Fractional harmonics?

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ericksonla
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Joined: Wed Jul 08, 2020 4:47 pm

Fractional harmonics?

Post by ericksonla »

I'm driving a source at a known frequency and measuring the frequency response with the built-in Picoscope 6 Spectrum Mode with a Blackman window function. I see many integer harmonics of the driving frequency as I would expect for my system. But I'm also seeing 1/5 integer harmonics (i.e., 0.2x, 0.4x, 0.6x, 0.8x, 1.0x, 1.2x, 1.4x, etc).

Can this be an artifact of the FFT signal processing, or do I need to be looking for a physical cause?

Gerry
PICO STAFF
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Joined: Mon Aug 11, 2014 11:14 am

Re: Fractional harmonics?

Post by Gerry »

Hi Erick,

A huge amount depends upon the detail that you're not telling us.

1/ When you say that you're "driving a source and measuring the frequency response", exactly what do you mean? I'm familiar with (a) a signal source being used to drive the input of a Device Under Test, and then the output of the DUT being used to drive an input of a scope, or (b) a signal source being used to directly drive the input of a Scope, and in both cases, the scope being used to measure the frequency response. So, do you mean (a) and that the DUT is your system, or do you mean (b) and that the signal source is your system?

2/ What is your signal source? Is it a squarewave at a specific frequency, a sinewave at a specific frequency, or something else at a specific frequency?

3/ It looks like the saying "a picture is worth a thousand words" would be justified in this case, so could you also post a psdata of what you're seeing.

Regards,

Gerry
Gerry
Technical Specialist

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