Issues with Band-pass Filter function amplitudes

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andyb1997
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Joined: Tue Jan 21, 2020 8:24 am

Issues with Band-pass Filter function amplitudes

Post by andyb1997 » Tue Jan 21, 2020 8:39 am

Hi
I'm relatively new to Picoscopes and am using a 5444D MSO to measure the noise at different points around a PCB. I've been using Spectrum Mode without issue to look at the frequency spectrum and have identified a region of 230-270kHz where there is an issue. As part of following up on this and to aid judging its impact on the device performance I returned to Scope Mode and used AC coupling to look at the peak to peak noise on the output signal. To try and focus in on the 230-270kHz region I used the Maths Channels to create a band-pass filter from 230-270kHz and observed the signal.This seemed fine but I then changed the time-base on the scope and noticed the amplitude of my filtered waveform changed with it. Further investigation into this has shown that increasing the time-base appears to increase the amplitude of the noise I'm measuring.
Am I making an obvious mistake somewhere? I apologise if this issue has been answered before but I've spent quite some time googling the issue and searching this forum and haven't found anything similar.
Thanks in advance.

Gerry
PICO STAFF
PICO STAFF
Posts: 594
Joined: Mon Aug 11, 2014 11:14 am

Re: Issues with Band-pass Filter function amplitudes

Post by Gerry » Thu Jan 23, 2020 11:20 am

Hi andyb1997,

How are you measuring the amplitude of the noise (manually using rulers, or automatically using measurement functions), and by 'amplitude' do you mean peak-to-peak or rms?

Regards,

Gerry
Gerry
Technical Specialist

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