Brainstorming some ways to measure phase noise in the 100MHz - 500MHz range. - Page 1
Forum section: electronics / rf-microwave

author: rf-fil
author_rank: Regular Contributor
timestamp: September 30, 2025, 06:03:01 am
content: I'm working on some 2m / 70cm gear.. hoping to check the synthesizer / VCO phase noise. Will be needing the noise floor to be somewhere around -130dBc/Hz @ 10kHz, or even better, ideally. The offset I'm looking at is from 1kHz to 1MHz-ish.I know of the crystal notch filter method, which is my current plan.Do I have any other options?The other method I know of is to get a proper signal source analyser which is a few orders of magnitude above my budget. Unless there is some rare analyser that I don't know of, on ebay?

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2025, 12:36:34 pm
content: I've seen a few DIY measurements using the self-correlation method, though you would still need an ADC at very high frequency and an FPGA to deal with the produced data, so in the range of $1k or so, if you buy off the shelf devboards and cobble them all together. There are some project examples with their included software, free and open source.One such project example here:https://www.eevblog.com/forum/metrology/phase-noise-measurement/msg3347292/#msg3347292I think there are a few more if you search on EEVblog.The true question is if you really need to measure the phase noise. If you only need to compare which VCO has the smallest noise, that is much simpler, because phase noise translates into time jitter, which can be easily observed with nowadays DSO.The improvised method I am using to evaluate the phase noise is to look with the oscilloscope at a signal edge, but to look much later after the trigger point, at 1ms to 1second later. So, set the DSO at max sample rate (mine is a Rigol DS1054Z with max 24million samples at 1GSa/s), trigger normally, say at the rising edge, then scroll the screen and observe the signal much later, at 1ms...1s later after the trigger point. You will see a lot of time jitter, and the visual observation alone might be enough to tell which oscillator has the smallest phase noise. The less the time jitter, the lower the phase noise.Of course, the phase noise of the oscilloscope's internal osc is included in the time jitter you observe on the screen, so this improvised method might not be good enough to compare oscillators in the ballpark of -130dBc/Hz.In theory, one can do 2 counters, one driven from the DUT, the other from a very low noise osc, and observe the time jitter difference only (so to skip all the expensive high speed ADC and FPGA part), but I never tried. Vaguely remember from my Rigol DG4102 AWG specs (price less than $1k) to have a phase noise around -115dBc/Hz at 10kHz, for a 10MHz signal. The AWG has a quartz and a PLL at about 1GHz internally, and a quartz osc is usually regarded as heaving low phase noise, so I guess -130dBc/Hz is much harder to achieve.I don't know anything about ham radio, so asking just out of curiosity: does a ham VCO really has to be so low phase noise, lower than the quartz driven PLL from my AWG?

author: shabaz
author_rank: Frequent Contributor
timestamp: September 30, 2025, 01:10:05 pm
content: I've verified synthesizers to that level of spec with the crystal filter method, it's a really cost-effective way to do it (granted it can only be at spot frequencies since it depends on the crystal). For occasional testing, I think it's a very attractive method as a result. Some day I'll try to make my own crystal filters (never attempted that), but I've not had that need come up currently.Also, on the plus side, modern freq synths ICs perform hopefully consistently, and can perform close to spec if care is taken with following the reference designs as much as possible, so for DIY projects, provided some basic checks are made, it can likely be assumed it is working to spec (at least from the few times I've measured using that method) - using judgement of course if that's acceptable, depending on the end use-case.

author: RoV
author_rank: Regular Contributor
timestamp: September 30, 2025, 05:12:41 pm
content: If you can afford two identical synthesizers, I would consider the TinyPFA, which is a NanoVNA-H4 with specialized firmware.https://www.tinydevices.org/wiki/pmwiki.php?n=TinyPFA.Homepage

author: Marsupilami
author_rank: Frequent Contributor
timestamp: October 01, 2025, 05:42:14 am
content: Cross correlation is one way. You need a two channel receiver that has uncorrelated sampling clock and LO (if mixed). The signal of interest is split equally then measured. The results of the two parallel captures then cross correlated which removes the uncorrelated contribution from the receivers and leaves you with the wanted signal magnitude. This then can be repeated and -since phase information is discarded- averaged.This way several 10s of dB improvement can be achieved over the single path phase noise of the measurement system.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 01, 2025, 07:40:50 am
content: What is the blue surface with holes (in the picture), please?Is that a repurposed optical-table, is it some sort of commercial RF/microwave "breadboard" I didn't know about, or is it some custom made item, or something else?

author: Marsupilami
author_rank: Frequent Contributor
timestamp: October 01, 2025, 01:06:56 pm
content: https://photos.app.goo.gl/cvjQV48BwFGm7o7MA3 sheets of laser-cut acrylic, a bag of nuts and anti-static coating. I would do it differently today, but I had this and came handy for that task.Bonus photo of RF Chicken.

author: G0HZU
author_rank: Super Contributor
timestamp: October 01, 2025, 09:50:13 pm
content: The PN2060C/D from BG6KHC looks interesting and I think it may still be possible to buy the hardware.https://qsl.net/bg6khc/pn2060c_phase_noise_analyzer.htmI was tempted to buy the PN2060C/D just to compare with what I have here. I've got quite a few ways to measure phase noise here including the crystal filter method, the crystal notch method and I have various signal/spectrum analysers that have an SSA mode built in. However, all of it is fairly dated in terms of the performance if the aim is to push boundaries.For example, I have one of the classic Agilent E5052A SSAs here but it is about 20 years old now and there are much better options available now for much less than the cost of a used E5052A. The E5052A is a poor choice (IMO) if you ever get interested in looking at phase noise very close to carrier, eg at 1Hz offset. The performance is quite disappointing in this respect. The B version (E5052B) is much better but these cost a fortune even when purchased used.I'm not sure how mature the PN2060D product is now because it always seems to be in a state of development. I'm still tempted to buy one but I'm not sure how easy it is to use.The E5052A is very easy to use and can easily meet your requirements but they rarely appear for less than $10k USD. I can show you a few plots from it if that helps? It also supports an optional baseband analyser which can be useful for other things.I think you should buy the PN2060D and then let me know if I should buy one as well

author: G0HZU
author_rank: Super Contributor
timestamp: October 01, 2025, 10:18:13 pm
content: A big advantage of the modern alternatives like the PN2060C/D is the small size and relatively low power consumption. The E5052A is huge at 43cm wide, 48cm deep and 28cm tall. It's a 21kg monster, and I have no room for it in my fairly small workroom so it lives on a trolley in another room and only gets wheeled in for special occasions. Otherwise, I use something else for phase noise measurements. This is partly why I'd like something like the PN2060C/D.The plot below is of a homebrew 10 MHz crystal oscillator that was designed to try and explore the limits of the E5052A. I think I'm quite close to its limits below when the correlation is set to 100. It doesn't take very long to get this plot with the E5052A as the correlation is only set to 100 and it's worth noting that some of the demo plots of the modern devices require several hours to produce. I typically don't like waiting more than a few seconds for a PN plot but I think the plot below took a minute or so. I really can't remember.

author: G0HZU
author_rank: Super Contributor
timestamp: October 01, 2025, 11:15:10 pm
content: Here's an old plot of my homebrew crystal notch filter at about 9.830 MHz. I designed this to have a deep but very narrow notch. The other design aim was low insertion loss and very high return loss (30dB typical). The plot below shows a 10 kHz span so you can see how sharp the notch is. This filter was designed using a simulator (with an auto optimiser) in order to get this level of performance.This allows phase noise measurements to within about 1 kHz of the carrier with fairly low uncertainty when used with a spectrum analyser that has a built in preamp. When used with my old Agilent PSA analyser, this means it can measure down to about -168 dBc/Hz close to carrier. I'd expect something like a Siglent analyser would manage about -158 dBc/Hz with this setup?I'll try and find some time to demo it tomorrow at 432 MHz using a mixer and a low noise LO. The aim would be to measure the phase noise of something like a Marconi 2024 sig gen at 432 MHz as this should be capable of about -120dBc/Hz at a 10 kHz offset. I haven't used this setup in ages but it should give impressive performance for something that costs peanuts to make. The crystals only cost about 20p each to make the filter. I keep meaning to make a 10 MHz version of this notch filter because I could probably sell a few of them on Ebay

author: rf-fil
author_rank: Regular Contributor
timestamp: October 02, 2025, 05:40:46 am
content: The PN2060C/D looks great.

author: G0HZU
author_rank: Super Contributor
timestamp: October 02, 2025, 10:37:09 am
content: The PN2060C will need some additional hardware (reference oscillators) as shown in some of the images in the link below but I guess this won't add a great deal to the overall cost.https://qsl.net/bg6khc/pn2060c_phase_noise_analyzer.htmIIRC the PN2060A/B cost around $900 but I don't know how much the C and D versions cost. I don't know how flexible the D version is in terms of the input frequency vs the reference frequency but maybe you could enquire about this. Otherwise, it may be the case that it won't quite do everything you expect.

author: G0HZU
author_rank: Super Contributor
timestamp: October 02, 2025, 04:57:35 pm
content: Here's a quick demo of the notch filter method. The aim is to downconvert to the 9.83 MHz notch frequency and this nulls the carrier but lets the noise sidebands pass through to a spectrum analyser.The performance is usually limited by the phase noise of the LO used to drive the mixer. In this case I've used an old Agilent ESGD sig gen E4438C and this has the low phase noise option fitted.The aim is to measure the phase noise of my Marconi 2024 sig gen and this has the OCXO option fitted. The phase noise of the Mi 2024 is shown below (when measured by the E5052A) and the notch method should give similar results.

author: G0HZU
author_rank: Super Contributor
timestamp: October 02, 2025, 05:00:36 pm
content: The phase noise of the E4438C (used for the LO that drives the mixer) is shown below. This sig gen is cleaner than an HP 8640B at offsets of 3 kHz, 10 kHz and 30 kHz when tested up at 433 MHz.Despite this, it is going to limit the noise floor performance, so I'm only going to measure at 3 kHz, 10 kHz and 30 kHz using the notch filter.

author: G0HZU
author_rank: Super Contributor
timestamp: October 02, 2025, 05:05:48 pm
content: Here's three plots showing the phase noise of the Mi 2024 sig gen at 3 kHz, 10 kHz and 30 kHz using the notch filter method.The agreement with the E5052A is quite close although the phase noise at 30kHz offset is starting to be affected by the contribution from the E4438C sig gen.However, this is still a good result I think as the result is within about 1dB in each case.The output level of the mixer (into the analyser) is about -6.5dBm and there is about 0.5dB typical passband loss in the notch filter so I have added a 7dB offset to the analyser and the marker therefore indicates dBc/Hz.The plots show-109.7 dBc/Hz at 3kHz offset (E5052A shows -108.9 dBc/Hz)-123.1 dBc/Hz at 10kHz offset (E5052A shows -122.8 dBc/Hz)-133.0 dBc/Hz at 30kHz offset (E5052A shows -134.2 dBc/Hz)