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I would like to evaluate an alternate TCXO used in a GPS receiver circuit. Our current TCXO works good but I'm looking to test a lower cost alternate.

The alternate part is Epson TG - 5035CJ / CG / CE (datasheet).

Import specifications like voltage, frequency tolerance, aging..., current consumption, and package size are acceptable according to our GNSS chipset manufacturer's recommendation.

I've done some basic GPS testing including Time To First Fix (TTFF) and Top 4 satellite CNo number comparison, as well as overall GPS performance. All of the tests I've done so far have been at room temperature.

I have access to a temp chamber, GNSS simulator, Vector network signal generator, and a decent Tektronix Oscilloscope. Our network analyzer is currently out of shop.

I do not have a test jig but I'm hoping to somehow objectively measure performance differences between our existing and new part.

EDIT

What specific tests can be done on both TCXOs in-circuit or out-of-circuit with the equipment I have available? Is it possible to measure jitter or are there any other performance characteristics that would be useful to measure?

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  • \$\begingroup\$ I am not so sure what you are after exactly. If you compare both tcxos directly to each other in isolation, all you can say is that they differ and by what amount. What is it that counts for your application? If you need GPS overall performance, it seems you have tested it. If you need to test frequency stability, you need to have a frequency that is guaranteed to be stable to do some comparisons. So for testing "A ist better than B in performance of X" what exactly should X be here? \$\endgroup\$ – PlasmaHH May 18 '15 at 20:56
  • \$\begingroup\$ @PlasmaHH Thanks, I updated question. I can provide additional updates this evening. \$\endgroup\$ – Samee87 May 18 '15 at 22:06
  • \$\begingroup\$ If you are able to read german, this article from the german AMSAT Journal has hand-on advice on how to evaluate oscillator stability. I looked for translations, but unfortunately none seem available. \$\endgroup\$ – Andreas Sep 23 '16 at 9:12
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I am going to make some assumptions here about the clocks in your measurement equipment being much better than both tcxos, otherwise it will be hard to tell which of those is causing deviations. Just for a simple illustration, say you have two unknown oscillators that should be 10MHz, but one is really off my much. You use one for your frequency counters reference, and measure the other. In one case the counter says 9.9MHz, in the other 10.1MHz. Not much gained here. Always keep this and similar issues in mind.

For a more in depth look, google up some guidelines about oscillator measurements, applications notes AN10007 and AN10033 from sitime seem to contain some useful tips for measurement setups. Also informing yourself about allan variance might be useful.

You always need to keep in mind what the requirements for your product are. Measuring one to deviate by 2.1ppm and the other by 1.8ppm doesn't mean much if your application is fine with anything under 10ppm.

For long term stability tests, you need something to compare to. Either you have a high end ocxo in one of your frequency counters, or you pick your best tcxo and discipline it by gps. Both should have very good mid-term stabilities, so you can then run your frequency counter with those as references, pick some high enough gate time, and collect the data of your frequency counter for long enough so you are confident that it will be meaningful for your application, then compare those for the new and old part.

For short term stability (which blurs over to jitter) you need to decide which kind of instability you are interested in, and there are tons of ways to measure jitter and similar, that all depend on your equipment abilities.

You could configure a spectrum analyzer for a rather long sweep time around the frequency you are interested in and compare the results of the tcxos. Phase noise will be visible as side bands here (if your analyzers LO is good enough). If your scope is good enough you can use its abilities to measure a certain amount of clock cycles and build a histogram out of that and compare both (some scopes have even a feature for histograms). Sometimes just using the infinite persistance of a scope can tell you something about the jitter.

Additionally it might make sense to compare the output waveforms and if they are usable or at the edge case of what your circuit can handle. A lot of cheapos on the market have rather weird clipped sine waves.

So since the sole responsibility of an oscillator is to generate a certain waveform (usually sine) with a precise frequency, there is not much more to measure here: Deviation from that intended waveform, as well as deviation from the frequency.

Of course, since you mentioned it, if your product requirements are such that it has to work within certain temperatures, make use of the temperature chamber, but only after you are confident in your measurement setup on the bench, and your abilities to read the data acquired.

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The ultimate metric for inherent oscillator quality (putting aside things like immunity to environmental influences and the like) is the sigma-tau-diagram. It shows Allan-Deviation (or other deviation metrics, see NIST Ch.5) against observation period. If your Tektronix is decent enough, you may able to measure such a diagram directly. Cheap Oscilloscopes will tend to measure their own crystal instead of the device under test.

An in-circuit test in a receiver can be used to gather some of this information, if the receiver gives you access to certain raw observables (which consumer grade receivers typically do not give). You are effectively comparing to a very high quality oscillator in the spacecraft.

Multiple error sources affect the channel between the spacecraft and your receiver, like scintillation, multipath, ionospheric delay, so not all of the observed deviations can be attributed to your oscillators imperfection. Yet, it is possible to gain some information, since f.e. the ionosphere does not change so quickly and you can mitigate multipath by measuring in a good location and using only high elevation SVs (rooftop or window is not a good idea).

The primary observables to go for are carrier phase and frequency. If you are able to get these readings at a high rate, say 1000/s, you will see the short term deviations in the phase and the longer term in the frequency (since the receiver will correct phase by altering the frequency).

This is a screenshot of a readout of these observables from my DIY-receiver, look for the blue carr_phase and carr_inc lines.GNSS3-Dash(this receiver uses a cheapo Rakon TCXO, the screenshot was taken while developing the tracking filter and, yes, I know the filter is not the best one)

If you have the opportunity to repeat this measurement with different oscillators, I can assure you, they will look different. The one that gives less noise energy in the carrier phase is the better one.

Environmental influences like shake or temperature variations can be seen in such a diagram (I do not have an example screenshot ready), but manufacturers usually do a pretty good job to compensate for them.

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