Message-ID: <005401c1a8c6$c6faca00$e32665d5@oemcomputer>
From: "Johan Bodin" <sm6lkm.jbeab@swipnet.se>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.0.20020128115929.00ab3b90@gemini.herts.ac.uk>
Subject: Re: LF: PSK31 generation
Date: Tue, 29 Jan 2002 14:12:07 +0100
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Dear Jim and the Group

Jim Moritz wrote:

> I tried some experiments with a scheme similar to that suggested by Johan - 
> The problem with this is that, while the square wave output contains 
> essentially no even harmonics, as soon as you change the duty cycle, or the 
> relative phase of the two totem pole outputs, a strong second harmonic 
> component appears.


I don't understand this... Perhaps I am missing something, but as far as I can see,
the waveform *will* be symmetrical if the drive to each totem pole has a 50/50 duty
cycle and the modulation is done only by varying the phase between the totem pole
drivers 0 - 180 degrees. For example, with 90 degrees shift it looks like this:

0, +1, 0, -1, 0, +1, 0, -1...

Curiousity made me write a simple program to simulate the signal. The program
generated an audio file containing the difference between two 50/50 square waves.
It starts with the two waves at 180 degrees. The phase difference is then gradually
reduced to zero at the end of the file.

The file was analysed whith Spectrogram - there was no sign of any even harmonic!

(This was easier than doing Fourier calculations :-)

As expected, the level of 3*F approaches that of 1*F when the phase shift
approaches zero.

As Andy mentioned, the fundamental voltage / phase ratio seems *very*
non-linear.

73
Johan SM6LKM