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

"Tom Holden" wrote in message
.. .
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


Tom,

I don't have the circuit, but have two diodes pointing down. Resistors
on both anodes to +V. Common cathodes with resistor to GND.

+V
___|______
| |
R1 R2
a_| |_b
| |
V V
- D1 - D2
| |
| |
__________
|
R3
|
|
GND

Changing the voltage at point "b" will cause "a" to also change. Another
way to look at it is that you steal different amounts of current away from
D1. Go too low at "b" and "a" won't change any more. It stops. However,
go higher on "b" and "a" keeps going up. It is a limiter.

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

Steve Nosko wrote:
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.
I don't have the circuit, but have two diodes
pointing down. [snip]
That's interesting, Steve but the circuit looks like this:

RF Amp----MXR------IF Amp
| |
R R
|------|---------|
| | D |
C R ---from AGC
| |
-----
|
GND

Just noticed an asterix by the 'delay' Diode that says it may be replaced by
a test-selected resistor - that's the way the DX-394 is done.

Tom

Steve Nosko wrote:
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.
I don't have the circuit, but have two diodes
pointing down. [snip]
That's interesting, Steve but the circuit looks like this:

RF Amp----MXR------IF Amp
| |
R R
|------|---------|
| | D |
C R ---from AGC
| |
-----
|
GND

Just noticed an asterix by the 'delay' Diode that says it may be replaced by
a test-selected resistor - that's the way the DX-394 is done.

Tom

"Tom Holden" wrote in message
.. .
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


Tom,

I don't have the circuit, but have two diodes pointing down. Resistors
on both anodes to +V. Common cathodes with resistor to GND.

+V
___|______
| |
R1 R2
a_| |_b
| |
V V
- D1 - D2
| |
| |
__________
|
R3
|
|
GND

Changing the voltage at point "b" will cause "a" to also change. Another
way to look at it is that you steal different amounts of current away from
D1. Go too low at "b" and "a" won't change any more. It stops. However,
go higher on "b" and "a" keeps going up. It is a limiter.

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

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.