Digital oscilloscopes for FM waveform capture?
 

There was an oscilloscope article written by Steven B. Warntjes about
sustained sample rate in digital oscilloscopes. I wanted to get a
verification from the experienced users on this group. Let's say I have
an oscilloscope with 25000 sample memory depth and a real time sampling
of 1 gigaSamples / sec with 200 MHz bandwidth (-3dB).

If I had an FM carrier at 100 MHz with a 1000 Hz (1 mS period) test
signal modulating it, does that mean that a digital oscilloscope with
the specs above will not be able to capture the waveform properly?

I was thinking that if the capture window (in seconds unit) is equal to
the memory depth (in samples unit) divided by the sample rate (in
samples / sec unit), then the maximum capture window that I can use and
still maintain the rated sampling rate would be 25 uS (micro-seconds).

So that means that if my carrier signal is deviating between 100MHz +/-
1kHz, then my capture window would at least have to be 1ms/div to
capture the deviation properly (?). With a capture window of 1ms/div,
my sample rate would decrease to 25 mega-samples/sec, which is not
enough to sample the 100MHz carrier frequency.

Is that right?

I know that I can just use a spectrum analyzer, but I wanted to capture
the waveform and duplicate the time-domain plot of frequency modulation
(amplitude vs. time) that I've seen in the books.

Thanks!

Digital oscilloscopes for FM waveform capture?
 

MRW wrote:
There was an oscilloscope article written by Steven B. Warntjes about
sustained sample rate in digital oscilloscopes. I wanted to get a
verification from the experienced users on this group. Let's say I have
an oscilloscope with 25000 sample memory depth and a real time sampling
of 1 gigaSamples / sec with 200 MHz bandwidth (-3dB).

If I had an FM carrier at 100 MHz with a 1000 Hz (1 mS period) test
signal modulating it, does that mean that a digital oscilloscope with
the specs above will not be able to capture the waveform properly?

I was thinking that if the capture window (in seconds unit) is equal to
the memory depth (in samples unit) divided by the sample rate (in
samples / sec unit), then the maximum capture window that I can use and
still maintain the rated sampling rate would be 25 uS (micro-seconds).

So that means that if my carrier signal is deviating between 100MHz +/-
1kHz, then my capture window would at least have to be 1ms/div to
capture the deviation properly (?). With a capture window of 1ms/div,
my sample rate would decrease to 25 mega-samples/sec, which is not
enough to sample the 100MHz carrier frequency.

Is that right?

I know that I can just use a spectrum analyzer, but I wanted to capture
the waveform and duplicate the time-domain plot of frequency modulation
(amplitude vs. time) that I've seen in the books.

OK, to begin with, why did you post this in an "antennas" group?

Your analysis seems about right, though you left out one important
thing: you will be very hard pressed to see the difference between
100.000000MHz and 100.010000MHz on a scope waveform directly. Note
that that's 10kHz difference: a normal signal on the FM broadcast band
has nominally 75kHz deviation on signal peaks, even if the modulating
audio is a 1kHz sinewave.

If you really want to see the effects of FM on a scope display, I have
a couple ways to suggest that you can reasonably do it. First, if you
want to see the "textbook" waveform, use a much lower carrier
frequency. Use a 50kHz carrier, and modulate it with a +/- 5kHz
deviation. That's a +/- 10% change in frequency, and it will be
relatively easy to see on your 'scope. But even at that, if the
modulating frequency is 1kHz, you'll need 50 cycles of carrier to see
one cycle of the modulation; so if you want to see the effect clearly,
pick a higher modulation frequency, say 5kHz. Then just ten cycles of
carrier represent a full cycle of the modulation. You can grok all
that in one snapshot of the display. In other words, instead of
looking for a scope that will work on an unrealistic signal, use a
signal that's realistic to display what you want. If you trigger the
scope on the modulating signal, you don't need a fancy digital scope;
an analog one will do just fine.

The second alternative: If you want to see the effect on a 100MHz FM
broadcast signal, use trigger delay. If you display the signal at
100nsec/division (normal 10 division wide screen), that's one cycle per
division. If you use a trigger delay of 10usec, then a 100kHz shift in
frequency will shift the displayed signal by one full cycle. That is,
10usec of 100.000MHz is exactly 1000 cycles, and 10usec of 100.100MHz
is exactly 1001 cycles. You will see the frequency deviation from
100MHz as a phase shift in the displayed section of the waveform,
1/100th of a horizontal division per kHz of deviation. You can use a
relatively cheap analog scope to do that; the main requirement is a
stable trigger delay. Also, you can look instead at the 10.7MHz IF
frequency commonly used in FM broadcast receivers, and that makes the
job even easier. It's actually not a bad way to check the deviation of
a broadcast signal, at least in a gross way.

Cheers,
Tom

Digital oscilloscopes for FM waveform capture?
 

If you have 1 ms period repetitive modulation, the repetitive waveform
you're generating repeats only once every ms, or more if the carrier and
modulation aren't synchronized. So it takes at least 1 ms to capture one
full cycle of your waveform. At 1 Gsample/sec, your sampling interval is
1 ns; 25,000 samples takes 25 us, way short of the 1 ms you need to
capture even one cycle of the waveform. Yet you need a sampling period
of 1/400 MHz = 2.5 ns to even hit the Nyquist rate, and around the 1 ns
sample period to get a practical reproduction of the waveform.

So yeah, you can't do it with that machine.

Roy Lewallen, W7EL

MRW wrote:
There was an oscilloscope article written by Steven B. Warntjes about
sustained sample rate in digital oscilloscopes. I wanted to get a
verification from the experienced users on this group. Let's say I have
an oscilloscope with 25000 sample memory depth and a real time sampling
of 1 gigaSamples / sec with 200 MHz bandwidth (-3dB).

If I had an FM carrier at 100 MHz with a 1000 Hz (1 mS period) test
signal modulating it, does that mean that a digital oscilloscope with
the specs above will not be able to capture the waveform properly?

I was thinking that if the capture window (in seconds unit) is equal to
the memory depth (in samples unit) divided by the sample rate (in
samples / sec unit), then the maximum capture window that I can use and
still maintain the rated sampling rate would be 25 uS (micro-seconds).

So that means that if my carrier signal is deviating between 100MHz +/-
1kHz, then my capture window would at least have to be 1ms/div to
capture the deviation properly (?). With a capture window of 1ms/div,
my sample rate would decrease to 25 mega-samples/sec, which is not
enough to sample the 100MHz carrier frequency.

Is that right?

I know that I can just use a spectrum analyzer, but I wanted to capture
the waveform and duplicate the time-domain plot of frequency modulation
(amplitude vs. time) that I've seen in the books.

Thanks!

Digital oscilloscopes for FM waveform capture?
 

K7ITM wrote:
OK, to begin with, why did you post this in an "antennas" group?

....Because I figured you guys/gals have a lot of experience with
oscilloscopes. :)

...snip...
Cheers,
Tom

Thanks, Tom!