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AGC signal/noise question...
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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 |
#3
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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. |
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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 |
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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. |
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