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pulse width modulation spectrum analysis

K

kristo

Jan 1, 1970
0
Hi,
I need some hints suggestion on calculating the spectrum of the a
pulse width modulated signal. I have a 100kHz pulse signal. The width
of the pulse is modulated by another low frequency sine wave (100Hz)
so that the pulse width of 100kHz signal varies from 10% to 90%. I
want to compute the magnitude of the average spectrum. The average
being taken over the time period of 10ms(i.e. period of 100Hz).
Kindly
suggest how to proceed with it. Any reference would be great start
for
me.
Thanks.
Kristo
 
A

Anonymous.

Jan 1, 1970
0
kristo said:
I need some hints suggestion on calculating the spectrum of the a
pulse width modulated signal. I have a 100kHz pulse signal. The width
of the pulse is modulated by another low frequency sine wave (100Hz)
so that the pulse width of 100kHz signal varies from 10% to 90%. I
want to compute the magnitude of the average spectrum. The average
being taken over the time period of 10ms(i.e. period of 100Hz).
Kindly
suggest how to proceed with it. Any reference would be great start
for
me.

I don't know the answer but if it were me, I'd start with the spectrum
of a square wave, which is pretty much lesson 1 in a course on Fourier
Analysis.

Now we have to consider a pulsed wave which is not square (ie, does
not have a 50:50 mark:space ratio). Calculate the spectrum with the
difference
from 50:50 as a parameter giving a mark of 50+a and a space of 50-a (this
will handle negative values OK to account for the shorter marks)

Now modify the calculation so that "a" is also dependent on "t" the time,
so that it is of the form a * sin (n * t / 1000) and you are well on the
way.

The factor of 1000 comes from there being 1000 pulses of 100kHz in the
cycle time of a 100Hz signal.

As I said, I don't know the answer and what I suggest above might
be bullshot, but it's the way I'd start off if it were my problem.
 
J

John Larkin

Jan 1, 1970
0
Hi,
I need some hints suggestion on calculating the spectrum of the a
pulse width modulated signal. I have a 100kHz pulse signal. The width
of the pulse is modulated by another low frequency sine wave (100Hz)
so that the pulse width of 100kHz signal varies from 10% to 90%. I
want to compute the magnitude of the average spectrum. The average
being taken over the time period of 10ms(i.e. period of 100Hz).
Kindly
suggest how to proceed with it. Any reference would be great start
for
me.
Thanks.
Kristo

Lots of engineering handbooks have tables of spectra of various
waveforms. The classic "Reference Data for Radio Engineers" lists this
exact waveform. They even have a version for the case of finite rise
and fall times.

John
 
B

Bob Masta

Jan 1, 1970
0
Hi,
I need some hints suggestion on calculating the spectrum of the a
pulse width modulated signal. I have a 100kHz pulse signal. The width
of the pulse is modulated by another low frequency sine wave (100Hz)
so that the pulse width of 100kHz signal varies from 10% to 90%. I
want to compute the magnitude of the average spectrum. The average
being taken over the time period of 10ms(i.e. period of 100Hz).
Kindly
suggest how to proceed with it. Any reference would be great start
for
me.
Thanks.
Kristo

Can't help on the calculations, but if you want to visualize it you
can download Daqarta and use the built-in signal generator and
spectrum/waveform/spectrogram analyzers.

Daqarta works with your sound card at audio frequencies, so you'd have
to scale the pulse and modulating frequencies appropriately.
(You may or may not want to listen to the results.)

The built-in PWM controls can use simple sine modulators, or
the output of a separate generator "stream" (streams 4 per channel).
The modulating stream can be any waveform, including any
arbitrary WAV file.

Note that there is no need to purchase Daqarta for this.
Even after the trial period expires, all the above features
remain available... you just can't view signals from the sound
card inputs.

Best regards,


Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!
 
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