[time-nuts] Re: 20221104: Help Requested Debugging SR620 Time Interval Counter Problem

[Magnus Danielson]

Hi Andrew,

On 11/4/22 19:44, Andrew Kalman via time-nuts wrote:
> Hi All.
>
> I have an SR620 that is misbehaving. I am working my way through the manual
> and schematics, but one consistent behavior has arisen: At a period of
> around 4-6s when measuring frequency, I get a "wrong number" that is
> consistent in scale across 10MHz and 1kHz (the REF frequency).
>
> I.e. when I measure a 10MHz signal, occasionally the display shows
> 9946187.4... Hz and when measuring REF (1kHz) the display occasionally
> shows 994.618757 Hz. This is independent of termination, and the A & B
> front-ends have passed their tests.
>
> Has anyone encountered the same issue, or have a suggestion as to where to
> look for the root cause?
>
> [image: 20221104_SR620_SN356_Frequency_error-1-small.jpg]
>
> [image: 20221104_SR620_SN356_Frequency_error-2-small.jpg]
> [image: 20221104_SR620_SN356_Frequency_error-3-small.jpg]

Since f = cycles / time, these two latest is actually about the same time.

time = cycles / f and cycles is nominal frequency times nominal 
time-base, so

t1 = 1E5 / 9946187.4882 = .0100541036 s

t2 = 1E1 / 994.618757 = .01005410357 s

So, both are really taking about the same time, just above the 10 ms the 
time-base is set for, 54.1 us or so.

I would guess that there is an issue with the stop-counter side, so it 
somehow keeps running. The difference you see is probably in the 
different start-time-stamps.

The time as used in above formula is really stop-time minus start-time 
to get the elapsed time, and the time-base triggers how far after the 
start-time that an attempt at a stop-time is taken. So, it's not 
unreasonable that we are just above 10 ms, it's the intended behavior, 
but the actual values is consistent with some issue to it.

I hope this gives you a good clue of what part of the good old SR620 
troubling you.

Sometimes I find that tweaking the trigger-points avoid these issues. 
That is part of the working that turns into part of a skill. However, 
when your trigger is in auto for normal good signals, you should not 
experience this. If messing with your trigger-point help you, it could 
be that you need to work on the trigger side of things to get stable 
operation as default properly. The 1 kHz output on loopback should 
always operate safely in auto-mode, yet you illustrate it's failure. I 
seem to recall calibration details about it.

Cheers,
Magnus