|
AGC signal/noise question...
Hello.
I'm wondering about a thing... When the AGC reduces the gain of an amplifier, composed by FETs (DG or SG) or BJT, the SNR remains constant? Or the performance of the amplifier may degrade/upgrade? I'm researching about the matter and I just read that, in a BJT for instance, emitter current is inversely proportional to the noise. So, if AGC reduces the gain (so current), SNR degrade? The question arises from a thing I just noted with a HF receiver... disabling AGC reduces (slightly) the noise (at least in FM reception)... Maybe just be a side effect, like noisy gain control signal... Ciao, AB .... Andrea Baldoni, 2002: messaggio non protetto da copyright. |
AGC signal/noise question...
From: Andrea Baldoni on Thurs, Aug 17 2006 1:34 pm
Hello. I'm wondering about a thing... When the AGC reduces the gain of an amplifier, composed by FETs (DG or SG) or BJT, the SNR remains constant? Maybe, maybe not... Or the performance of the amplifier may degrade/upgrade? Maybe, maybe not... I'm researching about the matter and I just read that, in a BJT for instance, emitter current is inversely proportional to the noise. So, if AGC reduces the gain (so current), SNR degrade? Not necessarily true. Noise, true natural noise, in a bipolar junction transistor is dependent on MANY things besides the emitter current. Foremost of all is the source impedance of the BJT's driving source: SNR will increase on both sides of the measured least-SNR current in some transistors. If you look at the characteristics/datasheet information on several hundred different transistors you will begin to get the idea of how many factors go into noise generation within the transistor. :-) Most complex Gilbert-cell ICs (Motorola MC1590, MC1350 as examples) will control gain by "stealing" current of a differential-input, common-emitter stage. While the example ICs can control gain over a 60 db dynamic range, the worst noise generated inside those bipolar-transistor architecture ICs occurs at minimum gain control input (maximum amplification). At higher and higher input signal levels, there would be more AGC control action but the SNR would also be higher and higher. The question arises from a thing I just noted with a HF receiver... disabling AGC reduces (slightly) the noise (at least in FM reception)... There isn't much FM on HF. What there is would be in narrow-band Data mode signals. Some of that Data is a combination of AM and PM similar to a wireline modem's modulation. Disabling the AGC would put the overall gain of the receiver at maximum. Naturally there would APPEAR to be "more noise" since the receiver would be picking up ALL noise available at the antenna input. Maybe just be a side effect, like noisy gain control signal... That's also possible, but unlikely there in my opinion. What is needed in an investigation of this is a reasonably- well-calibrated signal generator with a calibrated attenuator. On AM you could simply increase the signal generator output until the total signal plus noise reaches a certain level at the detector, usually 10 db above input of just noise alone (common definition of SNR). |
AGC signal/noise question...
Andrea Baldoni wrote:
Hello. I'm wondering about a thing... When the AGC reduces the gain of an amplifier, composed by FETs (DG or SG) or BJT, the SNR remains constant? Or the performance of the amplifier may degrade/upgrade? I'm researching about the matter and I just read that, in a BJT for instance, emitter current is inversely proportional to the noise. So, if AGC reduces the gain (so current), SNR degrade? The question arises from a thing I just noted with a HF receiver... disabling AGC reduces (slightly) the noise (at least in FM reception)... Maybe just be a side effect, like noisy gain control signal... Ciao, AB ... Andrea Baldoni, 2002: messaggio non protetto da copyright. You might also ask yourself whether it really matters or not. If your signal is strong enough to begin driving the AGC to limit the system gain do you really care what the noise level actually is? Once the AGC threshhold is reached, if the signal strength goes up faster than the noise contribution from the system goes up do you really care what the noise level actually is? tim ab0wr |
AGC signal/noise question...
tim gorman wrote:
You might also ask yourself whether it really matters or not. If your signal is strong enough to begin driving the AGC to limit the system gain do you really care what the noise level actually is? .... But you may very well care for the SNR on an interesting weak signal which you are trying to listen to, while a strong nearby signal activates your AGC... :) -- 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 |
AGC signal/noise question...
Regardless of an amplifier's gain, the signal to noise ratio at its
output remains the same as the signal to noise ratio at its input. Obviously - if its a linear amplifier! ---- Reg. |
AGC signal/noise question...
"i3hev, mario held" ) writes:
tim gorman wrote: You might also ask yourself whether it really matters or not. If your signal is strong enough to begin driving the AGC to limit the system gain do you really care what the noise level actually is? .... But you may very well care for the SNR on an interesting weak signal which you are trying to listen to, while a strong nearby signal activates your AGC... :) In that case though, you should be worried about that nearby strong signal dropping the gain of the receiver so you can't hear the weak signal, the fact that the noise level may increase won't matter because the lower gain will make the signal unreceivable anyway. Michael VE2BVW |
AGC signal/noise question...
Michael Black wrote:
In that case though, you should be worried about that nearby strong signal dropping the gain of the receiver so you can't hear the weak signal... may be you are not thinking of cw... :) Provided the SNR is good enough, you can filter and post-amplify your weak signal, e.g. with a good AF filter, or a dsp. Of course, if the IF stages are (reasonably) linear in response, you can disable the AGC, but this would be no answer to the original question ;) -- 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 |
AGC signal/noise question...
Reg Edwards wrote:
Regardless of an amplifier's gain, the signal to noise ratio at its output remains the same as the signal to noise ratio at its input. Obviously - if its a linear amplifier! I'm afraid this is quite wrong :( Your statement would be a correct one if and only if the amplifier is a non-noisy one - which, alas, is not a real case... -- 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 |
AGC signal/noise question...
"i3hev, mario held" wrote Reg Edwards wrote: Regardless of an amplifier's gain, the signal to noise ratio at its output remains the same as the signal to noise ratio at its input. Obviously - if its a linear amplifier! I'm afraid this is quite wrong :( Your statement would be a correct one if and only if the amplifier is a non-noisy one - which, alas, is not a real case... =================================== If an amplifier incorporates a noise generator or also behaves as a filter, then it is no longer just an amplifier. My statement is quite correct. ---- Reg. |
AGC signal/noise question...
Reg Edwards wrote:
If an amplifier incorporates a noise generator... ... then it is no longer just an amplifier. .... .... so, we must conclude that amplifiers do not exist! :) of course, there is no real amplifying device which does not generate noise; or, more precisely, there is no non-noisy real device at all :) If you choose not to call "amplifier" a device which amplifies signals if it has a non-unity noise figure, please feel free to do so - we live in a (at least partially) free world... but you will be alone! :)) Regards! -- 73 es 51 de i3hev, op. mario it.hobby.radioamatori.moderato http://digilander.libero.it/hamweb http://digilander.libero.it/esperantovenezia |
AGC signal/noise question...
i3hev, mario held wrote:
Reg Edwards wrote: If an amplifier incorporates a noise generator... ... then it is no longer just an amplifier. .... ... so, we must conclude that amplifiers do not exist! :) of course, there is no real amplifying device which does not generate noise; or, more precisely, there is no non-noisy real device at all :) If you choose not to call "amplifier" a device which amplifies signals if it has a non-unity noise figure, please feel free to do so - we live in a (at least partially) free world... but you will be alone! :)) Regards! Don't be too hasty. Reg has verified he's got some coax that's much lower loss than anyone else can buy (it actually meets the predictions of his coax loss program), and has verified it by measuring it many times. Maybe he also has a secret source of noise-free amplifiers. You never know. Roy Lewallen, W7EL |
AGC signal/noise question...
|
AGC signal/noise question...
Andrea Baldoni wrote:
. . . 2) build circuits with so high dynamic range that's completely impossible to have input signals overload them (what's the dynamic range one should normally expect at the antenna input, excluding obvious limit-case situations where the transmitting output is fed into the receiver input...?) . . . One night I heard audio in the background when listening to my direct conversion 40 meter receiver. It was designed specifically to be as immune as possible to AM demodulation, and since I had finished its optimization several years before, I hadn't heard any audio from demodulated AM. (It was common when I was using mixers with poorer balance and dynamic range.) It didn't take long to find the station with my home receiver. It was at about 7335 kHz, a religious HF broadcast station in San Francisco (about 600 miles from here). The broadcast was in Russian, so they were evidently beaming to Russia and I wouldn't be far off the main beam. Some careful measurements showed a signal strength of 250 mV RMS at my receiver terminals. (That's 74 dB over the typical S9 value of 50 uV.) I was using a vertical 4-square array, which isn't at all optimum for that path. I hooked the antenna directly to my oscilloscope and could see the carrier and modulation. I took the receiver on a visit to England, and heard the audio from a large number of AM stations in the background, so I believe the signal levels there from HF broadcasters commonly exceeded the 250 mV I saw only once at home. The problem can of course be reduced by use of very narrow filters, but they're often so close to the 40 meter band edges that even that wouldn't be enough in most cases. I've also encountered some staggeringly strong signals when operating Field Day, when a group with a high or even moderate power transmitter is on the next ridge or otherwise very close. The bottom line is that I'd be hesitant to trust just about any number for a "worst case" maximum signal strength. Be sure to test any proposed design on 40 meters for a while from your location in Europe. Roy Lewallen, W7EL |
AGC signal/noise question...
On Fri, 18 Aug 2006 16:33:25 +0200, "i3hev, mario held"
wrote: tim gorman wrote: You might also ask yourself whether it really matters or not. If your signal is strong enough to begin driving the AGC to limit the system gain do you really care what the noise level actually is? .... But you may very well care for the SNR on an interesting weak signal which you are trying to listen to, while a strong nearby signal activates your AGC... :) In theory the system bandwidth should not allow that strong signal to hit the AGC. Of course practical systems this may not be true. However, manual gain control helps if the stronger signal is not overloading the front end causing gain compression and intermodulation. Allison |
AGC signal/noise question...
On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held"
wrote: Michael Black wrote: In that case though, you should be worried about that nearby strong signal dropping the gain of the receiver so you can't hear the weak signal... may be you are not thinking of cw... :) Provided the SNR is good enough, you can filter and post-amplify your weak signal, e.g. with a good AF filter, or a dsp. Of course, if the IF stages are (reasonably) linear in response, you can disable the AGC, but this would be no answer to the original question ;) Since the SNR is established by the frontend the IF system can have a more relaxed SNR. However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison |
AGC signal/noise question...
From: Andrea Baldoni on Sat, Aug 19 2006 4:05 pm
wrote: : I'm researching about the matter and I just read that, in a BJT for : instance, emitter current is inversely proportional to the noise. So, : if AGC reduces the gain (so current), SNR degrade? : Not necessarily true. Noise, true natural noise, in a bipolar : ... : of how many factors go into noise generation within the : transistor. :-) So, if you have to engineer a (let's start with HF) receiver, do you think it may better to: There's no "engineering" involved, just a crunching of numbers AFTER you find the input levels versus AGC and how much noise is actually generated...and approximately WHERE this excess noise is coming from. 1) find a way to insert automatically a stepped attenuation (maybe using a diode switched resistor network) and leaving amplifiers without AGC, thus optimizing them for a particular gain I see no need of that at this point. "Getting fancy" with extra circuitry is rather useless without knowing what the problem all this fancy circuitry is supposed to cure. 2) build circuits with so high dynamic range that's completely impossible to have input signals overload them (what's the dynamic range one should normally expect at the antenna input, excluding obvious limit-case situations where the transmitting output is fed into the receiver input...?) That's NOT the issue here. Noise and signal-to-noise ratios are only important at LOWEST signal levels, not the highest. 3) use the usual AGC Why not? Decades of designs in many countries have successfully operated with "usual AGC." [voltage-controlled, sometimes current-controlled gain stages driven by a DC control line] ...I'm thinking the 1 could be a good solution if the demodulator had to be a digital one. That way, a calibrated attenuator simply add bits to the ADC. Hovewer, the 2 is very attractive, providing that all is analog, or the ADC dynamic range is better than the one that could come from the antenna... Experiment any way you want but I can't see that as your cure. I've read most use the 3, digitizing the AGC signal maybe with a second ADC channel, to have anyway a sort of more bits of resolution. So probably I'm wrong and the right solution is the 3... but only if adding an AGC never ruin amplifiers performance. A rather common (for decades of designs and production) AGC action is no more than 6 db change in output for 60 to 100 db of input signal (carrier) change. AGC should be approached from the standpoint of a servo loop. The "error signal" is the change in carrier level at the detector. The controlled items are the RF and IF amplifiers. The time-constant of the error feedback loop (what is commonly called "the AGC line") is quite slow but fast enough to try to keep detector level constant through flutter (rapid reflections at VHF and up) and ionospheric path variations. If "the AGC line" somehow has some noise in it, that noise is probably going to change RF-IF amplifier gain. However, the frequency of that noise is going to be low; it is band- limited by the usual AGC line decoupling. Let's look at SNR with low to higher antenna input levels: 1. Assume you have (for example) 1 uV of noise at no-signal. 2. If the RF signal is 3.16 uV then the signal-plus-noise to noise ratio is 10 db. 3. If the RF signal is 10 uV then the signal-plus-noise to noise ratio is 20 db. 4. If the RF signal is 31.6 uV then the signal-plus-noise to noise ratio is 30 db. The common (for about 40+ years, internationally) level of receiver sensitivity for AM mode signals is a 10 db signal- plus-noise to noise ratio. That's an easy test, done by connecting an AC voltmeter (that can measure RMS voltage) to the detector output. With no signal input, all you get is front-end noise; note that. Apply a known-level RF source to the antenna input, adjust that level to be 10 db higher than the noise level measured with no signal input. Note the RF source level; that is the "minimum sensitivity" level for the common "10 db S+N:N" criterion. For FM or PM it is a bit more complicated. FM and PM rely on quieting through the Limiter stages ahead of the FM detector. For most tests of FM/PM sensitivity you NEED a known-signal-source-level to determine the quieting. : There isn't much FM on HF. What there is would be in : narrow-band Data mode signals. Some of that Data is a : combination of AM and PM similar to a wireline modem's : modulation. In fact I was receiving 144MHz using a converter. That data was omitted. Have you checked out the converter insofar as adding noise? You can get a rough comparison by using another HF receiver. Have you checked your internal (to HF receiver) FM demodulator characteristics? Do you have the manufacturer's specifications on that? Since nearly all FM/PM demods use Limiters, they normally operate with AGC off. : What is needed in an investigation of this is a reasonably- : well-calibrated signal generator with a calibrated attenuator. Unfortunately I have only a Instek function generator, and I'm not very satisfied with any intrument I bought from this firm... Anyway, sooner or later I'll build a dds one... You can't work in the dark (without instruments) when trying to troubleshoot electronics. A DDS (Direct Digital Synthesis) signal generator gives you very precise FREQUENCY. For years there have been L-C oscillator based signal generators which have been stable enough in frequency to determine AGC action. What you really need to investigate the AGC is PRECISE RF ATTENUATION -and- a way to calibrate the maximum RF output. [an ordinary diode detector could do that if it was itself calibrated against a known RF source LEVEL] Ah, another question. I have a very precise digital voltmeter. Very precise, 6.5digits (this time from Agilent)... Unfortunately, it's absolutely unable to handle RF. I would like to build a "RF" frontend for it... Any ideas? I'm thinking to a precise rectifier built with an OP AMP followed by a OP AMP integrator... The usual method of making a "precise" RF voltmeter is to begin with a wideband video amplifier with gain controls setting the gain in the full-scale ranges desired. However, the BACK END needs attention, particularly if you want TRUE RMS measurement. The "less precise" HP3400A AC voltmeter could do that True RMS within 1% using an analog meter readout (mirrored scale on meter). The 3400A used a pair of matched heaters and thermocouples. Amplified AC heated one heater. A high-gain DC op-amp had inputs (opposing) from both thermocouples. Op-amp output heated the second heater. This was self-balancing. The AC Voltage indicated actually came from the DC op-amp output. If you are going to measure AC-RF volts of both sinewaves and noise, you need True RMS indication. Without that the noise (random stuff) read by simple averaging rectifiers will be DOWN by as much as 50% compared to a sinewave input. There are three basic types of AC voltmeters made: Rectify- average (common to handheld multimeters); Logarithmic (now a standard of high-end bench multimeters) using special ICs for True RMS conversion to DC; Thermal (now out of favor in new designs but using the first-principles of measuring the effective heating of a resistive load). Thermocouple sensors are reliable, can handle overloads, but a diode string biased for forward conduction can produce DC voltage changes of -2 mV / degree C heating. For some references, you can search the Internet for "RMS to DC" conversion, or begin at www.ednmag.com, go to their Archives button, select issue for May 11, 2000, and look at the "How It Works" article by Jim Williams of Linear Technology Corporation. LTC made an IC that was a dual heater-sensor, the LT1088, but that IC is now discontinued. The article shows a "front end" as well as the whole AC voltmeter circuit. |
AGC signal/noise question...
wrote: On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held" wrote: Michael Black wrote: However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison I have to disagree on the MC1350 and way back 30 years to its predecessor, MC1590. The prototype HF receiver presently on my workbench has a NF of 5.5 and that hardly rises more than that with AGC current applied to the AGC pin. BTW, that receiver, single-conversion with one IF at 21.4 MHz, uses only MC1350s up to the detector, including the one mixer stage. [ LO is a separate PLL board ] |
AGC signal/noise question...
On 19 Aug 2006 20:19:19 -0700, "
wrote: wrote: On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held" wrote: Michael Black wrote: However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison I have to disagree on the MC1350 and way back 30 years to its predecessor, MC1590. The prototype HF receiver presently on my workbench has a NF of 5.5 and that hardly rises more than that with AGC current applied to the AGC pin. Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the point where the gain cell has equal conduction on both legs the noise rises significantly. I duplicated the test fixture and yes, it's noisy, from around 6db to around 11db in my fixture when gain is reduced by 10db and that was at 16mhz. In a reciever that used it I went to two cascode stages using JFETs and the difference noise was notable for weak signals just into the agc range. I restrict the 1590/1350/ca3028 for lower perfomance recievers now. I also verified that the 1590 does same and also the CA3028 wired as differential AGC. Even tried three 2n3904s and same result. The agc range was good and at full gain the noise was ok but the noise increase at partial agc was surprizing. BTW, that receiver, single-conversion with one IF at 21.4 MHz, uses only MC1350s up to the detector, including the one mixer stage. [ LO is a separate PLL board ] I do most of my RX experimentation at 6/ 2M and 70cm SSB so noise and overload perfomance are important to me. Images are also a big problem as I'm near a lot of VHF/hf broadcast. Allison |
AGC signal/noise question...
From: on Sun, Aug 20 2006 9:06 am
On 19 Aug 2006 20:19:19 -0700, " wrote: wrote: On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held" wrote: Michael Black wrote: However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison I have to disagree on the MC1350 and way back 30 years to its predecessor, MC1590. The prototype HF receiver presently on my workbench has a NF of 5.5 and that hardly rises more than that with AGC current applied to the AGC pin. Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the point where the gain cell has equal conduction on both legs the noise rises significantly. I duplicated the test fixture and yes, it's noisy, from around 6db to around 11db in my fixture when gain is reduced by 10db and that was at 16mhz. In a reciever that used it I went to two cascode stages using JFETs and the difference noise was notable for weak signals just into the agc range. I restrict the 1590/1350/ca3028 for lower perfomance recievers now. Apparently I hit some nerve on my disagreement. My first experience with the MC1590 was in 1973 and a need to operate over 55-64 MHz. Electronic gain control was essential and it had to be fast. Motorola supplied some additional information which was later incorporated into appnotes. The MC1350 was marketed around '73 along with the MC1330 video detector as a TV IF system. It didn't sell that well in quantities (presumably) and both were dropped from active production (Lansdale acquired masks and now makes the MC1350). The 1350 (8-pin DIP) should use the same die in the metal can MC1590. While neither one was ever touted as a super-champ low-noise device, it is what I consider respectable as to NF. The fact that it has differential input and differential output is convenient from the standpoint of circuit design. Especially so when input impedances (each side) has a dependable 5K R in parallel with about 5 pF total capacitance. Gain of both begins to fall above 75 MHz with output loads of 100 Ohms resistive. I've found no noticeable difference between differential input v. single-ended. That IC is what I term a "double Gilbert cell" in that AGC control current affects both differential inputs equally (or very nearly so). Whether one connects to both inputs or just one shouldn't make any difference other than output gain. I also verified that the 1590 does same and also the CA3028 wired as differential AGC. Even tried three 2n3904s and same result. The agc range was good and at full gain the noise was ok but the noise increase at partial agc was surprizing. I've never encountered any "surprising" increase in noise at any AGC input to a 1590 or 1350 causing partial gain reduction. That is as true in 2005 as it was in 1973. If there is a SNR of 10 db at an RF carrier input of 3 uV and a gain reduction of 10 db for a 10 uV RF input results in 3 db more noise in the front end, the SNR with a 10 uV input is still higher than the one at 3 uV. What has been "lost" there? Let's look at the original problem starting this thread: There was a claim of "increased noise" with AGC on, but no quantifiable data. The sudden segue to stating that a certain IC is "bad" is a leap that defies good design practices to me. I'm not impressed that the ARRL had some test data in a publication; having been hands-on with this Motorola design for a number of years, I have a number of RCA lab notebook pages filled with my testing of it along with a patent involving it granted 1974...besides my own hobby notebooks. Low-noise input amplifier design is an entirely separate subject and there are a number of other active devices which can do lower NFs than 5. What was orignally needed was some way of getting some numbers and test configuration of Andrea's problem...to pin down a possible reason for alleged increased noise with AGC applied, presumably a "partial AGC" application. [I can't quantify "partial" as a numeric value...maybe others can?] I do most of my RX experimentation at 6/ 2M and 70cm SSB so noise and overload perfomance are important to me. Images are also a big problem as I'm near a lot of VHF/hf broadcast. [shrug I live about 6 miles from 50 KW KMPC on AM...] If we can get back to the original claim of "increased noise with AGC applied" we might be able to help Andrea some. We don't know what Andrea has for a main receiver and interjecting some "badness" remarks by the ARRL about a certain IC isn't going to help clarify Andrea's problem. |
AGC signal/noise question...
On 20 Aug 2006 12:46:25 -0700, "
wrote: shortend Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the point where the gain cell has equal conduction on both legs the noise rises significantly. I duplicated the test fixture and yes, it's noisy, from around 6db to around 11db in my fixture when gain is reduced by 10db and that was at 16mhz. In a reciever that used it I went to two cascode stages using JFETs and the difference noise was notable for weak signals just into the agc range. I restrict the 1590/1350/ca3028 for lower perfomance recievers now. Apparently I hit some nerve on my disagreement. My first experience with the MC1590 was in 1973 and a need to operate over 55-64 MHz. Electronic gain control was essential and it had to be fast. Motorola supplied some additional information which was later incorporated into appnotes. The MC1350 was marketed around '73 along with the MC1330 video detector as a TV IF system. It didn't sell that well in quantities (presumably) and both were dropped from active production (Lansdale acquired masks and now makes the MC1350). The 1350 (8-pin DIP) should use the same die in the metal can MC1590. While neither one was ever touted as a super-champ low-noise device, it is what I consider respectable as to NF. Wide open and at 20db reduction the noise figure is not bad at all. Only that the apparent increase is noteable. I've seen it occur with other topologies including simple bipolar or FET stages (even tubes). The fact that it has differential input and differential output is convenient from the standpoint of circuit design. Especially so when input impedances (each side) has a dependable 5K R in parallel with about 5 pF total capacitance. Gain of both begins to fall above 75 MHz with output loads of 100 Ohms resistive. I've found no noticeable difference between differential input v. single-ended. Those are the feature of the part that makes them desireable. That IC is what I term a "double Gilbert cell" in that AGC control current affects both differential inputs equally (or very nearly so). Whether one connects to both inputs or just one shouldn't make any difference other than output gain. I call that circuit an analog four quadrant multiplier for what it does not how it's made. The feature of the Gilbert cell that applies for the AGC use is the lack of DC shift at the output points keeping downstream DC coupled stages at their undistrubed bias points. However the active device that is in the agc control positions is still a noise generator (as are all active devices) and as agc increases it's contribution is additive to the RF path devices. Makes little difference if the node where outputs are combined see no DC shift the various diff amp transistor as individual pairs do see a significant shift (100% collector current to near 0). It's easier to see using the older MC1550 or CA3028 diffamps or even discretes in a diffamp with current source. I also verified that the 1590 does same and also the CA3028 wired as differential AGC. Even tried three 2n3904s and same result. The agc range was good and at full gain the noise was ok but the noise increase at partial agc was surprizing. I've never encountered any "surprising" increase in noise at any AGC input to a 1590 or 1350 causing partial gain reduction. That is as true in 2005 as it was in 1973. If there is a SNR of 10 db at an RF carrier input of 3 uV and a gain reduction of 10 db for a 10 uV RF input results in 3 db more noise in the front end, the SNR with a 10 uV input is still higher than the one at 3 uV. What has been "lost" there? The surprize is that I'd not considered the possibility that the SN+N/N could degrade unevenly due to applied agc. So I'd never paid attention until I was trying to improve an earlier reciever design (ca1978) of my own and at the same time aquired a copy of EMRFD and did some testing to verify their resuts. Since the design was optimized for low RF gain and high overload thresholds I was revisiting anything that could better the design without loosing those features. Note it's a single conversion system with high IF. The problem was a MDS of -136dbm but the 10db Signal+N/N point was around -110dbm and at ~121dbm it was worse than at -130! The front end was common gate RF amp (2n4416s) driving a pair of 4416s in a single balanced mixer. Low noise but limited gain for better overload performance. No agc before the IF. If needed there are switchable resistive attenuators (3, 6,12db or 19db total). Measured gain from antenna to IF is only 16db (after all losses). Disable the agc or increase the threshold and it wasn't as measurable or appeared to disappear.. The hunt was on. The results were a surprize as there is no data for noise output with no input or signal to noise with gain reduction. My Moto databooks go way back, as do my National, RCA and Signetics library. More current datasheets do not reflect any improved information. The revised RX used two stages of mpf102 Jfet in cascode plus a diferential pair of 2n3904s to resolve the 5-10V agc to be compatable with the new fet amp to replace the two MC1350s and the problem of decreasing signal to noise as signal increased with agc active disappeared. Not to say the fet amps did not do the same thing only that the rate of noise increase was a smoother curve from max gain to min gain. In retrospect a delaying AGC to the first of the two 1350s could potentially have the same effect but was not investigated. I may revisit it at some time as I still have the original if module in the junkbox. Let's look at the original problem starting this thread: There was a claim of "increased noise" with AGC on, but no quantifiable data. Thats a problem, the lack of data or information on the circuit. Also I've repaired a few commercial radios that due to component failure or "golden screwdriver" had the various operating conditions sufficiently altered as to cause a similar problem. The sudden segue to stating that a certain IC is "bad" is a leap that defies good design practices to me. I'm not impressed that the ARRL had some test data in a publication; having been hands-on with this Motorola design for a number of years, I have a number of RCA lab notebook pages filled with my testing of it along with a patent involving it granted 1974...besides my own hobby notebooks. That's nice but are the test results in error from two different sources? No. However, it's was a noteable weak point. But calling it bad is your words. It's a point that needs to be understood and allowed for. In a design with more RF gain and/or less mixer noise it many not have been a factor or less of one. Also in the case that brought it to a point for me even altering how agc is applied might have achieved a better result. Since the 1350 is at IF for most designs the noise is likely from front end causes should be investigated first. One would hope the design had secured the system noise performance before the IF. However in low gain systems or system with no gain before the mixer and first filters this may be problematic. Low-noise input amplifier design is an entirely separate subject and there are a number of other active devices which can do lower NFs than 5. What was orignally needed was some way of getting some numbers and test configuration of Andrea's problem...to pin down a possible reason for alleged increased noise with AGC applied, presumably a "partial AGC" application. [I can't quantify "partial" as a numeric value...maybe others can?] I do most of my RX experimentation at 6/ 2M and 70cm SSB so noise and overload perfomance are important to me. Images are also a big problem as I'm near a lot of VHF/hf broadcast. [shrug I live about 6 miles from 50 KW KMPC on AM...] 10 miles from the Needham towers in MA. Not less than 8 VHF broadcasters, then the usual crowd of UHF and now the HDTV-UHF broadcasters and no small party of FM broaccasters. Oh and WKOX 1200 AM three miles away. Then I have 9 hams within a 1 mile circle and two within 1500ft running KW level at VHF. RFI are us. I understand overload as +15dbm on coax is common here. It's an interesting design challenge to do low noise figure RX and at the same time be overload resistant in a harsh environment. If we can get back to the original claim of "increased noise with AGC applied" we might be able to help Andrea some. We don't know what Andrea has for a main receiver and interjecting some "badness" remarks by the ARRL about a certain IC isn't going to help clarify Andrea's problem. Having tested and understood the problem I would say the authors of EMRFD did a fair job of pointing out the points where a device needs better understanding. A blanket "it's great" is lore, testing it and understanding it is engineering. Having done the work to understand it better I can appreciate the perfomance of the part and it's limitations. I still use it and have a tube of them because it's a useful part. Just like the often reviled SA602 mixer. It's relevence is I've seen this before and understood it's origin and also elsehere. The other aspect is that if a commonly accepted part is not fully understood and can lead to undesired effects then, why not others. AGC is not a trivial thing to be tacked on and considered a problem solver. Protects the ears but it's place in the reciever is not always understood. It does not always solve things like gross overload at the front end or possibly further down or outside the agc detectors bandwidth. But a lack of information about his radio doesn't help us either. We do not know for instance what topology is used for RF and if any agc is even applied to it. Allison |
AGC signal/noise question...
|
AGC signal/noise question...
Roy Lewallen wrote:
: The bottom line is that I'd be hesitant to trust just about any number : for a "worst case" maximum signal strength. Be sure to test any proposed : design on 40 meters for a while from your location in Europe. Uh. Very interesting, Roy. Even a receiver with AGC has his own limits and probably what you experienced would have surely overload most commercial ones... Some numbers must be fixed, even if very high ones. So, how one could proceed? Ciao, AB .... Andrea Baldoni, 2002: messaggio non protetto da copyright. |
AGC signal/noise question...
On Mon, 21 Aug 2006 16:15:01 -0500, Andrea Baldoni
wrote: wrote: : If we can get back to the original claim of "increased : noise with AGC applied" we might be able to help Andrea : some. We don't know what Andrea has for a main receiver : and interjecting some "badness" remarks by the ARRL about : a certain IC isn't going to help clarify Andrea's problem. Sorry for the short answer. The receiver is a Yaesu FR-101 with his internal 2m converter. Actually, I'm doing some recalibration but I didn't found at now nothing really out. I just noted that, mostly noticeable in FM mode (by noise increasing) but present in all modes looking s-meter, the response of the IF was not a gaussian with a flat top. In the center there was a little decreasing area. If you tune it perfectly, you had a sligtly less signal than little up or down in freq. Dual conversion for HF and triple conversion of 2m. There would not be RF agc on the 2m converter. In FM modes you should see limiting so agc is behavour is different. So I started recalibrating. I had stopped because I had trouble with the noise blanker. In the manual, there is a indication of a test point where hook a voltmeter, but there is not any test point nor any indication on the schematic where the exact point is. The transformer is T116, maybe someone could help. Tomorrow I'll figure out what to do, I think I'll hook the voltmeter as to measure the voltage rectified, at the gate of the FET. If you have the schematic (it's freely downloadable in many places) maybe you could tell if I'm right. Likely a test point that is not marked on the board. I don't have a print handy. Anyway, thankyou very much you want to help troubleshooting my receiver, but I was using what I noted (increase in noise with AGC on) mainly to start a general discussion about AGC implementation and effects, and his future in digital receivers as well as in homebrew very high-end ones. It has started, and it's very interesting. Having experimented with recievers since before EE school and still many decades later I find it challenging. Always looking for and at new ideas. Allison |
AGC signal/noise question...
tim gorman wrote:
: Are you sure you are seeing an AGC problem? What you describe above, with : slightly less signal in the center, is typical of a *filter* with dip in I have the filter dip; it was not cured with realigning, but realigning was anyway helpful to gain a dB or two. I also see that disabling AGC cause less noise in FM while listening to 2m converted to HF by the internal converter of the Yaesu FR-101. I didn't check if enabling or disabling AGC cause any change in the filter dip, anyway I'll check and report soon. : Without knowing more about the receiver I can't make any guesses as to what : is in play here but I question if this is an AGC artifact. It uses a MC1496G as mixer and two CA3053 as IF. Plus some DG FETs, in first RF amplifier, after the mixer... What information do you need? Ciao, AB .... Andrea Baldoni, 2002: messaggio non protetto da copyright. |
AGC signal/noise question...
Andrea Baldoni wrote:
tim gorman wrote: : Are you sure you are seeing an AGC problem? What you describe above, : with slightly less signal in the center, is typical of a *filter* with : dip in I have the filter dip; it was not cured with realigning, but realigning was anyway helpful to gain a dB or two. I also see that disabling AGC cause less noise in FM while listening to 2m converted to HF by the internal converter of the Yaesu FR-101. I didn't check if enabling or disabling AGC cause any change in the filter dip, anyway I'll check and report soon. : Without knowing more about the receiver I can't make any guesses as to : what is in play here but I question if this is an AGC artifact. It uses a MC1496G as mixer and two CA3053 as IF. Plus some DG FETs, in first RF amplifier, after the mixer... What information do you need? Ciao, AB ... Andrea Baldoni, 2002: messaggio non protetto da copyright. A block diagram would be helpful. Do you know if the AGC is being derived from the audio chain or from a sampling circuit in the IF chain? Where are you turning off the AGC? In the FR-101? It sounds like you have a frequency converter feeding an HF receiver with an FM position. Is that correct? Could you just as easily tune in 2-m SSB as well as 2-m FM? I can't seem to grasp why turning off the AGC would result in *less* noise, especially on FM. The "noise factor" of the system is probably fixed by the converter, not the receiver. It would be like hooking an antenna to an HF receiver. Usually the noise in the receiver is set by the atmospheric noise the antenna picks up, not by the noise factor of the receiver. (On the higher HF bands, 15m and 10m, this may not always be the case) The same would apply for the 2m converter. Unless it is designed very well its contribution to the noise at the antenna of the receiver would probably mask the noise factor of the receiver itself. If you could kill the power to the converter you could probably test for this by just killing the power and seeing what happens to the noise out of the receiver speaker. If it goes down, then the noise factor of the receiver is irrelevant. If it doesn't change then the converter is contributing less noise than the receiver itself. If turning off the AGC causes less noise output then my first guess would be to look at what "turning off the AGC" is actually doing. Is it actually breaking the AGC loop so the AGC inputs to the CA3053 amps are left floating? Or does turning off the AGC actually mean putting a fixed bias on the CA3053's? Either case could potentially cause the gain of the CA3053's to actually go down with the AGC turned off and that might be what is going on. Can you actually monitor the AGC loop to see what happens to the AGC voltage when the AGC is turned off? tim ab0wr |
AGC signal/noise question...
tim gorman wrote:
: A block diagram would be helpful. Do you know if the AGC is being derived : from the audio chain or from a sampling circuit in the IF chain? The AGC is derived sampling the RF level at the last IF. In the same point, (using another diode, capacitor, etc.) there is the AM detector. The same RF signal go to a diode balanced modulator along with BFO signal for CW and SSB and to the input of FM IF amplifier (TA7061AP). : Where are you turning off the AGC? In the FR-101? It sounds like you have a : frequency converter feeding an HF receiver with an FM position. Is that : correct? Could you just as easily tune in 2-m SSB as well as 2-m FM? I have a FR-101 with the onboard 2m converter. Yes, I could tune 2m SSB but it's difficult to find SSB on 2m here usually so I didn't any test. The FR-101 is CW/SSB/AM/Narrow AM in segments of the HF range, plus you could buy (as a option) a onboard 2m converter, a onboard 6m converter and a onboard FM detector. I have all boards installed, so I eventually could use FM on HF as well as 2m or 6m. : If you could kill the power to the converter you could probably test for : this by just killing the power and seeing what happens to the noise out of : the receiver speaker. If it goes down, then the noise factor of the : receiver is irrelevant. If it doesn't change then the converter is : contributing less noise than the receiver itself. Turning on the 2m converter don't seem to change noise level, while instead turning on the 6m converter seems to double the noise. Maybe it needs realigning, I never use it so I don't know if it's working well. : If turning off the AGC causes less noise output then my first guess would be : to look at what "turning off the AGC" is actually doing. Is it actually The AGC line is derived from a fixed voltage using a 9V zener, then the RF GAIN pot permit to divide this voltage from 100% to ground and feed it (trough a resistor) to the first RF amplifier of the HF receiver (a DG FET) as well as the first RF amplifier of the 2m converter, and the same for 6m converter. It is also fed to the last but one CA3053. Other amplifiers are fixed gain I suppose. Everything in the receiver needs to reduce gain, lower this voltage by more or less shorting it to ground. For instance, the standby button shorts it to ground, silencing the receiver completely. The RF level at the last IF instead reduce it by means of common emitter transistor: the AGC voltage from zener at the collector and the rectified and filtered IF at the base. When you disable AGC, you disconnect the collector of this transistor, thus the signal is let alone to the level adjusted with RF gain pot (normally at maximum, so it is 9V). Ciao, AB .... Andrea Baldoni, 2002: messaggio non protetto da copyright. |
AGC signal/noise question...
|
AGC signal/noise question...
On Sat, 26 Aug 2006 14:50:32 -0500, Andrea Baldoni
wrote: wrote: : Dual conversion for HF and triple conversion of 2m. There would : not be RF agc on the 2m converter. In FM modes you should see : limiting so agc is behavour is different. By looking closer at the AGC circuits, there is RF AGC also in the 2m and 6m converters. However, the AGC involves not all amplifiers in this receiver, just the first and the last but one. I often see different configurations in receivers, where every amplifier is controlled. Maybe they thought it would suffice. Often not every amplifier needs to be controlled. It's a matter of van the overall gain change be achieved with fewest control points. : Likely a test point that is not marked on the board. I don't have : a print handy. I had calibrate it for the minimum voltmeter reading, because it appears that the signal is negative over a positive DC bias, supplied by the JFET. Probably there is another point where the signal is positive, anyway it should be the same. Generally will be. What I have seen in some cases is where the no signal resting point for gain control bias voltage is not correct and the gain can go up a bit before going down. Often seen on oder recievers where the large part of the radio is discrete devices and the various setpoints have drifited from age or componenet changes. : Having experimented with recievers since before EE school and still : many decades later I find it challenging. Always looking for and at : new ideas. I have read very interesting articles on QEX (by downloading them in PDF format from the site) following back the chain of cross references starting from "A software defined radio for the masses" to the R1 and R2. I've build R1, R2, MiniR2. I happen to like the miniR2 and use on with the matching T2 in 6m as my highest performing RX. Highest performing in this case is best dynamic range and lowest noise. SDR is an extension of that work. However since most of the digital work is done at low IF (under 50khz) or at baseband all of the frontend, filter and IF issues remain though the tradeoffs may be different. Every designer has his own ideas about AGC, dynamic range, et all. Often very different. While I never (as now) tried to design a receiver, I want to carefully understand the reasons behind every implementation. So I could build my own opinion and in future choose what I think better for a receiver project. Very true. What was considered best in class for 1960, 1970, 1980 and so on has changed considerably. However it's possible to use older topologies with newer devices and obtain perfomance unattainable back then. What is easiest to build and make perform is usually a low gain approach using amplifers that do not overload easily to get a balanced dynamic range and noise figure. I've seen too many chase for a high gain for sensitivity at low HF only to be overloaded with man made and atmospheric noise. A good example of a strong staple topology is the Elecraft K2. A very solid single conversion with medium high IF transceiver. The basic design topology is 25 years old (look up Progressive RX, QST) but, it's well executed using current tech parts. The manual is available on line and worth reading and reviewing. Another interesting argument is LO: DDS, PLL or DDS+PLL? I use Analog with premix for lowest close in noise. Though I also have a DDS with PLL tracking filter and a straight PLL system that has proven satisfactory at my favorite band (6M). In each case considerable care was taken to well shield and filter the signals used while using the best techniques. Executed with care and with an eye for how it fits into the system all work well. Of those (opinion follows) I find DDS has agility and stability with the very noticeable tradeoff for spurious outputs. Due to that I prefer to restrict raw DDS to narrow band systems or clean them up with a tracking PLL. The cost is power, and great care in shielding as the various oscillators and the concurrent need for a microprocessor to do the translation of knobs, buttons and tuning displays into the digital control for DDS can contribute to a lot of undesired signals (birdies). So the whole subject of the LO system can be as complex as the rest of the reciever and be a significant factor in it's total performance. Allison |
AGC signal/noise question...
Andrea Baldoni wrote:
..................................... The AGC line is derived from a fixed voltage using a 9V zener, then the RF GAIN pot permit to divide this voltage from 100% to ground and feed it (trough a resistor) to the first RF amplifier of the HF receiver (a DG FET) as well as the first RF amplifier of the 2m converter, and the same for 6m converter. It is also fed to the last but one CA3053. Other amplifiers are fixed gain I suppose. Everything in the receiver needs to reduce gain, lower this voltage by more or less shorting it to ground. For instance, the standby button shorts it to ground, silencing the receiver completely. The RF level at the last IF instead reduce it by means of common emitter transistor: the AGC voltage from zener at the collector and the rectified and filtered IF at the base. When you disable AGC, you disconnect the collector of this transistor, thus the signal is let alone to the level adjusted with RF gain pot (normally at maximum, so it is 9V). Ciao, AB Ok, have you checked the Dual Gate FET to insure that the bias supplied by the RF gain control puts the device at maximum gain when the AGC is off? Dual Gate FET's have transducer gain curves that are peaked curves. Depending upon the Gate 2 voltage, the transducer gain can actually go down as the Gate1 to Source voltage goes up. I would still be interested in knowing *exactly* what the AGC voltage on the gate of the DGFET is for 1) AGC on, no signal, RF gain wide open, and 2) AGC off, no signal, RF gain wide open. It would also be interesting to know what the Gate 1 bias voltage is for each state as well. I'll bet you'll find an interesting interaction between the bias voltages and the actual stage gain as the controls are manipulated. tim ab0wr |
AGC signal/noise question...
On Sat, 26 Aug 2006 21:30:47 -0500, tim gorman
wrote: Andrea Baldoni wrote: .................................... The AGC line is derived from a fixed voltage using a 9V zener, then the RF GAIN pot permit to divide this voltage from 100% to ground and feed it (trough a resistor) to the first RF amplifier of the HF receiver (a DG FET) as well as the first RF amplifier of the 2m converter, and the same for 6m converter. It is also fed to the last but one CA3053. Other amplifiers are fixed gain I suppose. Everything in the receiver needs to reduce gain, lower this voltage by more or less shorting it to ground. For instance, the standby button shorts it to ground, silencing the receiver completely. The RF level at the last IF instead reduce it by means of common emitter transistor: the AGC voltage from zener at the collector and the rectified and filtered IF at the base. When you disable AGC, you disconnect the collector of this transistor, thus the signal is let alone to the level adjusted with RF gain pot (normally at maximum, so it is 9V). Ciao, AB Ok, have you checked the Dual Gate FET to insure that the bias supplied by the RF gain control puts the device at maximum gain when the AGC is off? Dual Gate FET's have transducer gain curves that are peaked curves. Depending upon the Gate 2 voltage, the transducer gain can actually go down as the Gate1 to Source voltage goes up. I would still be interested in knowing *exactly* what the AGC voltage on the gate of the DGFET is for 1) AGC on, no signal, RF gain wide open, and 2) AGC off, no signal, RF gain wide open. It would also be interesting to know what the Gate 1 bias voltage is for each state as well. Tim, Your hitting the specifics of what I refered to earlier. I'll repeat it for emphasis. What I have seen in some cases is where the no signal resting point for gain control bias voltage is not correct and the gain can go up a bit before going down. Often seen on oder recievers where the large part of the radio is discrete devices and the various setpoints have drifted from age or componenet changes. I have seen this on older radios where parts have been replaced or the original parts used were at opposing ends of the allowable tolerence. Occasionally a part like a zener doide can drift form heating. The end result is the full gain voltage can be off or full gain for a single stage can be off (high or low from optimum). In one case it was a mechanical switch (agc/manual) causing difficulty (leakage path). Other suspect components seen in Japanese built radios are those commonly used ceramic disk caps for bypasses, they can and do go leaky(high resistance), or short and I've even seen microphonic. I have a reciever I repaired where the DGfet developed a substrate to gate2 short which casued all manner of unusual problems. The worst case by far was one that the agc bias point had drifted a bit high. When on manual agc the RX was hot. When agc was enabled the RX sensitvity would drop noticeably. The problem was the higher agc bias point had the IF and RF running harder and producing more noise and when agc was turned on it would see the noise and pull the agc voltage. That sounds ok save for the front end was more agc sensitivve than the overbiased IF and the front end would loose gain faster (it was 2 jfets cascode) rendering the reciever less sensitive. The fix was repairing the internal voltage regulator that fed 9V to most of the circuits (it was running at 11v due to open zener). Just a few examples of what can occur. I havent even gone into the golden screwdriver problems when pots are tweeked for "more". I'll bet you'll find an interesting interaction between the bias voltages and the actual stage gain as the controls are manipulated. I'd be inclined to agree. Allison |
AGC signal/noise question...
Andrea Baldoni wrote:
Roy Lewallen wrote: : The bottom line is that I'd be hesitant to trust just about any number : for a "worst case" maximum signal strength. Be sure to test any proposed : design on 40 meters for a while from your location in Europe. Uh. Very interesting, Roy. Even a receiver with AGC has his own limits and probably what you experienced would have surely overload most commercial ones... Some numbers must be fixed, even if very high ones. So, how one could proceed? If you really want to be rigorous about it, you could set up some kind of logging system, perhaps with an A/D converter and computer connected to a reference antenna and simple detector, to measure and log signal strengths over a long period of time. The tough part would probably be deciding what kind of filter to precede it with; maybe something typical of what you expect to use in a real receiver. Then you could do a statistical analysis on the logged signal strengths. Whether or not that's worth while would be up to you -- it would at least certainly make an interesting article. Or, you could build something and put a coarse step attenuator at the front end, noting how much attenuation you have to apply when operating in order to keep the spurs down. Roy Lewallen, W7EL |
AGC signal/noise question...
|
AGC signal/noise question...
On Sun, 27 Aug 2006 13:56:50 -0700, Roy Lewallen
wrote: wrote: . . . It's relevence is I've seen this before and understood it's origin and also elsehere. The other aspect is that if a commonly accepted part is not fully understood and can lead to undesired effects then, why not others. . . Very nearly 30 years ago, I was looking into "feed forward" circuits, a technique developed by someone at Tektronix for ultra-low distortion amplification. It turns out that the topology of the MC1350 is similar to what's needed, and a feed forward amplifier can be made from one plus just a few external components. But even by then, I'd learned that it's risky to use components for other than their intended purpose. So I collected 8 or 10 samples from various vendors (the part was widely sold then), and opened them up. Those in cans were easy, using a little can opener that worked like a tubing cutter. Some of the plastic DIP ones were more difficult, but one of the labs at Tek was able to dissolve the plastic while leaving the chip intact. Then I examined them carefully with an inspection microscope. Here's what I found: 1. There were at least three very different designs. The chip size of the largest was several times that of the smallest. 2. Some designs were inherently better balanced than others. Some had resistive "cross unders" where traces cross, which weren't the same on both sides of the circuit. Based on this, I decided it was too risky to make a design based on that part number, since a vendor could change chip suppliers or designs without notice. Interestingly, about six months later, I got a call from the component engineering group asking if I still had the chips. It seems that one or more of the vendors supplying that part (which was used for other applications at Tek) had changed their design, causing failure of some products and the shutting down of their production lines. Tek was big enough that vendors were often required to give advance notice before such changes, but they hadn't given any notice in this case. I'm bringing this up because I'm hearing the MC1350 being spoken of as though all are the same. It wouldn't surprise me if, after all these years, they're now all being made with one design from one foundry. But those ones in your junk box might be way more different than you think. This is almost certainly true of just about any IC. Roy, That is my engineering experience as well. At the time I did my testing I had Motorola, National and Hitachi parts Some fairly current date codes and a few from early 80s and and while the general behavour was similar I noted differences in gain, overall noise and DC balance as well. The noise increase was enough to be noteable in a particular case but on analysis understandable and to be expected. Then again I date back to when the Fairchild UA703 was a breakthrough gain block for RF. Allison KB!GMX |
| All times are GMT +1. The time now is 07:03 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com