2205 spectrum analy harmonic view incorrect

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richy

2205 spectrum analy harmonic view incorrect

Post by richy » Wed Apr 14, 2010 5:20 pm

My 2205 has this peculiar behaviour of not showing the correct harmonics. One varies the input level and the f2,3,5 (ref 1Khz) all shoot up/down as if the input to the A/D converter is showing harmonics related to amplitude. No it's anti-aliasing- The input is well below warning exclamation. Whats the reason: poor decoupling on 2205 ?

Other instruments don't have this varying harmonic representation with input level. So in the field it's impossible to get the true picture.
Help !

PeterF
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Re: 2205 spectrum analy harmonic view incorrect

Post by PeterF » Thu Apr 15, 2010 11:58 am

Hi,
I can't duplicate your problem here. Varying the input amplitude up/down causes the whole display to go up/down but all harmonics stay in the same proportion as they should. What settings are you using? What do you expect to see?
Regards,
Peterf.

richy

Re: 2205 spectrum analy harmonic view incorrect

Post by richy » Tue Apr 20, 2010 6:03 am

Peter, I've taken a couple of FFT's using the inbuilt sine generator with direct connection. I cannot post pics on this site so I will send them to the company main.

PeterF
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Location: Cambridgeshire

Re: 2205 spectrum analy harmonic view incorrect

Post by PeterF » Tue Apr 20, 2010 11:51 am

Hi Richy,
I can see what the problem is now. You are looking for harmonics which are at a level 50dB lower than the fundamental signal. Since the 2205 input, and the AWG are both 8 bit, the maximum resolution possible is about 48dB.
Please read the following explanation:-
http://www.picotech.com/applications/resolution.html
To do what you want, I feel you will need a 12 bit scope like the 4224.
Regards,
PeterF.

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