[AMRadio] Broadcast Processors

[Jim Tonne]

Regarding commercial broadcast processors for ham use.

> The transmitter "Frequency response must be less than 
> 3dB down at 0.15Hz, and less than 0.1dB down at 9.5 kHz."

That figure of 0.15 Hz may sound odd and arbitrary but is
based on a direct-coupled modulator (i.e., flat down to DC)
with a one-second time-constant on an input blocking
capacitor and resistor.  

That figure of 0.15 Hz is sometimes specified in the contract
between the manufacturer and the purchaser.

Another way the low end is sometimes specified is that if
the transmitter is modulated with a 50 Hz square wave that
the tilt on the resultant modulation is to be less than 2%.

Why this "nonsense" you may ask.   It is because the 
commercial broadcast processors  generally deliver an
output waveform that is somewhat clipped, although it
may be clipped only on transients.   For them to control
the modulation requires that the following audlio stages
(the modulator) have such a flat response.

Another area that is of concern in the transmitters is the
insistence by the purchaser that the response be flat on
the high end and yet have no overshoot on a 2 kHz
squarewave.   Can't do that and when the manufacturer
sits down with the purchaser and explains the math,
the purchaser comes away enlightened and happy.

- Jim Tonne   used to design BIG PWM rigs at CEC