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Measurement problems on 3406B

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Measurement problems on 3406B

Postby CoyoteDavid » Fri Jan 18, 2013 9:27 am

I seem to have problems with the measurement on the 3406B. I am using PicoScope 6.6.50.3. The setup is watching a single channel A only, 500 us/div, 5 volt range, channel A on the compensation signal from the scope, so it is about 1 kHz. Trigger at 1 V in middle of display. I then enabled two measurements, A High Pulse Width and A Cycle Time. The measurements vary significantly based on how many samples I take. For example, some measurements I get by varying the number of samples:

Request 50 kS ==> 508.1 us and 1000 us (ave High Pulse Width and Cycle Time respectively)
Request 100 kS ==> 470 us and 900 us
Request 500 kS ==> 240 us and 500 us
Request 1 MS ==> 139 us and 270 us
Request 2 MS ==> 61 us and 133 us
Request 3 MS ==> --- and --- (measurement reading seem to stop)
Any request for more samples that this does not get a reading. Also, as the number of requested samples goes up, the readings start to bounce around more and more. For example, when requesting 1 MS, the cycle time "Value" can be seen anywhere in the range of 190 us to 375 us when you do single traces.

In all cases, the 2 volt square wave from the compensation output is displayed OK, with 5 cycles on the display.
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Re: Measurement problems on 3406B

Postby Martyn » Mon Jan 21, 2013 8:52 am

Can you repeat the test with AC coupling selected and the trigger level at 0V
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Re: Measurement problems on 3406B

Postby CoyoteDavid » Mon Jan 21, 2013 9:23 pm

I repeated the test as you indicated. Also, in order to be more reproducible I reset the default settings. The setup is a single probe in channel A connected to the probe compensation signal. No external electronics are used. (This was not how I discovered the problem, I was just trying to show an easily reproducible scenario). To setup the picoscope, I did the following:

1) Setup hardware with 1 probe in channel A connected to compensation signal, ground clip connected to trigger ground. Probe in X10 mode.
2) Load PicoScope 6.
3) File-->Startup Settings-->Reset Startup Settings
4) Shutdown PicoScope
5) Load PicoScope again (to get startup settings)
6) File-->Startup Settings-->Load Startup Settings
7) Select 500 us/div, x1, 10 kS, Channel A at Auto, AC. Channel B,C,D are off.
8) Trigger on "Auto", A, Rising, 0V, 50%
9) Single measurement on channel A of "Cycle Time", "Whole Trace". All other parameters for measurement default.
10) Take readings while varying the number of requested samples.

My results at different number of samples while measuring the compensation signal on AC mode:
REQ ACTUAL RESULTS(Value, Min, Max, Average)
10k 9926 1 ms, 999.9 us, 1 ms, 1000 us
1M 625,005 1 ms, 750 us, 1 ms, 1 ms (Min varies between 750 us, 873 us and 1 ms)
2M 1,250,005 1 ms, 1000 us, 1 ms, 1 ms
3M 2,500,005 ((blank)), ---, ---, ---, --- (Readings go blank, or indicate "---")
5M 5,000,005 ((blank)), ---, ---, ---, --- (Readings go blank, or indicate "---")

If you put the trigger in single mode, so you see each trace, and request 1 MS, then the cycle time indicates 750 us some of the time. The waveform looks perfect on the display, the measurement calculation is sometimes wrong. The value is not wrong all the time. It gets it wrong about 1 of 5 tries. The value when incorrect is either 750 us or 873 us, so it is not a small error, it is a gross error. Also, when the cycle time reads 873 us, if you add a "High Pulse Width" measurement, it reads 406.5 us (normal value is 508.1 us).

So, in conclusion, it appears that it works differently in AC mode, but still gives bad readings. It still fails completely at requested samples of 3 Meg and higher, and only occasionally at 1 MS request. Regardless, it means that if I use the measurement function, I have to verify any values with a manual measurement before trusting Picoscope's value.
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Re: Measurement problems on 3406B

Postby Martyn » Tue Jan 22, 2013 7:37 am

I originally tried with the signal generator using a BNC to BNC cable and the values were rock solid across all sample settings, I found odd values when using the compensation signal possibly due to the DC component moving the zero crossing point used by the software measurements, but when I changed to AC and set the range appropriately it was once again solid. I will carry out some more tests and report back.

Please try using x1 on the probe.
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