[ARC5] Product detectors - the name.

Wed May 10 10:36:28 EDT 2017

Les,

Yes, you're right.  I also believe the name derives from that very math
product.  I just don't know why the term become commonplace only when the
more linear detector types were employed because all mixers whatever we
call them obey the same basic math whether linear in response to their
inputs or not.  If they didn't there wouldn't be any of the sum and
difference frequencies of which we are so familiar.  I think part of the
confusion stems from the many names our field has given mixers over the
decades.  It sounds like there are many different principles are work.

There also seems to be some confusion about how to reconcile the need for
non-linearity to produce mixing action with the desire for linearity in
response to input signal.  The device or devices producing the mixing have
to be driven into their non-linear regions by a strong signal (such as the
BFO in the case of a detector) for that trig identity to show up within a
rather long math equation.  Linearity means the circuit is capable of
producing a desired output that faithfully follows the amplitude and phase
of the input signal. Double the input and the output doubles. No
distortion.

Dennis AE6C

On Tue, May 9, 2017 at 11:59 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

> 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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