[ARC5] Product detectors - the name.

[Leslie Smith]

Hello Dennis,
I always believe a product detector followed the trig function:

sin(A) * cos(B) = 1/2[sin(A+B) + sin(A-B)].  

It's the PRODUCT of two trig functions to give the sum and difference.
I have no idea why I believe this, but I suspect it's true (for a
product detector, or product mixer)
Put in two signal and get the sum and difference.  Isn't maths amazing!!
Isn't electronics amazing!!  If anyone knows more than this - I'm
interested!

   73 de Les Smith
   vk2bcu at operamail.com

On Wed, May 10, 2017, at 03:43, Kenneth G. Gordon wrote:
> 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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