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

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.

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

That's the meaning I know -- and it's probably more appropriate for the
purposes of keeping the receiver noise figure up.

"Doug Smith W9WI" wrote in message
...
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.

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

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

73
JM
----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/

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

On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden"
wrote:


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."

Mentioning a certain voltage level doesn't really make so much sense
when you don't know what the rest of detector stages looks like. Drake
2-B has 5Vpp from 6BA6, and R-4C somewhat less, but not too important.
This is only a reference which may not apply to any other receiver
because it is another AGC amplifier which also amplifies IF to the AM
detector for 2-B, and product detector fed via a voltage divider. For
R-4C everything is totally different, and the 4-5Vpp level seems more
chosen for economical reasons. Another English LF communication
receiver I checked had 80-100V pp IF output. The level to choose
depends on good AGC characteristic, and usually the IF level should be
certain level above the background noise, possibly 10-20dB for good
operation. Some receivers have not particularly good AGC, Atlas 210X
is one, and my Yaesu FT-902 is another, even worse is FT101B because
carrier oscillator leaks into the IF and AGC threshold must be set
10dB above the level you would want it to to start, simply because it
can't operate properly below it.
It was a surprise to learn that the Lorenz 6P203 receiver operated
nicely on SSB when BFO level was increased, in spite that it has only
6AV6 detectors, but IF is split, and it has two different 6BA6's with
different diode detectors, one for AM/CW/SSB and one for AGC.
http://home.online.no/~la8ak/b71.htm
Siemens Rainbow receiver also had some improper connection between
product detector and BFO, and could detect SSB well when this was
corrected http://home.online.no/~la8ak/b72.htm
Heathkit SB300/301 have bad AGC, but may be easily improved a lot by
using 1N4148 AGC detector diodes (voltage doubler), simply because the
original diodes have too much capacitance and do not rectify properly.
I did some experiments using Drake 2-C type and R-4B type AGC
detectors in my Kenwood TS-500, later in 2-B, and could set the
detector output variation from AGC threshold to as little as 1dB, but
it seem no real point since the subjective sound seem best with at
least 6dB variation, but wasn't too important if it was kept as
original 10dB for 2-B, it is only important when you have a lot of
advanced measuring equipment in the shack, but not at all for the
radio operator.

73, LA8AK


----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/

Jan-Martin Noeding, LA8AK wrote:
Mentioning a certain voltage level doesn't really make so
much sense when you don't know what the rest of detector
stages looks like. [snip]

Thanks for the interesting examples, Jan-Martin. The quote was from the 2004
ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical
superhet receiver with AGC applied to multiple stages of RF and IF". The RS
DX-394 bears some resemblance to this with a resistor in place of the "Delay
Diode". It beats me how the AGC in this HB example actually controls the RF
gain - the diode appears to block control. A second one in parallel in the
opposite direction seems to me to be needed.

Tom

Jan-Martin Noeding, LA8AK wrote:
Mentioning a certain voltage level doesn't really make so
much sense when you don't know what the rest of detector
stages looks like. [snip]

Thanks for the interesting examples, Jan-Martin. The quote was from the 2004
ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical
superhet receiver with AGC applied to multiple stages of RF and IF". The RS
DX-394 bears some resemblance to this with a resistor in place of the "Delay
Diode". It beats me how the AGC in this HB example actually controls the RF
gain - the diode appears to block control. A second one in parallel in the
opposite direction seems to me to be needed.

Tom

"Jan-Martin Noeding, LA8AK" wrote in message
...
On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden"
wrote:


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."

Mentioning a certain voltage level doesn't really make so much sense
when you don't know what the rest of detector stages looks like.
73, LA8AK


Jan-Martin,

I think the issue is that it is NOT a time delay, but a signal level
delay. The AGC does not begin to reduce receiver gain until the signal
reaches some defined LEVEL.

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

"Jan-Martin Noeding, LA8AK" wrote in message
...
On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden"
wrote:


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."

Mentioning a certain voltage level doesn't really make so much sense
when you don't know what the rest of detector stages looks like.
73, LA8AK


Jan-Martin,

I think the issue is that it is NOT a time delay, but a signal level
delay. The AGC does not begin to reduce receiver gain until the signal
reaches some defined LEVEL.

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