Choosing an SA for phase noise measurements - Page 1
Forum section: electronics / testgear

author: bson
author_rank: Unknown
timestamp: December 18, 2015, 11:28:32 pm
content: I'm looking for an instrument to perform phase noise measurements on carriers in the 5-20MHz range. All of AC sinusoidals, DC sinusoidals, and DC square wave (HC through LVC family clock signals).The spectrum analyzer method looks very appealing (side slope integration, as a ratio to the center, or an approximation). I see an Agilent 8561A can be had for not too crazy coin, and they can be upgraded with software to do both spur and phase noise measurements. This is appealing because the software can be cognizant of the instrument's own noise floor and receiver limitations. But beyond that I don't really know the detailed mechanics used for the measurement: for example simple trace data analysis (which is a downmixing receiver sweep I presume), or by stepping in CW mode. I do understand it's not a real-time spectrum but is going to be a sweep of some kind. But if it can identify the center frequency it could sit in CW mode in the sideband, maybe by activating the multiple receivers at different side frequencies, for a real-time continuous sample series at multiple points in the spectrum. (Exactly how to do it is beyond my understanding.)Anyone gotten into this? Is it usable? The 8561A looks like a pretty nice instrument to have on hand anyway, and even though my uses for a SA are limited, I'd probably put it to use for various similar "signal purity" type uses. I'm not a ham person or particularly interested in wireless comms. (At least not per se.)Any advice? Thoughts before I press the 'buy' button?

author: Theboel
author_rank: Frequent Contributor
timestamp: December 18, 2015, 11:45:10 pm
content: I don't think phase noise measurement with an SA (direct one) can be easy.try to ask John Miles (Id name: KE5FX) he know about phase noise measurement far better

author: bson
author_rank: Unknown
timestamp: December 18, 2015, 11:56:09 pm
content: Agilent has a nice little summary on measurement techniques...http://www.keysight.com/upload/cmc_upload/All/PhaseNoise_webcast_19Jul12.pdf

author: KJDS
author_rank: Super Contributor
timestamp: December 19, 2015, 12:39:32 am
content: Whilst the spectrum analyzer method is the easiest way of measuring the phase noise, it's also about the least accurate in terms of bang for buck.Build two oscillators at the same frequency, mix them and run the baseband through an audio analyzer.

author: bson
author_rank: Unknown
timestamp: December 19, 2015, 02:07:51 am
content: Yeah, I considered the detector/discriminator, but not sure what kind of specs I need. For example, this onehttp://www.minicircuits.com/pdfs/ZLW-11.pdfshows LO-IF isolation of >67dB for the frequencies I'm interested in. Do I care more about this than LO-RF isolation? I just don't have the knowledge to answer that.However, I'm actually more interested in the shaped clock than the sine XO, and the impact (or lack thereof) of various trace geometries. I think this would be a non-issue if I wasn't dealing with a clock. I could distribute it as a HS diff pair, but then again need to measure the actual benefit of doing so. I could shape it closer to the consumer, but then I'm still back to measuring the benefit.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 19, 2015, 11:31:04 am
content: I made a couple of videos that may be of interest to you. This first one shows post processing using Labview compared with Teledyne/LeCroys JTA2 software. Also shown is data compared with a Wavecrest. All very old but may give you some idea what could be done using Labview.https://www.youtube.com/watch?v=Bg45FuoeHZk&index=4&list=PLZSS2ajxhiQBcHhIaGpmm9GyZQfrCzqkvThis video shows the SA.https://www.youtube.com/watch?v=GP6B_ImnoII&index=1&list=PLZSS2ajxhiQBcHhIaGpmm9GyZQfrCzqkvHope it helps.

author: awallin
author_rank: Frequent Contributor
timestamp: December 19, 2015, 01:26:09 pm
content: Quote from: bson on December 18, 2015, 11:28:32 pmThe digital phase noise test sets are very good for this specific purpose. Microsemi/Symmetricom 5115A, 5120A (lower noise floor), or the newer 3120A.Similar techniques are now emerging using significantly cheaper SDR hardware and GNU-radio. This is more of a DIY-hack currently but might become more mainstream quickly.Both the digital phase noise test sets and the SDR method *compare* two oscillators REF and DUT. They digitize the signal at 78MS/s or so and are limited to 30MHz RF signal input. You will need at least two, preferably three reference oscillators that have similar or better noise than the DUT you are measuring.A typical SA will measure phase-noise against its own internal oscillator, which may be a more or less good OCXO. The SA is more flexible as it can measure phase-noise at e.g. 100MHz or any frequency the LO can reach. The new SAs combine analog techniques at high offset and a digital method for the close-in I think.Anders

author: KJDS
author_rank: Super Contributor
timestamp: December 19, 2015, 03:01:56 pm
content: Quote from: awallin on December 19, 2015, 01:26:09 pmYou've missed a critical point. If the SA was comparing the input signal with it's internal OCXO then that would be great, but it's not, it's comparing what's coming out of the last IF, by which time it's being compared with it's first, second and third LOs. That first LO will be a broadband oscillator and the broader the sweep of an oscillator the worse it's phase noise is.

author: cncjerry
author_rank: Unknown
timestamp: December 19, 2015, 08:32:54 pm
content: Search ke5fx phase noise. I use his code with my 8568b SA. His code supports a number of Spectrum analyzers and is quite simple to use.

author: bson
author_rank: Unknown
timestamp: December 20, 2015, 01:09:40 am
content: The SA method is appealing to me because of the low barrier to entry. I think I'll start with that - I really don't want a side project. However, I can easily see experimenting with mixer methods in the future. However, I think I still want the SA method for sanity checking and a sort of baseline, even if not as accurate. John Miles' GPIB toolbox looks great for this and I think I'll get an HP 8566B as a starting point. The only thing drawback is I run Windows in a VM (under VMware Workstation 11 on Linux or in a VM on OS X depending on whether it's a desktop or laptop), and my GPIB bus is on an ICS8065 ethernet bus controller. I assume Windows is no problem, but the KE5FX GPIB software really wants just a GPIB device address and I don't see (off hand) any way of specifying a full tcpip VISA address. (Although it seems to use a file name connect.ini to configure itself, maybe this can be edited to make it work with a VXI-11 based bus controller.)

author: TheSteve
author_rank: Unknown
timestamp: December 20, 2015, 01:18:15 am
content: If nothing else works you can always grab a Prologix USB to GPIB adapter, it should work fine in MacOS and works great with John's software.

author: bson
author_rank: Unknown
timestamp: December 20, 2015, 02:33:14 am
content: Quote from: TheSteve on December 20, 2015, 01:18:15 amTrue! Excellent point!

author: KE5FX
author_rank: Super Contributor
timestamp: December 20, 2015, 03:53:49 am
content: Quote from: bson on December 20, 2015, 01:09:40 amLike TheSteve said, the Prologix USB-GPIB adapter should do a good job for you in this application. 8566Bs are good for indirect measurements as well since they go down to 100 Hz (which will also be your minimum measurement offset, based on the 8566B's 10 Hz RBW filter.) The 8568B may be even more cost-effective, but its 1500 MHz upper end makes it somewhat less versatile than the 8566B.One cool thing about the 8566B/8568B approach is that they use the same display section, and the RF decks are very inexpensive when you buy them by themselves. (Onebadthing is that the interconnect cables cost a fortune, so be sure you end up with at least one good set.)For low(er) noise measurements you can add a quadrature PLL to the analyzer and use it with a procedure similar to the last FAQ entry athttp://www.ke5fx.com/gpib/faq.htm. By themselves the 8566B and 8568B are adequate for measuring a lot of VHF-microwave sources, but low-noise crystal oscillators are impossible to characterize without more hardware. Spend some time surfing the app notes in the links in the FAQ entry, especially Wenzel's.You can always specify TCP/IP addresses in connect.ini (see the comments in the file, as well as the FAQ) but VISA addresses aren't currently supported. I recently bought a handheld SA that only supports VISA TCP/IP connectivity, so this may change pretty soon...

author: cncjerry
author_rank: Unknown
timestamp: December 20, 2015, 06:48:55 am
content: Quote from: TheSteve on December 20, 2015, 01:18:15 amI use this adapter with my 8568B. Works perfectly, can be quickly swapped to a tablet like a MS Surface pro for portability. I also have an NI PCI GPIB that I bought for $50 for a guy in Sweden.

author: G0HZU
author_rank: Super Contributor
timestamp: December 20, 2015, 12:02:05 pm
content: QuoteAssuming that you want to measure phase noise at offsets of maybe 100Hz to 50kHz on a clock based on a typical crystal oscillator at a few MHzthen the HP865x analyser is going to be at least 30dB noisier than the thing you are trying to measure. eg at 500Hz offset I'd expect the clock to be better than -135dBc/Hz but a typical HP856x analyser might only achieve -105dBc/Hz here. The 8568 can achieve -112dBc/Hz here but it is still a long way off the performance of the thing you are trying to measure.One way to overcome this is to use the clock as the phase detector reference frequency for a microwave PLL and then look at the phase noise inside the PLL loop bandwidth on the HP856x analyser.The clock noise will multiply up by 20logN and so assuming you use a wide loop BW and N is a big numberthen you would be able to see differences in the phase noise of the various clocks you used as the reference for the PLL. i.e. if you design the system correctly you can judge the noise performance of the 20MHz oscillator by the close in phase noise of the microwave oscillator. You could then use the HP856x analyser here although it will still be a bit marginal in terms of what noise levels you will be able to measure down to.

author: Rupunzell
author_rank: Frequent Contributor
timestamp: December 20, 2015, 06:02:57 pm
content: SA, being the most basic is often not the best choice.Spend some time with this hp app note.http://www.hparchive.com/seminar_notes/Scherer_Art_of_PN_measurement.pdfExample in sampled data systems.http://www.analog.com/media/en/technical-documentation/application-notes/589324855748812403694448557703434217045254275316215330254506699244016AN741_0.pdfBernice

author: cncjerry
author_rank: Unknown
timestamp: December 20, 2015, 06:12:12 pm
content: Yesterday after reading this thread I went back and tested several different sources including my SA calibration oscillator. I then ran a plot without a signal to see what the lowest noise floor possible would be. I have the 85685A Preselector so I ran the test with/without. Both came in at -117dB at 1K dropping to about -125dB past 10Khz without a signal.I then tested a qualcom oscillator, the 20Mhz calibrator, and two crystal oscillators. I can clearly see noise stacked above my baseline plot. So at least for comparison purposes it looks like you can characterize oscillators using a relative plot.John, I found a chart that plotted SA baseline plots, I thought it was on your site, can't find it now, but wouldn't the noise have to be above the plots I made without a signal?

author: awallin
author_rank: Frequent Contributor
timestamp: December 20, 2015, 06:28:48 pm
content: Here's what we have at work.Rhode&Schwartz FSW8 SA in phase-noise measurement mode with a 10MHz lab reference connected at the back REF-in port (not that it makes much difference for this measurementt, AFAIK). Same lab-reference 10MHz fed to SA input input.3120A noise floor measured by taking the 10MHz lab reference through a 50ohm Mini-circuits splitter and feeding one branch to REF and the other to DUT.If someone has data for other SAs I could easily plot them in this same graph. Or PM me if someone wants the data in CSV from this graph.

author: G0HZU
author_rank: Super Contributor
timestamp: December 20, 2015, 06:56:49 pm
content: Quote from: cncjerry on December 20, 2015, 06:12:12 pmI don't think your test method above is valid because you have to inject a clean signal** into the analyser to display the analysers own phase noise levels. Testing/comparing without a signal isn't meaningful I'm afraidAssuming you are using an old HP8568B then the phase noise performance of this analyser is well documented and at the bottom end of the analyser range the phase noise profile within the loop bandwidth is fairly flat at about -112dBc/Hz from about 300Hz out to offsets of 50kHz or so.This isn't low enough to measure a typical 10MHz crystal oscillator directly. It's going to be about 30-40dB too high.** By clean I mean inject a test signal source that is (ideally) at least 15dB cleaner than the analyser's own phase noise profile.

author: G0HZU
author_rank: Super Contributor
timestamp: December 20, 2015, 07:16:22 pm
content: Here's a phase noise plot of an old Marconi 2024 sig gen set to output 10MHz. This is the yellow trace and was measured a few years ago at work using an E5052A SSA from Agilent. Note that this is 10MHz from the regular RF output port of the Mi 2024 and not a direct measurement of the internal 10MHz reference.I've hand drawn in the phase noise performance of a typical HP8568B in blue and I've also hand drawn in the phase noise of a decent 10MHz OCXO in green. I would expect that a typical/basic/low cost clock oscillator will sit somewhere between the yellow and the green traces and this is much, much lower than the HP8568B can measure directly.

author: G0HZU
author_rank: Super Contributor
timestamp: December 20, 2015, 07:36:40 pm
content: Here's the phase noise performance of the HP8568B according to HP's literature. I've highlighted the relevant trace with blue dotsYou can see that when it is used down near 20MHz the phase noise is about -112dBc/Hz from 300Hz to about 50kHz offsets.I have a good example of an HP8568B here and I have checked/measured its phase noise performance numerous times in the 25 years I've been using this particular HP8568B. It agrees very closely with the 20MHz trace in the graph below.I don't have access to an HP8561 but I would expect it to be slightly noisier than the HP8568B across offsets of 300Hz to 10kHz when used down at 20MHz. The HP8568B used a clever comb system in the PLL that achieved very low close in phase noise when used down in the HF>VHF region. So I'd expect it to be maybe 6dB better than other HP856x analysers here.

author: KE5FX
author_rank: Super Contributor
timestamp: December 20, 2015, 07:37:24 pm
content: Quote from: cncjerry on December 20, 2015, 06:12:12 pmWhen you run a plot with no input signal, the result is based on a combination of the spectrum analyzer's reference level and its front-end noise figure (seeQ. Is it possible to use PN.EXE to determine the RF noise floor of the spectrum analyzer by making a measurement with no carrier present?in the GPIB ToolkitFAQ.) The composite noise performance floor of most spectrum analyzers is determined by the phase and/or amplitude noise of their first LO, but if no signal is applied to the first mixer, the noise from the first LO has no way to make it into the plot.QuoteThere's a comparison of a couple of HP and Tektronix analyzers on page 14 of this deck (http://www.ke5fx.com/ke5fx_mud2010.pdf). The plot of the 8568A agrees nicely with the one from G0HZU's document. One of the all-time greats!Note how it shows its taillights to an 8560E built 15 years later.Another plot on page 34 includes traces from an HP 3585A and a very early TimePod/Microsemi 3120A prototype with two ADCs.

author: TheSteve
author_rank: Unknown
timestamp: December 21, 2015, 08:58:01 pm
content: Quote from: KE5FX on December 20, 2015, 03:53:49 amAnything exciting such as an Agilent E7495?

author: KE5FX
author_rank: Super Contributor
timestamp: December 21, 2015, 10:33:07 pm
content: Quote from: TheSteve on December 21, 2015, 08:58:01 pmNo, an N9340B. Although that E7495 does look nifty!

author: Electro Fan
author_rank: Super Contributor
timestamp: December 21, 2015, 11:20:25 pm
content: Somewhat off topic but since we have a good assembly of SA-ers, how much does the difference in sweep time result in the different signal attributes displayed in the images below? The top is from a 8568B and the bottom is from a 8561E. It's hard to know what the differences might have been in the actual signals - but assuming they were both 20 MHz sine waves or square waves what causes the different shape where the signal meets the noise? Is that something (a lower noise floor?) that is revealed as a result of the faster sweep time of the 8568B or is that a result of some other capability present in the 8568B that is not present in the 8561E? (Both units are set to 10kHz for RBW and VBW with a span of 1 MHz.)Maybe there is some type of hard(er) filter in the 8561E that cuts off a more graceful transitioning view shown by the 8568B?Or maybe it's noise in the signal - and the signal on the 8568B simply had more noise which shows up around -75dB? Maybe the signal was cleaner on the 8561E and it doesn't show noise until about -80dB?Net, net: I'm just trying to understand how you would interpret these signals, in particular in the area where the signal meets the noise.... Thx

author: G0HZU
author_rank: Super Contributor
timestamp: December 21, 2015, 11:37:47 pm
content: Not sure what you mean but a 1MHz span on the HP8568B is the place where there is a significant change in the way the analyser performs a sweep.For spans =< 1MHz the analyser PLL is in lock for the whole sweep. But above 1MHz span it goes into 'lock and roll' mode where the PLL is 'locked' at only one part of the sweep and the YIG oscillator sweeps (or 'rolls') for the rest of the sweep.Try changing the span on the 8568B ever so slightly to 1.01MHz and watch the noise profile change in the area you are looking in. The PLL is now no longer locked here and the noise close to the carrier will appear less or narrower. This is because the PLL loop BW is very wide for spans of 1MHz or less and the noise kind of looks like it has shoulders. These shoulders disappear for spans above 1MHz because the PLL is not locked across the sweep.In other words, for spans of 1MHz or less the signal will show the combined noise of the YIG oscillator and also the PLL noise with its wide loop bandwidth (or wide shoulders) of noise. But above 1MHz span the noise will be that of the free running YIG oscillator alone so it will look a bit narrower or cleaner in terms of noise because the wide PLL noise will be gone.Also, the HP8568B log amp only has an effective range from 0 to 90dB or so and therefore the noise will appear to shrink in intensity as the trace gets near to -100dB because the log amp isn't working properly down here. On these old analysers you are not supposed to trust the accuracy of any measurements of noise near the bottom of the screen

author: TheSteve
author_rank: Unknown
timestamp: December 21, 2015, 11:48:40 pm
content: Quote from: KE5FX on December 21, 2015, 10:33:07 pmExcellent - if you do update your software I'm happy to try it out with an N1996a

author: Electro Fan
author_rank: Super Contributor
timestamp: December 22, 2015, 12:03:31 am
content: Quote from: G0HZU on December 21, 2015, 11:37:47 pmOk, so maybe this signal just happened to be near the span threshold on the 8568B and it would look more like the signal on the 8561E if the span had been a few hertz higher?I don't have a 8568B - the image was just something I found so I can't try it. FWIW, I did adjust the span on the 8561E from about 900 kHz to about 1.1 MHz and it didn't seem to have much effect on how the signal and noise look on the 8561E. Presumably the PLL and YIG are different on the two SAs? So, in order to interpret what you see on a SA you might need to know something more about it's architecture than just the frequency range it covers?

author: G0HZU
author_rank: Super Contributor
timestamp: December 22, 2015, 09:12:25 pm
content: Here's 1.000MHz and 1.001MHz spans on my HP8568B. I've used a different reference level to the other plots but I don't think this matters.You can see the difference in the noise response between having the PLL locked all the time or when lock and roll mode kicks in.QuoteIdeally, I think you need to know quite a bit about a spectrum analyser's performance limitations before you can begin to trust what it displays.

author: Electro Fan
author_rank: Super Contributor
timestamp: December 22, 2015, 10:25:13 pm
content: Quote from: G0HZU on December 22, 2015, 09:12:25 pmRoger that, Thanks