[ARC5] A discussion on AGC.

[Kenneth G. Gordon]

On 9 May 2017 at 9:38, Dennis Monticelli wrote:

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

Good. Thank you.

> 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.

Yes. I completely agree with you and I have been saying the exact same thing for years.

> 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

Correct.

> 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.

Well, yes, but in addition, the AGC voltage was taken from some place in the circuit OTHER 
THAN the detector. Separating the two makes a huge difference.

>  For some historical 
> reason unknown to me this type of detector became widely known as the product detector.

Supposedly because the output was the "product" of the two inputs, and not a straight simple 
addition. Supposedly also if the BFO signal is missing, output is zero, so an AM signal 
applied to the input of a true "product" detector would result in zero output.

However, I have never believed that.

One of the simplest product detectors, first used by Collins in the KWM-2, a simple triode, 
demodulates AM just fine.

Heathkit used that PD in almost all of their rigs.

I also wonder about the term, "linear mixer": how can any "mixer" be "linear" since it must be 
"un-linear" in order to work?

Even so, the so-called "product detector" is about the best detector available for SSB and 
CW.

Ken

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