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

[Phil Alexander]

On 19 Jul 2006 at 21:31, Burt I. Weiner wrote:

> 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.

Hmmmmm ... my dim memory may be playing tricks but it seems to me
we once had a little black thingy called a 6H6 that was flat a lot
further than that when connected to the right RF transformer. <g>


---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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