EDITOR'S COMMENTS ON THE SYNCHRONOUS DETECTOR DOCUMENT COLLECTION:

This collection was compiled by an experienced designer who has been designing and building synchronous detectors to demodulate AM DSB signals for over 30 years.  It includes a number of documents that may be helpful to someone investigating or building synchronous detectors, including DIY construction articles, documentation of some commercial synchronous detectors, and datasheets for components used in some of the circuits or potentially useful for constructing synchronous detectors.  (None of the circuits in the collection are the editor's designs.)

Something can be learnt from each circuit in this collection.  Of course, some of them perform better than others, while some hardly perform at all.

By far the best commercially available synchronous detector is the one built into the Drake R8B receiver (Note: R8B only, not R8A or R8).  Anyone who is really interested in experiencing a state-of-the-art synchronous detector should find a way to test-drive an R8B.  Unfortunately, for someone building his or her own synchronous detector, it operates at the R-8B's 50kHz IF frequency and would need to be re-engineered to work at the far more common 455 or 500kHz IF frequencies.  Just one of the nice features of the R8B synchronous detector is that it will demodulate the upper sideband only, the lower sideband only, or both sidebands (USB/LSB/DSB).  The USB and LSB modes are useful when there is QRM on one sideband.  When there is not, the DSB mode reduces noise by 3dB and reduces the effects of sideband fading (because the same audio frequencies will usually not be faded on both sidebands at the same time).  The R8B circuit is so much better than everything else out there that it may make sense to build a downconverter to convert a radio's IF frequency to 50kHz, and run that into a copy of the R8B circuit.

For someone building a DIY synchronous detector to operate at 455/500kHz or thereabouts, the Drake SW-8 circuit is the best-performing circuit in this collection (indeed, it is BY FAR the best 455kHz synchronous detector that is publicly known, to the best of my knowledge).  Using lower-noise opamps in the phase-shift network and lowering the impedance of the RC pairs (multiplying the values of C1, C2, C3, C4, C6, C7, C8, and C9 by 5 or 10, and dividing the associated resistors -- R6 through R9, R12 through 15, and R30 through 37 -- by the same factor) can bring the noise down by 10 dB or more.

The Trevor Brook/Surrey Electronics synchronous detector works quite well.  The S041P (used as a limiter in that circuit) is difficult to find today, so an alternative may need to be found.  PC artwork is provided.  Note that the original 6-page document contains a few errors.  The 3-page Electronics & Wireless World article from 1989 corrects these.  

The Nagel vacuum tube design looks sound, although I have never tried it.


CONSTRUCTION PROJECT SUGGESTIONS:

If you want to build something from existing plans that just works, build the Surrey Electronics synchronous detector.  You can even make a PC card just like the original using the included artwork.  It is still possible to find the S041P IC if you look hard.  Otherwise, you will need to come up with an alternative limiter circuit.

If you want to do a bit more work, including a layout, build the Drake SW-8 synchronous detector.  If you cannot find the uPC1037 ICs, you will need to improvise.  The NE/SA/602 or SA612 will work with appropriate redesign.  You can look at the SW-8 synthesizer schematic for ideas in this regard (the synthesizer schematic shows NE612s used to replace uPC1037s as the first and third mixers, U1 and U2 -- see notes on the schematic for Revision E13).

If you are looking for something simple that works OK but not great, to satisfy your curiosity or determine if you think investing more time and effort into synchronous detection may be worthwhile, you might want to try the OH2GF circuit (originally presented in QST in 1993 and reprinted in some editions of the ARRL Handbook).


Enjoy!


