Re: [IRCA] frequency measurement accuracy

["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.

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.

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?

Everybody else bored yet?


Chuck

-----Original Message-----
From: irca-bounces@xxxxxxxxxxxxxxxx [mailto:irca-bounces@xxxxxxxxxxxxxxxx]
On Behalf Of Nick Hall-Patch
Sent: Saturday, September 02, 2006 8:54 AM
To: Mailing list for the International Radio Club of America
Subject: [IRCA] frequency measurement accuracy

At 06:03 02/09/2006, you wrote:
>There is an issue of calibrating.  Because of that, I just use it to
>determine activity and measure the relative difference between the
>signals. I measured a spike earler this week at 738.004 right before
>Sunrise.
>
>I am using spec lab. Maybe I need to figure out a better way to
>calibrate it.
>

I'm using Spectran, Bill (and Chuck), but when one sets up a receiver 
for a, say, 400 Hz het, what guarantee do we have for the accuracy of 
the receiver local oscillator?    If a station is right on channel, 
then Spetran/Spectrum Lab should read 400.0 Hz on the het, but that 
is unlikely to be the case with commercial receivers (Al Lehr uses an 
inherently more accurate precision frequency measurement system, 
that's why I deferred to him.  See IRCA reprint T079 for a 
description.  See Lee Freshwater for a CD of all IRCA reprints!)

We can check IBOC stations, which are supposed to be spot on 
frequency, and apply corrections based on that.   For example, KSL 
should be on 1160.0000kHz, but my R8 and Spectran say that when I 
tune to 1160.4 kHz, that a 395.54 Hertz results, so the R8 must be 
nearly 5 Hz in error on that channel....Unfortunately, with the R8 
it's not just an offset; there's a slope correction also (there's 
different offsets depending on the frequency you're tuned to), so I 
need to check several known frequencies.   And that needs to be done 
right at the time of the measurement of the unknown channel to 
hopefully eliminate drift problems.   It's not straightforward, but a 
spreadsheet I made up helps.
The results on off frequency stations have been within a few tenths 
of a Hertz with Al's published frequencies on the MWoffsets group, so 
I'm hopeful.

Comments please?



****************************************************************************
Nick Hall-Patch
Victoria, B.C.
Canada

****************************************************************************


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