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

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w3jn:
Don, after re-reading your post I think I see where your confusion comes from.

The audio output of a Costas loop, or a sync detector like the YRS-1, or even a sideband slicer like the Central Electronics, is a function of how the I and Q channels are combined in the matrix output.  The matrix can be configured to reject USB, reject LSB, or combine both for USB+LSB. 

The CE slicer has all such options, but its BFO isn't phase-locked - it's free running.  It would probably be fairly easy to convert the CE slicer to a sync detector -either Costas or phase locked to the carrier.

Frank - that's certainly one option, but I coulnd't hear much, if any, increased distortion with the wider loop.  It would be nice to automagically switch between PLL and envelope detection if lock is lost.

k4kyv:
Since sometimes with selective fading you may lose carrier but retain both sidebands and at other times sideband information may fade while the carrier is retained, a syncing system that simultaneously sampled both the sideband phase info and the carrier, and used the information from one or both to set the carrier would more reliably maintain lock than either the Costas Loop or the carrier PLL system used alone.

w3jn:
The time constant of the PLL is slow enough to ride thru loss of the carrier.  The slowest time constant of the SE-3 is about 10 seconds (the FLUTTER position).  This mode is for copying signals with significant phase distortion due to ionospheric disturbances, according to the manual.  I suspect a well-designed Costas loop would use a similar slow time constant to ride thru loss of one or the other sidebands.

In actual use I haven't had it lose lock on a fading signal, even listening to some of the PW tropical band broadcashers around 4900-5000.

You sound outstanding all the time, Don, but the other night when we had that QSO the SE-3 really drove home the fact that your audio is extremely clean and undistorted.

AB2EZ:
Questions about synchronous detectors

For synchronous detection of AM, I think one would be more concerned (in terms of subjective performance) about frequency "lock" (i.e., within .1 Hz) than phase lock. Is that correct?

If so, and given that broadcast stations (as opposed to vintage amateur stations) have very tight frequency control, wouldn't you want the sync detector (when listening to a broadcast station) to stay right on the frequency it was last locked to (assuming the sync detector has a very stable frequency reference) in the event of a carrier drop out? I.e. you would push the "search and lock" button only when tuning in a new station. When the carrier is present, the detector would phase lock. When the carrier drops out, the detector would keep its locally generated, surrogate carrier frequency right where it currently is.... until the carrier returns.

When listening to a vintage amateur transmitter in the presence of selective fading, but no significant interfering carrier(s), I would think you would use a carrier discovery algorithm whose search range increases the longer the carrier has dropped out. If the dropout is less than 1 second, the algorithm might keep the frequency of the surrogate carrier right where it is. Beyond that, it might begin to look for the carrier over a wider search range (e.g., +/- 50 Hz).

When listening to an amateur transmitter with a very stable frequency... I would think you would revert back to the algorithm used when listening to broadcast stations.

When listening to a vintage amateur transmitter in the presence of a significant interfering carrier... I would think that you might fall back on yet another algorithm.

So... wouldn't a modern synchronous detector... which uses a demodulation method that comes to life on the combination of a digital signal processor and a general purpose microprocessor (software defined)... have much more sophisticated and much more effective algorithms than any vintage or even relatively recent sync detector?

Stu

w3jn:
Stu - no, you need phase lock.  .1 Hz difference will cause an annoying buildup and fade of the audio due to periodic incoherence with the carrier.

Due to the slow PLL time constant, the sync detector will stay pretty much on frequency during no signal intervals.  So if someone comes back and is almost (but not quite) on freq the SE-3 will immediately lock.

You don't need to hit the lock switch during tuning intervals at all if you're not bothered by heterodynes as you tune around.  Just tune close to the signal and the SE-3 will lock right in.

If you're offset tuning a signal (to escape interference, or to put the whole BW of your filter towards one sideband) the SE-3 has a switch whereby it will maintain lock (ie speeds up the PLL time constant) as you tune.  As soon as a signal comes on frequency, the PLL will attempt to lock to it.  How fast it does so is entirely dependent upon the PLL time constant, and how far off frequency the sync detector's LO is from that signal.

Stu, you're overthinking this with all these algoritms  ;D  In practice, the stock SE-3 works very well if everyone in the QSO is within +/- 150 Hz.  If they're a KC or two off, then the PLL time constant needs to be shortened so it can capture the off-freq signal quickly.  The mod described in the Receivers section of the Online Handbook accomplishes this well.

In a typical QSO you don't even notice the modified SE-3 locking/unlocking.

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