[time-nuts] Newbie with questions

lymex at vip.sina.com lymex at vip.sina.com
Thu Jul 28 11:41:11 EDT 2005

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 


An example is at: http://www.timing.com/products/testsets.php

it says ADEV <5x10-14 at 1 second for TSC 5110A

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. 

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?  

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)

It is easy to say that:

1) the lower the reading, the better the frequency source for

tau = 1sec

2) compact Rubidium is worse than OCXO in short time stability.

3) the ADEV of the SR620 (if it applies) is better

than any of the above readings

4) the ADEV of the SR620 is perhaps not too much

better than the lowest reading of above, 1.0E-11 is my estimate.

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.

Q1D: Is the ADEV and the SDEV always the same for frequency 

counter noise?

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.

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)

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.

correct me if I am wrong.

Thank you for your attention.

Lymex, BG2VO


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