BC-348 AVC

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W2JBL:
clamp your HS-53 cans real tight to your head. tune in WSM on 650KC. key your interphone and say "captain i've got some great music coming from the states- do you want me to patch it over the interphone?" last time the touring B-17 and B24 came by the local airport (a few months ago) i stopped while touring the plane, sat down and cranked that BC-348 to 3885... then while nobody was looking i also slammed the 80meter tuning unit into the '17's festering BC375. if anybody out there tours the Collings Foundation's B-17 "909" and finds the rig on 75AM it was only me...

k4kyv:
Quote from: w3jn on November 13, 2006, 02:30:17 PM

THat's one of the reasons BFO signals are very lightly coupled to the IF.  You can improve SSB reception by increasing the coupling, but at the expense of decreased sensitivity due to the AVC action (resulting from the BFO signal) decreasing the gain of the receiver.

The other reason, as I noted, is to prevent the BFO from phase-locking to the incoming signal.

The first problem can be addressed by turning off the AGC.

I discovered the second phenomenon while living in Houston in the late 70's.  The HRO Sr. had too little BFO injection with that little fabricated coupling capacitor consisting of a chip of bakelite separating  two small rectangular metal plates.  It worked ok for weak-signal cw, but there was not enough injection for strong cw signals, and it would not demodulate SSB very well.  So I used the injection  capacitor as a terminal strip and soldered a larger cap - something like 50 pf in parallel with it.

With increased  capacitance it demodulated SSB almost as well as a product detector, and with AGC off, the sensitivity on SSB/CW did not decrease.  Even  with the stock coupling capacitor as I recall, that receiver never worked well with the BFO and AGC on at the same time, so I always used the classic procedure of turning the BFO on, AGC off, audio gain near maximum, rf gain near minimum, and advancing the rf gain for the most comfortable copy.  The result was a crystal clear cw or slopbucket signal with absolutely no "pumping" sound.

One night I noticed that with a stable AM carrier, I could turn on the BFO, set the carrier to the centre of the passband, and zero-beat.  With careful tuning, I would  hit a point where the AM audio, especially from a marginal station, suddenly seemed to jump right up above the QRM/N and out  the speaker.  I had found the point where the BFO phase locked with the AM carrier.  This was  my first experience with a crude, but effective SYNCHRONOUS DETECTOR!

I used the receiver in that mode to copy weak AM signals until I replaced it with the 75A4.  Of course, when copying SSB or CW, if the signal was too  strong, the BFO would FM, resulting in gibberish or chirpy cw.  Reducing  the rf gain always solved that problem.  But I found, that with a stable AM carrier, carefully zero-beating would result in phase-lock, even with the weakest AM signals.  Usually there was enough drift that I  had to touch up the tuning every 30 seconds or so to restore lock.  But what else could be expected with this crude, brute-force method?

There is a construction article in one of the old GE Ham News flyers for building a phasing-type "sideband slicer."  For USB and LSB and AM it uses a product detector.  For SSB and CW the phasing method is used to reject the unwanted sideband.  For AM, a (tube type) phase-locked loop circuit phase locks the BFO to the AM carrier, but the sideband rejection feature is turned off, so that true DSB reception is achieved.

The advantage of the synchronous detector is that both sidebands are received simultaneously, with a 3 dB gain in audio output due to the vector addition of the two sidebands.  The output of the heterodyne  detector is simply the product of the BFO and the two sidebands (or one sideband with SSB).  With a conventional envelope detector you have the heterodyne product of the AM carrier and the sidebands, but also of the QRM and the sidebands, the AM carrier and the QRM, and every discrete bit of QRM heterodynes against every other discrete bit of QRM.  Basically, everything within the passband intermodulates with everything else within the passband.  Together, all these intermodulation products decrease s/n ratio and intelligibility.  Of course, the intermodulation between the AM carrier and the two sidebands is how the detector functions.

Most the apparent "superiority" of SSB over AM is due to the advantage of the product detector at the receiver, and is not inherent to the modes themselves.

With the synchronous detector, DSB retains the primary advantage of SSB, reception with the product detector.  The problem with receiving AM on a product detector is that the BFO signal must be not only exactly zero-beat with the AM carrier, but it must be precisely in phase or 180° out of phase with the original carrier.  Thus the injected BFO signal is synchronous to the AM carrier.

I have seen the description of "synchronous detectors" on the market within the past decade or so that not only lock onto the carrier, but reject one of the sidebands of the AM signal.  These are  not synchronous detectors at all; they are nothing more than classic sideband slicers.  Disregarding possible differences in audio fidelity, you can do equally well by simply copying the AM signal in conventional USB or LSB mode if the receiver has a product detector.  A true sync  detector takes advantage of the vector additive effect of the two sidebands plus the advantages of the product detector.

w3jn:
GE marketed that sync detector commercially as the YRS-1.  It's very interesting in that it will sync up to a DSB suppressed carrier signal - ituses the phase differences between the sidebands to phase lock the BFO so it doesn't need a carrier.  It has 2 separate detectors, and both I and Q channels which are then added/subtracted to select USB or LSB.

I have one, but I haven't been able to get it to phase lock.... another one of the million projects on the bench.

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