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PAR vs. EVM - RF Cafe Forums

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 Post subject: PAR vs. EVM
Posted: Fri Feb 19, 2010 4:35 pm 
 
Captain

Joined: Mon May 04, 2009 6:47 pm
Posts: 22
Confused!!!

I'm designing a 1W OFDM power amplifier for 500~700MHz. The signal PAR is about 12.5dB and ESG reading is about the same, which means the PA P1dB should be better than (30+12.5) = 42.5dBm. In my design P1dB rolls off to 38.5dBmat each end, which is 4dB compressed, however EVM measurement is better than -39dB.

Can someone help me understand this? Could a compressed PA provide doog EVM?


 
   
 
 Post subject: Re: PAR vs. EVM
Posted: Wed Feb 24, 2010 4:03 pm 
 
Site Admin
User avatar

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Usually the EVM can not be good if the P1dB is lower than the signal's PEP.

Maybe it has to do with the filter the signal has (Raised-Cosine etc), and other signal parameters like clipping which affect the CCDF. This can affect (improve or degrade) the EVM.

But this is just my guess...

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 Post subject: Re: PAR vs. EVM
Posted: Mon Mar 01, 2010 2:43 pm 
 
Captain
User avatar

Joined: Wed Apr 08, 2009 7:53 am
Posts: 12
The signal has 12.5dB of PAR, but that doesn't exactly means that your power amplifier should have the same back-off. Note that in the CCDF curve the 12.5dB peaks have very low probability. Remember that EVM is an average value and perhaps your can effort clipping the signal once a while having no grater effect on EVM.
For some signals, clipping affects first the ACPR and then the EVM. You can check this on your VSA, by looking if this good EVM output signal that you have has degradation on its ACPR.

This was my guess.

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