[R-390] Tuning SSB

[Jim Whartenby]

The link seems to me to be blurring terms.  The product detector is described as equal to coherent detection which I believe may be better described as John Costas' synchronous detection.  I don't see how a product detector oscillator is coherent with the transmitted carrier.  The product detector frequency may be very close to being spot on with the transmitted carrier frequency but not so with respect to phase.  

DSBSC reception using synchronous detection has advantages over SSB in the presents of jamming according to Costas and is much less complicated to transmit.  There is no need for linear power amplifiers or high Q filters.  The original question was a comparison of the product detector with the envelope detector but it seems to have quickly become, at post #3 and after, about synchronous detectors vs envelope detectors.

Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Monday, September 30, 2024 at 10:46:03 PM CDT, Jacques Fortin <jacques.f at videotron.ca> wrote:   

 Hello Giovanni,

Noise reduction of an amplitude-modulated signal by a product demodulator vs an envelope detector is a verifiable fact.
I have found many explanations for this, but I believe that the thread from the following shortcut will provide a valid answer to your question:
https://www.edaboard.com/threads/envelope-dectector-vs-product-modulator.262958/

For the R-390 and R-390A cases, the AGC system action time needs to be "speeded up" when SSB is received, without changing the global AGC behavior VS the AM case.
In other words, we need to keep the same AGC characteristic for both, because the global receiver behavior and the signal meter calibration depends on it.
Many attempts were made up to now (and by many people) to "improve" the AGC system of these receivers and the search for the best solution continues.
Some already available solutions uses external "add-on" circuitry to achieve the necessary goals, but simultaneously implies some compromises vs other receiver features, like the use of the MGC mode and the Noise Limiter circuit when the product detector is activated.
What we try to develop for some time is a self-contained solution with no "compromises"...
In that respect, I still have some (more or less simple) AGC circuits to try.
Larry Haney, on his side, have put many efforts on improving the tube-based product demodulator using a 6BY6 that replaces the 6BA6 BFO oscillator.
What we usually call a "Lee" product demodulator.
And he is actually testing a noise limiter circuit compatible to both AM and SSB reception modes.
Maybe someday we will combine all of our circuits together.

Stay tuned...

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la part de Ing. Giovanni Becattini via R-390
Envoyé : 30 septembre 2024 18:10
À : Larry Haney <larry41gm2 at gmail.com>
Cc : R-390 Forum <r-390 at mailman.qth.net>
Objet : Re: [R-390] Tuning SSB

Hi & Thanks.

"Another important 'feature' of using a product detector is its natural reduction of interfering noise coming in on your antenna.  The amount of noise reduction depends on the type of noise it is, but can be from 40% to 75%.”

Would you please explain to me why it reduces the noise? Because the BFO signal returns back?


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