maintaining selectivity even if I get *more* than what I want. The superhet's conversion mixers/filters/amps seem to be considered sub-steps of "IF stages", but I find it easier to think of the mixer/filter steps as "stages of tuners interlaced with IF amplifier stages". ok, then. Since the final conversion step may represent a detection, the idea of "selectivity" as being interlaced with IF amps has a more tidy representation in my mind. Comments, criticisms, corrections, caveats - are always welcome. |
Intermediate Frequency
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Intermediate Frequency
ok, call these things whatever you want, but you are making up new names
for existing specs. I have never seen an 'exclusivity' db rating for a radio, and your desired signal remaining in the passband has little to do with selectivity: try listening to a weak signal next to a SW broadcaster, then you find selectivity. It is invisible until you need it. end Hamateur wrote: Hamateur wrote: wrote: On 7 Sep 2006 13:17:19 -0700, "radio_rookie" wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison I can easily agree that an IF amp's job is to cleanly and efficiently amplify a specific, modulated, carrier frequency and to allow for gain control feedback. But I don't see how "selectivity" should be considered a function of an IF amp (other than they're not amplifying what they shouldn't amplify). It seems easier to think of "selectivity" as a property of a tuner or several tuner stages. It's hard for me to think of IF "stages" as improving tuner selectivity when my homebrew 40m DC recvr seems to be selective enough so that when I listen to CW the pitch will not change audibly. The frequency may fluctuate a little, but certainly not enough to loose a signal, and it does not drift monotonically enough to worry about (except perhaps as a matter of pride). this is not selectivity, this is stability. selectivity is filtering nearby strong signals, which direct conversion has more trouble with, especially as they get closer. the IF allows cheap, narrow, lower freq filters, which will have great side skirts to remove the nearby strong signals. I tend to think of "stability" more in terms of random fluctuations. Instability may or may not effect selectivity. As long as my desired selection remains decipherable, I would say that selectivity has been accomplished regardless of whether there's any kind of instabilty. Filtering strong nearby signals seems more about "exclusivity" than "selectivity". I would rather say that IF stages maintain selectivity while they are excluding undesired mixing products and other signals. As long as a recvr includes my desired frequency and it does not drift out of my receiver's bandwidth requiring me to retune, then I would say that the recvr is maintaining selectivity even if I get *more* than what I want. The superhet's conversion mixers/filters/amps seem to be considered sub-steps of "IF stages", but I find it easier to think of the mixer/filter steps as "stages of tuners interlaced with IF amplifier stages". ok, then. Since the final conversion step may represent a detection, the idea of "selectivity" as being interlaced with IF amps has a more tidy representation in my mind. Comments, criticisms, corrections, caveats - are always welcome. |
Intermediate Frequency
Hamateur wrote:
. . . I tend to think of "stability" more in terms of random fluctuations. Instability may or may not effect selectivity. As long as my desired selection remains decipherable, I would say that selectivity has been accomplished regardless of whether there's any kind of instabilty. Filtering strong nearby signals seems more about "exclusivity" than "selectivity". I would rather say that IF stages maintain selectivity while they are excluding undesired mixing products and other signals. . . . You're certainly free to make up interpretations of words any way you choose. But if you want to communicate with others, that is, to have them understand what you're saying and for you to understand what they're saying, it's necessary to use common terms in the way they're widely understood to mean. In this context, "selectivity" is universally understood to mean the ability to pass some signals and reject others, on the basis of their frequencies, and is quite independent of stability. If this isn't what you mean by "selectivity", you should use some other word or make up a new one and define it, if your objective is to understand and be understood. In a superhet receiver, most of the selectivity is achieved in the IF stages, for a number of good reasons. One of the reasons is that it prevents off-frequency signals from being amplified to a high level where they can cause intermodulation and other problems. In a direct conversion receiver, all the selectivity (other than relatively broad selectivity from any bandpass filtering ahead of the mixer) is achieved by audio filtering. Properly done, this filtering is near the input of the high gain audio amplifier. Neither is inherently better than the other at the basic job of providing selectivity. Roy Lewallen, W7EL |
Intermediate Frequency
Henry Kiefer wrote: You mentioned FM. I have a general question about the Tayloe mixer. Is it possible to receive NBFM or FSK from VHF with it? I'm a little lost in the question how to use the for outgoing phases or I/Q to demodulate FM. All I found on the Net was doing shortwave SSB demodulation. Provided you implement the Tayloe mixer with sufficient baseband bandwidth (probably about 20 KHz for NBFM), you can mix, say, a 2 meter FM signal to baseband and demodulate it with an audio discriminator (some sort of audio frequency to voltage conversion scheme). Mix the signal to be either exclusively in the upper or lower sideband of the output. So, for example, a NBFM sig at 146.000 MHz mix with the Tayloe mixer set to 145.990 MHz and select the upper sideband. The upper or lower sidebands are, as you probably know, selected by phase shifting and summing circuits following the Tayloe mixer (implemented in software in SDRs). Note that SDRs like the Flex-Radio SDR 1000 do not process incoming signals near 0 Hz anyway, but mix the desired signal centered around about 11 KHz (I think). Are there other analog switches going higher than the mentioned 70MHz for FST3253 or 74HC4066? Perhaps video switches? What is better: Tayloe 4-phases or a switched mixer with one output only? In the latter case I would need a conventional IF filter after the switching mixer? The Tayloe mixer is a passive mixer terminated in large capacitors, and similar performance can be obtained at VHF, UHF and microwaves with two FET ring mixers driven with quadrature signals and also terminated with capacitors (that is, no wideband transformer on the mixer outputs, but capacitors followed by HiZ input differential audio amps). I've done this with the Peregrine Semiconductor FET mixers. If I need a DSP at the baseband doing math with the phases or I/Q I would think of an www.wavefrontsemi.com DSP AL3101/2CG DSP-1K will suffice? Wolud it suffice? It is a little simple DSP mainly for doing FIR - but FAST and with 24-bits including audio AD converters ip to 50KHz. The DSP runs with 50MHz up to 1000 instructions long until it repeats the prog. The nice think is a very low pin-count package and cheap too. There is maybe a middle way with a device like the www.cypress.com PSoC family of mixed-mode Microcontroller with programmable analog cells. There even exists a PSoC app note describing a heterodyne FSK receiver for 130KHz. Maybe a www.microchip.com dsPIC is better? Only familiar with the dsPIC, though the others sound okay. The dsPIC would work, though its 12 bit A/D doesn't give a lot of dynamic range. Plenty for NBFM though, especially since you can run the signal(s) through a limiter first. For NBFM sixteen bit DSP is sufficient. Regards, Glenn |
Intermediate Frequency
On Sat, 09 Sep 2006 19:05:08 -0000, Hamateur
wrote: wrote: On Sat, 09 Sep 2006 15:13:15 -0000, Hamateur wrote: wrote: On 7 Sep 2006 13:17:19 -0700, "radio_rookie" wrote: Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Images of the same signal may be a source of interference. Does the IF stage conditions the incoming signal? Yes, in many ways. The most significant are amplification and selectivity. What are the advantages of the IF stage? Less amplification needed at the recieved frequency. Gain at a frequency removed from the recieved frequency. Selectivity is easier to obtain at lower frequencies. Gain control can be applied if needed. Allison I can easily agree that an IF amp's job is to cleanly and efficiently amplify a specific, modulated, carrier frequency and to allow for gain control feedback. But I don't see how "selectivity" should be considered a function of an IF amp (other than they're not amplifying what they shouldn't amplify). First that last is the central description of what an IF should do. And the word that defines what should or should not be amplified is selectivity (or bandwidth). That come from the former use of distributed selectivity in IF stages, AKA those old IF cans. Since stages were coupled with tuned circuits it was possible to add both gain and selectivity. However in modern designs the IF is preceeded by a crystal filter giving lumped selectivity. In the end the when people talk about an IF, gain, gain control and selectivity are central parameters of that circuit block. It seems easier to think of "selectivity" as a property of a tuner or several tuner stages. Usually image rejection is perfomed there. Selectivity as in 3khz bandwidth would be difficult to do at 50mhz! It's hard for me to think of IF "stages" as improving tuner selectivity when my homebrew 40m DC recvr seems to be selective enough so that when I listen to CW the pitch will not change audibly. The frequency may fluctuate a little, but certainly not enough to loose a signal, and it does not drift monotonically enough to worry about (except perhaps as a matter of pride). DC gets its slectivity at baseband using bandpass or peaking filters. Also if it's not a image reject design it sees images making it's selectivity effectively twice the bandpass filters width. Example of DC at 7.1mhz... if the desired signal is 7.1 and offending signals at 7.101 and 7.099 what do you hear? That is where selectivity is important. Drift is a seperate issue and with care very managable. I would say as long as the desired baseband signal remains within the received bandwidth, selectivity has been accomplished. Your misapplying standard terms to describe RF system behavour. I agree DC receivers tend to have poor resolution, but this Again, if anything ther is no reolution issue unless you applying it to the frequency dial/display being used to tune in a signal. An example of poor resolution would be a dial that reads to the nearest Khz when you need to read to the nearest .01khz (10 cycles). cannot be corrected by filtering the baseband signal by sending it through a parametric audio equalizer. Any selectivity of basebands has to be accomplished before detection. I'm sure that's not what you meant- but what you said could be interpreted that way. If I wanted 7.101 I wouldn't detect first and then try to filter out 7.101 and 7.099. Assume a DC RX. Lo at 7.100 for a CW tone of 1khz what frequency is the recieved signal? It could be 7.101 or 7.099! I agree some DC receivers seem like Michaelangelo trying to scuplt David with a sledge hammer. You may still receive the message but it will be impressionistic and so contain many other messages. Not at all and bad example at best. The superhet's conversion mixers/filters/amps seem to be considered sub-steps of "IF stages", but I find it easier to think of the mixer/filter steps as "stages of tuners interlaced with IF amplifier stages". I'd prefer to not hear that. It muddies the functional description of what the stage does. It is better to think of RF, Mixer, IF and detector as distinct systems with functional goals even though the raw parts used could be very similar. You use "tuners" in ways that are better described with different and more specific terms. For example a tuned circuits at 14.000mhz even with very good Q will be broad compared to the desired signal. In fact it's barely narrow enough if the IF is 455khz to suppress the images (lo at 13.545 and image at 13.090). However, at an IF of 455khz with four tuned circuits of decent Q will give enough selectivity for an AM signal but marginal for close spaced SSB signals. Q loses meaning when the desired frequency does not lie within the relevant bandwidth. A tuned component can have a very high Q and yet be very totally unselective of a desired frequency. You do not understand what Q means then. A tuned component can have a very high Q and yet be very totally unselective of a desired frequency. Meaningless misstatement! A tuned component can have a very high Q and yet be insufficiently selective of a desired frequency. Would be a correct application. What you are talking about is not what I would call selectivity. I would call it "exclusivity" since it is more about excluding than about selecting. I can acknowledge that if the exclusions aren't done correctly at any point in the chain then selectivity could be be lost. Selectivity is measured in bandwidth and DB. These terms are standard and meaningful. Exclusivity is marking hype at best and never applied when refering to selectivity. For me the difference between selectivity and exclusivity seem alot like the difference between accuracy and precision. I can be very precise but inaccurate at the same time, I can be very accurate but imprecise at the same time. So I find it easier to think that IF stages are more about precision (exclusivity) than about accuracy (selectivity). Get a dictionary. But I quibble and realize "selectivity" is often used to mean both accuracy and precision. It just seems to me that IF stages are more about precision than about accuracy. It means neither. Precision is tied to resolution as a concept. Acccuracy is a matter of calibration or using the same scale. Selectivity is a matter of what is in or out and the measurements for radios includes a in or out by how much. For example a filter with 3khz bandwidth at 6db down with a shape factor (usually measured at 6 and 60db on the slopes) 2:1 is 6khz wide at -60db. A filter that is 2 khz wide at 6db down with a shape factor of 3 is also 6khz wide at -60db. However, they will not sound the same in a given radio nor will the rejection of undesired signals be the same. This is one of the metrics of how radios are specified and discussed. To do so any other way is like specifing the speed of you car in furlongs per fortnight. A lack of accuracy in language will alway reduce the precision in the discussion. Allison |
Intermediate Frequency
i3hev, mario held wrote:
Dave Platt wrote: As I understand it, in order to do SSB via direct conversion, you must use a modern version of the old IQ phasing technique. ... that's correct :) the problem here is that you need a phasing filter offering a constant (and precise) 90 degrees phase shift all over the receiver band, and that can be tricky to do... These days, of course, you can apply the phase shift by converting the two baseband signals to digital format... well, you're opening doors to digital radio :) But, AFAIK, the actually attainable dynamics does not seem to incite enthusiastic greetings... ;) IQ phasing detection (AKA image reject mixers) are only necessary if you want to build a single signal receiver (a good idea). Many modern DDS chips provide a way to generate perfect quaditure outputs and by using DSP you can combine the resulting quaditure af outputs into a single signal. One way to do this on the cheap is to have a pc sound card sample the two signals. |
Intermediate Frequency
Thanks for responding Glenn!
Provided you implement the Tayloe mixer with sufficient baseband bandwidth (probably about 20 KHz for NBFM), you can mix, say, a 2 meter FM signal to baseband and demodulate it with an audio discriminator (some sort of audio frequency to voltage conversion scheme). Mix the signal to be either exclusively in the upper or lower sideband of the output. So, for example, a NBFM sig at 146.000 MHz mix with the Tayloe mixer set to 145.990 MHz and select the upper sideband. The upper or lower sidebands are, as you probably know, selected by phase shifting and summing circuits following the Tayloe mixer (implemented in software in SDRs). Note that SDRs like the Flex-Radio SDR 1000 do not process incoming signals near 0 Hz anyway, but mix the desired signal centered around about 11 KHz (I think). So the phase shifter is NOT for flatten the group delay variance (source is the Tayloe mixer low-pass)? Is multiplying I with Q enougth to demodulate FM as in a quadrature demodulator? I cannot find a suitable theory page to look for. The Tayloe mixer is a passive mixer terminated in large capacitors, and similar performance can be obtained at VHF, UHF and microwaves with two FET ring mixers driven with quadrature signals and also terminated with capacitors (that is, no wideband transformer on the mixer outputs, but capacitors followed by HiZ input differential audio amps). I've done this with the Peregrine Semiconductor FET mixers. Thank you for given the link to Peregrine. I read the datasheet. How do you mix it? At the moment I prefer the Tayloe mixer because of it's simplicity. BTW: They have a nice low-power consumption PLL being compatible to National. At TI I found nice tinylogic capable of switching like a 4066 down to 500ps .... Should be possible to run the mixer at 150MHz with it. Only familiar with the dsPIC, though the others sound okay. The dsPIC would work, though its 12 bit A/D doesn't give a lot of dynamic range. Plenty for NBFM though, especially since you can run the signal(s) through a limiter first. For NBFM sixteen bit DSP is sufficient. How much dynamic range do I need? I thought about 100dB? If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy. regards - Henry |
Intermediate Frequency
Henry Kiefer wrote: So the phase shifter is NOT for flatten the group delay variance (source is the Tayloe mixer low-pass)? If I understand your question correctly, that is true. The IQ output from the Tayloe mixer (or any IQ mixer) is not upper and lower sideband. To get those you must do further signal processing, which usually involves shifting the I and Q channels ninety degress with respect to each other then summing or subtracting the channels depending on which sideband you want. Is multiplying I with Q enougth to demodulate FM as in a quadrature demodulator? I cannot find a suitable theory page to look for. I can't either. The IQ signals multiplied do not make an FM quadrature detector as the phase does not really shift much over the NBFM bandwidth. There may be a clever way to extract FM more direcly from the IQ channels without first detecting a sideband, but I don't know it. Thank you for given the link to Peregrine. I read the datasheet. How do you mix it? Maybe the wrong part? I'm talking about the PE4140 FET ring mixer. You use two, driven with LOs ninety degress out of phase with respect to each other. At the moment I prefer the Tayloe mixer because of it's simplicity. BTW: They have a nice low-power consumption PLL being compatible to National. At TI I found nice tinylogic capable of switching like a 4066 down to 500ps ... Should be possible to run the mixer at 150MHz with it. Yeah--digital just keeps getting better. Good luck. How much dynamic range do I need? I thought about 100dB? If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.techniques, You need good dynamic range up to the limiter. This can be done with analog circuits as discussed. After the limiter an A/D converter is not even needed in theory--a fast-running timer hooked to a digital port could do the trick. I don't remember any loss by removing AM in the limiter (since all the the information is contained in the frequency of the signal), but my theory is in the distant past, back when FM meant 'funny math'. Regards, Glenn |
Intermediate Frequency
On 12 Sep 2006 13:34:58 -0700, "MadEngineer"
wrote: Henry Kiefer wrote: So the phase shifter is NOT for flatten the group delay variance (source is the Tayloe mixer low-pass)? If I understand your question correctly, that is true. The IQ output from the Tayloe mixer (or any IQ mixer) is not upper and lower sideband. To get those you must do further signal processing, which usually involves shifting the I and Q channels ninety degress with respect to each other then summing or subtracting the channels depending on which sideband you want. Is multiplying I with Q enougth to demodulate FM as in a quadrature demodulator? I cannot find a suitable theory page to look for. I can't either. The IQ signals multiplied do not make an FM quadrature detector as the phase does not really shift much over the NBFM bandwidth. There may be a clever way to extract FM more direcly from the IQ channels without first detecting a sideband, but I don't know it. FM the problem is not responding to amplitude changes but frequency changes. So any system that can count and measure frequency and render a pattern based on changing frequency. Synthetic PLL or simple a software PLL. would do it. The synthethetic PLL approach means a software oscillator locked to a varying frequency external signal. The error word generated is the demodulated signal (apply to D/A or use raw). How much dynamic range do I need? I thought about 100dB? If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.techniques, Dynamic range is one of those the more the better but, many things like noise eat away at it. These days a radio with 80db is good and 90DB excellent, 100 Db is attainable. You need good dynamic range up to the limiter. You need good dynamic range up to the first selectivity that can remove offending close in signals. Then you can limit hard. If you limit before or without adaquate selectivity you will have intermodulation problems. This can be done with analog circuits as discussed. After the limiter an A/D converter is not even needed in theory--a fast-running timer hooked to a digital port could do the trick. I don't remember any loss by removing AM in the limiter (since all the the information is contained in the frequency of the signal), but my theory is in the distant past, back when FM meant 'funny math'. If the signal is limited then zero crossings are enough. That could be expressed as 1bit. Your now working in the time/frequency domain. Allison |
Intermediate Frequency
radio_rookie wrote:
Hello, I want to know the importance of intermediate frequency in any receivers. IF was used in Superhet transceivers. My question is why doesn't anyone use zero IF now a days. What is the problem of brining the RF signal directly to baseband? Does the IF stage conditions the incoming signal? What are the advantages of the IF stage? Just confused. Can anyone throw some light on this? Thanks. Direct conversion is used in nearly all modern mobile phones, because it is cheaper (no IF filters), and because the baseband amplifiers use less current than the old IF amplifiers used to. The RF performance is not necessarily as good as a well designed superhet. It is difficult to make the receiver immune to "AM detection" which is a problem caused by receiving a strong interfering signal with amplitude modulation on it, at a frequency other than the one that you are trying to receive, but which somehow gets turned into a baseband frequency signal coming out of the mixer, even though it shouldn't. There are plenty of reasons why this can happen, such as second-order nonlinearity in the mixer, meaning that a strong interferer coming into the RF port of the receiver can mix with itself in your mixer and end up on top of the wanted signal. Another cause could be if there is coupling between the LO generation circuit and the RF input (in either direction, both are bad.) Also it is common to get large DC offsets coming out of direct conversion receivers, and for some modulation formats where you're interested in frequencies down to DC, that can be a pain. People have pretty much solved these problems in phones, after a lot of work. Chris |
Intermediate Frequency
Hi Glen -
Is multiplying I with Q enougth to demodulate FM as in a quadrature demodulator? The IQ signals multiplied do not make an FM quadrature detector as the phase does not really shift much over the NBFM bandwidth. There may be a clever way to extract FM more direcly from the IQ channels without first detecting a sideband, but I don't know it. direct way: I read about something like: phase = (I * diff Q + Q * diff I)/(I*I + Q*Q) but I cannot remember it exactly. Thank you for given the link to Peregrine. I read the datasheet. How do you mix it? Maybe the wrong part? I'm talking about the PE4140 FET ring mixer. You use two, driven with LOs ninety degress out of phase with respect to each other. Yes, the correct part! But I cannot see a great benefit using your concept. What is better than others? How much dynamic range do I need? I thought about 100dB? If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.techniques, You need good dynamic range up to the limiter. This can be done with analog circuits as discussed. After the limiter an A/D converter is not even needed in theory--a fast-running timer hooked to a digital port could do the trick. I don't remember any loss by removing AM in the limiter (since all the the information is contained in the frequency of the signal), but my theory is in the distant past, back when FM meant 'funny math'. Everywhere I read about the necessity to remove AM. If the band is used for FM only, why then remove the not found AM in it? I ran a simulation with Spice doing FM demodulator concepts comparision. The difference between limited and not limited FM product detector was not of significance. Doing the correct low-pass filtering after the detector was of much higher importance. - Henry |
Intermediate Frequency
FM the problem is not responding to amplitude changes but frequency
changes. So any system that can count and measure frequency and render a pattern based on changing frequency. Synthetic PLL or simple a software PLL. would do it. The synthethetic PLL approach means a software oscillator locked to a varying frequency external signal. The error word generated is the demodulated signal (apply to D/A or use raw). A digital detector will have problems if the SNR is to low! If the SNR is high enought, the all-digital system is just simpler to realize and consumes lower power. You need good dynamic range up to the first selectivity that can remove offending close in signals. Then you can limit hard. If you limit before or without adaquate selectivity you will have intermodulation problems. Superposition prinzip. |
Intermediate Frequency
"Henry Kiefer" ) writes:
Everywhere I read about the necessity to remove AM. If the band is used for FM only, why then remove the not found AM in it? The limiting has nothing to do with receiving FM, it has everything to do with getting rid of QRN. Armstrong pursued FM because he wanted some system that was more noise free than AM. Since FM does not have an amplitude componenet, that means you can limit to clear out the QRN and some of the fading. The limiting is what makes FM so appealling. Take out the limiter and the FM demodulator will still work fine, but there'll be little point in switching to FM. Michael VE2BVW |
Intermediate Frequency
"Michael Black" schrieb im Newsbeitrag ... "Henry Kiefer" ) writes: Everywhere I read about the necessity to remove AM. If the band is used for FM only, why then remove the not found AM in it? The limiting has nothing to do with receiving FM, it has everything to do with getting rid of QRN. Armstrong pursued FM because he wanted some system that was more noise free than AM. Since FM does not have an amplitude componenet, that means you can limit to clear out the QRN and some of the fading. The limiting is what makes FM so appealling. Take out the limiter and the FM demodulator will still work fine, but there'll be little point in switching to FM. OK Michael. So it is for fight against QRN and fading, not primary for being immune to AM. As my Spice simulations suggest; the FM demodulator works almost completely comparable in analog (=not limiting) and digital (=limiting) mode. What about the performance comparision of PLL demod and quadrature demod concepts? Seems no much difference? regards - Henry |
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