[Premium-Rx] real basic question on stability etc.

[mikea]

On Thu, Sep 01, 2005 at 03:08:56PM +0000, Eugene Hertz wrote:

> Ok, here's a novice question. I know that receiver stability is a
> big topic of discussion. The more stable the better. But it seems to
> me stability for a receiver is only as useful as the stability on the
> transmitter?! Of course, if both Tx and Rx are drifting, then unless
> they are drifting in the same direction at the same rate it certainly
> would be better to have only one node drifting as opposed to both.

> I recently acquired a receiver (not premium) P-1524(P)/WRR submarine
> countermeasures receiver 30Mhz-1,000Mhz with plug in RF tuning stages
> and it has this neat feature (probably common, but new to me) where
> it has AFC automatic frequency control. It works pretty well (to my
> unsophisticated mind). Essentially adjusts the LO up or down to match
> the carrier of the transmitter. I would imagine a feature like that
> to perhaps be more valuable than stability. Who cares what the LO of
> the receiver actually is, as long as its locked onto the transmitter.
> Imagine being able to track the tx no matter how much it is drifiting.
> That to me should be a great design goal. Now, I would imagine AFC
> would be darn near impossible for ssb. But FM, AM and maybe even CW
> could benefit.

> I am sure I am missing something. Why the emphasis on stability and
> not tracking?

Because typical synthesizers nowadays have very low drift rates, and  
most transmitters (excluding some of the glowbug stuff) use these     
synthesizers. That being so, the transmitter's signal will tend to    
stay in the receiver's passband for long periods.                     

I'm not sure what the P-1524's designers had in mind when they
added AFC to the feature suite, but suspect that it had to do with
listening to a particular sort of signal source, possibly one that was
"interesting" but known to drift. Nowadays, the only thing I can think
of that would make AFC really useful would be communications with
satellites not in synchronous orbits: the Doppler shift from, say, a
QSO with the ISS makes retuning pretty much necessary, and it would be
nice to have that automated.

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin