[BC] Bessel Nulling an FM Transmitter

[stanleybadams]

All you need to know is the Bessel null chart to keep you equipment
calibrated.  You can see the carrier null on any basic receiver no matter if
you are looking at RF or IF.  You simply will see a lot of FM sidebands and
the carrier will be gone.

And as stated you can calibrate your own equipment to that function.
There are plenty of null charts around, look in engineering books or the
web.

If you really want to be 1000% accurate, use you will need an extra low freq
counter that is accurate to about a hundredth of a cycle and that is what
you use to parallel across the audio genset that you are going to use.  Use
your regular dBm console meters or a more accurate indicator if you have one
across the output line and with the right freq and the right percent you
will see the carrier null and it will appear that the station has gone off
the air.

Here is a simple chart:

% of Modulation       Null           Mod Frequency
100                    2              13.587 kc
75                     2              10.190 kc
50                     2               6.793 kc
40                     2               5.435 kc
30                     1               9.355 kc
10                     1               3.125 kc

This is related to the Modulation Index table where

1st null equals a modulation index of 2.405
2nd null equals a modulation index of 5.520

I have posted a nice Agilent/HP primer on Harold's web site for AM/FM
Measurements, it will be good for the younger set on the list and the older
ones too. Barry has been sent a copy.  It is in the free domain for
education.

As for checking deviation or even noise on an FM baseband you can do it as
mentioned in a couple of the notes on our list and that is use a RF genset
and a calibrated VTVM or its modern equivalent and first measure your demod
signal and then use the genset to give you proper deviation, they should
about equal if they don't you got trouble.  This is the way the old timers
had to measure AM noise in FM and TV transmitters.  There were no nice
demods in those days or spec anal's.  You did it by the substitute method.


Stan
Memphus