Frequency measurements with oscilloscope - what accuracy to expect? - Page 1
Forum section: electronics / testgear

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 10, 2016, 07:00:59 am
content: I'm validating an oscillator and clock generator, which should output a 250 MHz clock from a 40 MHz crystal. Internally, a PLL is used (black box IC). My 13 GHz infiniuum scope shows that one oscillator runs at 39.985971 MHz with output frequency 249.40278 MHz. The other oscillator is 40.00921 MHz for an output (after PLL) 249.42577 MHz. The multiplier is 6.23725 and 6.23421 times, respectively. The correct multiplier (250/40) should be 6.25 so something is wrong. However, I'm not sure I can trust the accuracy of the oscilloscope although a high-performance unit. How would one go about verifying the scopes functionality with a "frequency reference standard" of some kind? Which one to buy? How would one get best accuracy when measuring frequency with oscilloscope? Currently, I just set the averaging to 16. The scope additionally, calculates "mean". It is an Infiniuum DSO81304B.http://www.keysight.com/en/pd-734419/infiniium-high-performance-oscilloscope-13-ghz?cc=US&lc=engAlso, would a frequency counter be more accurate? It would need to be high-impedance to not disturb the oscillator, I suppose. How to connect to DUT?Thanks.

author: Psi
author_rank: Super Contributor
timestamp: May 10, 2016, 07:03:44 am
content: Most scopes calculate the frequency in software from a subset of the sample data. (not very accurate)Some scopes do it from the pixel data on screen (terrible)Others have a hardware frequency counter built in (good)In general the frequency measurements on a scope are not that accurate.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 10, 2016, 07:07:11 am
content: How exactly are you measuring frequency with the scope?If you use the scope's memory to store a single complete cycle, and use the cursors to measure the period as accurately as possible (by zooming in to position each one precisely), then you should get a much more accurate result than if you just rely on built-in measurements.

author: borjam
author_rank: Unknown
timestamp: May 10, 2016, 07:22:21 am
content: Rigol DS1000Z scopes have a buit in frequency meter, it seems. I have checked it against known sources and the accuracy is quite good.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 10, 2016, 07:54:43 am
content: My scope has a 13 GHz analog bandwidth and a 40 GS/s sample rate so I would assume that the time-base is sufficiently accurate to measure correct frequency of a 40 MHz and 250 MHz sine wave. I simply use my 2.5 GHz active probe and press the "quick meas" button on the front of the scope. It shows various statistics about the signal (period, frequency, including mean period and frequency). I essentially want to know that what I see on the scope is the true frequency. Assuming that it is, there is lots of jitter in the clocks and the oscillator is way off. Perhaps bad crystals? They are both right next to ICs that have been reworked many times. Perhaps the crystals have been damaged during multiple heating cycles (260 degrees C).I have attached some scope pictures.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 10, 2016, 08:06:38 am
content: Perhaps.You should check your scope's accuracy against a known accurate standard at the frequency range of concern, for absolute measurement accuracy. But since you are aslo seeing a relative difference between the two oscillators under test, you could be dealing with both scope inaccuracy and DUT variation.In general a dedicated frequency counter with an oven controlled crystal oscillator or rubidium internal reference will be more accurate than an oscilloscope at measuring frequency. Your scope is giving you eight digits of precision which is pretty good for a scope, but you can get nine or ten digits or even more from an actual frequency counter.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 10, 2016, 09:04:21 am
content: 1) If you do several sequential measurements with your scope - are results the same every time?2) A good frequency counter will be much more accurate. For frequencies above few Mhz, it would be wise to use a matched impedance cable into 50 Ohm input on the meter and a resistive divider (say, 1K/51 Ohm) directly on the output of the circuit you are trying to measure.CheersAlex

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: May 10, 2016, 09:11:59 am
content: Quote from: John_ITIC on May 10, 2016, 07:54:43 amIt isn't. Your DSO81304A (which I assume is what you have) has a timebase accuracy of+1ppm, which isn't great for frequency measurements. You can connect a 10MHz reference source (i.e. GPSDO) to increase the accuracy, usually by several magnitudes.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 10, 2016, 09:27:02 am
content: Quote from: Wuerstchenhund on May 10, 2016, 09:11:59 amThe timebase accuracy of the scope tells you very little about the frequency measurement accuracy, especially if the frequency is measured from the acquired waveform (meaning that the amplitude errors are converted to frequency errors and the results are barely good to something like 1000ppm). look at the frequency min/max results on the screen shots - i.e. 39.898/40.058 MHz. Even an RC oscillator should have a better frequency stability! Unless the scope has a hardware frequency counter it is pretty useless for accurate frequency measurements IMHO.CheersAlex

author: rf-loop
author_rank: Super Contributor
timestamp: May 10, 2016, 09:55:53 am
content: Quote from: John_ITIC on May 10, 2016, 07:00:59 amIf think just numbers in your message it can clearly see that your scope is "far" away from accuracy what you need. And in this scope is quite good freq reference +/-1ppm. But for your frequency measurements it is not at all enough.There is external reference input in your scope. You can use external reference and get much more accuracy and also stability of course. For someone who may think some Rigol cheap bottom entry level scope have "good" accuracy. Well... +/25ppm is +/- 2500 Hz at 100MHz. Is it good accuracy?OP scope have +/- 1ppm class reference. Is it enough? IMHO not. It is still +/- 100Hz @ 100MHz.From scope screen (or acquisition memory) automatic measurements from trace. It do not give anything acceptable for this purpose, just nothing, how ever it do.Also many oscilloscopes HW counters may have severe things what affect badly for accuracy, even if scope reference is good.For middle level accurate frequency measurements I will recommend real frequency counter what use known accuracy reference.Example something like HP53131A or something in this class of meters with known accuracy reference.

author: nctnico
author_rank: Super Contributor
timestamp: May 10, 2016, 09:59:33 am
content: I agree. For frequency measurements nothing beats a frequency counter. Use the scope to see if the signal is suitable and use the frequency counter to measure the frequency.

author: wraper
author_rank: Unknown
timestamp: May 10, 2016, 10:15:25 am
content: Isn't there hardware frequency counter available in the scope (many scopes have)? Measurements based on the waveform may be not very accurate.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: May 10, 2016, 10:47:20 am
content: Quote from: Alex Nikitin on May 10, 2016, 09:27:02 amThe timebase accuracy tells you that you what basic uncertainty you'll have for your measurements, i.e. it won't be any better than 1ppm.QuoteThat's another problem, but to what extend depends on the signal and how the actual measurements are performed.I agree that a frequency counter would be a better choice but you can do pretty accurate frequency measurements with a scope which is connected to an accurate reference source.

author: rf-loop
author_rank: Super Contributor
timestamp: May 10, 2016, 10:48:55 am
content: Quote from: wraper on May 10, 2016, 10:15:25 amJust short time ago:https://www.eevblog.com/forum/testgear/frequency-measurements-with-oscilloscope-what-accuracy-to-expect/msg937581/#msg937581And also some other members comments.Most today oscilloscope have HW counter. Just this accuracy is question. And accuracy is what here is needed. (related to OP and numbers there.Measuring from captured waveform, who even try 1ppm or better what here is needed.

author: Someone
author_rank: Super Contributor
timestamp: May 10, 2016, 11:35:03 am
content: Quote from: John_ITIC on May 10, 2016, 07:00:59 amThe timebase accuracy of your scope is already quite good and similar to a dedicated frequency counter, as discussed above its how you're resolving the measurement that is the problem. One useful method since you are looking for small deviations from a known frequency is to set the pre trigger far off by say 1000 cycles (25us) and measure the phase offset, you can keep multiplying this pre trigger out for finer and finer measurement as limited by the timebase accuracy.

author: Someone
author_rank: Super Contributor
timestamp: May 10, 2016, 11:41:26 am
content: Quote from: Psi on May 10, 2016, 07:03:44 amI'm wondering what the memory depth was for the measurements shown in this thread, its possible to make high accuracy period and/or frequency measurements with scopes and I've had no problem getting jitter data below 0.001UI from the realtime measurement on an Agilent MSOX.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 10, 2016, 11:44:38 am
content: Accuracy and precision are not the same thing, and both are required in this case.

author: VK5RC
author_rank: Unknown
timestamp: May 10, 2016, 12:13:40 pm
content: Isn't the answer in the data sheet, about 1/3 the way down, in "Delta time measurement accuracy"?http://literature.cdn.keysight.com/litweb/pdf/5989-4604EN.pdf?id=20515580.9pS averaging disabled is pretty darn hot for any counter over 5 cycles but the accuracy for frequency (esp at 40-250MHz) is getting back to the limit of the crystal from a few of my back of the envelope calculations. e.g. 5 cycles at 2.5 e-8 / 1 e-12 approx 1 part in 10e5. Averaging helps a lot but then the time reference would really be the limit of accuracy, resolution would be better I presume.KE5FX - the time nut -who often visits eevblog - could provide some useful comment I am sure.

author: nctnico
author_rank: Super Contributor
timestamp: May 10, 2016, 12:27:24 pm
content: Quote from: alsetalokin4017 on May 10, 2016, 11:44:38 amActually absolute accuracy isn't required here since John is comparing two frequencies to determine whether he programmed the PLL correctly. A simple crystal referenced frequency counter will do just fine if the measurements are taken within a couple of minutes. It also depends on the programming steps of the PLL. If it can do 6.0, 6.25 or 6.5 then 6.23 is close enough to say it is OK. If it can do much finer steps then the measurement needs more significant digits. The question boils down to: how many significant digits does the measurement need?Another thing to look for is whether the PLL has a stable lock. For that purpuse it is a good idea to use FFT or a spectrum analyser to look at the spectrum of the output signal.

author: MT
author_rank: Super Contributor
timestamp: May 10, 2016, 01:10:34 pm
content: Simple frequency counters may be based on a simple crystal, quality frequency counters are based on a aged ovenized crystalsand some people use NISThttp://www.nist.gov/pml/div688/grp40/wwvb.cfmQuoteTo do this he needs a "known" accurate time base.Quotehttp://www.thinksrs.com/products/SR625.htmhttp://www.thinksrs.com/products/FS725.htm

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 11, 2016, 12:33:32 am
content: Thanks all for the input. So, I could (for instance) buy an Agilent 53131A/53132A with GHz option and possibly a higher precision time base built-in. And then use the 1K/51 resistive divider to connect to the 50 ohm input on the counter?I'm currently reading in on the Agilent 53131A/53132A to get familiar with its feature. It looks like the 10 MHz reference on the rear panel can be output for use by my DSO81304A's "ext reference" input so I don't have to use a dedicated external 10 MHz reference?What external 10 MHz reference would you recommend, if I need/want to buy a separate one?I also have a stack of my books on the subject to read in on the theoretical details...

author: AndyC_772
author_rank: Super Contributor
timestamp: May 11, 2016, 07:08:25 am
content: What frequencies do you get if you measure carefully and manually using the cursors, rather than relying on built-in measurements?

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 11, 2016, 08:26:02 am
content: Quote from: AndyC_772 on May 11, 2016, 07:08:25 amBased on the above first scope screen shot, there is exactly 5 x 5ns between the x-axis transitions, which works out to exactly 40 MHz. But that is only for a single cycle. Variation in the measured period is large. Possibly due to spread-frequency clocking, which I'm not sure how to measure. I still have yet to read in on if/how a time-base improvement would improve the max/min/mean frequency reported by the scope.I'm quite close to buying a HP 53131A with the 2.5E-9 (0.0025 PPM or 2.5 PPB) OCXO built-in. I can then also use this oscillator to drive the scope's time base to see if it changes the result. I then have two ways to validate the frequency. I doubt I will need a rubidium reference. The specs for the system that will use the 40 MHz oscillator states +/- 50 PPM. The one thing that it a mystery is why my scope, which is supposed to handle 1PPM accuracy, measured some 2000 PPM error in the oscillator frequency. It seems to indicate a flakey oscillator, possibly due to bad crystals after multiple reflow cycles. The HP 53131A should give me more data points...

author: tggzzz
author_rank: Super Contributor
timestamp: May 11, 2016, 08:57:10 am
content: Quote from: John_ITIC on May 11, 2016, 08:26:02 amYou can use a hammer to put screws in, but a screwdriver is usually preferable.In this case, use a spectrum analyser, which is the traditional tool for measuring clock quality. It will, when properly used, show both jitter (a.k.a. phase noise) for a nominally single-frequency clock, and also the deviation for a modulated (e.g. spread spectrum) clock.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 11, 2016, 09:08:37 am
content: Sorry, John, I think you're in danger of throwing additional equipment at a problem which your scope is perfectly capable of solving.Set your scope to trigger on the rising edge of the clock, and set its time base such that you get one complete period on the screen, with the trigger point near the left edge of the graticule. Set the vertical scale and position such that the mid-point of the rising edge (ie. the trigger threshold) is in the middle of the screen, and such that the signal occupies most of the height of the display.Set one X cursor precisely at t = 0, and the other cursor on the next rising edge, which should be near the right edge of the display.Now, adjust the horizontal position and time base settings so you can see that second rising edge in detail, ie. so it completely fills the display. It should look like a diagonal line from bottom left to top right. Using this zoomed-in view, position the second X cursor as precisely as you can on the point where the trace crosses the trigger threshold.This should give you a much more accurate indication of the period of one cycle.If the trace isn't steady, then this indicates either noise or the deliberate use of spread-spectrum techniques. In this case, try turning on display persistence to see the extent of the variability, and take measurements to each extremity.Another technique you can use is to measure over multiple periods. So, rather than placing the second X cursor on a point which is one cycle after the trigger, put it on, say, the 10th rising edge. As before, zoom right in to that edge, place the second cursor as precisely as you can on the point where the rising edge crosses the trigger point, then divide the measured X distance by 10.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 11, 2016, 09:15:31 am
content: Quote from: John_ITIC on May 11, 2016, 08:26:02 amMost likely it is the limit of the scope's accuracy when measuring frequency from the acquired waveform. The vertical resolution is limited (8-9 bits) and the amplitude errors will translate into timing errors when frequency is calculated. A hardware counter uses an analogue trigger so the timing resolution is much (thousands of times) better. There are ways to improve the accuracy of the scope for that kind of a measurement, however even a half-decent frequency counter will always give a more consistent and accurate result.CheersAlex

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 12, 2016, 01:21:41 am
content: I tried the cursor approach but the resolution of the scope screen does not allow me to place the cursors with more than about 0.5 ps resolution. I also played around with averaging (256 waveforms) and infinite persistence and the period of the oscillator seems to be spot on (25ns). With the built-in measurements (frequency and period jitter), the scope shows a too low frequency, although a 'mode' (statistically) that is very close to 40.000MHz. Turning off the averaging shows lots of noise, which could be related to the probe ground lead inductance picking up noise from a nearby switched mode regulator. I will study the problem in more detail, try better probe ground attachment etc.I did today bought (Ebay) a HP 53131A Frequency Counter with 3GHz (030) and High stability OCXO (010) options. I also bought (Ebay) a FE-5680A Rubidium standard so I can make sure the scope and frequency counter shows correct frequency/time. The scope was (supposedly) calibrated when I bought it (6 months ago) but I'm not sure about the frequency counter.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 14, 2016, 04:24:11 am
content: Okay, I now think I know what is going on. I have about 5% power supply noise that is affecting the oscillator in the form of phase jitter. My Agilent scope has a cool feature that allows me to display the trend of the frequency jitter. The first attached scope screen shot shows the oscillator frequency in the upper waveform and the frequency jitter trend in the lower waveform. The lower waveform shows that the total jitter frequency spikes to around 1.85 MHz p-p every 14.5us. I then measured the 1V8 power rail (2nd attached screenshot), which confirms that the 1.8V power rail has about 171mV p-p noise every 14.5us (80 kHz). This works out to 5% p-p, while the datasheet for the IC says less than 10%. The jitter trend feature of the Infiniium was very helpful in figuring this out. This is discussed in this Agilent document:http://cp.literature.agilent.com/litweb/pdf/5989-4198EN.pdfWhen designing this board, I likely incorrectly assumed that as long as the p-p noise spec is met for the analog oscillator/PLL IC pins are met, it was okay to tie the analog and digital power rails together rather than separate with ferrite bead. I now suspect that the PSRR of the oscillator is not very good and that a re-spin of the board is needed to create a separate analog power rail.This opened up a new research area so I'm now side-tracked with some in-depth study of crystal oscillators, circuit noise and the finer details of analyzing jitter with Agilent scopes!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 14, 2016, 09:22:25 am
content: How good the frequency measurement is with the scope depends on the software and the waveform. In principle the hardware could allow for really good frequency measurements, possibly even better than many counters. A DSO input stage can very well be superior to the comparator input stage of a counter - especially if the signal is a sine or similar and not a steep digital one.The 8 Bit vertical resolution could be used for extra interpolation to get timing way better than sampling rate and good / adaptive suppression of noise. So even relatively noisy signals are not such a problem.I did frequency measurements with not so perfect signals (decaying sine with sometimes quite some noise) with a digitizer card. Though not that great hardware the frequency resolution was really good often limited by the signal source and the crystal used. Even a version for a sound-card could give 8 digits in 1 second at 1 kHz (sine). Easy to see warmup drift of crystals / generators.For analyzing a PLL a spectrum analyzer would be the instrument of choice - though expensive. Some basic function could be done through the FFT function of some DSOs.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 17, 2016, 10:58:36 pm
content: Quote from: Kleinstein on May 14, 2016, 09:22:25 amThanks all for your input. I have now cleaned up the power supply and I no longer have any deterministic jitter caused by power supply switching noise. However, when measuring jitter on my oscilloscope (now halved from before so clearly improved by cleaner power) it is still higher than I "think" the datasheet for the IC specifies. I say "think" because the specs are not actually there for the internal oscillator, only for an optional external oscillator that can be used instead of the on-chip oscillator with an external XTAL. I'm trying to confirm with the chip vendor what the acceptable phase noise of the internal oscillator is but so far I have not received any final word on this (web based support is very slow). I'm therefore continuing my independent research to figure out if my phase noise seems "ball-park" acceptable.I have analyzed the oscillator output with my HP 8590B spectrum analyzer. The clock is at 40.00 MHz and there are no spurious signals. However, the phase noise seems too high. I have measured and calculated-95 dBc @ 10 MHz, which apparently is not too stellar. Other numbers for "commodity" oscillators state around -120 dBc @ 10KHz. The noise floor of the 8590B is at -80dBm and the clock signal is at -7.7dBm. Lab setup and SA results per attached picture. Note that the minimum resolution bandwidth of the 8590B is 100 KHz so I'm doing a correction per the below paper:http://www.crystek.com/documents/appnotes/impactultralow.pdfI'm not sure whether the normalization from dBc/1000 hz to dBc/hz in the article is correct (subtract 10 log (RBW)). Can anyone confirm that my phase noise measurement is correct?Thanks,/John.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 17, 2016, 11:50:36 pm
content: Also, I'm seeing multiples of my 40 MHz oscillator frequency on the SA. I assume this is some normal side-effect of how the oscillator is designed on-chip? The answer surely is in my books but perhaps someone could clue me in?In the attached image, the 40 MHz clock is to the left and then there are multiples at these frequencies"80 MHz -55 dBm120 MHz -60 dBm160 MHz -65 dBm200 MHz - 58 dBm240 MHz -65 dBm280 MHz - 58 dBm320 MHz -65 dBm360 MHz -61 dBm400 MHz -62 dBm440 MHz -60 dBm480 MHz -63 dBmAfter this the multiples seem to taper off. Is this typical of crystal oscillators?Thanks!

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2016, 12:30:32 am
content: Those are the harmonics and pretty much to be expected. What does the oscillator's datasheet say about 2nd and 3rd harmonics?

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 18, 2016, 01:26:13 am
content: Quote from: nctnico on May 18, 2016, 12:30:32 amBut a sine-wave doesn't have harmonics (unless imperfect)? Is this explained by how the oscillator is implemented? Or, is it the PLL, which generates the 250MHz clock, that feeds back these harmonics into the oscillator clock?The datasheet doesn't say anything regarding oscillator performance. It only says: "connect a crystal with 50 PPM frequency tolerance/stability". The oscillator is only used internally by this chip so they likely figure nobody needs to know the required phase noise. At this point, I don't know whether the crystal is not good enough, if the oscillator is not good enough. I will have to wait to hear back from the vendor what the phase noise requirements are.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 18, 2016, 05:49:46 am
content: "Only God can make a tree"---------the same would seem to go for perfect crystal oscillators .50 to 60 dB down on the fundamental is not marvellous,but it's not unusual,either.A LPF or a resonant circuit at 40MHz will soon attenuate those harmonics into the noise floor.

author: rf-loop
author_rank: Super Contributor
timestamp: May 18, 2016, 06:46:12 am
content: Quote from: John_ITIC on May 18, 2016, 01:26:13 amI do not know any real world sinewave what is perfect. Perfect sinewave exist only in theory books.Yes, it can be more or less pure but never perfect. There do not exist perfect square wave, not perfect produced sine. Many (even good) RF signal generators have this amount of harmonics.Also in practice there is not perfect accurate frequency or perfect accurate time.

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2016, 10:28:58 am
content: Quote from: John_ITIC on May 18, 2016, 01:26:13 amI doubt it is the PLL. The harmonics can be caused by cross-over distortion in the 40MHz clock. As vk6zgo wrote commercial RF generators usually don't produce a perfect sine wave either (some have 2nd order harmonics at -30dB referenced from the fundamental frequency). It shouldn't be a problem for the PLL because it only locks to the fundamental frequency which basically means it is -sort of- filtering the harmonics away from it's input.

author: G0HZU
author_rank: Super Contributor
timestamp: May 18, 2016, 08:06:07 pm
content: QuoteThere are various issues with the way you have tried to measure the phase noise but none of it really matters because the noise performance of the 40MHz oscillator is going to be much, much better than the converter section of the HP8950 analyser. So you are only going to 'self measure' the noise performance of the analyser and not the oscillator.Even a fairly basic LC oscillator at 40MHz can be built that would achieve a ballpark -140dBc/Hz at 10kHz offset. Your analyser is probably limited to measuring about -95dBc/Hz phase noise at a 10kHz offset.If we ignore this for a moment I'll list some of the other issues with your measurement process and I'll assume that you are (now) trying to 'self measure' the noise performance of the analyser. You have selected the peak detector and used a standard marker in an attempt to measure noise. This is going to introduce system errors because the peak detector can't represent noise accurately and also the log amp in the analyser won't respond to noise correctly. You ultimately want to measure/predict the rms voltage in a 1Hz BW and the peak/envelope detector and the log amp can't do this without some additional correction factors. These correction factors for the detector, the log amp and the chosen RBW filter are included for you in the 'noise marker' function in your analyser (assuming it can do this as I've not used the HP8590 model) and this gives you a more accurate reading of noise in a 1Hz BW. It isn't as simple as doing 10*log(BW) to get a correct reading of noise power in a 1Hz BW because of the need to (also) correct for the detector and the log amp issues and the noise marker is designed to include all these fudge factors.Also, you have tried to measure noise in the bottom box on the analyser display where the log amp will be out of spec anyway. Normally you are supposed to adjust the ref level to bring the noise at least 10dB (prefer 14dB) above the bottom of the display. Also you ideally need to select a lower RBW filter here but I'm not sure if your HP8590 has the option for a lower RBW?You can fix some of the above errors/issues by selecting the 'noise marker' function in the analyser but you also need to get the noise inside the log range of the analyser and this means getting it up maybe 14dB above the bottom of the display.But your analyser's first LO is going to be way too noisy to allow you to measure a decent 40MHz crystal oscillator at 10kHz offset. Probably by 40dB.

author: G0HZU
author_rank: Super Contributor
timestamp: May 18, 2016, 10:42:16 pm
content: FWIW your harmonic distortion measurement plot may be prone to a fair degree of measurement uncertainty because the HP8590 doesn't boast a very good 2HI spec.You appear to be putting -18dBm into the first mixer (- 8dBm via the internal 10dB attenuator) and this will self generate a 2nd harmonic inside the first mixer at a level that may be able to interact and ultimately introduce some uncertainty in the measurement.Ideally, you are supposed to make sure that any self generated harmonic distortion is 15-20dB LOWER than the distortion you are trying to measure/display. Otherwise you can get a false reading on the analyser that might be higher than reality or maybe a lot lower than reality. If you obey the 15dB rule you can manage the uncertainty down to about a dB or so.The 2HI spec for your analyser seems to be quite poor but I suspect that the typical performance will be a lot better than this published data. But even if this is the case you may be outside the recommended 15-20dB margin.It's worth getting to know the distortion limits of a spectrum analyser and these limits will be different for different parts of the frequency range. This is where there is no substitute for experience when using a particular model of spectrum analyser. I'm not familiar with the typical 2HI performance of the HP8590 at 40MHz so I can't advise if you are breaking the 15dB rule or not.To put this into some perspective you ideally need to get the internally generated harmonic distortion 20dB lower than the 'genuine' distortion level of the device under test because you can then get the uncertainty effect it introduces down to about +/-1dB.If you obey the 15dB rule I think you get within about +/- 1.8dB for the uncertainty of the displayed harmonic but that is just an approximation

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 18, 2016, 11:19:50 pm
content: Quote from: G0HZU on May 18, 2016, 08:06:07 pmThank you for the information; it is much appreciated since I'm mainly a digital engineer venturing into the RF world by necessity!I was not aware of the "noise marker" function but I tried it and now get -107 dBm @ 10KHz offset from the carrier (see attached). I played around with the reference level and attenuation but the same -107 dBm is reported. Based on various information from newer Agilent spectrum analyzers, they all seem to have a inherent phase noise level of about the same -105 dBm (the worst -89 dBm and the best around -132 dBm).In order to additionally pull the signal out of the instrument's noise level, could I connect an external pre-amplifier such as the HP 8447D? It is supposed to give 25dB gain from 100 KHz to 1.3 GHz. Or is there something else in the spectrum analyzer that contributes to phase noise except the general "noise level" of the SA? I may not be able to measure down to -140 dBc/hz but, perhaps, I can get half-way there without having to buy a new SA...I also found this detailed description from Agilent regarding the "noise marker" functionality:http://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=703435&nid=-11143.0.00&id=703435EDIT: I'm not sure if I can measure the 10 KHz phase noise directly like done above. The -107 dBm is not in relation to the carrier (40 MHz clock) but an absolute value so if I subtract the signal level of the carrier (around 7dBm) I get around -100 dBc. But I'm not sure if I can do the subtraction since the carrier power level is measured with a different RBW (100 KHz minimum) and via standard marker.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2016, 11:33:00 pm
content: Maybe you will find these helpful.

author: G0HZU
author_rank: Super Contributor
timestamp: May 18, 2016, 11:43:32 pm
content: The high phase noise of the local oscillator in the analyser will be the issue here. By the time your crystal oscillator signal has passed through the RF converter section of the analyser and it reaches the detector section in the analyser it will have adopted the same (much higher) noise characteristic as the local oscillator in the analyser.Very few spectrum analysers will be able to measure a decent 40MHz crystal oscillator at 10kHz offset because they will all suffer from this issue to some degree. You would ideally need to use something dedicated to phase noise measurement like an E5052A signal source analyser and these are very expensive!One cheaper way around this is to mix your 40MHz crystal signal with a known ultra low phase noise source such that the mixer output produces a signal that is 10kHz away from the centre of a narrow crystal filter.This way you can dodge the effects of the noisy LO in the HP8590 analyser and just measure the chunk of noise you are interested in. Obviously you also need to know the carrier power in order to calculate dBc/Hz but this can be done by shifting the mix such that the carrier passes through the crystal filter.I've not explained this very well but this is a way to measure devices with ultra low phase noise 'on the cheap' if you don't mind a fair bit of uncertainty in the measurementIn the image below I'm showing the phase noise from a (noisy) Agilent ESGD sig gen compared to the stored trace of a much cleaner signal source. I've used a narrow 10.7MHz crystal filter to attenuate the carrier part of the signal by about 60dB but NOT the wanted chunk of noise 5-10kHz away. This method dodges the limitation of the phase noise of the analyser to a significant degree.The cleaner sig gen is the lower noise trace. It's about 45dB better than the ESGD and the noise marker says -146dBm/Hz at an offset of a few kHz.I can't remember how I set this test up but I probably set the test signal at +4dBm power level so that the (4dB?) loss in the crystal filter meant that the noise marker reading is close to being correct in terms of dBc/Hz because 4 - 4 = 0..Hope this makes sense, but the HP8566B analyser used in this test probably only has about -100dBc/Hz phase noise at 5kHz offset (at 10.7MHz) yet by using a crystal filter to null the reciprocal noise from its local oscillator it can measure down another 46dB lower.The ESGD sig gen has a phase noise level of about -100dBc/Hz typical and you can see how high this noise appears with this method. It's trace is stored as the higher plot and it is over 45dB higher than the low noise LC oscillator. You can 'see' the response of the crystal filter in the noise plot of the ESGD sig gen and this helps demonstrate how this system works. The carrier on the right is about 60dB lower than it should be because it is attenuated by the crystal filter.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 19, 2016, 12:23:23 am
content: Thank you Gentlemen. I think it is fair to say that I'm going too much into detail in the oscillator quality and that the issue I'm trying to solve is somewhere else. Essentially, I have trouble with data lock on a USB 3.0 PHY and I was trying to find out whether a jittery clock was to blame. If the phase noise level is so low that a standard spectrum analyzer cannot measure it then I suspect it is good enough. However, what's good enough is not easy to know since no data from the chip vendor's datasheet. I'm also afraid that going to "method 2" and mix with known good reference clock and measure the difference is too much of an effort at this point. I will have to prove that the current clock is no good before I go down that route!One thing I'm considering trying is to swap out the current +/- 50 PPM crystal to a +/- 10 PPM one. Would this have any effect on the phase noise or is the PPM number merely related to long-term stability and frequency accuracy? Or would a +/- 10 PPM crystal generally have better phase noise characteristics?

author: G0HZU
author_rank: Super Contributor
timestamp: May 19, 2016, 01:09:26 am
content: I don't know what type of circuit your oscillator uses so I can't comment but I don't think you will see a significant difference.One other cheapo option is to mix two of these 40MHz oscillators together and look at the audio beat/difference frequency on a PC sound card.If you have a significant issue with low frequency warble in this oscillator (maybe due to power supply issues?) then you would see this easily on a PC running some spectrum analyser software. The beat frequency between the two oscillators only has to be a few kHz and presumably this oscillator can be nudged a few kHz away from 40MHz?You would get a fairly decent realtime display of any warble effects and you could store and replay the results as a wav file.The other option is to explore the jitter analysis options that are available with your scope series? I'm definitely NOT an expert in this area as I've only used an Infiniium 80404B a few times but these do offer some advanced jitter analysis options?

author: G0HZU
author_rank: Super Contributor
timestamp: May 19, 2016, 01:16:28 am
content: The other option is to look at the 250MHz oscillator when locked to the 40MHz oscillator because any close in phase noise in the 40MHz reference will get multiplied by 20*logN (=16dB?) at the 250MHz oscillator.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 19, 2016, 02:25:18 am
content: Thanks. I tried looking at the 250 MHz clock but it has spread spectrum enabled so looks terrible (picture attached)! I will have to remove the chip, rework the BGA site (add a 5 mil trace to strap differently, fix up solder mask, reball the IC, reflow it back) to try without SSC.My scope does have all the advanced jitter measurements options. I will read in on this too to see if I can use it to get finer details. One thing I'm unsure about is a spec in the PHY datasheet that says that if an external oscillator it used (optional via pin strap) it should have the below specs. It seems reasonable that, when using the internal oscillator (with external XTAL), the specs should be similar.I have attached screenshots of the frequency and period jitter of the 40 MHz oscillator. This is after the 1.8V switched power supply was replaced by a quiet external P/S. The PHY also has a 1.1V switched voltage rail but the SA only showed a 5uV spur at the switching frequency so I doubt any phase noise is due to the 1.1V switching noise. I could try to put in an additional 1.1V external P/S...Edit: One thing that confuses me is that the below specs state "Reference clock jitter 50 psec (absolute p-p)". Peak-peak jitter includes random jitter, which by nature is unbounded. Therefore, it seems impossible to meet a 50ps TJ requirement. My scope (see attached image) shows the total p-p jitter to be 272 ps. The Std Dev. value is under 50ps (33ps) so perhaps should be used? I find the terminology and various measures confusing. How to interpret the below oscillator specs and compare with what I see on my scope?5.2 Clock Source Requirements5.2.1 Clock Source Selection GuideReference clock jitter is an important parameter. Jitter on the reference clock will degrade both thetransmit eye and receiver jitter tolerance no matter how clean the rest of the PLL is, thereby impairingsystem performance. Additionally, a particularly jittery reference clock may interfere with PLL lockdetection mechanism, forcing the Lock Detector to issue an Unlock signal. A good quality, low jitterreference clock is required to achieve compliance with supported USB3.0 standards. For example,USB3.0 specification requires the random jitter (RJ) component of either RX or TX to be 2.42 ps (randomphase jitter calculated after applying jitter transfer function - JTF). As the PLL typically has a number ofadditional jitter components, the Reference Clock jitter must be considerably below the overall jitterbudget.5.2.2 OscillatorIf an external clock source is used, XI should be tied to the clock source and XO should be left floating.Table 5-1. Oscillator SpecificationPARAMETER MIN TYP MAX UNITS CONDITIONFrequency tolerance ±50 ppm Operational temperatureFrequency stability ±50 ppm 1 year agingRise/Fall time 6 nsec 20% - 80%Reference clock RJ with JTF (1 sigma)(1) (2) 0.8 psec(1) Sigma value assuming Gaussian distribution(2) After application of JTFCopyright © 2010–2012, Texas Instruments Incorporated DESIGN GUIDELINES 29Submit Documentation FeedbackProduct Folder Link(s): TUSB1310ATUSB1310ASLLSE32E –NOVEMBER 2010–REVISED JULY 2012www.ti.comTable 5-1. Oscillator Specification (continued)PARAMETER MIN TYP MAX UNITS CONDITIONReference clock TJ with JTF (total p-p)(2) (3) 25 psecReference clock jitter 50 psec (absolute p-p)(4)(3) Calculated as 14.1 x RJ + DJ(4) Absolute phase jitter (p-p)5.2.3 CrystalEither a 20-MHz, 25-MHz, 30-MHz, or 40-MHz crystal can be selected. A parallel, 20-pF load crystalshould be used if a crystal source is used.Table 5-2. Crystal SpecificationPARAMETER MIN TYP MAX UNITS CONDITIONFrequency tolerance ±50 ppm Operational temperatureFrequency stability ±50 ppm 1 year agingLoad capacitance 12 20 24 pF30 DESIGN GUIDELINES Copyright © 2010–2012, Texas Instruments IncorporatedSubmit Documentation

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2016, 10:55:49 am
content: Does USB3.0 allow spread spectrum clocking?In the past I had a problem with a USB2.0 PHY which got a clock which wasn't stable (too much frequency wander) enough so I'd look in that direction first.

author: bson
author_rank: Unknown
timestamp: May 19, 2016, 12:58:05 pm
content: The 40MHz reference clock may itself be a red herring; it could still have a significant amount of coupled RF noise riding on it that causes jitter in the first-order PLL it drives. You might not see this noise when it drives a 50ohm load like the SA since its source (coupling) is much higher impedance. What if you add something like a 330R load-side termination near the PLL input pin for the reference clock?

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 19, 2016, 08:56:52 pm
content: Yes, USB 3.0 supports SSC as do all newer serial protocols such as SATA and SAS. I suppose wander would show up on the spectrum analyzer similar to spread spectrum clocking?I have difficulties adding resistors to the oscillator but since this is per IC vendor's design spec it seems reasonable that the XTAL and load caps should be sufficient for the chip to work.I today received my HP 53131A frequency counter. Both PHY oscillators are too high by 28 vs. 34 PPM but would be within spec since the PHY datasheet states the crystal should have +/- 50 PPM frequency accuracy/stability. I'm still somewhat disappointed that the 40MHz oscillator clock is off by 1,365 Hz per the attached picture. Would a +/- 10 PPM version of the crystal tighten this up? I'll order a few to try this out.

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2016, 09:09:54 pm
content: Can you disable the spread spectrum clocking just to try? It seems unlogical to me to hunt for a small offset in a base clock (40MHz) while the resulting clock moves several MHz continuously.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 19, 2016, 09:26:35 pm
content: Quote from: nctnico on May 19, 2016, 09:09:54 pmYes, but it is not easy. I have to remove the BGA chip, rework one pad to connect somewhere else, re-ball the IC and reflow again. I may go down that route if all else fails.Note that I have SSC enabled on both PHYs and one PHY consistently can lock onto the USB 3.0 data. The other one (which has the slightly higher frequency) cannot. If SSC was to blame, I would assume non of the PHYs could data lock. I usually look for differences and the only thing I can see here is the slightly higher frequency of the oscillator of the non-working PHY.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 20, 2016, 10:10:41 pm
content: Quote from: joeqsmith on May 18, 2016, 11:33:00 pmYes, thank you. Much interesting on your YouTube channel!I noticed you were using the Standard Deviation value for measuring the jitter. My datasheet (TUSB1310A TI USB 3.0 PHY) above states that if an external oscillator is used, its "absolute p-p" clock jitter should be less than 50 psec. This seems impossible since the p-p jitter is unbounded and the actual built-in oscillator measures around 300 ps p-p. Likely they mean "std dev"? And what is this business with "reference clock TJ with JTF (Jitter Transfer Function) total p-p" of 25ps? The note says calculated as 14.1 x RJ x DJ. That would require (RJ + TJ) to be < (25ps / 14.1) or less than 1.77psAll this is like walking in a hall of mirrors...I'm still reading in on the subject and I will look into the EZ-Jitter Plus software installed on my Agilent scope. But I really would like to understand this analytically.On a brighter note: I figured out why my oscillator was some 1.5KHz too high. The external load capacitance was incorrectly calculated. When increasing to 33pF, which meets the crystal's total load capacitance of 18pF, the frequency is spot on (within 1PPM). However, it drifts some 200 Hz down when the board heats up from 25 degC to around 50 degC. This is expected since the XTAL used only has a +/- 50PPM temperature stability. I have ordered one with +/- 10 PPM on both frequency and stability over temp spec. Hopefully this will reduce the temp drift.On a negative note: My PHY still doesn't lock. Next test would be to rework the board to disable the SSC to see if it makes a difference.

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2016, 10:31:44 pm
content: Quote from: John_ITIC on May 20, 2016, 10:10:41 pmAre you using NPO caps?Just checking.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 20, 2016, 11:58:55 pm
content: Quote from: tautech on May 20, 2016, 10:31:44 pmYes, I'm currently using these capacitors, which I had available in my lab. These have +/- 5% tolerance :http://www.mouser.com/ProductDetail/Murata-Electronics/GRM1885C1H330JA01D/?qs=E%252b5dfF6xfG3shwmI3hm0tg%3D%3DFor my production boards, I'll likely use these, which are +/- 1%http://www.digikey.com/product-search/en?keywords=490-10715-1-ndRegardless, the frequency is now clearly well within the 50PPM specified so not much need to fine-tune further (although it is fun!).

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 27, 2016, 01:12:30 am
content: Time Interval Error Jitter Histogram gives vastly different result depending on ns/div setting.I'm trying to measure oscillator clock jitter with my DSO81304B. The datasheet for the IC (http://www.ti.com/lit/ds/symlink/tusb1310a.pdf) states on page 32 that a reference clock should have a phase jitter p-p of < 50ps. I have done research and I understand "phase jitter" is the same as TIE jitter? When measuring, I get very different result depending on the horizontal time scale used on the scope. It appears the best (correct) setting is whatever results in a histogram "x scale" of "5.00 ps /". Anything higher will make the reported p-p jitter many times larger. What is going on here?Thanks.