[Premium-Rx] Synchronous detector for the WJ and Harris receivers

[Michael O'Beirne]

Hi all,
    This is my first go at mailing you, having been a member of your group for all of a week.  I am dead keen on quality reception and I have spent a great deal of time in researching the topic.  Here are some thoughts.
    The synchro design of the Racal RA1792 / RA6790 is excellent and relatively simple and has none of the locking problems of some PLL designs.  However, in my view, the very best will be obscure for non-UK members but is worth thinking about.  The design is by Trevor Brook of Surrey Electronics published in the UK's Electronics & Wireless World in September 1989.  This is a highly respected professional journal, not a hobby mag.  It works from an IF of 455kHz and features two separate detectors:-
    1.    A precision envelope detector.
    2.    A fairly complex synchronous detector using an SO41P high performance limiter for very low AM-to-PM conversion and good limiting action down to very low input levels.  This IC is made by Siemens and is the only part likely to be tricky to locate.  All the other components are pretty standard - eg 3N201, TL 084, BC549, TBC549, BC309 2N2369, 4013, 4066, and 7508 regulator.
    The output of the limiter is squared and feeds a balanced modulator.  
    Op amps form the main loop filter. There are three switchable loop bandwidths of 12kHz,  2kHz and 60Hz.
    A Colpitts oscillator runs at 1820kHz (ie 4 x IF) and the output is divided by 4 and is fed in quadrature to drive the balanced modulator and demodulators.  The outputs are so phased so as to give:
a.    DSB
b.    LSB
c.    USB
d.    ISB/Stereo spread with LSB to the left and USB to the right, which gives interesting effects on fading signals.
e.    Quadrature which gives a null on the audio from the strongest station on the band, thus improving the audibility of any background station.  This position can also receive NBFM or any other phase modulation.
    The power requirement is 10 to 16v at 50mA
    The envelope detector and synchro modes all yield a total harmonic distortion below -44dB (0.6%) at 100% modulation at any frequency, and the response is 20Hz to 7kHz at +/- 0.5dB.  The lack of steep mechanical or crystal filters reduces considerably the phase distortion that such filters introduce, particularly early narrow Collins filters.  
    If the carrier fades badly, the loop remains very close to its last frequency indefinitely and re-acquires lock within a few milliseconds of the signal reappearing.
    I listen to the output direct into decent Sony headphones.
    It will of course demodulate CW and FSK with ease, and the oscillator is reasonably stable for long periods.
    The circuit is far more complex that the brief outline, but should not be too difficult to build.
    I have the author's original article and pcb top layout and I copy it to anyone who may be interested via slow mail.
    This design was going to go into production (I have a pre-production model), but Trevor hit patent problems from another UK manufacturer and went no further.  However, the design is published and is reproducable on an "amateur" basis.
    I hope this helps.  
    You all seem a great lot.
    73s
    Michael O'Beirne
    G8MOB
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