Re: [time-nuts] windows for FFT measurements of phase noise
> I was thinking more of the sidelobes: if you're looking at a quiet > oscillator (e.g. -140dBc @ 100Hz) , with a 1 second epoch, and you want > to measure the noise at, say, 100Hz out, the window function needs to be > down 140 dB at that bin. > > WIndows like uniform and Hamming are probably only down 50 dB that far out. The segmented FFT helps with that. Ideally you have enough segments that there's rarely more than 30-40 dB of flatness variation within any one of them, which is why HFT95 works well and Hann is still usable. Except in the presence of very strong spurs, most of the energy in the narrowband segments is going to reside in the first few bins. It's the HPF prior to each FFT stage that keeps that close-in noise from spreading, more than the choice of window function. -- john, KE5FX Miles Design LLC ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] windows for FFT measurements of phase noise
Hi > On Jun 11, 2016, at 11:42 AM, jimlux wrote: > > On 6/10/16 4:09 PM, John Miles wrote: >>> What sort of windows do folks use for making FFT measurements of >>> phase noise. >>> >>> Say you have 1 second of sampled data (so the FFT resolution is 1 >>> Hz). If you're interested in the noise power at, say, 10 Hz away, >>> a rectangular window isn't going to be very far down, unless you >>> have a LOT of points in the FFT. >>> >>> Grove's paper from 2004 doesn't mention this detail. >> >> As Bob suggests, a multisegment FFT chain is the usual approach. By >> the time you're displaying noise down to 10 Hz, you should have quite >> a bit more than 1 second worth of data to draw from. >> >> For measuring noise the choice of window function doesn't matter very >> much as long as you correct for the noise bandwidth of the function >> you use. > > I was thinking more of the sidelobes: if you're looking at a quiet oscillator > (e.g. -140dBc @ 100Hz) , with a 1 second epoch, and you want to measure the > noise at, say, 100Hz out, the window function needs to be down 140 dB at that > bin. Ummm ….. e … not so much. If you are looking at phase noise, you are doing it with a system that has already taken the carrier out of the picture. Either you quadrature lock two oscillators (the 3048 approach) or you do an SDR to DC (TimePod) approach. The only thing you have to handle is the noise slope in the region you are working in. Bob > > WIndows like uniform and Hamming are probably only down 50 dB that far out. > > I did find a reference to some Blackman-Harris windows that are pretty wide > for the main lobe, but the sidelobes are 100 or 150 dB down. > > > > However, for spur detection there are major >> window-dependent differences that need to be considered. There is >> only one reference that's worth looking at, and that's the paper by >> Heinzel, Ruediger, and Schilling. (Google the authors' names and >> it'll come up.) > > That's a real nice report.. Excellent > > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] windows for FFT measurements of phase noise
On 6/10/16 4:09 PM, John Miles wrote: What sort of windows do folks use for making FFT measurements of phase noise. Say you have 1 second of sampled data (so the FFT resolution is 1 Hz). If you're interested in the noise power at, say, 10 Hz away, a rectangular window isn't going to be very far down, unless you have a LOT of points in the FFT. Grove's paper from 2004 doesn't mention this detail. As Bob suggests, a multisegment FFT chain is the usual approach. By the time you're displaying noise down to 10 Hz, you should have quite a bit more than 1 second worth of data to draw from. For measuring noise the choice of window function doesn't matter very much as long as you correct for the noise bandwidth of the function you use. I was thinking more of the sidelobes: if you're looking at a quiet oscillator (e.g. -140dBc @ 100Hz) , with a 1 second epoch, and you want to measure the noise at, say, 100Hz out, the window function needs to be down 140 dB at that bin. WIndows like uniform and Hamming are probably only down 50 dB that far out. I did find a reference to some Blackman-Harris windows that are pretty wide for the main lobe, but the sidelobes are 100 or 150 dB down. However, for spur detection there are major window-dependent differences that need to be considered. There is only one reference that's worth looking at, and that's the paper by Heinzel, Ruediger, and Schilling. (Google the authors' names and it'll come up.) That's a real nice report.. Excellent ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] windows for FFT measurements of phase noise
I remember cobbling together a home brew phase noise system 35 years ago using an HP3582 FFT analyzer. It had several available windows: IIRC: flat top, rectangular, and hamming (hanning? I can never get them straight). Basically, if you are looking for spurs, you need to use the flat top. If you are looking for noise, you use the hamming, or whatever it was. For most practical frequency sources, the slope of the phase noise is gradual enough that the windowing isn't a big issue. If you think it is an issue in your case, as you say, you can use a lot more points and see if the answer changes. This is similar to decreasing the minimum time step in SPICE. I am now putting together a modern homebrew phase noise system. I am planning to use a ZRPD-1 phase detector driven by AMC-123 amplifiers. The phase detector will drive a SpectraDAQ-200 digitizer with SpectraPlus-RT FFT software. I am not currently planning to put a preamp between the phase detector and digitizer, as it isn't needed for the particular measurements I am planning to make. However, if you have a candidate for this preamp, chime in anyway. I'm kicking around possible calibration modes. I am thinking of inserting a passive phase modulator. It needs to be able to revert to a mode where it contributes no phase noise of its own. The digitizer shown is the cheapest digitizer with believable performance. The really cheap ones don't even use BNC connectors; they use screw terminals :-(. Much more expensive ones have bandwidth I don't need, but fewer bits, that I do need. I would appreciate any comments pro or con about this configuration. I haven't bought any of this stuff yet, so if you have a better idea, fire away! I did a little looking on Ebay for bargains, but only no-name stuff was cheap, and it was always "we don't have any way to test it". Name brand stuff cost real money (or was "call for quote"; you know that is always going to be expensive). Does anyone know of a used equipment dealer in Silicon Valley? Rick N6RK On 6/10/2016 12:42 PM, jimlux wrote: What sort of windows do folks use for making FFT measurements of phase noise. Say you have 1 second of sampled data (so the FFT resolution is 1 Hz). If you're interested in the noise power at, say, 10 Hz away, a rectangular window isn't going to be very far down, unless you have a LOT of points in the FFT. Grove's paper from 2004 doesn't mention this detail. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] windows for FFT measurements of phase noise
> What sort of windows do folks use for making FFT measurements of phase > noise. > > Say you have 1 second of sampled data (so the FFT resolution is 1 Hz). > If you're interested in the noise power at, say, 10 Hz away, a > rectangular window isn't going to be very far down, unless you have a > LOT of points in the FFT. > > Grove's paper from 2004 doesn't mention this detail. As Bob suggests, a multisegment FFT chain is the usual approach. By the time you're displaying noise down to 10 Hz, you should have quite a bit more than 1 second worth of data to draw from. For measuring noise the choice of window function doesn't matter very much as long as you correct for the noise bandwidth of the function you use. However, for spur detection there are major window-dependent differences that need to be considered. There is only one reference that's worth looking at, and that's the paper by Heinzel, Ruediger, and Schilling. (Google the authors' names and it'll come up.) The TimePod and 3120A allow the user to choose between the HFT95 function from the Heinzel paper -- which was essentially reverse-engineered from the HP 35670A -- as well as the usual (von) Hann(ning) window. HFT95 is the default, with good sidelobe rejection and high amplitude accuracy for spurs regardless of where they fall in their FFT bin. The Hann window can be selected when frequency offset accuracy and/or resolution of closely-spaced spurs is more important, but it can underreport their amplitude due to scalloping loss. I would suggest using one of the HFT windows unless/until you have a specific reason not to. Heinzel also describes several flattop variants with higher sidelobe rejection than HFT95, in the unlikely event you need them. In my experience it's better to stick with flattop windows and increase your bin density if you need better frequency resolution, rather than put up with scalloping loss. -- john, KE5FX Miles Design LLC ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] windows for FFT measurements of phase noise
Hoi Jim, You know, you have this peculiar way of asking seemingly simple questions that are very hard to answer? :-) On Fri, 10 Jun 2016 12:42:13 -0700 jimlux wrote: > Say you have 1 second of sampled data (so the FFT resolution is 1 Hz). > If you're interested in the noise power at, say, 10 Hz away, a > rectangular window isn't going to be very far down, unless you have a > LOT of points in the FFT. Maybe the book [1] can help you. It's chapter 2 contains infromation on how to get spectral components from samples. And one point is how windowing influences the result. Sorry, I cannot help you more than that, I barely understand the topic myself. Attila Kinali [1] "Spectral Analysis of Signals", by Petre Soica and Randolph Moses, 2005 http://user.it.uu.se/~ps/SAS-new.pdf -- Malek's Law: Any simple idea will be worded in the most complicated way. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] windows for FFT measurements of phase noise
Hi If you sit and watch most commercial gear, they go for the “we need more data” approach. Put another way, you don’t get 10 Hz data up on the screen in one second. It’s more like 10 to 100 seconds (some gear can be painfully slow). Bob > On Jun 10, 2016, at 3:42 PM, jimlux wrote: > > What sort of windows do folks use for making FFT measurements of phase noise. > > Say you have 1 second of sampled data (so the FFT resolution is 1 Hz). If > you're interested in the noise power at, say, 10 Hz away, a rectangular > window isn't going to be very far down, unless you have a LOT of points in > the FFT. > > Grove's paper from 2004 doesn't mention this detail. > > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] windows for FFT measurements of phase noise
What sort of windows do folks use for making FFT measurements of phase noise. Say you have 1 second of sampled data (so the FFT resolution is 1 Hz). If you're interested in the noise power at, say, 10 Hz away, a rectangular window isn't going to be very far down, unless you have a LOT of points in the FFT. Grove's paper from 2004 doesn't mention this detail. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
