So, I sat down in front of the scope and tried the 100Hz modulation of a 20MHz signal - nothing like a real example, right?
It seems to me that the digital scope behaves pretty much like an analog scope in this case. The slow sweep speed does not seem to detract from the ability to display an outline of the 20MHz signal (even without Peak Detection enabled).
Obviously, filtering out the 20MHz would destroy the information as displayed here.
(note: normal mode, not peak detect)
(Attachment Link)
Thank you for a very nice example. As you correctly note, it's obvious that you cannot sensibly filter out the carrier and not kill the side bands.
If the signal were down converted to DC - 500 Hz with the carrier at 250 Hz followed by a 500 Hz LP filter it would be very accurately displayed in the frequency domain provided the programmer took the frequency translation into account. A mere change in the carrier frequency would not change the picture unless the ratio of carrier and the modulation were very small. And that would only be in the time domain.
Have Fun!
Reg
I can see how the down-conversion won't damage the frequency domain display, but the time domain looks terrible...
Below, I used a 250Hz carrier modulated by 100Hz to demonstrate the downconverted case.
It doesn't really look good in the time domain. To make it look similar to the 20MHz image, I had to lower the frequency of the modulation and increase the sweep time.
Using my limited math skills -
The first example with the 20MHz carrier could have been sampled at 40MHz for a sweep of 50ms, so would consist of 2,000,000 samples.
The downconverted 250Hz carrier is samled at 500Hz for 4,000 seconds, with a modulation of 0.0015Hz. This also has 2,000,000 samples and looks exactly the same.
It seems to me that to display this waveform, we end up with the same number of data points needing to be plotted even after downconversion?
Example with 250Hz carrier, but 0.5Hz modulation (scope cannot sweep slow enough to show 0.0015Hz)