[BC] Finding a Bessel Null without a Spectrum Analyzer...

[Burt I. Weiner]

You can still generate a precision +/- 75 kHz signal by using a 
narrow bandwidth communications type receiver.  The receiver should 
have selectivity on the order of 7 or 8 kHz and not much more since 
you need to resolve 13.5 kHz sidebands.  Without any modulation, 
pilot, SCAs,  whatever, tune the receiver to the carrier.  If the 
communications receiver will not tune to the FM band you can tune it 
to the 10.7 MHz IF of a FM tuner and use that.  Stuff a 13,587 Hz 
tone into the exciter and starting with zero (nothing) bring the tone 
up until the level adjustment centers on the 2nd null of the 
carrier.  At that point you are deviating precisely +/- 75 kHz.  You 
can use that signal (at the 2nd null) to calibrate a scope connected 
to the "precision" demodulator prior to any de-emphasis.  Your 
"precision" demodulator should be flat to better than one-tenth dB 
over the frequencies of interest, typically 50 Hz to 100 kHz.

Burt

At 06:46 PM 7/19/2006, you wrote:

>It's also possible to obtain highly accurate results of the competition even
>if you do not have access to a spectrum analyzer for the Bessel function.
>If you calibrate against your Tx monitor, and you are reasonably certain as
>to its accuracy, then other stations can be measured relative to your own.
>Even the older Belar FMM-1 series was a good reference, since it was pretty
>tough for the internal calibrate function to go awry, even after many years
>of use.

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at earthlink.net
K6OQK