Jan-Martin Noeding, LA8AK wrote:
On Mon, 23 Feb 2004 10:28:19 GMT, Doug Smith W9WI
wrote:

Tom Holden wrote:
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".

I always thought "delayed AGC" means that there's no
gain reduction unless the strength of the incoming
signal reaches a certain threshold. It's not a delay in
time, but in amplitude.

Yes, this is correct for the proffesionals, a certain
amplitude level must be reached before the AGC threshold
is hit, while amateurs started to talk about time delay,
rise and fall times when AGC was optimized for SSB
reception. But even receivers designed before WWII had
some degree of mode-dependent time delay optimization,
fast for AM and somewhat slower for CW
I experienced the importance of proper time constant when
I tried to improve the Collins 51-S, see
http://home.online.no/~la8ak/b35.htm

I seized on the word "delayed" and linked it to the inclusion of a longer
time constant in the AGC to the RF stages of the DX-394 than the time
constant in the AGC to the IF stages. The Handbook example actually says "As
an option, the AGC to the RF amplifier is held off, or 'delayed', by the
0.6V forward drop of the diode so that the RF gain does not start to
decrease until larger signals appear. This prevents a premature increase in
the receiver noise figure. Also, a time constant of one or two seconds after
this diode helps keep the RF gain steady for the short term."

I think 'delay' is a misnomer if what we have in effect is a higher
threshold of signal strength for activation of RF AGC than for IF AGC.
'Two-step' AGC might be a better description. 'Delay' seems a more
appropriate term for the way AGC is implemented in the DX-394. RF stage
attack speed is slower (extra RC time constant probably on order of 100ms)
than that for the IF stage and is clearly 'delayed' in reaching steady
state, no matter what the signal level change is, as long as the AGC is
activated by the higher, later level. Release speed of the RF AGC is
similarly slowed or 'delayed' relative to IF AGC, certainly not by the 1 or
2 seconds in the HB example.

Having removed the 'delay' capacitor, I have not noticed any deleterious
effect. What should I be looking for? Would there be some advantage in
revising the RF AGC to achieve a 2-step AGC?

73, Tom