[time-nuts] Fluke/Pendulum PM6680/CNT-80 measurement instability/outliers

[Attila Kinali]

On Thu, 14 Dec 2017 22:08:01 -0600
Bill Byrom <time at radio.sent.com> wrote:

> I have a few questions about your setup:
> (1) What is the waveform shape you are measuring? Are you measuring a
> square wave (or some other duty cycle waveform with fast rise and
> falling edges)? Or is it a sinewave with low harmonic content?

Sine, +7dBm

> (2) How is the counter voltage threshold set with respect to the peak
> excursions? Is it set to the 50% point or near the bottom or top of the
> waveform?

Auto-set to mid-range.

> (3) How are you connecting the source to the counter input? Is it a 50
> ohm RG-58 cable with BNC connectors? Are there any TEE connectors or
> feed-through terminations? What is the cable impedance? You can easily
> find 75 ohm cables with 75 ohm BNC connectors which will mate (with a
> bit of difficulty in some cases) with 50 ohm female connectors. If you
> have a 50 ohm counter impedance, a 75 ohm cable can cause difficulty.

Short (15cm) piece of RG-172 (IIRC) with SMA connector, all 50 Ohm

> (4) What is the output impedance of the source you are measuring?

50Ohm

> (5) What is the input impedance of the counter (as it is set up for your
> test)?

50Ohm

But you are right. I should have suspected the oscillator side instead of
the instrument or its reference. I just checked the time-stamps of the outliers.
All of them are during times someone was in the office. None during nights
or weekends. So it's likely that there is a bad contact somewhere and
I or someone else caused that to jump from time to time.

Thanks for your time and help!

				Attila Kinali

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