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[dsp] PWM/phase relationship

David Willmore willmore at optonline.net
Wed Jan 25 21:37:46 UTC 2006

> Scott wrote:
> Not a DSP expert by any means, but I've had good luck with using PWM for
> Bell 202 and PSK31/BPSK modulation.  Both used waveforms from lookup tables
> that could just have easily been voice samples.  I think Bell 202 especially
> is likely to be more sensitive to any phase error than human voice.

I'd guess the ratio of sample rate to modulation rate would be important
here.  I did some hand generated PSK31 with an SX microcontroller and
noticed some improvement when I went from single sided to dual sided
PWM--and my sample rate was even higher than yours--but, I wasn't generating
an AF signal, I was generating RF directly, so maybe the ratio is sample
rate to carrier frequency?  I've just been told that the human ear is
sensitive to subtle phase variations--especially at higher frequencies.
So, I'm a little concerned.

> In the Bell 202 example, I'm using a PWM clock rate of 7.3728 MHz.  With 4
> bits of resolution (16 step PWM) that comes out to a frequency of 460 kHz.
> A simple RC filter removes the high frequency component.  I'm really not
> sure about any phase shift.  The sample rate is 57.6 kHz, so in this example
> the PWM register stays at the same setting for 8 periods in a row.

Hmm, why did you chose to do it that way?

If the resolution of your calculation is greater than those 4 bits, there
are a few tricks you can do to expand it to have those 8 samples be a bit
more meaningful.

> The BPSK code uses 8 bit resolution, so the PWM carrier is 28.8 kHz.  That's
> still easy to filter for a 4 kHz audio passband.

That seems like a pretty good sample rate to data rate (or carrier frequency)
ratio, so you're probably not going to see problems.

> > All of this is for a 'next gen' laser communicator--
> > inspired by my first laser contacts in this last
> > VHF contest. :)
> Sounds cool.  Are you sending PWM with the laser, or just using that for
> audio output?

The idea is to send PWM encoded voice--so as not to have to modulate the
intensity of the laser--which takes some clever work and doesn't buy us
much.  On and off are about all I can trust it to do safely. :)  I don't
want to get into the subtlties of modulating laser diodes.  I'll take
the output of the laser, put it into a photo cell, and watch the output
on a scope.  As long as the output looks right, I'm happy.  PWM is fairly
forgiving in that sense.

I intend the laser signal to have plenty of the PWM 'carrier frequency'
in it.  I'll use that on the receive side as a sort of Super-audio tone
squelch. :)  I'll send a filtered version of the signal to a speaker--or
headphones.  I'll take the received signal and send it through some kind
of signal processing to compare it with our local 19.53KHz reference to
decide if we're hearing anything real or just listening to a lightbulb.
>From there, I'll drive a bargraph or something as a RSSI display.

> I'd like to experiment with pulse position modulation.  On a high-altitude
> ballooning list we were talking about the possibility of using a xenon
> strobe to send position and telemetry data.  The same thing should work with
> a pulsed laser.  what makes it interesting is the very high ratio of peak
> power to average power.  Data rate's not very high, though.  Say the strobe
> has a half-second recharge time, and you have millisecond timing accuracy.
> You'd start with a reference pulse, and the window starts .5 seconds later.
> The next .5 seconds are divided into 256 slots, so the slot during which the
> pulse arrives determines the value of an 8-bit byte.  That gives you a data
> rate of 1 byte per second.  Not good for voice, but you can get position and
> altitude in 10 seconds.

Hmm, that may work as you'll have plenty of gain from directivity--I assume
you'll keep a telescope on it as the 'receive antenna'.  You may want to put
a yellow filter over the lens to act as a 'blue block' to filter out any
sky noise--which should mostly be blue. 

I'm hoping to find a reasonably good red filter to remove a lot of
interference on my receiver.  Most of the receivers I've seen so far
have no optical filtration at all--but most laser contacts take place
'face to face', so it's not really an issue.  Expecially since they're
sending CW, so you don't need a huge SNR to receive it.  I'm a little
more concerned with voice. :)

David n0ymv

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