Test and Measurement Forum

4224 as Spectrum is not scaling properly with RANGE

Post any questions you may have about our current range of oscilloscopes

4224 as Spectrum is not scaling properly with RANGE

Postby 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
xdsl
Newbie
 
Posts: 0
Joined: Wed Mar 21, 2012 8:19 pm

Re: 4224 as Spectrum is not scaling properly with RANGE

Postby 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 Specialist
Martyn
Site Admin
Site Admin
 
Posts: 2347
Joined: Fri Jun 10, 2011 8:15 am
Location: St. Neots


Return to USB PC Oscilloscopes

Who is online

Users browsing this forum: No registered users and 1 guest