[BC] FM Stereo Question

[Grady Moates]

>> It just barely shows up on my Inovonics modulation monitor.
>> Maybe one or two LED's (and flashing) whatever they mean.

    Several things about the Inovonics mod monitors.

    I really like my model 530, but it has a coupla gotchas:

[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,
[2] the 530 has no "AM Noise" measurement available,
[3] while it _does_ have a "multipath" reading, I believe that
    it is looking at pilot amplitude disturbances, rather than
    RF envelope amplitude disturbances, and
[4] on a model 530, the bottom LED on the "multipath" scale is
    -39 dB; if this lowest LED is blinking at all when connected
    to a direct RF sample, your AM is too high.

        Pilot modulation is useful when attempting to aim
    a directional antenna for minimum AM, but doesn't give
    you a good indication of how consistent your FM carrier
    envelope is.  Tuning issues with the antenna or tuned
    networks within the transmitter can cause AM slope-
    modulation, and power supply defects can _really_ cause
    problems that won't show up without a "real" AM noise
    measurement.

STORIES are sometimes THE BEST way to convey an idea:

    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.

    The station had 20% AM at 60 Hz!

    The problem turned out to be a mis-wired low-voltage power
supply, putting 60 Hz ripple right onto the negative power supply
pins of the opamps in a circuit that was intended to deliver a
control voltage to an "RF Gain Control" between the exciter and
the intermediate power amplifier.  The 60 Hz ripple was appearing
right at the output of the opamp, going into the RF gain control
device, and amplitude modulating the FM carrier with the ripple.
I found the wiring error, fixed it, and the AM noise dropped to
around 3%, primarily modulation-related.  A quick tune-up brought
that down so low that the scpoe no longer gave meaningful information.
At that point, a mod monitor I had brought in measured about -50.
Coverage increased dramatically.

    Another station, up in Michigan, was also having serious
coverage problems.  Again, I hooked a 'scope to the forward power
RF sample port, and saw 120 Hz AM at about 15% amplitude modulation.
It turned out that the 3-phase power supply for the IPA stage had
lost a phase right at the power transformer output terminals, and
once again the power supply ripple was showing up on the FM carrier
as AM.  $4.00 worth of new bolt/nut/splitwasher/flatwasher hardware,
and the transmitter's power-supply-related AM became so low that a
quick tune-up to get rid of slope modulation in the transmtter's
tuned circuits doubled the station's useful coverage.

    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?

Grady