Tom Holden wrote:
I'm looking for some advice/guidance on the design of AGC detection and
timing circuits, prompted by some level of frustration with a modification I
have been doing to a DX-394 SW radio. My questions, though, probably apply
to receiver design generally. I have a problem with stability - the receiver
gain oscillates at medium and fast release speeds.

Previously I had done a mod that pretty successfully provided 3 release
speeds for the DX-394 but fell short of what I thought was the ideal: an
attack time of ~1 millisecond, independent of the release time. That was
based on a survey of receivers from which I concluded that the attack should
be less than 13 ms and that 1 ms seemed to be the goal. Release speeds
should probably be on the order of 30ms, 300ms and 3 seconds, for fast,
medium and slow, respectively, although there seems to be lots of scope for
subjective preference. My mod required a rather large capacitor for Slow
release so my Slow was more like 1.2 seconds and the attack was slowed to
maybe 50-70 ms for the slow release..

The objectives of the enhanced mod are to:
a) improve the attack speed to better less than 13ms for all release speeds
b) extend the Slow release using smaller cap
c) reduce the loading of the AGC detector on the output of the 2nd IF amp
and also possible distortion due to the AGC and AM/Product detectors fed in
parallel

I used a JFET to buffer between the IF amp and the diode detector and an
emitter follower between the attack R-C circuit and the release R-C circuit,
dc coupled to the stock AGC amplifier. On the release side, about 1/10 the
capacitance vs the earlier mod is required for slow release and the attack
does seem to be similarly less affected by the release network.

However, at the fast and medium release settings, the receiver gain
literally oscillates at a rate that seems to be a function of attack and
release time constants, manual RF/IF gain setting, AGC gain setting and
signal strength. The depth of this gain modulation is affected by AGC and RF
gain. In order to get stability, it seems that I have to slow down the
attack (and/or release) time constant and carefully tweak the AGC gain
between the onset of oscillation and receiver peak distortion caused by not
enough gain reduction.

Have I completely misunderstood the meaning of attack/release speeds? My
'ideal' attack circuit has a R-C time constant of 1 ms, which means it will
even respond substantially to 1kHz modulation. That seems high. The R-C time
constant for my target fast release of 30 ms means that it will
substantially follow a 30Hz signal. I have had to pad these out to ~20ms
attack, 50ms release for stability or tolerably low gain oscillation depth
at medium and lower signal strengths. With this slower attack, stability is
much improved with the 500ms medium release speed.

The target attack/release of 1ms/30ms is not good for AM reception anyway as
it causes considerable distortion on heavy bass modulation - it is for data
services on steady carriers, e.g., PSK, FSK, DRM. But if the AGC causes
oscillation, then that's interference of another kind that would adversely
affect error rates. Several, including myself, have noted that DRM SNR is
improved by defeating AGC, on a wide variety of receivers.

Is this a typical problem for receiver design? Would 'hang' AGC stabilise
the AGC loop? Are my design objectives reasonable?

Comments from experienced radio designers/builders/experimenters much
appreciated.

Tom


Here is a way out solution.

To avoid the pumping try the following as a last resort:

1. Increase the agc filter cap. value. This by itself will decrease the
agc attack time, but if you lower the charging source resistance you can
again decrease the attack time. This change may require two emitter
flowers in series to lower the charging source resistance. (If 2 x .7
volts is two high of an offset try npn followed by pnp or select the
optimum fet.) The object is to move the filter corner down in frequency
and stop the pumping. At this point you have fast attack and very slow
release.
2. Now add a second agc circuit in parallel with the first. It should
have the attack and release times that you want. Compare the two agc
voltages and when the second one is lower than the first by some small
amount, activate an active pull down to discharge the large cap in the
first agc filter.

This may be one way to isolate and stabilize the agc feedback .