[time-nuts] Re: time interval measurement on SR620

[Bob kb8tq]

Hi

There are indeed “issues” with converting the reference to 1 pps. 
If you use a modern D flip flop at the output of the divider these
issues are quite small. There are similar issues with coax cable, 
buffer amplifiers, and even the input on the counter. Eliminating / 
characterizing them is “fun”. Even the counter’s conversion of the 
reference to 1 KHz is also likely to have issues ….

Bob



> On Mar 3, 2022, at 9:16 AM, Mete Balci <metebalci at gmail.com> wrote:
> 
> Thanks Bob. I was also thinking something similar but on the SRS pdf I
> mentioned (high res. freq. measurements), it also mentions Allan variance
> and as an example the noise floor data is collected also with a sample size
> of 1000. The setup is not like I mentioned but normal ChA/start ChB/stop
> driven from the same source but with REF OUT as EXT trigger and I guess
> because REF OUT is 1kHz, sample size is set to 1000 to get 1 sample per
> second.
> 
> I also wonder if there is no effect of converting 10MHz to 1 PPS in order
> to measure Allan variance (when the reference is GPSDO PPS). Isnt it at
> least conceptually the same thing if not exactly the same (that it
> eliminates the variations in a 10MHz signal) ?
> 
> Mete
> 
> From the pdf mentioned: "(This data was collected by connecting one 10MHz
> out of an FS725 to the SR620 reference
> input. A second 10MHz output from the FS725 went to the input of an RF
> splitter (Minicircuits
> 15542 Splitter ZSCJ-2-1). The two splitter outputs were connected with
> identical 3 inch BNC
> cables to channel A and B of the SR620. The 1kHz REF out of the SR620 was
> connected to the
> EXT input of the SR620. The SR620 was set up for time interval
> measurements, starting on
> channel A using EXT +/-time gate/arm, with a sample size of 1000. 57600
> 1-second
> measurements were collected, starting on 3/2/04 4:15pm The modified root
> Allan variance was
> computed to give the graph shown above.)"
> 
> On Thu, Mar 3, 2022 at 2:17 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> The main disadvantage (if you are after ADEV) is the pre-processing
>> that takes out the noise you are trying to measure. Getting the repeat
>> rate so that you actually got samples once a second (after all the overhead
>> and processing) would be a challenge.
>> 
>> Bob
>> 
>>> On Mar 3, 2022, at 2:28 AM, Mete Balci <metebalci at gmail.com> wrote:
>>> 
>>> Hello,
>>> 
>>> Is there any disadvantage of using SR620 with TimeLab as described in
>> high
>>> res freq. measurement document of SRS ?
>>> 
>>> 1) DUT (10MHz) is connected to External Clock In
>>> 2) REF OUT (1kHz) is connected to ChA
>>> 3) GPS receiver PPS output configured as 1kHz and is connected to ChB.
>>> 4) Sample size 1000
>>> 
>>> Can this value be used in the same sense as in a (normal) single
>>> measurement (Ch A: GPS PPS vs. Ch B: DUT output converted to PPS) ?
>>> 
>>> If not, is there still a benefit of maybe converting REF OUT (not DUT) to
>>> PPS and using it still like the above configuration but with sample size
>> 1 ?
>>> 
>>> Mete
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