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

[Ben Dover]

At 03:08 PM 9/1/2005 +0000, you 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.      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.     Why the emphasis on
>stability and not tracking?
>
>Eugene  


The choice between stability and tracking depends on what you're trying to
do.

Others have already mentioned the main point; both transmitter & receiver
stability are already on a high enough level for most purposes. In some
applications (coherent detection schemes) even higher levels of stability
are needed, and can be achieved without a whole lot of trouble, tho it
tends to get to be cumbersome.

However, tracking is useful in at leat onecommon application that I know
of; synchronous detection of AM and related modes.

One of my receivers (RA-1772) has an AFC option in it; using it, that rig
is a killer receiver for AM BCB DXing! Another place where it's proven to
be useful is in experiments done by Radio Havana with SSB broadcasting. On
occasion they've sent out SSB with a low level "pilot" carrier that the
AFC can lock onto.

Another of my receivers (Drake R8) includes a sync detector, but I find it's
not nearly as effective as the AFC in the Racal; it loses sync lock much 
more easily.

The idea of using a pilot carrier is an old one, used with a LOT of modes
where very high precision is needed; FDM (frequency division multiplex) and
the ISB are prime examples; the old Voice of America "feeder" ISB channels
also used the pilot carrier idea. I just wish I'd had my 1772 back when those
were still in heavy use!


73's,