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IMD Measurements Using Picoscope 3206

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IMD Measurements Using Picoscope 3206

Postby DL8OV » Sun May 10, 2015 10:52 am

I want to setup my 3206 to do some IMD measurements on a transmitter. All of the FFT screens start at 0 Hz so I can, for example, scan from 0-50 MHz or 0-100 MHz. How do I set the instrument to scan from a non-zero frequency, say 44,9 MHz to 45,1 MHz?

I know I can do this using the zoom function but this wastes bandwidth.

Software in use is Picoscope 6, latest version.

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Re: IMD Measurements Using Picoscope 3206

Postby AndrewA » Thu May 14, 2015 12:14 pm

The FFT calculations work by using the full bandwidth from 0 to the displayed frequency which is based on the sample rate.
Even if picoscope software could be set to display say from 40-60MHz it would still use the 0-60MHz bandwidth of the scope. So you need to use the zoom function. In your case you could type in a zoom of x100 into the zoom box, and scroll to 44-45MHz, with the frequency on 200MHz.
You can display sampling and FFT information by opening the properties window ("Views" menu, "View Properties")
Here is some more information about the spectrum mode -
At low frequencies, the update rate is slower than at high frequencies as it takes longer to capture the required number of data points for a particular frequency resolution. There is a trade-off between update time and frequency resolution on screen. A high number of "Spectrum Bins" selected (16384?) which will give you, for instance, a frequency resolution (Bin Width) of 305.2mHz on the 5kHz range and a refresh rate (Time Gate) of 3.277 seconds. To get a faster refresh rate, please got to the spectrum options pull-down menu (to the right of the spectrum button) and change the number of bins to, say, 1024. This will give you a refresh rate of 204.8ms, and a frequency resolution of 4.883Hz You may find it useful to keep the "Properties" window open where all these parameters will be permanently displayed.
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Re: IMD Measurements Using Picoscope 3206

Postby DL8OV » Thu May 14, 2015 6:05 pm

OK, thank you for the detailed reply Andrew. My knowledge of FFT mathematics is very basic so I am wondering if it would be possible to include spectrum analysis using either a variable center point or start point/end point in a future update?

Last week we had a demonstration of spectrum analyzers at our local ham radio club. The surplus HP unit we used could go to 2 GHz but it was a two-man lift, then I brought my 3206 out of my laptop bag. There were some impressed people but the ability to limit scan width was top of the wish list.

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Re: IMD Measurements Using Picoscope 3206

Postby Martyn » Wed May 27, 2015 8:17 am

I am not sure how feasible this would be but will pass the information on to the development team. Zooming, as suggested, would be the obvious option at present.
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Re: IMD Measurements Using Picoscope 3206

Postby Jeff » Thu Jun 04, 2015 10:18 am

It may be that you need a technique called "zoom FFT". It's possible using National Instrument LabVIEW (see http://zone.ni.com/reference/en-XX/help ... _analysis/) but not in PicoScope 6. The NI article says that you need to acquire multiple blocks of data to achieve the desired frequency resolution, so it might not be any faster than computing a full-band FFT using PicoScope and zooming in, and it wouldn't work on single-shot signals.

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