What's going on with this FFT? Keysight 1102g - Page 1
Forum section: electronics / testgear

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 22, 2022, 08:27:34 pm
content: 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!

author: electr_peter
author_rank: Unknown
timestamp: October 22, 2022, 09:34:18 pm
content: 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.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 22, 2022, 11:23:02 pm
content: 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!

author: bdunham7
author_rank: Super Contributor
timestamp: October 22, 2022, 11:51:00 pm
content: 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?

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 23, 2022, 12:23:34 am
content: Quote from: bdunham7 on October 22, 2022, 11:51:00 pmHi, 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!

author: bdunham7
author_rank: Super Contributor
timestamp: October 23, 2022, 02:35:55 am
content: I'm not sure what is going on with your Keysight. What bandwidth does it have? Not that I have any idea why I'm asking, I'm just curious. And baffled.As a comparison, I had a DS1054Z upgraded to 100MHz and I was able to quite easily detect a signal from a garage door opener that I was working on right at about 315MHz IIRC. So I also don't know why you can't see it on yours, although perhaps your signal level is just too low. Can you boost the signal in each instance?

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 23, 2022, 04:28:56 am
content: I'm using Bud's liberated firmware with everything unlocked except the memory, so 200MHz bandwidth and 1M memory. The signal level is very low. This is the module I'm using, and i'm just hooking the probe tip on the antenna connection, and I'm seeing about 200mV pk-pk. I don't have a way to boost the signal. I fiddeld a bit more with the rigol and was able to get a usable FFT, the counter isn't even close, but the frequency measure from the side menu is ok.Thank you!

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 24, 2022, 05:03:09 am
content: I haven't been able to find anything documenting something similar, if anyone could think of any reason for why this happens, letting me know would be appreciated, thank you!

author: Bud
author_rank: Super Contributor
timestamp: October 24, 2022, 02:15:26 pm
content: Can you test a good quality reasonable amplitude sinewave source signal from a signal generator or something?

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 24, 2022, 11:27:12 pm
content: Quote from: Bud on October 24, 2022, 02:15:26 pmHi! the only generator I have is the EDU33212A. using its arb capabilities, the best i can get is 125MHz at also 200mV pk-pk. I have an FPGA board that runs ar 400MHz, I'm still just learning the basics of FPGA but i wonder if I could get it to toggle an output pin fast enough, then i would get a 2Vpp square wave (measured 2Vpp at one of the built in LEDs).

author: Bud
author_rank: Super Contributor
timestamp: October 25, 2022, 12:03:42 am
content: 100MHz is good enough.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 25, 2022, 12:38:17 am
content: with a 100MHz signal, centering the FFT at 100MHz, i can go up to 27µs/ before it starts doing something weird, and 32µs/ before it disappears. When at 50µs/ it shows up at 50MHz. This is all so strange

author: Someone
author_rank: Super Contributor
timestamp: October 26, 2022, 12:46:51 am
content: Quote from: Anthocyanina on October 25, 2022, 12:38:17 amIncreasing the timebase (horizontal) at some point reduces the sampling rate as the scope only has so much memory. More memory is good, high sample rate is still available at slower timebases. But the missing bit....The FFT is only calculated from 64K points, spread across the screen and decimated from the acquisition data. 64k/20us = 320MHz sampling, 160MHz nyquist. That signal reappearing at longer captures is from aliasing.Leave the span equal to twice the centre frequency and it should all be pretty self explanatory as you change the timebase.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 27, 2022, 02:17:38 am
content: oooh, thank you, this seems to be what makes the most sense, but I still wonder, since the sampling rate of the scope remains 2GSa/s at up to 20µs/, wouldn't the FFT still use the same points as it did at faster timebase? or is the FFT sample rate something that doesn't depend directly on the time domain sample rate? (if that question even makes sense)Thank you!

author: Someone
author_rank: Super Contributor
timestamp: October 27, 2022, 03:36:40 am
content: As above, the sampling rate for the FFT is 64k points across the current timebase for most (all?) conditions.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: October 27, 2022, 03:42:30 am
content: Quote from: Someone on October 27, 2022, 03:36:40 amOoh, I misread that, now going to try what you said about setting the span twice the center frequency and changing the timebase. Thank you!