Post subject: Capturing FM waveform on digital oscilloscope
Posted: Fri Nov 17, 2006 12:56 pm
Joined: Fri Apr 07, 2006 6:10 pm
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
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.