[BC] FM Stereo Question

[Kirk Harnack]

Grady wrote several things that are key for proper FM transmitter
setup/tune-up:

> [1] (most important) you cannot make meaningful AM noise
>     measurements anywere except at the transmitter site,
>     connected to a direct RF sample source, prefereable a
>     sniffer slug in a line section,

And this is true of an mod monitor that purports to read AM Noise.  You must
be at the transmitter site, connected to an output sample port, after the
low pass filter.

Grady continued:

>     I once went to a station with a red transmitter.  The
> owners had never had good coverage, but no one had been able
> to find a cause.  The first thing I did was hook an oscilloscope
> up to the forward-power RF sample port with a 50-ohm cable,
> with a "t" connector at the scope so I could put a 50-ohm load
> there to make the coax happy.

You can do so many things with an oscilloscope.  Checking and tuning for
minimum AM Noise on an FM transmitter is certainly one of them.

Another technique is to use the scope's X-Y mode.  Feed Y (vertical) with a
diode-detected transmitter RF sample.  Feed X with a 2nd output of the
composite signal feeding the exciter.  Adjust for meaningful display that
will look like a fat diagonal line or a fat, but likely tilted, umbrella
curve.  Adjust transmitter for untilted, symmetrical display and flattest
"umbrella".  This should be lowest AM noise.

Marvel at the increased "coverage" and clearer sound.

If you transmitter has a tube IPA or uses a 1/2 wave cavity, repeat the
above procedure more often than you would think necessary.  

Grady summated:

>     Rule number two in the engineer's handbook is:
> * an analog-only AM transmitter should have no FM component in
>   its output signal, and
> * an analog-only FM transmitter should have no AM component in
>   its output signal.
> 
>     Rule number one, of course, is:
> * is the thing plugged in?

Amen, Brother Grady!

Kirk