Re: [IRCA] frequency measurement accuracy

["N. Hall-Patch" <nhp@xxxxxxxx>]

Quoting Chuck Hutton <charlesh3@xxxxxxx>:

> Is the main AM carrier locked to GPS with IBOC, or is GPS used to set the
> subcarrier frequencies, spacing, guard time, etc? My impression is the
> latter, at least for the installed base of legacy equipment. I bet Barry
> McLarnon can verify that.

Would like to know also about this.  Have been working on the belief that the 
carrier frequency among others is locked to GPS, and that IBOC stations use 
the same frequency determination whether they're using IBOC or not.
> 
> Now back to the comparison of Bill's number with yours: the numbers were
> taken 4 or 5 month apart, right? Were you both calibrated right before the
> numbers were taken? If not, there's uncertainty raised by receiver drift,
> and for that matter we couldn't rule out transmitter drift although I'd
> expect that should be smaller.

Not home right now, and the topic's probably getting old for most readers, but 
quickly....the R8 needs to be calibrated at the time of the measurement.  This 
is an instrumentation technique that long predates my own work involvement 
with such things.  

> I wonder how much drift there is in the R8? Were you to tune to 10,000.400
> and use Spectran to calibrate the R8 against WWV, how long does it take for
> the R8 to drift 1 Hz? 2 Hz? How long had the R8 been on and been stabilizing
> before the 2 numbers were taken?


During the calibration period (which can be on the order of minutes, as some 
frequencies are repeated), there seems to be little apparent drift (i.e. < 0.1 
Hz as measured using Spectan).  I've been surprised to see the R8 stabilize 
within less than half an hour, but will look carefully at this upon my 
return.  Incidentally, I try not to use WWV above 5 MHz due to possible 
Doppler effects depending on geomagnetic conditions.

Oh yes, transmitter drift...some do, though back in the early days of 
precision frequency measurement, the IBU used to log the big Euros (do I 
remember right, Charlie?) and drift was minimal; hence the point of PFM I 
thought...you "ID" the station through its repeatable carrier frequency.
> 
best wishes,

Nick



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