Re: Sherwood SE-3 Sync Detector mod for ham use

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k4kyv:
Do those demodulate in true DSB mode? 

Some of the so-called "sync detectors" I have seen advertised and reviewed are little more than SSB product detectors that eliminate one sideband or the other by the phasing method.  In other words, it is essentially the old "sideband slicer" concept with a PLL carrier lock feature added.  You could do almost as well by simply copying an AM signal using any receiver equipped with a product detector, in SSB/CW mode.  The only advantage of these devices is that the BFO locks onto the carrier to eliminate the tuning error. 

A true DSB sync detector receives both sidebands at the same time, and locks the BFO exactly on frequency and in phase with the original carrier, so that you get the full advantage of coherent double-sideband reception plus the advantage of demodulating with a product detector.  In most sync detectors the lock is achieved using a PLL to lock onto the existing AM carrier.  But there is also the Costas Loop, which uses the relationship between the mirror-image sidebands to establish lock, ignoring the carrier altogether.  In fact, that is the only kind of detector that can properly demodulate DSB suppressed carrier signals.

I seriously doubt that the Sherwood SE-3 uses the Costas loop, but I am wondering if it really demodulates in a true DSB mode.

Are they still being sold new, and how much does (or did) one cost?

w3jn:
Nope, the SE-3 doesn't use a Costas loop, nor are the sidebands selectable.
It is a true PLL sync detector.

BTW just because a sync detector uses the phasing method to select sidebands doesn't mean it isn't a sync detector.  The GE YRS-1 is one such example.

The signal is split in two and feeds two 6H6s, which are fed with BFO signals 90 degrees out of phase.  The DC output of the I 6h6 feeds a reactance tube, which phase locks the BFO to the incoming signal.

The I and Q channels each go thru phase shifters and a matrix network cancels the unwanted sideband.

My SE-3 (the later ones may differ) is similar to the sync detector project in the early 90's ARRL handbook.   Uses a NE604 and several NE602 mixers.

k4kyv:
Quote from: w3jn on April 12, 2008, 03:50:25 PM

The I and Q channels each go thru phase shifters and a matrix network cancels the unwanted sideband.

So what is this "unwanted sideband" business? 

If there is always an unwanted sideband when it is demodulating DSB, it sounds to me like it is actually working  like a sideband slicer.  There shouldn't be any "unwanted" sideband, since the detector should be demodulating both sidebands simultaneously with each one contributing 50% of the audio voltage at the output, just as in the  case of a regular envelope detector.

According to the Costas article in the 1956 IRE Proceedings, a true DSB synchronous detector can be configured to reject the interference on one sideband or the other while both sidebands are still contributing to the detector output.  But if the only option is to simply blanket reject everything on the USB or LSB, without the option of receiving both sidebands of the desired signal, it is basically copying AM as a SSB signal with a PLL BFO, and is not a true synchronous detector.

w3jn:
True, Don, I guess I was referring to the sideband reject feature which is very handy if you have QRM above or below.

A Costas loop won't get rid of in-passband interference, as far as I know.  The Costas loop as you noted detects the audio phase difference of the I and Q channels and the error signal drives the BFO.  The matrix is identical regardless of whether the BFO sync comes from the audio or the carrier.  It's also possible to receive USB and LSB in different channels, and feed them into a stereo amp for a quite interesting effect.

THe SE-3, and others like it, are true synchronous detectors in that they sync with the carrier.  A different approach, but still synchronous.

WA1GFZ:
I wonder if you could make this automatic with a monitor on the phase locked signal if you can get at it. Say it loses lock for over .5 seconds then the loop is made wider until it sees a lock again. Then it switches to tight loop mode. Sounds like you could do it with bus switches or analog switches. I built a synthesizer once that worked like that.

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