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Interesting article on fading distortion
http://www.kongsfjord.no/dl/dl.htm
Dallas Lankford has done some serious research on the cause and cure of/for the distortion caussed by ionospheric "hops". All of his PDFs are informative, but his "Elliptic Low Pass Audio Filters" series are must reads. His conclussions in his "Elliptic Low Pass Audio Filters (Amplified) - Simplified and improved", 29-MAY-2006, are downright fascinating. I have been playing with his design and a couple of steep cut off 3600Hz filters. I don't have the tools to duplicate his research. I knew from the "get go" that my remote/weak signal source with which I use to test detector, and to a lesser degree, antenna/feedline combination suffered from the major weakness that there was no multipath effects. All of my experiments were local ground wave and I couldn't, and still can't, "messure" the effects of such fading. I would love to have a Drake R8B, but I was forced to deal with the receivers I do have. I used R390, R392, R2000(modified), R2000(stock),(borrowed)AOR7030, and a DX398 for some simple tests. I am just out of the ground wave for several MW stations and around dawn and dusk I get serious and nasty fading. So for the last few weeks I have been comparing stock, ie non-synchronous, detectors with synchronous detectors, and the addition of a brick wall 3.5KHz low pass fitler. As Mr. Lankford concludes a synch detector is only (and that might ought to be "may") be slightly better then a AF LP good filter. It is rather frustrating to have spent the last 18 months building an outboard synchronous detector to find that a simple LP filter offered so much improvement. Don't get me wrong, a synch detector is a usefull addition, but not the end all I had hoped. I will disagree with his use of a simple bipolar 2W AF amp. The one thing I have descerned is that after AF detection, any additional distortion rapidly degrades intelligibility. I found MOSFETs, and vacuum tubes, amps allowed me to understand signals better then 6dB down from a "good" bipolar AF amp. "My" third R2000 was siezed by my wife. I had added a MOSFET amp, redesigned the treble cut to a tone-tilt control and just completed adding switchable 3KHz/4KHz Filters as designed by Mr. Lankford. My wife has been doing some casual SWL for the last few days and agrees the filters are very good for nasty band conditions. Email is abandoned and dead. Terry |
Interesting article on fading distortion
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Interesting article on fading distortion
How can any audio filter make up for severe distortion?
-- Brian Denley http://home.comcast.net/~b.denley/index.html |
Interesting article on fading distortion
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Interesting article on fading distortion
Brian Denley wrote: How can any audio filter make up for severe distortion? -- Brian Denley http://home.comcast.net/~b.denley/index.html ----------------------------------- Please read the pdf at: http://www.kongsfjord.no/dl/Audio/On%20The%20Causes%20And%20Cures%20Of%20Audio%20Dis tortion%20Of%20Received%20AM%20Signals%20Due%20To% 20Fading%20II.pdf Lots of nifty formulae and even has FFT trasform screen captures to show his reasoing. Terry |
Interesting article on fading distortion
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Interesting article on fading distortion
wrote in message ups.com... Brian Denley wrote: How can any audio filter make up for severe distortion? -- Brian Denley http://home.comcast.net/~b.denley/index.html ----------------------------------- Please read the pdf at: http://www.kongsfjord.no/dl/Audio/On...ures%20Of%20Au dio%20Distortion%20Of%20Received%20AM%20Signals%20 Due%20To%20Fading%20II.pdf Lots of nifty formulae and even has FFT trasform screen captures to show his reasoing. Terry Am I reading the nifty formulae wrong? It looks to me like he's deriving the distortion of a diode detector from the modulation index only. My sense of these things says that a 50% modulated signal at a tenth of a volt is going to have much more distortion than a 50% modulated signal at 10 volts. Frank Dresser |
Interesting article on fading distortion
Frank Dresser wrote: Am I reading the nifty formulae wrong? It looks to me like he's deriving the distortion of a diode detector from the modulation index only. My sense of these things says that a 50% modulated signal at a tenth of a volt is going to have much more distortion than a 50% modulated signal at 10 volts. Frank Dresser Very few radios drive the detector with anything near 10V. The R390 and R392 have the highest diode drive voltages I have seen and I think they are less then about 3V. Most modern, IE "solid state", receivers I have measured have less 1V. All that I have seen that use discrete diode detectors as oppossed to ICs, have farily high AF gain stages. I didn't post this as an attemp to claim that "Synchronous detectors" are a hoax, but to offer another viewpoint that is backed up by what appears to be valid engineering to me. ASCII text is not my choice for this arcane topic because of the great difficulty in expressing meaningfull equations. This is merely another tool to be used in trying to receceive fading signals. His filters work much better then I expected. I found that by forward biasing the detector in my R2000 I got a much cleaner, ie lower distortion, signal. This was difficult to manage over very modest temperature changes. A full wave "improved AM detector" gave even better results. http://www.amwindow.org/tech/htm/alowdisdet.htm A synch detector in an outboard detector gave even better results. But the simple improved AM detector with a 4000Hz LP filter is a pretty close match to the synch detector at 1/100 the effort. The above link goes into the math, this link starts with simpler math and may help the none engineers enter the fray. http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page2.html Another unusual but good detector can be seen at: http://www.pan-tex.net/usr/r/receivers/elrpicamdetect.htm Tom Holden's Synch detector group has a link to a very detailed math examination of "detection". I lost the link to that paper so you will have to ask Tom or join his group. And please note Mr. Lankford is not merely slapping a 4000Hz LP AF filter in the audio chain, he is offset tunning, with good narrow IF fitlers, to eliminate one sideband. Terry |
Interesting article on fading distortion
On 27 Jul 2006 17:09:16 -0700, "N9NEO"
wrote: Terry, Frank, et al. And David, what was that comment about the Drake receivers? Do you know what type of filters they use? ''The R8 IF filters hark back to the models 1, 2 and early R4, is they are LC tuned circuits, not crystal filters. Crystal filters have gotten more expensive over the years (is the world running out of quartz, too?), and apparently LC filters are now more cost-effective. Some receiver users claim that LC filters provide better audio response for listening to broadcast stations. From a performance standpoint, I wouldn’t have known the R8 had I.C. filters if I hadn’t read about them in the Owne r 's Manual.'' http://www.dproducts.be/drake_museum/qst-r8.pdf |
Interesting article on fading distortion
N9NEO wrote: Terry, Frank, et al. Yes, an interesting article. Thanks for bringing it up to the front burner again. I don't understand his representation of the fading term as it has no link to the actual fading frequency. I would think the distortion would have to be created by the fade. The only frequencies discussed are w_carrier, and w_audio. Also his depiction of instantaneous fourier spectrum at the point of deepest fade leaves me guessing. I would have liked to see him strobe the fade in and out at a fixed frequency and see the spectrum output of that. All in all a good article. I read it a few weeks ago briefly, but haven't had the time to get into it. I'll for sure make the time knowing there might be some meaningfull discussion here. Usually the threads degenerate at warp speed. My boss just bought a smokin nice spectrum analyzer that I might be able to share with him. And David, what was that comment about the Drake receivers? Do you know what type of filters they use? regards, Bob N9NEO One big problem with recreating real world HF propagation is the random nature of multipath. Lankford's sweeping RF notch recreates a simple single fade, but can't produce the multiple "comb filter" like effect I have noticed. I built a simple HiFer "beacon" that allows me to check a "real world" RF signal for just how much SN effected intelligibility. I found that many non obvious things had a big effect. One of the simplest source of nastiness is the post detection AF chain. I had doubted that minor things like the type of capacitor could "really make any diference". My whole goal was to improve my best radio as much as I could for as little money as possible. A synch detector is one improvement. But even though a "good" synch detector with a phasing fitler is supposed to reject non signal out of band signals. see :http://home.worldnet.att.net/~wa1sov/technical/sync_det.html It does help, in some conditions. I found the biggest jump in intelligibility for the least money came from an "Improved AM detector". See: http://www.amwindow.org/tech/htm/alowdisdet.htm To qoute "There are no intermodulation terms to contend with. In fact, functionaly, this is identical to synchronous detection. The negative half wave signal is derived in a similar fashion." For clear signals, with no or minimal fading this detector is cleaner then any other detector I have played with. With this detector, a decent 4KHz IF filter, and using offset tuning on AM with a 4KHZ AF LP fitler, the result is very close in performance a Synch detector. If you wish to follow my "saga", please look up the various threads from last summer. IF I were doing this project over I would likely go with the better Kiwa fitler module and the improved AM detector. Along with an improved AF chain. My wife's R2000 has a 4HKZ mechanical fiter in the AM-N/SSB slot, the improved AM detector and a home built 2W MOSFET audio chain. And she loves it. Terry |
Interesting article on fading distortion
wrote in message oups.com... Frank Dresser wrote: Am I reading the nifty formulae wrong? It looks to me like he's deriving the distortion of a diode detector from the modulation index only. My sense of these things says that a 50% modulated signal at a tenth of a volt is going to have much more distortion than a 50% modulated signal at 10 volts. Frank Dresser Very few radios drive the detector with anything near 10V. The R390 and R392 have the highest diode drive voltages I have seen and I think they are less then about 3V. The range is extreme, but not outlandish. Most modern, IE "solid state", receivers I have measured have less 1V. All that I have seen that use discrete diode detectors as oppossed to ICs, have farily high AF gain stages. But I'd expect considerably less distortion at 3V rather than 1V. And I'd also expect that no radio really uses a square law detector to detect the audio. Real detectors try to linerize a diode's operation by lightly loading the detector with a reletively high resistance and trying to minimize operation in the diode's "square law" area. Both voltage and AC/DC impedance are important considerations in determing diode audio detector distortion. I suspect the term "square law detector" is the same sort of term as "first detector" -- what's now known as a mixer. I know I've been tripped up by these archaic terms before. Frank Dresser |
Interesting article on fading distortion
In article
, "Frank Dresser" wrote: wrote in message oups.com... Frank Dresser wrote: Am I reading the nifty formulae wrong? It looks to me like he's deriving the distortion of a diode detector from the modulation index only. My sense of these things says that a 50% modulated signal at a tenth of a volt is going to have much more distortion than a 50% modulated signal at 10 volts. Frank Dresser Very few radios drive the detector with anything near 10V. The R390 and R392 have the highest diode drive voltages I have seen and I think they are less then about 3V. The range is extreme, but not outlandish. Most modern, IE "solid state", receivers I have measured have less 1V. All that I have seen that use discrete diode detectors as oppossed to ICs, have farily high AF gain stages. But I'd expect considerably less distortion at 3V rather than 1V. And I'd also expect that no radio really uses a square law detector to detect the audio. Real detectors try to linerize a diode's operation by lightly loading the detector with a reletively high resistance and trying to minimize operation in the diode's "square law" area. Both voltage and AC/DC impedance are important considerations in determing diode audio detector distortion. I suspect the term "square law detector" is the same sort of term as "first detector" -- what's now known as a mixer. I know I've been tripped up by these archaic terms before. I'm not a radio circuit designer but detectors circuits are designed for a certain situation and will not produce the expected output if the expected input conditions do not exist. All RF carrier and sidebands (tones) are an alternating wave forms. To recover the AM modulated information the sideband tones are rectified and averaged, which is the low frequency audio modulation. The sideband tones are usually much lower than the carrier but the detector rectifies all of these signals. For the detector design a minimum signal level is required for it to rectify the side band tones and the designs have depended on the carrier to be there so that the detector is switching on and off into the liner region of the diode. If the carrier is not there then the sideband tone signal is switching the diode on and off resulting in a lot of distortion. The sync detection uses a PLL circuit to lock a local oscillator to the received carrier and that is summed with the received carrier and side band tones so that when the received carrier disappears due to selective fading the locked local oscillator signal is enough to keep the detector operating in its liner region with just the side band tones present. The same thing happens using a BFO or when you switch to SSB mode on a radio but here the local oscillator is not locked to the received carrier and you have to tune the radio very carefully to get it spot on the received carrier frequency so the side tones are reproduced at the original modulation audio frequencies. Before sync detection circuit designers would use diodes with smaller non-liner switching regions using germanium for example with lower forward voltages. These diodes would need less signal power to turn on and off into the liner region of it operating curves so less energy from the carrier would be needed to keep the detector in its liner region. This is a help when the received carrier only fades a little but does not help if fades a lot or disappears. Some detector designs would use a DC bias on the diode to put it on the edge of its liner region to improve its small signal sensitivity. The optimum bias voltage will depend on the diode characteristics. -- Telamon Ventura, California |
Interesting article on fading distortion
David wrote: On 26 Jul 2006 23:00:58 -0700, wrote: It is an interesting idea, but nobody builds LCR filters. Rather, you use the LCR filter as a prototype, then build a leapfrog active filter from signal flow graphs based on the physical LCR filter. Really? What kind of filters does the Drake R8 series use? As a demod filter? I would image a low order active filter to clean things up. Remember, this is the audio band, not RF. I've seen some write ups on 455khz IFs being done with active filters. Kiwa sells an active filter for 455Khz http://www.kiwa.com/kiwa455.html Note the AR7030 has "tone controls", so certainly it has an active filter past the demod. The problem with building LCR filters in the audio band is they are bulky, not to mention often inaccurate. With active filters, you have more flexibility over component values. |
Interesting article on fading distortion
wrote: As a demod filter? I would image a low order active filter to clean things up. Remember, this is the audio band, not RF. I've seen some write ups on 455khz IFs being done with active filters. Kiwa sells an active filter for 455Khz http://www.kiwa.com/kiwa455.html Note the AR7030 has "tone controls", so certainly it has an active filter past the demod. The problem with building LCR filters in the audio band is they are bulky, not to mention often inaccurate. With active filters, you have more flexibility over component values. For an example of what I consider to be a very usefull filter, please look at: http://members.tripod.com/roymal/ReverbTone.htm With minor component value chages, it is easy to get more or less cut/booast and/or frequency range. Please note that I had nothing to do with this page, I found it usefull. Terry |
Interesting article on fading distortion
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Interesting article on fading distortion
"Carter, k8vt" wrote: wrote: Dallas Lankford has done some serious research on the cause and cure of/for the distortion caussed by ionospheric "hops". Interesting! In the RTTY world, this is known as selective fading. Gee! In the SWL'ing world, this is known as selective fading. Who woulda thunk it. It's multipath distortion. dxAce Michigan USA |
Interesting article on fading distortion
Well, it works.
I have been playing with the ELPAF since last autumn; first on my R-390A, which, despite having done the AF Deck mod, does have its quirks with regard to audio quality. The ELPAF cleaned up audio admirably. Mostly doing MW DX then. Then, this summer together with a modified IC-703 mostly on SW. It practically eliminates the distortion caused by fades, as well as high-frequency hiss and noise giving an audibly better signal to noise ratio. The trade-off is of course a more limited audio response. Personally I can live with that - I never use bandwidths wider than 6-7 kHz anyway. My ELPAF has a bypass switch so it is easy to compare audio quality. I used to have an SE-3 as well, and enjoyed the excellent audio it produced. The ELPAF does little less with regard to audio recovery. I am thinking about doing an A-B comparison between the two later on. BM |
Interesting article on fading distortion
"Telamon" wrote in message ... [snip] Some detector designs would use a DC bias on the diode to put it on the edge of its liner region to improve its small signal sensitivity. The optimum bias voltage will depend on the diode characteristics. There's a linear region in the usual model of a semiconductor diode (a fixed voltage drop with a series resistance), but that model is only an approximation. The other model, the square law model, is also just an approximation, although it's supposed to be close enough over small parts of the curve. However, the diode doesn't have to be linear in order to have a fairly linear diode detector circuit. Imagine we have a diode whose forward resistance drops in a square law with the voltage. At 0.1V the forward resistance is 1 meg. At 0.2V the forward resistance is 1K. At .0.3V the forward resistance is 32 ohms. At 0.4V the resistance is 5.6V, and so on. Now, let's put this nonlinear diode in series with a linear load resistance and decide that the circuit is pretty much linear once the diode resistance drops to 10% of the load resistance. Well, it's obvious that diode detector circuits which work into higher resistance loads will linearize themselves at lower voltages than diode detectors which work into lower resistance loads. Below a certain voltage, the diode's non linear characteristics will dominate the detector. Low voltage signals will have much more of their waveform in this funky reigion than high voltage signals, even at the same modulation index. So, as I see it, there's alot more to know about a diode detector's audio distortion than only the modulation index. There's the actual characteristics of the diode, the resistance of the load and the signal voltage the detector is operating at. There's also the RF filtering, which will tend to "sawtooth" the audio a bit, much as the rectifier and capacitor do in a power supply. There's also some resistances/capacitances in the AVC line. But I could be wrong. If so, let me know! Frank Dresser |
Interesting article on fading distortion
wrote: wrote: As a demod filter? I would image a low order active filter to clean things up. Remember, this is the audio band, not RF. I've seen some write ups on 455khz IFs being done with active filters. Kiwa sells an active filter for 455Khz http://www.kiwa.com/kiwa455.html Note the AR7030 has "tone controls", so certainly it has an active filter past the demod. The problem with building LCR filters in the audio band is they are bulky, not to mention often inaccurate. With active filters, you have more flexibility over component values. Drake uses an LC filter in the IF. They "Get away" with it becuase of the lower IF they use. R390s, original not the R390A, and the R392 use several staged of LC filters and have excellent skirts. Except we are not talking about IF filters .The "fading" filter is at the end of the chain, i.e. past the demod. The Kiwa filter you refference is not a "active filter", but a ceramic filter with amplification. To me active filter means opamp or norton amp with feedback to control pass/reject charactoristics. The premium Kiwa unit is nearly as good as a crystal or mechanical filter and MUCH easier to connect. I installed one in a friends R2000 and was impressed by the quality and how well it worked. A big advantage of passive LC filters is they are much less "fussy" then active filters. I like not having to mess with power and proper bypassing. And if you are willing to wind your own torroids, it is pretty easy to get the L very close to what you want. The C can be built with standard value caps in parallel. Except this is at audio frequencies, where the component sizes are much larger. Again, this is not at IF frequencies. The Tone-Tilt filter I used in all 3 of our R2000s is active because it would be VERY difficult to use LC filters effectively. Terry |
Interesting article on fading distortion
In article ,
"Frank Dresser" wrote: "Telamon" wrote in message .com... [snip] Some detector designs would use a DC bias on the diode to put it on the edge of its liner region to improve its small signal sensitivity. The optimum bias voltage will depend on the diode characteristics. There's a linear region in the usual model of a semiconductor diode (a fixed voltage drop with a series resistance), but that model is only an approximation. The other model, the square law model, is also just an approximation, although it's supposed to be close enough over small parts of the curve. However, the diode doesn't have to be linear in order to have a fairly linear diode detector circuit. Imagine we have a diode whose forward resistance drops in a square law with the voltage. At 0.1V the forward resistance is 1 meg. At 0.2V the forward resistance is 1K. At .0.3V the forward resistance is 32 ohms. At 0.4V the resistance is 5.6V, and so on. Now, let's put this nonlinear diode in series with a linear load resistance and decide that the circuit is pretty much linear once the diode resistance drops to 10% of the load resistance. Well, it's obvious that diode detector circuits which work into higher resistance loads will linearize themselves at lower voltages than diode detectors which work into lower resistance loads. Below a certain voltage, the diode's non linear characteristics will dominate the detector. Low voltage signals will have much more of their waveform in this funky reigion than high voltage signals, even at the same modulation index. So, as I see it, there's alot more to know about a diode detector's audio distortion than only the modulation index. There's the actual characteristics of the diode, the resistance of the load and the signal voltage the detector is operating at. There's also the RF filtering, which will tend to "sawtooth" the audio a bit, much as the rectifier and capacitor do in a power supply. There's also some resistances/capacitances in the AVC line. But I could be wrong. If so, let me know! I don't anything wrong with what you wrote but you seem to think that the diode used makes no difference because you can make it up its deficiencies with an amplifier whose input impedance and gain adjusts for it. Basically that is true that you can use a less efficient diode but you will have to provide higher signal levels to it and weak signals will still be distorted due to compression. I suppose you could use a logarithmic type amplifier following the detector in order to make up for the compression. If you look at the diode curves germanium has one of the better forward current to input voltage ratios of several diode types. Not being a radio designer my approach would be to use a diodes fairly liner region with a better forward current to input voltage ratio where the least distortion and compression would be due to it and therefor the least needed correction to be made up for by a amplifier with a fixed correction. Another reason to use a more efficient diode besides the signal level power needed is the power the diode itself burns when you bias the diodes with larger forward voltage junctions. -- Telamon Ventura, California |
Interesting article on fading distortion
"Telamon" wrote in message ... I don't anything wrong with what you wrote but you seem to think that the diode used makes no difference because you can make it up its deficiencies with an amplifier whose input impedance and gain adjusts for it. I don't think we disagree on anything important, but I wanted to say that, after a point, it won't make any practical difference to the distortion of the detector if a diode has a linear region or a very non linear square law region. The resistance of the load soon dominates the characteristiscs of the circuit. The rest of my reply was mostly aimed at the original article's contention that a diodes distortion level can be derived from only from a diode's presumed square law characteristics and the modulation index. Basically that is true that you can use a less efficient diode but you will have to provide higher signal levels to it and weak signals will still be distorted due to compression. I suppose you could use a logarithmic type amplifier following the detector in order to make up for the compression. I suppose, but I don't see any need. The distortion of the diode detector can be quite low if it's driven at a proper level to minimize the the amount of the waveform in the non linear region of the detector. If you look at the diode curves germanium has one of the better forward current to input voltage ratios of several diode types. Right. A germanium diode would generally give less distortion and better sensitivity than a silicon diode. More than that, there used to be a bunch of specialized germanium diodes intended for radio audio detection, video detection and such. It seems now it's 1N34A types. Not being a radio designer my approach would be to use a diodes fairly liner region with a better forward current to input voltage ratio where the least distortion and compression would be due to it and therefor the least needed correction to be made up for by a amplifier with a fixed correction. Another reason to use a more efficient diode besides the signal level power needed is the power the diode itself burns when you bias the diodes with larger forward voltage junctions. Efficiency is a bigger consideration with crystal sets. Frank Dresser |
Interesting article on fading distortion
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Interesting article on fading distortion
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Interesting article on fading distortion
wrote: wrote: snip Except we are not talking about IF filters .The "fading" filter is at the end of the chain, i.e. past the demod. snip Except this is at audio frequencies, where the component sizes are much larger. Again, this is not at IF frequencies. Mr. Lankford's main concept is that by using a narrow enough IF filter, a narrow filter with with a step attenuation skirt, and by offset tunning to only get the carrier and the desired sideband, and with a following suitable AF LP fitler can do wonders. It is not magic, and doesn't work with every receiver and under every condition. In the ret of this, and all future posts, I will simply call it "ELPAF". I can say is that it is a usefull technique, and will even help when used premium receiver like an AOR7030 or R390, the filter can really reduce the effects of "fading". With a modest receiver like the R2000 that has been upgraded with a suitablely narrow IF filter, the results are impressive. With a "marginal" receiver like the ATS909/DX398 the results are nothing short of amazing. Almost sounds like the 'syncho-phase' detector on the R7/A. Use of a narrower filter and tilting the passband control to either USB/LSB. dxAce Michigan USA |
multipath distortion
Sorry for being such a RX newbie here. I guess I shoulda stayed awake
more during signals class. My expertise is very large switching and resonant power supplies and transmitters. So the multipath distortion causes fading of the carrier only?? This makes some sense to me. A small set of the lower sideband frequencies would also cause phase cancellation, but since the audio spectrum is moving around so fast no one notices. I think I'm on the right track here. So use another carrier slaved to the received carrier and you get better reception during fade. Even if it wanders a few cycles during fade you probably don't hear anyway. I guess that is how a sync detector works. I Imagine it would be a chore to build a sync detector from the ground up, but I would also think it must have already been put into an ic chip, no??? I like the link R2000swl posted to AmWindow for the precision full-wave rectifier. I think I'll stick it on a pc board and give it a try. If anybody wants a board let me know. They are very inexpensive. bm, or anyone else, if you have good link to ELPAF or alternative RX circuits then maybe I could throw that down on same board if not too much room. Details of board size at www.expresspcb.com I do the small one double side and no silkscreen 3 boiards 60bucks. You can contact me off board at 73 Bob N9NEO bm wrote: Well, it works. I have been playing with the ELPAF since last autumn; first on my R-390A, which, despite having done the AF Deck mod, does have its quirks with regard to audio quality. The ELPAF cleaned up audio admirably. Mostly doing MW DX then. Then, this summer together with a modified IC-703 mostly on SW. It practically eliminates the distortion caused by fades, as well as high-frequency hiss and noise giving an audibly better signal to noise ratio. The trade-off is of course a more limited audio response. Personally I can live with that - I never use bandwidths wider than 6-7 kHz anyway. My ELPAF has a bypass switch so it is easy to compare audio quality. I used to have an SE-3 as well, and enjoyed the excellent audio it produced. The ELPAF does little less with regard to audio recovery. I am thinking about doing an A-B comparison between the two later on. BM |
multipath distortion
Back to the Future movie is on Radio tv.See if your radio has a Flux
Capacitor.According to the doc,that's what makes time travel possible. Calling Art Bell,calling George Noory.I found the secret to time travel. cuhulin |
multipath distortion
On 30 Jul 2006 09:28:29 -0700, "N9NEO"
wrote: Sorry for being such a RX newbie here. I guess I shoulda stayed awake more during signals class. My expertise is very large switching and resonant power supplies and transmitters. So the multipath distortion causes fading of the carrier only?? This makes some sense to me. A small set of the lower sideband frequencies would also cause phase cancellation, but since the audio spectrum is moving around so fast no one notices. I think I'm on the right track here. So use another carrier slaved to the received carrier and you get better reception during fade. Even if it wanders a few cycles during fade you probably don't hear anyway. I guess that is how a sync detector works. I Imagine it would be a chore to build a sync detector from the ground up, but I would also think it must have already been put into an ic chip, no??? I like the link R2000swl posted to AmWindow for the precision full-wave rectifier. I think I'll stick it on a pc board and give it a try. If anybody wants a board let me know. They are very inexpensive. bm, or anyone else, if you have good link to ELPAF or alternative RX circuits then maybe I could throw that down on same board if not too much room. Details of board size at www.expresspcb.com I do the small one double side and no silkscreen 3 boiards 60bucks. You can contact me off board at 73 Bob N9NEO http://users.adelphia.net/~alexmm/Prod_det/detector.htm |
multipath distortion
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multipath distortion
David wrote: On 30 Jul 2006 09:28:29 -0700, "N9NEO" wrote: http://home.att.net/~wa1sov/technical/sync_det.html While this synchronous detector works quite well, there is a link http://home.worldnet.att.net/~wa1sov/technical/allpass/allpass.html to a filter that allows improved reception by "rejecting" signals other then the desired signal. While this does improve reception, it isn't quite as effective as some literature would suggest. If you really interested in synch detectors, Tom Holden has an excellent page, http://groups.yahoo.com/group/Synch_AM/messages that has some links to very good resources. Terry |
multipath distortion
N9NEO wrote: So the multipath distortion causes fading of the carrier only?? This makes some sense to me. A small set of the lower sideband frequencies would also cause phase cancellation, but since the audio spectrum is moving around so fast no one notices. I think I'm on the right track No, a simple 2-path is effectively a comb filter with the separation between frequencies of constructive and destructive interference determined by the difference in propagation delay. With enough delay, this separation can be much less than the bandwidth of the desired signal, causing multiple cancellations within the passband. The delay difference is not a constant due to the roiling ionosphere so the frequencies at which destructive interference occurs and their separations are constantly changing. The severest form of distortion is when a cancellation occurs at the carrier frequency but if you have ever heard of 'flanging' in the recording industry, you'll know what the interference effect can be when a cancellation occurs in the sidebands. It is is very noticeable. So use another carrier slaved to the received carrier and you get better reception during fade. Even if it wanders a few cycles during fade you probably don't hear anyway. I guess that is how a sync detector works. That's basically it. Tom |
multipath distortion
Thanks Terry, I just signed up.
regards, Bob wrote: David wrote: On 30 Jul 2006 09:28:29 -0700, "N9NEO" wrote: http://home.att.net/~wa1sov/technical/sync_det.html While this synchronous detector works quite well, there is a link http://home.worldnet.att.net/~wa1sov/technical/allpass/allpass.html to a filter that allows improved reception by "rejecting" signals other then the desired signal. While this does improve reception, it isn't quite as effective as some literature would suggest. If you really interested in synch detectors, Tom Holden has an excellent page, http://groups.yahoo.com/group/Synch_AM/messages that has some links to very good resources. Terry |
Interesting article on fading distortion
In article .com,
wrote: wrote: snip Except we are not talking about IF filters .The "fading" filter is at the end of the chain, i.e. past the demod. snip Except this is at audio frequencies, where the component sizes are much larger. Again, this is not at IF frequencies. Mr. Lankford's main concept is that by using a narrow enough IF filter, a narrow filter with with a step attenuation skirt, and by offset tunning to only get the carrier and the desired sideband, and with a following suitable AF LP fitler can do wonders. It is not magic, and doesn't work with every receiver and under every condition. In the ret of this, and all future posts, I will simply call it "ELPAF". I can say is that it is a usefull technique, and will even help when used premium receiver like an AOR7030 or R390, the filter can really reduce the effects of "fading". With a modest receiver like the R2000 that has been upgraded with a suitablely narrow IF filter, the results are impressive. With a "marginal" receiver like the ATS909/DX398 the results are nothing short of amazing. Any time you narrow the frequency range with a filter you lower the noise floor possibly making the faded carrier large enough so the detector does not greatly distort the audio. -- Telamon Ventura, California |
Interesting article on fading distortion
wrote: wrote: snip Except we are not talking about IF filters .The "fading" filter is at the end of the chain, i.e. past the demod. snip Except this is at audio frequencies, where the component sizes are much larger. Again, this is not at IF frequencies. Mr. Lankford's main concept is that by using a narrow enough IF filter, a narrow filter with with a step attenuation skirt, and by offset tunning to only get the carrier and the desired sideband, and with a following suitable AF LP fitler can do wonders. It is not magic, and doesn't work with every receiver and under every condition. In the ret of this, and all future posts, I will simply call it "ELPAF". My point still holds in that nobody serious builds LCR filters for auido (speaker crossovers exempted). His LCR filter IS in the audio chain, not the IF. I can say is that it is a usefull technique, and will even help when used premium receiver like an AOR7030 or R390, the filter can really reduce the effects of "fading". With a modest receiver like the R2000 that has been upgraded with a suitablely narrow IF filter, the results are impressive. With a "marginal" receiver like the ATS909/DX398 the results are nothing short of amazing. Terry |
Interesting article on fading distortion
wrote: wrote: snip My point still holds in that nobody serious builds LCR filters for auido (speaker crossovers exempted). His LCR filter IS in the audio chain, not the IF. Your comment that "nobody serious" is so off base as to be asiniine. Dallas Lankford is clearly one of most serious DXers alive today. Why not take a look at all the technical information at http://www.kongsfjord.no/dl/dl.htm before jumping to silly conclussions. Terry |
Interesting article on fading distortion
wrote: wrote: wrote: snip My point still holds in that nobody serious builds LCR filters for auido (speaker crossovers exempted). His LCR filter IS in the audio chain, not the IF. Your comment that "nobody serious" is so off base as to be asiniine. Dallas Lankford is clearly one of most serious DXers alive today. Why not take a look at all the technical information at http://www.kongsfjord.no/dl/dl.htm before jumping to silly conclussions. Terry I've done (as in been paid for) filter design in telecom/datacom applications, including elliptic filters, delay equalizers, etc. I know of what I speak. There is a leapfrog design on this page if you want to educate yourself: http://www.filter-solutions.com/active.html |
Interesting article on fading distortion
wrote: I've done (as in been paid for) filter design in telecom/datacom applications, including elliptic filters, delay equalizers, etc. I know of what I speak. There is a leapfrog design on this page if you want to educate yourself: http://www.filter-solutions.com/active.html I give up, you win. Only idoits and fools would bother to build any passive AF fitlers. Please build all the active ELPAF filters your heart desires. I made the mistake of assuming you where a serious SWL. I bet you would find the thought of building a clipper to limit the AF level to a set of earphones with something as simple as a couple of parallel 1N4004 diodes and a series resistor so repulsive as to nearly make yo puke. I can hear it now, "Oh My God, how simple." I bet you could whip up a active limiter that would achieve +/-0.0001dB clipping accuracy. Too bad you make Cuhulin seem reasonable. I can't PLONK you with Google beta, but I damn sure will ignore your posts. Terry |
Interesting article on fading distortion
Hey, is idoit something like an idiot?
Your reaction is bizzare to say the least, but plonk away. wrote: wrote: I've done (as in been paid for) filter design in telecom/datacom applications, including elliptic filters, delay equalizers, etc. I know of what I speak. There is a leapfrog design on this page if you want to educate yourself: http://www.filter-solutions.com/active.html I give up, you win. Only idoits and fools would bother to build any passive AF fitlers. Please build all the active ELPAF filters your heart desires. I made the mistake of assuming you where a serious SWL. I bet you would find the thought of building a clipper to limit the AF level to a set of earphones with something as simple as a couple of parallel 1N4004 diodes and a series resistor so repulsive as to nearly make yo puke. I can hear it now, "Oh My God, how simple." I bet you could whip up a active limiter that would achieve +/-0.0001dB clipping accuracy. Too bad you make Cuhulin seem reasonable. I can't PLONK you with Google beta, but I damn sure will ignore your posts. Terry |
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