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Picolog maximum frequency measurement

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Picolog maximum frequency measurement

Postby Tim Stinchcombe » Mon Apr 04, 2005 4:58 pm

I've just about given up trying to find the answer for myself, so hopefully someone else can put me right. What is the maximum possible frequency that Picolog can measure when using 'real time continuous' mode, 1 second sample intervals, and an ADC200/50? What ever I try I cannot get it to sensibly measure more than about 7kHz: I'd like just a bit more, and in any case I'm only interested in audio frequencies, which surely must be within the device's 25MHz capability?

I have tried:
- looking in both the help files and the pdf user guide,
- searching this forum
- and much experimentation, by altering the 'scan time' on the 'edit measurement' pop-up, which suggests that indeed considerably higher frequencies should be possible: all it does is go beserk (apparently due to aliasing effects) at frequencies greater than 7k.

Am I missing a trick somewhere?

I downloaded the latest Picolog version 5.12.0, but still no joy, and the user guide I've got is plw044.pdf, dated 21/6/04 (and this doesn't seem any different from what's in the help file anyway).

Thanks,
Tim
Tim Stinchcombe
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Postby Sarah » Tue Apr 05, 2005 9:49 am

Hi

Thank you for your post.

I would suggest that this is probably the highest you will achieve in Picolog. With the number of samples you are taking, it will start to aliase above this point.

Instead, if you wish to measure frequency there are two ways you could do it - either to use Picoscope (although you cannot log frequency with this) or use a frequency to voltage converter as this makes it much easier to measure.

Hope this helps

Best Regards
Sarah
 

Postby Guest » Thu Apr 07, 2005 9:03 pm

Thanks for the response, but unfortunately it raises more questions than it answers.

Previously I had been assuming that the ADC200's entire buffer was being filled during the 'scan time', and that this happened at every 'sampling interval' (once a second in my case). But if the max frequency is about 7kHz, then this suggests that the buffer is being filled over the entire second of the sampling interval. Since to be able to estimate AC and frequency measurements you need multiple data points, if this is the case then over the second I'm getting very 'averaged' readings, and since my signal does a step change once a second, the readings are probably unreliable (and hence Picolog is unusable). In any case the appearance of the 'scan time' on the 'edit xxx measurement' pop-up is completely misleading and should be greyed out/removed, and it would also help if both the user guide and the help files actually explained just how the device works.

If I increase the sample interval, sure enough the max frequency that seems to get logged correctly does decrease, supporting what you say, but when I slowed it to once every 5 seconds, it was quite clear that what I see on the main recorder window was getting refreshed much quicker than once every 5 seconds, which is contradictory since from what you say it shouldn't be calculating the frequency until the whole buffer is full, i.e. once every 5 seconds? Still very confused as to what Picolog is actually doing, not helped by the less-than-adequate (and sometimes rather woolly detail in the) documentation.

I'm not going to resort to using Picoscope because I don't want to spend the time recording large amounts of data by hand.

Thanks,
Tim
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