Re: Demodulating AM pulse width coding (long post)

On May 1, 9:03 am, Jim Thomas <jtho...@xxxxxxxxxxxx> wrote:
rickman wrote:
On Apr 28, 10:07 am, Jim Thomas <jtho...@xxxxxxxxxxxx> wrote:
rickman wrote:
Thanks for your comments. I appreciate it.
I was involved in an IRIG-B demod about 20 years ago. As I recall, the
first thing we did was to remove DC, then square the signal, and run
that through a 2KHz notch filter. Since IRIG-B is primarily a 1KHz
signal, squaring produced a double frequency component (the sum), and a
DC component (the difference). The notch filter IIRC, was a 4-tap
boxcar (really a comb), and that knocked out the double-freq component.
We had a nice square wave at the output of the notch filter, which we
then needed to slice and dice to get the ones, zeros, and position pulses.

That is interesting. Was the 4 tap comb just a running sum of the
last four samples? If you were sampling at 8 kHz that would give you
a full cycle of the frequency doubled carrier and would give you
square waves with a rise time of one clock cycle.

Yes, we were sampling at 8K, and the 4-tap comb was a running sum.



In terms of processing, this is the same as what I was doing; squaring
the incoming signal vs. multiplying it by a reference carrier followed
by a sum over a carrier cycle. I am planning to sync to the zero
crossing when calculating the sum. Then instead of getting 8 values
per cycle, I get one and the rise time is one sample.

I suppose using the squared input eliminates the need to sync to the
carrier, but I have to do that anyway since the carrier is part of the
timing reference. The PLL gives me a reference, phase locked to the
carrier, that I can use to determine the zero crossing of the carrier,
without interference from the noise. I guess I can combine both
methods. Use the PLL to provide the phase reference and use the
squared signal to measure the amplitude.

Rereading my post, I realized that I mispoke (it's not just for
politicians anymore!). I should have said the risetime is one cycle
of the frequency doubled sine wave. That would be four clock cycles.
Still, that is not bad.

I have been letting this perk in my mind the last couple of days and
unfortunately I have not been able to work on it due to having to
chase problems with a board fab house. They had some quality control
issues on my first order, but the second order for the test fixture
has been a complete disaster! They accepted the order and then
canceled it without telling me. So on the day I expected delivery of
the boards, I find out they had nothing for me at all! Now I am in
the second day of trying to get the order back online and they are
being very retarded about making it work at all, much less in an
expedited manner! I just can't believe how bad a company can be. If
things are not straightened out by the end of the day, I will have to
place the order elsewhere and take a larger hit...

Thanks for your comments. This has been a big help.

Rick
.

Relevant Pages

  • Re: Demodulating AM pulse width coding (long post)
    ... The notch filter IIRC, was a 4-tap ... We had a nice square wave at the output of the notch filter, ... the incoming signal vs. multiplying it by a reference carrier followed ...
    (comp.dsp)
  • Re: Demodulating AM pulse width coding (long post)
    ... We had a nice square wave at the output of the notch filter, ... square waves with a rise time of one clock cycle. ... the incoming signal vs. multiplying it by a reference carrier followed ...
    (comp.dsp)
  • Re: Demodulating AM pulse width coding (long post)
    ... and modulated in AM on a 1 kHz carrier. ... single cycle at a time, starting with the zero crossing point. ... will need to scale Y to compensate for the fact that the modulated pulse ...
    (comp.dsp)
  • Re: alternating-polarity Amplitude-Modulated tone?
    ... The part I am stuck on is how to get the phase to reverse after each ... cycle of the AM envelope. ... carrier frequency 1000 Hz ... how to get the initial phase of both frequencies to be zero? ...
    (comp.dsp)
  • Re: Demodulating AM pulse width coding (long post)
    ... decision about whether that particular cycle is a binary '1' or a '0'. ... and modulated in AM on a 1 kHz carrier. ... Given the 3x modulation factor, ... pulse width is not 50/50 duty cycle on the average. ...
    (comp.dsp)
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