In article , Tim Brown
wrote:

I seem to be hearing RCI's 9515 kHz transmission up above the assigned
frequency all over the place. Anyone else notice?

Tim Brown

After posting this originally, I went back to the receiver to check
their modulation envelope with my 'scope to see what was going on.

What I noticed was that peak negative modulation was -not- reaching 100
percent. Maybe they were getting 75 to 80 percent peak negative
modulation, indicating that the carrier was not getting cut off. No
harmonic emission was noted at the 2nd harmonic of the 9515 carrier
frequency (19030 kHz).

My guess is that something blew in the transmitter's pulse step
modulator unit causing modulation steps to be missing, resulting in the
lack of complete or full modulation in the negative direction. This
would cause emission to occur at frequency multiples of the missing
"steps" period. I don't know what the switching frequency of the PSM
unit in the ABB transmitter is (anyone know?).

While I was watching the 'scope, it sounded like someone up there was
futzing with modulation levels apparently trying to correct the
situation. If it was my operation I would have immediately pulled the
plug due to the substantial interference it was causing. I guess this
underscores the necessity of using a spectrum analyzer to monitor your
transmitter's output if it employs a pulse type modulator. Mod monitors
are nice but they don't tell you what is going on off frequency.

Tim Brown