Tom,

The term "Delayed AGC" originally referred to the fact that the AGC
action (reduction of gain) does not occur until the received signal reaches
some signal level and NOT to a time delay. I believe this is to allow the
signal to get up to a desirable level at the detector before any gain
reduction occurs, otherwise it won't get to "maximum" level at the detector.
AGC time constant is quite a personal preference and also depends on
different conditions. I prefer slow AGC to prevent pumping and the annoying
bursts of noise between SSB syllables, but if QSB is fast, this can loose
syllables.


--
Steve N, K,9;d, c. i My email has no u's.

"Tom Holden" wrote in message
.. .
In researching desirable AGC characteristics that might be applied to the
RS
DX-394 over a year ago, I came across the terms 'delayed' and 'hang'.
Thought they were interchangeable but on reading the ARRL 2004 Handbook,
it
seems that 'delayed' means that the attack speed on the RF stages is
slower
than on the IF stages or is relatively delayed. According to the HB, "This
prevents a premature increase in the receiver noise figure". On the
DX-394,
I found and removed an electrolytic capacitor in the AGC line of the RF
stages that has the effect of slowing the attack relative to that of the
IF.
Can't say I hear any difference with it out. I would have thought that we
would want the AGC attack to be fast on all stages in order to avoid
momentary overload.

I concluded the following to be good targets for AGC behaviour after
surveying a number of radios - corroboration or otherwise appreciated.

Attack: 1-13ms
Release:
- fast: 25ms
- medium: ~300ms
- slow: 1.8-3 seconds

I thought the fast release to be too fatiguing for human listening to SSB
speech and ICW code but desirable for machine decoded data formats to
minimise loss of data. Also, with audio derived AGC, the distortion on
heavy
bass modulation of all AM modes would be excessive.

In applying mods to the DX-394 by others and some designed by myself,
stretching the release time towards the 'slow' target has the side effect
of
lengthening the attack time to potentially a few hundred milliseconds. My
version is the fastest so far with an attack of about 100 ms on a release
of
2 seconds. I'm wondering if there is much to be gained by struggling to
bring that down to the target of 1-13 ms.

Comments on my assumptions, logic, conclusions and questions most welcome!

TIA,

Tom