M
Michael Terrell
Guest
On Thursday, February 20, 2020 at 7:05:26 PM UTC-5, upsid...@downunder.com wrote:
You only want limiting in the final IF amp, not every stage.
You've never built a Telemetry system that had to follow a satellite from the ground, until it was in space, where there wasn't enough land to provide decent physical seperation. One of our turnkey systems for the ESA was more than 40 dB too hot for the LNA. The only solution was a mechanical shutter that opened, as the signal level dropped. Guess how that was controlled. We didn't have to do that to the portable earth station, since it was parked about 20 miles away. These systems were for a contract of just under a half million dollars, not some stereo system. Our company was consider the top in the industry at that time.
On Thu, 20 Feb 2020 12:19:09 -0800 (PST), Michael Terrell wrote:
On Thursday, February 20, 2020 at 1:18:35 PM UTC-5, upsid...@downunder.com wrote:
On Thu, 20 Feb 2020 07:14:05 -0800 (PST), Michael Terrell wrote:
These days, when the IF gain is
very cheap, there is enough IF gain that the band noise or receiver
front end noise will drive the last IF into saturation. On stronger
signals, the last few IF stages will go into saturation, not just the
last. Thus, effectively, this limiting action works like "AGC"
delivering a few volt constant voltage to the FM detector.
You only want limiting in the final IF amp, not every stage.
The important thing is that all selectivity filtering must be done
prior to the first IF stage that would start to saturate on very
strong signals. This is not much of an issue with ceramic filters.
Just make sure that the RF-amplifier and mixer gain in front of the
ceramic filter has enough gain to mask the filter losses.
The situation was different in the tube era with distributed LC
bandpass IF transformers between each amplifying tube, so the final
selectivity was achieved just at the last IF stage.
Try it, you might even like it!
But why would I use AGC for the RF amplifier ? Is the mixer so weak
that it is overloaded by a strong signal amplified by the RF
amplifier? Use a stronger mixer so that full RF gain can be applied
before the mixer. A strong mixer can be a problem with low power
battery powered equipment, but using a weak mixer in a mains powered
receiver is simply incompetence.
AGC in the front end prevents overdriving the mixer in areas with excessive RF, and producing Intermod.
If you use AGC to reduce front end gain due to some strong signals in
some nearby frequency, it will also reduce the gain for a weak signal
that you want to listen, thus the weak signal may be swamped in front
end noise when a strong nearby signal is present. A sharp filter
between RF stage and mixer helps getting rid of some strong signals.
Since the filter needs to be tunable, realistically the bandwidth is
at least 1 MHz (Q=100), so it helps only in reducing strong signals a
few MHz away.
A few MHz? It could be 200KHz away, which a single FM channel in the US.
The adjacent channel filtering is done in the IF filter. A capacitance
diode tuned filter between the RF amplifier and mixer will pass 1-2
MHz to the mixer. The mixer _must_ handle any strong signals 200 kHz
away, which is then handled by the IF filter. If the RF amp/mixer
can't handle that strong signal without gain reduction, the receiver
can't be used to receive a very weak signal only 200 kHz away from a
strong signal, no mater what fancy IF filtering is used.
A gain reduction in the RF amp will degrade the noise figure and it is
no longer capable of masking the noise of subsequent stages. In a
good receiver design, the total receiver noise figure is determined by
the noise figure of the first RF stage _provided_ that the gain is
high enough to mask the noise of the following stages.
Show me any FM tuner without a tuned RF stage.
I said nothing about omitting the RF tuned filters, but they are much
wider than the channel spacing. If some dynamic range claims are
given, look carefully at what frequency separation is used. Most
likely a few MHz is used, i.e. outside the RF filter bandwidth.
Of course, if you have a fixed tuned receiver, you could use a 1/4
wave filter with Q=500, but these are big (75 cm) and hard to tune
This would have a 200 kHz RF bandwidth.
I still maintain that applying AGC to the first RF stage is a bad idea
and a receiver design requiring it is a suspect, possibly crap design.
Some equipment might have no AGC, but that would be the $20 portable crap.I have worked with this most of my life, both broadcast and deep Space Telemetry.
In space applications, you usually have a fixed gain LNA at the focal
point to compensate for feeder losses as well as receiver added noise
figure. In such case, reducing the indoor unit RF stage gain is not a
very bad thing.
Television also uses AGC in the RF. All of these use the same AGC in the IF and RF stages.
Analog requires accurate AGC so that the synch pulse levels are
normalized after the diode detector. Feeding AGC into multiple stages
helps keeping the level constant. In a TV receiver, the needed IF gain
is quite low, thus only a few stages are used and you may have to use
AGC on all IF stages as well as on the RF stage.
I serviced CATV Hetrodyne signal processors, including one crap brand that used all PNP RF transistors in the signal path. That isn't a problem other than having to buy the 1000 at a time. Luckily, that company went bankrupt. I serviced hundreds of designs and all used AGC for RF and IF.
That must have been a very old design , from the old times when
gain and selectivity distribution was not well understood .
You've never built a Telemetry system that had to follow a satellite from the ground, until it was in space, where there wasn't enough land to provide decent physical seperation. One of our turnkey systems for the ESA was more than 40 dB too hot for the LNA. The only solution was a mechanical shutter that opened, as the signal level dropped. Guess how that was controlled. We didn't have to do that to the portable earth station, since it was parked about 20 miles away. These systems were for a contract of just under a half million dollars, not some stereo system. Our company was consider the top in the industry at that time.