> Hi everyone, > > I'm very glad to be here for the first time. > Thank you John for manually registered me in. > Thank you Tom Van Baak for putting my e-mails here and also > for your great answer which clear up most of my confusions. > Sorry for my poor English because its not my mother tongue. > > I come from a place where there are few people who are > interested in time and frequency so I learn most of the > subject by myself mainly. Until one day I came across an > article by Tom and then his leapsecond.com which I read > with great interest, then I found this mailing list. After > read most of the archives I understand I belongs to here. > > I have a lot of questions now (and probably more in the future). > Here is my first set of questions. > > We all know that Allan Variance/Deviation (ADEV) is one of the > common ways to measure time domain noise of frequency sources. > It seems to me that the noise of many time-interval equipment > can also be described in this way. > > Q1A: Is ADEV could be used for all time-interval measuring > equipments? > An example is at: http://www.timing.com/products/testsets.php > it says ADEV <5x10-14 at 1 second for TSC 5110A
Yes, that is the noise floor specification for the TSC 5110A - the minimum resolution that it can measure. Every oscillator has a certain specified, or measured, ADEV plot; the same is true for any test equipment that does timing measurements. The resolution specification is not always listed as ADEV. Some counters advertise the "number of digits per second", or the single-shot resolution, or the LSD (least significant digit). But it's all essentially the same thing. > If so, or partly so, > > Q1B: Could it be easily measured/confirmed in general? > I even have difficulties measuring the ADEV of SR620 which has > the ADEV function build in. I made wrong assumption in my last > mail to Tom, so I re-measure it below. You can indirectly measure the ADEV of the test equipment if you know the ADEV of each of the two oscillators you are comparing with that piece of test equipment. If, for example, you have two nice OCXO that you know have an ADEV of, say, 1e-12 at 10 seconds and your counter tells you their relative ADEV is 1e-11, then you can be sure it's the counter that is the limiting factor, not either of the sources. In general, the ADEV you measure is the RMS of the stability of each source and the TI counter itself. And so in the special case that the OCXO perform identically and the TI counter is much better than the OCXO the true ADEV of each of the OCXO is sqrt(2) of the measured ADEV. Does this make sense? > Q1C: I actually measured the ADEV of my frequency counter > SRS SR620 below (for tau = 1 sec), anything wrong? > > SR620 is able to take many sample frequencies and calculate the > ADEV and standard deviation (SDEV). What happened if it measure > the reference frequency? This is a sort of self-test and it gives you a rough measure of the jitter in the system. > Using a good 10MHz as a external reference for SR620, also > Tee this frequency and let the counter measure it at the same > time. Ideally it will read 10MHz exact. However, because of > the jitter (and some other reason) of the counter, the reading > is not perfect. Set the gate time to 1 second, number of > samples to 100, the reading for Jitter:Allan is as follows: > the counter at tau=1 second. > 1.5E-11 for HP Z3801A (no GPS) > 3.7E-11 for SR625 (Rubidium time base) > 1.6E-11 for HP 58540A (GPS locked) > 1.3E-11 for HP 58540A (Holdover) > 1.1E-11 for Trimble Thunderbolt (GPS locked, time const=400s) > 1.1E-11 for Trimble Thunderbolt (Holdover) > 1.4E-11 for SR620 its own internal reference (TCXO) > (during above measurement, any reading are taken for at least > three times, I keep steady, and no cables are touched) This is a nice set of data. > It is easy to say that: > 1) the lower the reading, the better the frequency source for > tau = 1sec Correct. > 2) compact Rubidium is worse than OCXO in short time stability. Yes, this is often true. Many OCXO have much better short-term stability than compact Rb. > 3) the ADEV of the SR620 (if it applies) is better > than any of the above readings I'm not sure you can say this. It might be equal to some of the readings. I would guess that one or more of your sources is much better than 1e-11. The fact that all the readings are above 1e-11 suggests that the SR620 itself is the limiting factor here. And, based on my earlier email, this is to be expected -- 1e-11 is 10 ps per second. I can't see the SR620 getting much better than this > 4) the ADEV of the SR620 is perhaps not too much > better than the lowest reading of above, 1.0E-11 is my estimate. Correct. > 5) this 1E-11 is equivalent to 10ps jitter which is good > because SR620 has 100ps uncertainty for frequency measurement > as specified. > 6) SR620 cannot measure ADEV of much lower than 1E-11 for > tau = 1 sec. because of its own noise floor. This makes it > useless to measure ADEV of good OCXOs. Yes and no. It is useless to measure ADEV of OCXO for short tau. It should still be very useful for longer tau. The resolution is approx 1e-11 / tau so at 100 seconds the SR620 can produce data that gives you ADEV down to 1e-13. It is not uncommon to use multiple methods to achieve a full ADEV plot, from sub-second to multiple days or weeks. A single reference or a single counter is just not good enough to cover the entire spectrum. Some people, for example, use their best OCXO for tau 0.1 s to tau 500 s, and use GPS for tau greater than about 1000 s. The SR620 is one of the best, but it does have its limits, as does every instrument. Measuring ADEV of good OCXO (1e-12 to 1e-13) is not that hard; but you have to use methods other than simple frequency or TI counters. That's why I mentioned the heterodyne (DBM) method. > Q1D: Is the ADEV and the SDEV always the same for frequency > counter noise? Mostly this is true. When dealing with white noise the two will be similar. Where ADEV and stddev diverge is when there are drift effects; this is usually something that happens in the mid- to long-term, not short term. If you ran a TI counter for a day you would definitely see periodic, drift, or thermal effects in the data. stddev sweeps all this data into one calculation while ADEV(tau) can resolve different parts of the time spectrum. > I ask this because on my SR620, it almost the same reading > for Jitter:Allan and Jitter:STD DEV regardless of the gate > time and the frequency source. Tom has answered this question > by saying that it will be the same in his previous mail. For white noise this will be the case. With the SR620, you will start to see Jitter:Allan and Jitter:stddev differ for longer runs. > Q1E: which is better as for noise, Agilent 53131A or SR620? > For Agilent 53131A, only the SDEV function is available. > I made the similar measurement as above (gate time=1s, N=100 > samples). The SDEV is worse than those obtained by SR620. > For instance: > > 2.3E-11 for HP Z3801A (no GPS) > 2.1E-11 for HP 58540A (GPS locked) > 2.3E-11 for Trimble Thunderbolt (GPS locked, time const=400s) > 2.4E-11 for SR625 (Rubidium time base) > 3.1E-11 for 53131A its own internal reference (TCXO) The SR620 should be much better than a 53131A, or even a 53132A. It will be about the same as a 5370. > Yes, these readings are quite the same for external references. > It seem to me that the 53131A uses external reference directly > while SR620 uses the external frequency indirectly by phase > locking an internal VCXO to it. This 2.3E-11 deviation is > equivalent to 23ps of jitter which is very good comparing to > the 500ps error spec. for frequency measurement. Not sure on this one. It could be that the time constant of each PLL is different, or the TCXO in each counter is of different quality. > correct me if I am wrong. > > Thank you for your attention. > > Lymex, BG2VO /tvb _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts