[time-nuts] Stanford Research SR620 Measurement Bias

Ulrich Bangert df6jb at ulrich-bangert.de
Mon Sep 28 07:08:21 UTC 2009


I own a SR620 as well and mine behaves the same (while with a different
number of "counts") so your's is surely not defective! My understanding a
few years ago when I acquired the counter was that this bias is not due to a
"adjustment" in the normal sense but to set a parameter in the counter's
eeprom to a correct value. Since this parameter is limited to a certain
precisison you can make some effort in minimizing this bias but you should
not expect to make it exactly zero. The manual explains how to do but you do
not find the procedure where you would have expected it.

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Stan
> Gesendet: Sonntag, 27. September 2009 18:15
> An: time-nuts at febo.com
> Betreff: [time-nuts] Stanford Research SR620 Measurement Bias
> I just acquired an SR620 that looks to be in excellent 
> condition. I've noticed something in its operation and I'm 
> not sure if it represents a problem, a calibration issue, or 
> if it's just a "feature."
> Yesterday, after letting the counter warm up for an hour, I 
> calibrated the 10 MHz timebase and did a couple of Auto-Cals 
> (just to be sure), but even after all of that, there's still 
> a bias of somewhere between 40 and 70 counts in the frequency 
> readout, independent of the gate time. For example, if I 
> measure the 5 MHz output of my 5065A, and using a sample size 
> of 1, here's what I see (these are representative 
> readings-the values are bouncing up and down by 10 or 20 
> counts and I'm "visually averaging" them). Increasing the 
> sample size has no effect other than to reduce the 
> variability of the measurements, but the bias is still the 
> same magnitude:
> 1 sec Gate:     5,000,000.00054 Hz
> 0.1 sec Gate:   5,000,000.0060 Hz
> 0.01 sec Gate:  5,000,000.055 Hz
> 1 Period Gate:  5,004,7__ Hz
> I've had several HP 5370A/B counters, but never one of these. 
> As I recall-and it's been a while-there's an adjustment in 
> the 5370A/B for the interpolators that can remove this sort 
> of bias. Is there a similar adjustment in the SR620? I've 
> been through the manual and don't see anything obvious.
> Thanks for any advice!
> Stan
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