> Dear Tom Van Baak, > > Sorry to bother you in this way but I cannot get registered on time-nuts mailing list.
Thanks for the mail. Very pleased to meet you. I'll cc time-nuts and John can add you to our list manually. > My name is Lymex Zhang (or Zhang Limin, if direct translate from Chinese), I am one of a few time-nuts in China. > > I have read you home page many times with great respect - many of your equipment and the way you are dealing with them are simply my dreams. > > I own a SRS SR620 frequency counter plus SR625 Rb time base, an Agilent 53131A, a HP58540A GPS, a Trimble Thunderbolt GPS, and many OCXOs. Recently, I acquired two HP Z3801As and became confused in measuring the stability of their OCXOs. That's a very nice set of equipment. Yes, you are a time nut; acquiring and confusion are two key elements ;-) > My primary goal is to find a purist, most stable frequency source as I can afford. One good criterion seems to be the Allan deviation (of many tau intervals). This is a great goal, one that most of us are trying to achieve one way or another. > Z3801A seems to use HP 10811-60158 OCXO (http://www.realhamradio.com/GPS-oven-journey.htm). From the spec of 10811-60158(http://www.febo.com/time-freq/hardware/HP10811-Specs.pdf, page 21), it says <5E-12 for 0.1s and 10s, <9.8E-13 for 1s. This is the best time domain stability I have ever seen for frequency source. However, by your article (http://www.leapsecond.com/pages/z3801a-osc/), many of the Z3801A are not satisfying this. Is it might be that not all Z3801A use the same crystal? Or because the crystal degraded as many years have passed by? All Z3801A use the same model crystal. The 1 s noise should not change much over time. That's an interesting question. It seems all 12 of those Z3801A that I tested meet spec for every tau except for the 9.8e-13 spec at 1 second. It seems most of them are slightly above this at 1 second. One theory is that the OCXO itself meets the spec but we are not really measuring the bare OCXO - we are measuring the 10 MHz BNC output of the Z3801A and there could be added noise in the output buffering circuitry or something. This is something I could check. I'll let you know the result. Also the "9.8e-13" is one of the stranger specs that I have seen. Sometimes you see 5e-13, or 1e-12, or 2e-12, but "9.8"? It seems a very odd number and there must be a story behind it. > The other concern for me is to how actually measure the Allan deviation. I use some of my standards(mentioned above) in turn, and make calculation afterwards to cancel the instability caused by the frequency source of its own( reading^2 = f1^2 + f2^2, where f1 is the Allan dev of the measured freq. and f2 is the reference freq.). Even though, the noise of the frequency counter is a big problem. I use the SR620 to measure SR625 (also Tee the output of SR625 as a reference), here are the results for Jitter: > Interval, Allan dev., Std. dev., N > 0.1sec, 3.9E-10, 3.9E-10, 200 > 1.0sec, 3.7E-11, 3.8E-11, 100 > 10.0sec, 3.9E-12, 4.0E-12, 100 > > It seems that: > 1, the floor noise is very high, higher than many OCXOs. > 2, the noise is inverse proportional with interval > 3, Allan deviation and standard deviation are almost the same in this case. 1. Good observations. You should not use the SR625. It is sold as a low drift reference, and yes, rubidium has very low drift. But it has poor short-term noise. Almost any OCXO will beat a compact rubidium, short-term. If you want to calculate ADEV for tau 1000 to 1 day then use Rb. If you want to calculate tau much less than that, then use a good quartz standard as a reference. The SR625 spec is 2e-11 at 1 s. This is 10x to 50x worse than a really good OCXO. http://www.thinksrs.com/products/SR625.htm 2. When you see noise inversely proportional to tau it's the -1 slope on the log-log ADEV chart. It usually means you are seeing measurement noise. This makes sense. A SR620 is sensitive to about 20 ps at one second, or 20e-12, 2e-11 so it cannot measure stabilities much less than 2e-11 at one second. 3. Yes, ADEV and stddev will be pretty close to each other for many measurements, especially those containing noise. Where ADEV begins to be less than stddev is when there are slow trends in the data, trends (like frequency offset or drift) that rise above the noise. > I also did the similar measurement with my 53131A. It shows that Std. dev. is about the half of the SR620. However, no Allan dev. function is available for 53131A. For the purpose of comparing all your standards against each other using the HP53131A stddev is perfectly fine. > I understand that TSC-5110A is the best for this purpose (the floor noise is 2.5E-14 for 10MHz). But, is there another way to measure the low Allan deviation? Yes, the typical way is to beat the two sources against each other and measure the stability of the beat note. This technique gives you orders of magnitude improvement in short-term resolution. If you can manually tune any of your sources 0.1 Hz or 1.0 Hz off frequency then mix that and your UUT in a DBM (double balanced mixer) and low-pass filter the output (a simple RC filter will work). You will see a beautiful 10 Hz or 1 Hz sine wave. If you then measure and record each period of the sinewave to, say 1 us, then you will have a measure of the relative stability of your two sources down to parts in 10^13th. I can go into this in more detail if you wish. > Thank you for your patient in advance. > > Best regards, > > Lymex Zhang > > p.s. I'm also a HAM(call sign is BG2VO) /tvb http://www.LeapSecond.com _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
