[ARC5] A discussion on AGC.

[Dennis Monticelli]

Guys,

I feel compelled to contribute to this discussion on the impact of the
detector upon the AGC.

First of all, the detectors we are discussing are just special purpose
mixers and like all mixers they generate the mathematical products of their
inputs.  Regardless of what we call them....detectors, product detectors,
converters, etc.....the math is the same.

Now, we don't want all of those math products because some of them can
cause trouble.  If we put a strong BFO signal into a simple detector in
order to extract a relatively low distortion CW or SSB demodulated output
then the BFO energy must be substantially stronger than the incoming signal
and that energy can easily overwhelm any downstream AGC pickoff point.

When SSB came into being the linearity of the detector became more
important than ever because even small amounts of voice distortion are
irritating.   To overcome this problem the detector was made balanced with
respect to the incoming BFO.  Properly executed this balance reduced the
amount of BFO energy bleeding into the signal path by a lot.... about
40dB.  Now one could pump in lots of BFO, enjoy low distortion, and not
mess up the AGC.  For some historical reason unknown to me this type of
detector became widely known as the product detector.

Dennis AE6C

On Tue, May 9, 2017 at 8:15 AM, Kenneth G. Gordon <kgordon2006 at frontier.com>
wrote:

> On 8 May 2017 at 22:53, K5MYJ wrote:
>
> > Modern CW/SSB rig us PRODUCT DETECTOTS
>
> True, but those have nothing to do with AGC. In my opinion, the almost
> universal switch to
> product detectors was a very significant advance in receiver technology. I
> really like product
> detectors.
>
> > and the AVC is generated by rectified
> > audio. Not from the rectified RF.
>
> Not true for most modern rigs.
>
> Audio-derived AGC has some, few, advantages, but very significant
> dis-advantages. For one
> thing, it is too slow to react.
>
> All SSB transceivers I have been working with for the past many years use
> an IF derived
> AGC. This includes all of Heathkit's SSB products (SB-100/101/102,
> HW-100/101,
> HW-12/22/32, SB-300/301/303 receivers) among other makers stuff. Collins,
> Drake, Swan,
> Kenwood, Yaesu, Icom, all use IF derived AGC. In fact, I don't know of ANY
> major
> manufacturers of SSB equipment which use audio-derived AGC, although I am
> sure there
> are some.
>
> The "secret" is to take your AGC voltage from some place which is NOT
> effected by the BFO,
> and most specifically NOT from the detector. Usually it is taken from some
> stage well ahead
> of the detector.
>
> In the AN/ARC-5s, that appears to be the case with those receivers which
> derive the AGC
> voltage from the diode in the 12SF7.
>
> I always thought it was very interesting that someone (ARC?) prevailed on
> someone else
> (military, tube makers?) to make or have-made a special tube which was
> (apparently) ONLY
> used in the AN/ARC-5 receiver, the 12SF7.
>
> I know of no other piece of equipment which ever used one, although I am
> sure there were
> some, probably all military.
>
> Which reminds me: I told someone here some time ago I would send two NIB
> 12SF7s to
> him. So many things have intervened in the meantime that I have forgotten
> who that was. If
> you know who that was, please remind me.
>
> They have been sitting on my operating bench in their white boxes for many
> months.
>
> Ken W7EKB
>
>
>
>
> ---
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