4224 as Spectrum is not scaling properly with RANGE

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xdsl
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Joined: Wed Mar 21, 2012 8:19 pm

4224 as Spectrum is not scaling properly with RANGE

Post by xdsl » Wed Mar 21, 2012 8:55 pm

When using the PicoScope 4224 in Spectrum Mode I am measuring the downstream power of an ADSL line at the DSLAM. I am bridge on to the cable pair and so far everything looks normal when the Spectrum Range is set to 2 Mhz

See file
ADSL_2Mhz.gif

Even when I change the Spectrum Range to 1Mhz, it still looks quite normal
See file
ADSL_1Mhz.gif

The problem is when I reduce the range lower, I see an error in the presentation.
See file
ADSL_500Khz.gif

The pictures will explain it all.

I grabbed another Spectrum Analyzer to compare, and regardless of the ragne I use on it, I always see what I would expect as in the ADSL_2Mhz.gif. Even when I reduce the range to 100Khz.


This is not good, any help will be greatly appreciated.

thanks in advance
Attachments
ADSL_2Mhz.gif
Looks Good - see the downstream power on ADSL
ADSL_1Mhz.gif
Reduced Range - still looks good
ADSL_500Khz.gif
Reduced Range - BAD

Martyn
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Re: 4224 as Spectrum is not scaling properly with RANGE

Post by Martyn » Fri Mar 23, 2012 2:05 pm

Assuming the 2MHz spectrum shows a true representation of the input signal, it contains frequency components over 500kHz and so in a 500kHz spectrum (sample rate 1MHz) these will be aliased (Nyquist sampling theorem) back into the 0-500kHz band.
A PicoScope does not contain any anti-alias filtering (it would need a separate filter for every sample rate - not practical) so in order to avoid aliasing you must ensure the frequency components of the input signal fall within the displayed frequency range.
A benchtop spectrum analyser is not normally FFT-based (they use a swept receiver) so does not exhibit aliasing.
Martyn
Technical Support Manager

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