Mysterious 25 Ghz signal! - Page 1
Forum section: electronics / beginners

author: testtube44
author_rank: Regular Contributor
timestamp: May 14, 2019, 02:37:17 am
content: I have a 200Mhz oscilloscope and I'm seeing some strange signals in FFT mode at 25Ghz with peaks at 12.5 and 37.5 respectively. There are definitely peaks at those frequencies, they appear, stay on for several 10s of seconds and then occasionally vanish. I've also seen stuff fluttering around in the 2.4 GHz range before, is this a bug or are there MMW signals in my room? Is my scope oversampling, and if so by how much?

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2019, 02:44:37 am
content: 2.4 GHz is WiFi and it comes and goes.Are you using a cell phone nearby or is it in your pocket ? If so shift it well away.25 GHz seems too high for a 200 MHz scopes FFT to resolve so discount it as real IMO.

author: wraper
author_rank: Unknown
timestamp: May 14, 2019, 02:56:28 am
content: Should be artifacts. Such scope cannot catch signal nowhere near to this frequency. Even if you bypass analog frontend and push it straight into ADC.

author: rf-loop
author_rank: Super Contributor
timestamp: May 14, 2019, 08:05:33 am
content: Quote from: testtube44 on May 14, 2019, 02:37:17 amYour scope (if it is same as in your some previous message in EEVblog), Hantek 5202P have maximum real time sampling speed 1GSa/s (and it is also done using multiple ADC's interleaved.Maximum theoretical FFT is 500MHz (fNyquist)In this mode, if FFT frequency band go over 500MHz is totally designer error.But then it have also Equal Time Sampling mode. I do not remember if ETS max is "25GSa/s" or "50GSa/s"Is it so that FFT can also enable when in ETS mode? If so, then..If it have max ETS 50GSa/s then FFT is 0-25GHz based to sample speed. In theory and practice it can work but it need extremely good "nearly ideal" timing and so on. And in this Hantek, this is not at all - nearly worst possible. Sampling quality is totally junk for this purpose and also its analog front end can not handle these higher frequencies, so thes what you see are not from outside of scope.But, it must say, what ever 20GHz or something like it you can see there, they are total bullshit, generated by sampling system and FFT math itself. Think there is many separate ADC very poorly interleaved, Then in ETS mode these acquisitions are repeated many many times. How are they positioned and are there really picoseconds class real timing accuracy. Really not, not at all. There is ADC clock's jitter and also levels are not well balanced between adc's, there is trigger jitter etc. In real world these samples are shot like a shotgun. After then they are arranged like they are in line at regular intervals. FFT math input data is garbage and result is garbage^kThen more, afaik it have 1k sample points for FFT math.About ETS sampling and FFT:http://literature.cdn.keysight.com/litweb/pdf/5988-4368EN.pdfand there page 16Also FFT without Equal Time Sampling mode is problematic because partially same reason. Many interleaved ADC with poor timing and level balancing.But, of course it can use and it is much more than no FFT at all. But user need understand its limits for avoid confusion between true world and scope display.

author: radiolistener
author_rank: Super Contributor
timestamp: May 14, 2019, 01:26:48 pm
content: Quote from: rf-loop on May 14, 2019, 08:05:33 amactually you can go over 500 MHz with FFT, but it requires to capture signal at different sampling rates which are not multiple to each other. It allows to identify exact Nyquist zone for each peak with math processing.

author: Bud
author_rank: Super Contributor
timestamp: May 14, 2019, 01:52:31 pm
content: Sampling rate is irrelevant here for a simple reason - the scope analog front end is not capable of that high frequencies.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 14, 2019, 04:50:58 pm
content: Quote from: radiolistener on May 14, 2019, 01:26:48 pmany literature on this? afaik FFT is only for equal time interval samples. or at least rearrange the "random" sampling rate samples into apparently or mimicking equal time interval samples what we know as equivalent sampling rate. jitter is another issue causing funny business on theoritical FFT, leakage etc. anyway, is OP 200MHz scope has that different/random sampling rate mechanism? ps or fs jitter spec? as Bud's says. AFE is only 200MHz, 25GHz will be far from reach, otherwise i will be interested in such a scope to put the AFE into my 20GSps scope, 200GSps eqv, maybe i can see a 100GHz signali will say that is a 25GHz ghost in the shell.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 14, 2019, 05:38:39 pm
content: Yeah, this is definitely an artifact. There are several ways you can generate mysterious artifacts with a scope FFT.One comes from the equivalent time sampling. What this does is combine multiple acquisitions together with different offset between the sample clock and trigger. This creates a reconstructed waveform with much higher sampling rate than the base rate. One of the downsides to this is that any signal that is periodic but not synchronized to the trigger can be misinterpreted.Another way to generate spurs is from the interleaving of the ADCs. If you have 4 ADCs each operating at 250 MS/s, ideally you want the delay between them to be 1 ns. However, if the first two are 1.05 ns apart, that will create spurs because the sample rate is non-uniform.These are some of the reasons that oscilloscope FFTs are not substitutes for a true spectrum analyzer designed for good frequency domain performance. You still get weird spurs and aliases on the best spectrum analyzers because they are almost impossible to avoid, but they are much rarer and typically quite small.I would be doubly skeptical of any FFT data past the Nyquist frequency of the base sample rate, or 500 MHz in your case. Equivalent time sampling has its place, and it can make time domain waveforms look a lot more sensible, but it has serious drawbacks when trying to compute a spectrum.

author: radiolistener
author_rank: Super Contributor
timestamp: May 14, 2019, 08:24:57 pm
content: Quote from: Mechatrommer on May 14, 2019, 04:50:58 pmUnfortunately I don't know literature about this technology. But I know device which use it. It has 100 MHz ADC AD9288 and allows to measure spectrum up to 1000 MHz. It captures input signal with 4 different sampling rate: 200.00 MHz, 194.(4) MHz, 187,0968 MHz, 181,25 MHz. And then reconstruct spectrum up to 1 GHz by finding minimum on unfolded Nyquist zones from 4 FFT. Since they are captured with different sample rate, each of these 4 FFT has different Nyquist zone borders. It allows to combine unfolded Nyquist zones of each FFT by finding minimum.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 14, 2019, 08:28:08 pm
content: i guess its sort of sampling scope? what brand and model? so people can study about it.

author: radiolistener
author_rank: Super Contributor
timestamp: May 14, 2019, 08:45:00 pm
content: Quote from: Mechatrommer on May 14, 2019, 08:28:08 pmit's a sort of stroboscopic oscilloscope, but for frequency domain.The same as stroboscopic oscilloscope uses several captures in order to reconstruct waveform with higher equivalent sampling rate, it also uses several captures in order to reconstruct spectrum above first Nyquist zone and to remove images.The device which use that technique is here:http://www.osa103.ru/en/main-page/This device implements both - stroboscopic oscilloscope with equivalent sample rate up to 10 GHz and combined FFT up to 1 GHzIt's analog frontend bandwidth is 400 MHz, but it can work up to 1 GHz with reduced amplitude.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2019, 04:03:46 am
content: Quote from: Mechatrommer on May 14, 2019, 04:50:58 pmI think the HP 71500A/70820A Microwave Transition Analyzer works that way. It has a 20 MS/s sample rate but 40 GHz bandwidth.