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What's going on with this FFT? Keysight 1102g
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Anthocyanina:
I got some cheap low power RF communication modules today, and not having anything else to check them with, i connected the output directly to the scope input using the stock probe. It is a 315MHz ASK TX module. I wanted to see the spectrum of it, so i set up the FFT for it, center at 315MHz and a span of 20MHz. To try and get better resolution on the FFT i zoomed out to 10µs/ from 5µs/, but then the FFT seemed to disappear. I tried the FFT from the math menu, and same seemed to happened, so i just moved the center of the FFT to higher and lower frequencies to see if anything was showing up, and i find this. When at 10µs/, the FFT centered at around 185MHz showed the expected spectrum, but, at an incorrect frequency. I was inputing a 1MHz signal to the RF module, and that was properly showing up in both cases, at 5µs/ with the correct center frequency, and at 10µs/ with the incorrect center frequency.

Anyone has any idea what is going on here? why changing the timebase to something a bit slower completely confuses the FFT and "moves" it down to much lower frequencies, which are not really present at the displayed frequencies?

Thank you!
electr_peter:
At 5us/div scope manages to have higher sample rate than in 10us/div setting (i.e. memory is not enough to run at full sample rate with such timebase). Sample rate probably is 2x smaller to 1GS/s, RF signal got "folded" down around 500 Mhz (500 - 315.06 = 184.96 Mhz - matches exactly)

Scope FFT has limitations and you have to find best suited setting for viewing your signal with FFT. FFT resolution and frequency limits are related to time base division, sampling rate and available memory. FFT at 10us/div shows misleading results for RF signal.
Anthocyanina:
Thank you, I at first thought it might be something like that, but this scope still does 2GSa/s at up to 20µs/. but that thing about it folding to 500MHz does seem to be what's going on. Does this happen on other scopes as well? I've tried on the rigol 1104z but it doesn't properly display the signal, and the counter doesn't recognize it as 315MHz, but the FFT on the rigol at least doesn't go away or displays peaks at other frequencies.

Thanks again!
bdunham7:
If you set it up at 5µs/div so the the 315MHz peak is showing and then go down and look for the 185MHz peak without changing anything else, is that 185MHz signal still there?
Anthocyanina:

--- Quote from: bdunham7 on October 22, 2022, 11:51:00 pm ---If you set it up at 5µs/div so the the 315MHz peak is showing and then go down and look for the 185MHz peak without changing anything else, is that 185MHz signal still there?

--- End quote ---

Hi, this is what it looks like, same 1MHz signal at the input of the transmitter, one picture shows the cursor settings, the other one the FFT settings, while there is a lot of content below 315MHz, there isn't a similar peak to what it shows when the timebase is 10µs/.

Thank you!
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