DIY Phase Noise measurement. How to calibrate? - Page 1
Forum section: electronics / projects

author: erikka
author_rank: Regular Contributor
timestamp: January 21, 2024, 04:50:37 pm
content: I'm trying to build my own phase noise analyzer using the Quadrature method explained in this document:http://www.ke5fx.com/David_Owen_PN_measurement_practice.pdfAs FFT analyzer I'm using a PC audio input (24 bit, 192kHz sample rate)The uncertainty is in the translation to dBc/Hz of the noise level.For calibration a DDS was used to generate 10MHz with white noise phase modulation at a -110dBc/Hz level. This level was confirmed using a real spectrum analyzer with noise markers.Measuring the same DDS output for calibration of the DIY phase noise analyzer gave -108dBc/Hz, so seems close enough (see first thumbnail)The drop off above 20kHz is caused by the audio input which, despite a 192kHz sample rate, rolls of above 20kHz. This was confirmed using a sine wave phase modulation scanning the whole 10Hz to 100KHz range which showed the same roll of.Is this is a correct way to check the noise level calibration of a phase noise analyzer?I did read about offsetting the two input frequencies into the mixer to get the 0dBc level but this does not help with understanding the effective noise bandwidth of the FFT buckets.Then I did some measurements.second thumbnail: phase noise of a DDS 10MHz output (without phase modulation)third thumbnail: phase noise of a OCXO. There are two broad peaks at 1.7kHz and 3.5kHz of which the origin is unknown. The other spurs are mostly mains related (50Hz country)Do these measurements make any sense?

author: bson
author_rank: Unknown
timestamp: January 22, 2024, 07:25:54 pm
content: What is the question? The output frequency is the difference between the inputs. So if you have a 10MHz reference source and an input S, and you have a 3.5kHz spur on the output, then you have a 3.5kHz frequency difference. You won't know the sign, but for an RMS noise measurement this doesn't matter. The easiest way to measure PN should be to adjust the reference (if it's a DDS) until there are no fixed spurs, then measure the signal strength (average, std dev etc).Looking at the paper it doesn't say a whole lot, just a few basic diagrams and plots. Nothing on how to actually turn this into correction (error) terms and actual measurements. Think of the mixer and reference as a notch filter; it removes the fundamental leaving you with the noise skirt (downconverted and absolute around the fundamental), which you then measure the signal strength of relative to the noise floor.

author: RFDx
author_rank: Regular Contributor
timestamp: January 23, 2024, 12:24:35 am
content: Quote from: erikka on January 21, 2024, 04:50:37 pmYes, injecting noise with known power density is a valid calibration method. Another approach to calibrating your homebrewed phase noise test set would be the beat note method. A DDS generator is imo not a good signal source because of the numerous spurs. These are normally buried in noise and not visible on a spectrum analyzer but can be seen on a phase noise test set. The noise spectrum generated by the DDS for calibration purposes doesn't look very good (flat) either.https://www.bartelsos.de/_media/messtechnik/phasenrauschmessplatz/dk7jb_oscillatorphasenoisecalibration.pdfTake a look at the "AudioMeter" software from T. Baier (DG8SAQ) seen in the PDF file above. Compared to ARTA it is much better suited for noise measurements (e.g. phase noise).https://dg8saq.darc.de/AudioMeter/index.shtml

author: erikka
author_rank: Regular Contributor
timestamp: January 23, 2024, 12:55:20 pm
content: Many thanks for the feedbackUsing Audiometer I tried to replicate the calibration as described in the document.Setup was similar.Input 10MHz at 10dBm before resistive combiner.Noise source at -90dBm/Hz with 20dB attenuator before resistive combiner.Resistive combiner with two 3dB attenuatorsNoise is calculated to be at -123dBm/HzSee measurement belowRed trace is not so good oscillator onlyBlue trace is oscillator combined with noiseAll seems ok, however I do not understand where the data length comes in the calculation.The second plot shows the difference in noise level when using 8k or 131k data length.In the calculation in the document there should be somewhere the impact of the FFT bin width but I can not find it.The measurement in the document describes a data length of 131k and a sample rate of 192ks/s This seems to be about 1.3Hz bin width so this could explain a small difference.Another problem could be the compression of the mixer inputUsing the DDS with phase modulation with noise I tried to understand this.With the mixer I'm using there is no compression of the phase noise signal till about 0dBm of the phase modulated 10MHz signal. The measured phase noise varies in measured level with the power of the 10MHz signal.Above 10dBm the compression is about 3dB and above 20dBm the mixer input level has no longer any impact on the measured phase noise. Only the Phase Deviation of the phase modulation has an impact.Hope you can help me better understand these complicated measurements.

author: erikka
author_rank: Regular Contributor
timestamp: January 23, 2024, 07:05:54 pm
content: The measurements in relative way seem to work fine.Here a comparison between the noise floor of the used LNA (green), a rather stable OCXO (red) and the output of a DDS (blue).The many spurs of the DDS are very visible and there are still some mains spurs, despite various isolation transformers to break the ground loops.

author: RFDx
author_rank: Regular Contributor
timestamp: January 24, 2024, 03:56:43 am
content: Quote from: erikka on January 23, 2024, 12:55:20 pmCorrections for the chosen data length/bin, FFT window function etc. come into play for calibrations done with the beat note method.Quote from: erikka on January 23, 2024, 12:55:20 pmThe settings can be changed anytime but you also have to twiddle afterwards with the offset until the correct noise level is displayed again.Quote from: erikka on January 23, 2024, 12:55:20 pmThe combined carrier level of your DUT and the level of the added noise (with a power density based on 1Hz bandwidth) have to be measured/calculated as accurately as possible. After applying a 3dB correction for (assumed) 50% AM noise and another 3dB because of SSB phase noise you arrive to the final value in dBc/Hz relativ to the carrier power. Adjust the offset in AudioMeter to get the same value (in dB). After switching off the noise generator you should now be able to measure the phase noise of the DUT at any frequency offset from the carrier.

author: erikka
author_rank: Regular Contributor
timestamp: January 24, 2024, 09:34:51 am
content: Quote from: RFDx on January 24, 2024, 03:56:43 amI think I understand. As long as the mixer is in the linear region the measured level of the noise is not impacted by the strength of the carrier. With the mixer in compression the measured noise level only depends on the ratio of the power of the carrier and the power of the noise thus the measured noise level becomes a dBc measureTo test this I tried with different levels of carrier. See attached picture.Below 0dBm into the resistive combiner the measured noise level is independent of the level of the carrierAbove 0dBm compression starts but I'm not sure if the step from +20dBm to +23dBm (the maximum I could generate) resulted in a -3dB step of the noise level signaling full compression and true dBc measurement of the noise levelIs this correct? Will I need to drive the mixer even harder?

author: RFDx
author_rank: Regular Contributor
timestamp: January 25, 2024, 06:36:37 am
content: Hard if not impossible to tell what part is or could be compressing and what power levels would be safe to apply without knowing any details of your setup.The Bartels (DK7JB) paper shows what happens if you keep the DUT at a constant level and start increasing the noise level used for calibration. At some point the noise spectrum will not follow in a linear fashion anymore. As the phase detector (high level diode ring mixer) in his PN test-set is connected in reverse and is using the LO-port (typ. 7...20dBm) as input for the DUT, it seems the high gain, low-noise amp following the mixer and not the mixer itself is getting into compression when the applied noise level reaches substantial levels.The second part of the paper (not translated yet) shows what happens when the noise level is kept constant and the DUT carrier level varries. As phase noise measurements are relative measurements (dBc), the noise spectrum obviously has to increase if the power of the DUT decreases.You could maybe also try out the beat note calibration method for comparison with the noise calibration method. Both methods need to deliver the same or at least very close to the same results.

author: erikka
author_rank: Regular Contributor
timestamp: January 26, 2024, 07:43:04 pm
content: Again thanks for the feedbackThe beat note currently is not possible due to the LNA getting overloadedBut I did the calibration using a 20dBm carrier combined with -100dBm/Hz noise.Next a measurement using a DDS generating a signal with -110dB/Hz phase noise (confirmed using an SA) and the measurements where correct so it all seems to work wellThe advantage of the dds output is the flat phase noise than can easily be checked with an SA

author: RFDx
author_rank: Regular Contributor
timestamp: January 26, 2024, 09:27:02 pm
content: Glad to hear that the noise method works to your satisfaction.The voltage of the beat note is normally measured at the (buffered) output of the phase detector, the LNA is not involved.Everything else needed for the beat note method, like the gain of the LNA, the FFT bin width, the effective noise bandwidth of the windowing function etc., is known.

author: cncjerry
author_rank: Unknown
timestamp: January 28, 2024, 02:13:47 pm
content: I built a phase noise test system designed by Andrew Holme. I had a rough time getting rid of powerline grid noise and found that i had to keep they test set 2ft or more away from a transformer. Even then i had noise and hash in that area. I switched to lt3045 and lt3094 regulators and then ran my entire project except for the fpga on Dewalt tool batteries. Noise is gone unless i got close to a powered transformer.There was another person who built something like this and was posting on the time-nuts reflector. He had a problem with grid spurs too. Once in the system they will interact with every clock including the ADC clock. The ADC clock had a lot to do with spurs in our case up to over the 30th harmonic or more. They were square wave oscillators. We were using a spreadsheet to multiply out the spurs to find reference clock and ADC clock harmonics to keep them outside the DUT harmonics. We used a high speed ADC with a clock originally at 150Mhz. So testing 5Mhz and 10Mhz oscillators fell within that area and we switched to 146Mhz. What I'm getting at is there are many things to consider for spurs and you need control of every clock in the system.

author: erikka
author_rank: Regular Contributor
timestamp: January 28, 2024, 03:37:23 pm
content: Mains spurs is indeed a problem, below you can see how they are visible in my, somewhat primitive, setup.The first chart is using a 20Hz FFT bin width. This is easy for measuring noise levels as it is fast but the bins are so large that mains spurs are averaged out and distort the readings below 1kHz a lotThe second chart uses a 1.5Hz FFT bin width. The spurs are clearly visible and also the real noise levels are easier to observe.Be aware of the difference in the frequency scales between the two charts.I've not done anything to reduce the mains spurs except breaking ground loopsBrown: DDS generated 10MHz with -110dBc/Hz phase noise (confirmed by measuring with SA method)Green: DDS generated 10MHz, no phase modulationRed: OCXO generated 10MHzBlue: Noise floor with no inputThe drop above 20kHz is due to the audio input card which is running at 192kHz, 24 bit sampling but it basically filters out anything above 22kHzThe loop of the PLL locked to the fundamental has a bandwidth of about 4 Hz which seems to explain the drop below 10Hz in the narrow bin chart.

author: cncjerry
author_rank: Unknown
timestamp: January 28, 2024, 06:29:01 pm
content: On the one I built using the fpga, it uses cross correlation and we run the test for hours, up to two days. I'll have to read how this one works. Do they notch out the fundamental or just translate it to DC and amplify it?Every part in the chain has to be the best and lowest noise. If you haven't tried them, get the LT3045 and LT3094 (if negative is needed) modules off ebay or build them yourself using small smd carrier boards, about half the cost. You run them after your supply and they made a worthwhile difference. You have to clean every spur because of all the mixing. We had a big interaction between the ADC clock and the dut frequencies. I dont know how you would fix that here. Also, more data and averaging.I still have to come to terms with the dynamic range required for a sound card. Iur noise floor was down around -185dB. 24bit capture is what, 144dB.Its an interesting project, well below the cost of the fpga itself in mine. Then all the amps and splitters cost a lot, a good reference is very expensive, you need to run the signal at the maximum ADC input level so we amplify and then attenuate. We use qbit 188 amps.Have fun.

author: erikka
author_rank: Regular Contributor
timestamp: January 28, 2024, 06:43:49 pm
content: This design cancels the fundamental by locking a vco at 90 degrees to the incoming clock and sending both the incoming clock and the locked vco output into a double balanced diode mixer. The output of the mixer is zero volt + phase noise when in lock.The lower limit is determined by phase noise of the vco, the spurs and the noise of the amplifier after the mixer.The hw is simple and cheap, except for the vco as this has to be very low phase noise. I may even build two so I can do cross correlation to go below the phase noise of the used vco's, but for now it is good enough.A clear disadvantage is it can only measure 10 MHz sources because of the vco range.

author: cncjerry
author_rank: Unknown
timestamp: January 28, 2024, 09:42:31 pm
content: Quadrature with DBM is addressed a lot in the EMRFD book published by the ARRL. It is basically a downconverter. Maybe there are some noise mods in there that can apply. I remember there was a lot done to address termination of the mixers. Try doing a quick test on batteries. These days 90% of the oscillators I test are 10Mhz so I don't see a drawback.I don't know how you would calibrate it other than have a known plot maybe? Possibly get specs for an oscillator. The best deal in 10Mhz are the Wenchel sprinters, the 500 and 501 class. Only difference between them are one is a custom part that generally has no specs published and the other does. Can't remember which is which but the cost delta is obvious. I've been drooling over the ULNs but i need a pair of them.