BPSK strange peaks with spectrum analyzer - RF Cafe Forums

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lurker
 Post subject: BPSK strange peaks with spectrum analyzer
Posted: Wed Mar 25, 2009 3:40 pm 
 
Lieutenant

Joined: Wed Mar 25, 2009 3:33 pm
Posts: 1
Maybe someone can help me:

I used spectrum analyzer with very big rbw to view bpsk signal in peak mode(not RMS).
the data was random

I saw the familiar sinc envalope with nulls at N*Rb (N-integer) .
what's strange is that also I saw strange peaks at LO and multiples of Rb/2.
If I lower the RBW or put detector in RMS the peaks disappear.

I verified it with 2 different signal generators.

It's wither a charectaristic of the signal or spectrum analyzer related phoneomena.

PLEASE HELP ME

confused from Israel....
 
   
 
Dexter
 Post subject: Re: BPSK strange peaks with spectrum analyzer
Posted: Sat Apr 11, 2009 11:42 am 
 
Captain
User avatar

Joined: Thu May 04, 2006 7:37 am
Posts: 8
Lurker:

Sounds like LO leakage from improper DC cancellation. You might want to check your DC levels on I/Q data input to the modulator.
 
   
 
RDO-RF
 Post subject: Re: BPSK strange peaks with spectrum analyzer
Posted: Sat Apr 11, 2009 8:14 pm 
 
Captain
User avatar

Joined: Wed Apr 08, 2009 7:53 am
Posts: 17
Hello Lurker,

It is normal to have different measurement results if you read power to a digital modulated signal with different RBW.
Perhaps the best way to analyze peaks on a signal is to trace CCDF curves.
Check if you still have those peaks near LO when filtering a CW or without any signal.. If the tones disappear, then perhaps you have some DC filtering problem.. if they still there, then is something on the synthesizer design..

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Posted  11/12/2012