[AMRadio] Monitoring Modulation Accurately


Jim Tonne Tonne at Comcast.Net
Fri Nov 15 17:03:07 EST 2013



W2XJ wrote in part:

 > A good test is how well it can pass a square wave

[ snip ]

It might be of interest to know how most shortwave broadcast
transmitters are tested.   Generally written into the contract is
a little item which is intended to show that the transmitter can
handle somewhat-clipped audio.   What is written is that the
transmitter must handle a 50 Hz squarewave input with less
than 5 percent tilt.   This is done by having the low end go
down to way less than 1 Hz at the 3 dB point.

On the high end we used a 2 kHz squarewave to verify that
the overshoot was less than 5 percent.

Tilt is a problem on the low end and overshoot is a problem
on the high end.

If you are going to use low-level clipping in your audio
processing chain then that kind of testing should be used.
The usual ham rig just can't handle a clipped 100 Hz
waveform and the result is that the output of the speech
amp will have to be turned down to avoid overmodulation.
A decent PWM system can sidestep this situation.

- Jim Tonne  W4ENE





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