[AMRadio] Monitoring Modulation Accurately


W2XJ w2xj at w2xj.net
Fri Nov 15 18:26:17 EST 2013


We agree here as I also said typical ham and military rigs do not do very good AM but a modified BC rig and some home brew PDM rigs will.

Sent from my iPad

> On Nov 15, 2013, at 2:03 PM, Jim Tonne <Tonne at Comcast.Net> wrote:
> 
> 
> 
> 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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