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/