[R-390] Best approach for SSB mod on R-390A

[James A. (Andy) Moorer]

This is a complex subject, but I will try to give 2 short answers:

(1) In a fully-analog design, what you do is design a PLL with multiple 
FET-selectable loop bandwidths. Generally 2 (narrow and wide) is enough, but 
I have seen as many as 5. The trick is deciding when to switch from "hold" 
(very narrow) to "capture" (very wide). There is generally some kind of 
signal-to-noise detector that compares the amount of signal in a wide band 
to the amount of signal in a narrow band, but the circuitry gets pretty 
baroque very quickly. It is hard to make this design work reliably over a 
wide range of operating conditions. The PLL tends to get sucked into nearby 
hot stations, or gets stuck holding in noise after the transmitter drifts 
off one way or the other.

(2) The classy way to do it is put a computer and DSP in the loop where the 
oscillator is tuned with a D/A converter from the computer. The computer and 
DSP do digital spectral analysis and look at the signals all around, then 
decide what number (frequency) to stuff into the D/A, and consequently what 
the oscillator frequency should be. This can be made arbitrarily smart, 
although more often than not it is still pretty dumb. On some of the $$$ 
receivers, this "digital detector" is pretty good, like the high-end R&S 
offerings these days. Not for the casual listener, of course. If you have 
enough DSP horsepower, you do it all digitally with no hardware PLL at all. 
Being a DSP-head, this is my preferred implementation, but it takes a lot of 
careful DSP programming to make it work well.

James A. (Andy) Moorer
www.jamminpower.com



----- Original Message ----- 
>
> If you were designing a PLL synchronous detector that attempted to 
> maintain phase coherence during a selective fade lasting several seconds, 
> what loop bandwidth would you choose?
>
> Given such a loop bandwidth, how would you arrange to achieve lock after 
> tuning the receiver without waiting an inordinate period of time?
>
> Not that I'd ever spring for a Sherwood SE-3.
>
> Best regards,
> -Steve
>
> -- 
> Steve Byan <stevebyan at me.com>
> Littleton, MA 01460
>