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

This sounds like a classic negative feedback oscillation. You sense the
signal is too large, so you send a signal to kill the gain, and then
you sense the signal is too small, so you send a signal to increase the
gain.

Having different attack and release time means you have two different
time constants My guess is the quick attack leads to the instability,
since it is the lesser damped system. If this is true, then you should
concentrate on the attack time, i.e find how slow it has to be for the
sytem to be stable.

Of course this is really had to do without seeing the circuitry in
action.

I agree with you and slowing the attack is the only way that I have
been able to approach stable operation with a fast release. But 20ms or
longer attack runs counter to what I understand to be the objective -
an attack speed of less than 13 ms and ideally about 1 ms. So, unless I
have this wrong, how do other receivers accomplish similar speeds
without self-oscillation?

The way my circuit operates (I think) is as follows (I'd be happy to
send a schematic to anyone who is interested) :
a) assume an impulse of signal of duration very much longer than the
attack time
b) the rectified signal is filtered of RF by a series-parallel R-C
attack network whose adjustable output feeds an emitter follower
c) the emitter follower pumps current as a low resistance source into
the release R-C network so the attack is not greatly slowed - its
output feeds the AGC driver amp
d) at some point, equilibrium should be reached - the current flow
through the release resistor and AGC driver base should equal the flow
though the emitter follower - but maybe the emitter follower pinches
off and that could be a cause of instability?
e) the signal drops, the attack network discharges at attack speed and
shuts off the emitter follower, so the release capacitor discharges
through its parallel R at release speed, the voltage to the AGC driver
falls so the AGC bias rises at roughly release speed to increase RF/IF
gain.

Having written that out, I have an idea or two I will try.

Tom

Tom wrote:
I agree with you and slowing the attack is the only way that I have
been able to approach stable operation with a fast release. But 20ms or
longer attack runs counter to what I understand to be the objective -
an attack speed of less than 13 ms and ideally about 1 ms. So, unless I
have this wrong, how do other receivers accomplish similar speeds
without self-oscillation?

The way my circuit operates (I think) is as follows (I'd be happy to
send a schematic to anyone who is interested) :
a) assume an impulse of signal of duration very much longer than the
attack time
b) the rectified signal is filtered of RF by a series-parallel R-C
attack network whose adjustable output feeds an emitter follower
c) the emitter follower pumps current as a low resistance source into
the release R-C network so the attack is not greatly slowed - its
output feeds the AGC driver amp
d) at some point, equilibrium should be reached - the current flow
through the release resistor and AGC driver base should equal the flow
though the emitter follower - but maybe the emitter follower pinches
off and that could be a cause of instability?
e) the signal drops, the attack network discharges at attack speed and
shuts off the emitter follower, so the release capacitor discharges
through its parallel R at release speed, the voltage to the AGC driver
falls so the AGC bias rises at roughly release speed to increase RF/IF
gain.

Having written that out, I have an idea or two I will try.

Tom


Tom,
Another option may be to reduce the gain in the loop. This may reduce
the oscillation without significantly slowing the loop.

Craig

Craig, the dilemma is that reducing the AGC loop gain results in IF or
AM detector or audio overload; increasing it aggravates instability! So
far, I can only avoid these two adverse effects by slowing the AGC
attack and release speeds to well below what I am aiming for.

Tom

Hi Tom,

You might find some good info from reading the description and
data sheets for the Analog Devices SSM2165 and/or SSM2166
at www.analog.com. They address the issues of having a threshold
which can be adjusted and then varying the amount of compression
or limiting asymetrically. Perhaps you could modify your circuit to
emulate some of these features - or perhaps just use the devices
described?

Bill

wrote in message
oups.com...
This sounds like a classic negative feedback oscillation. You sense the
signal is too large, so you send a signal to kill the gain, and then
you sense the signal is too small, so you send a signal to increase the
gain.

Having different attack and release time means you have two different
time constants My guess is the quick attack leads to the instability,
since it is the lesser damped system. If this is true, then you should
concentrate on the attack time, i.e find how slow it has to be for the
sytem to be stable.

Of course this is really had to do without seeing the circuitry in
action.

Thanks for the reference, Bill. I did learn something of value from it but
the devices are clearly intended for audio frequency although one might
actually support 0dB gain at 455kHz! However, they are a closed loop system
and it's not obvious that one could bring out the required control voltage
to drive the receiver AGC.

Regards,

Tom

"Netgeek" wrote in message
...
Hi Tom,

You might find some good info from reading the description and
data sheets for the Analog Devices SSM2165 and/or SSM2166
at www.analog.com. They address the issues of having a threshold
which can be adjusted and then varying the amount of compression
or limiting asymetrically. Perhaps you could modify your circuit to
emulate some of these features - or perhaps just use the devices
described?

Bill

wrote in message
oups.com...
This sounds like a classic negative feedback oscillation. You sense the
signal is too large, so you send a signal to kill the gain, and then
you sense the signal is too small, so you send a signal to increase the
gain.

Having different attack and release time means you have two different
time constants My guess is the quick attack leads to the instability,
since it is the lesser damped system. If this is true, then you should
concentrate on the attack time, i.e find how slow it has to be for the
sytem to be stable.

Of course this is really had to do without seeing the circuitry in
action.

In article ,
"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.

Snip

I don't know receiver design but I have a RX340 that uses the following
settings. Attack is in dB/mS, Hang in seconds, and Decay are in dB/S.

Attack Hang Decay
Fast .8 0 1600
Medium .8 0 100
Slow .8 0 25

Programable attack .01 to 1 dB/mS
Programable hang 0 to 99.9 seconds
Programable decay .01 to 99.9 dB/S

--
Telamon
Ventura, California

Telamon, those are interesting numbers, expressing the action of a more
sophisticated, programmable, digital AGC. Classic analog AGC speeds are
expressed as the length of time it takes to reach a certain percentage
or within a few dB of the desired gain setting, i.e., similar to and
based on RC time constants as that was the foundation of the classic
AGC control system. With an RC derived control, whether the gain change
is 10 dB or 100 dB, it takes the same time. With your digital control
in 'Fast' mode, attack would be 8ms for 10 dB and 80 ms for 100 dB;
release would be hang time plus 6ms or 60 ms respectively. It's
interesting how these compare with my target of 1-13 ms attack, 25-50
ms release.

I'm wondering how your RX340 behaves when you program to 0.01 dB/ms
attack, 0 hang, and 1600 dB/s decay (but I see that the programmable
decay is limited to 99.9? probably for good reason!). That would
correspond to my Fast target when you tune from no signal to S9+50.

Regards,

Tom

On Tue, 24 May 2005 23:08:51 -0400, "Tom Holden"
wrote:


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.


Tom,

The problem is the system (AGC) is a servo loop and the loop lacks
damping so it overshoots then rings at the loop time constant.

Try adding a simple RC of something like 1000 ohms and 100uf in the
place where your putting a time constant. The high series resistor
will allow fast change but the series capacitor will absorb some of
the rate of change. You may have to emprically tweek the values.

Also Experimental Methods in RF Design (ARRL Press) is a good
reference and has examples plus discussion on the subject..

Allison