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Picoscope low frequency accuracy

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Picoscope low frequency accuracy

Postby swetterlin » Wed Jan 21, 2004 2:29 am

What is the low frequency limit of picoscope? I have analyzed a circuit that I know drops off below 50 Hz, but no matter what I do it shows up with a flat response on picoscope. I have used long sweep times (5 sec) and short sweep times--same result.
swetterlin
 

Postby markspencer » Wed Jan 21, 2004 8:22 am

Hi,

I have used Picoscope to measure frequencies as low as 3 Hz, it will definetly measure 50Hz. What happens when you place a frequency above 50 Hz, do you see anything. If a 1.5 battery is placed across the input what do you see.

What unit are you using?
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Mark Spencer
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low frequency

Postby swetterlin » Wed Jan 21, 2004 3:40 pm

I am measuring a noise generator circuit using the ADC212. I have made all sorts of measurements, and it seems to operate fine. But the noise circuits output drops off at the high end and the low end. The high end (1MHz) dropoff is shown perfectly, but the low end (100Hz and below) shows a flat output (that is, it shows output at the same level as for the midrange frequencies), even though it has to drop off below 50Hz or so because of coupling capacitors. I used the 190Hz setting to get good detail, and used various numbers of FFT samples, various FFT windows, and various sweep times (up to several seconds). I can't say it is impossible that something is awry in the circuit, but I wanted to see if there is something special I need to do to get accurate low frequency readings.
swetterlin
 

Postby markspencer » Thu Jan 22, 2004 8:25 am

Hi,

I have used the ADC-212 to measure 50 Hz with a signal generator with no problems. Try to do this is possible, if you are still having problems when you try this send me the result to:

tech@picotech.com

The .psd files so that I can view the traces you are getting.

Best regards,
Regards,

Mark Spencer
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low-frequency--the problem is ALIASING

Postby swetterlin » Sun Jan 25, 2004 9:20 pm

After much experimentation, I have discovered why the spectrum is alway flat at low frequencies. I thought I could narrow the bandwidth setting in order to narrow in on the low end; but apparently this limits the number of samples and allows frequencies above my selected bandwidth to be aliased into the narrow range, causing a display of, say, 390 Hz to be a summation of the ranges 0-390, 391-780, 781-1170, etc. Big problem. If I put in a filter to eliminate everything above 10kHz, then I can select a bandwidth above 10kHz and get a good display, although I can still not narrow in on 100Hz as well as I would like.

The solution would be to select oversampling to raise the frequency threshold at which aliasing will occur. According to the manual, there is a "max oversampling" option in the same dialog as the ETS oversampling rate. I can find the ETS oversampling box, but there is no "max oversampling". Where did that go?
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Postby markspencer » Mon Jan 26, 2004 9:03 am

Hi,

The box that you are looking for is labelled as ETS oversample and can be found by the following:

Settings | Options | Advanced

Best regards,
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Mark Spencer
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