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Re: [Amps] IMD

Subject: Re: [Amps] IMD
From: Dan Mills <>
Date: Tue, 03 Jan 2012 01:21:32 +0000
List-post: <">>
On Mon, 2012-01-02 at 19:57 -0500, Gary Schafer wrote:
> Hi Dan,
> I don't know about a DSP radio but I do know that if you modulate a regular
> old fashion SSB rig with a square wave you will not get anything out of it
> at RF that resembles a square wave. Looking at it in the time domain.

Bear in mind that a communications SSB rig is only a decade wide (300Hz
to 3K give or take), and a square wave is odd harmonics, so at most you
will fit a 300Hz fundamental, plus 4 harmonics, which is not enough to
give anything that remotely looks like a square wave (Especially as the
radio is unlikely have particularly constant group delay).... 

Inputting at less then 300Hz does not help because you loose the
fundamental, and increasing the frequency just means that you get ever
fewer harmonics, above about 1Khz you just get a sine wave as all the
harmonics are cut away. 

an input at much less then 300Hz will indeed give a series of spikes,
these being the components that fit into the radios passband. 

This is hard to demonstrate as most radios use one of the IF filters to
implement the bandwidth control, so you cannot easily see that the
envelope really does follow the input to the modulator as long as the
input results in RF that lies within the passband of the downstream

This is why you must band limit the clipper output, and ensure that the
band limiting does not reintroduce overshoot, tedious but it can be

73, Dan.

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