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Better driver, worse system performance??? - RF Cafe Forums

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lna
 Post subject: Better driver, worse system performance???
Posted: Thu Jun 03, 2010 6:49 pm 
 
Captain

Joined: Mon May 04, 2009 6:47 pm

Posts: 22

--------------------------------------------------------------------------------

I'm looking for help for a PA design.

My project is a balanced 1W(average) OFDM PA at 600MHz, droven by a half watt class A driver. The original design is very close to system requirements, except marginal fail EVM, which I think caused by the driver. So I found a drop in replacement driver and evaluated it. With new match circuits, the new driver alone performancs very good--2dB higher P1dB and 1~2dB better EVM.

What confused me is that system EVM drop by 2~6dB!!

I don't think the driver change would impact the final stage because there is a 50ohm splitter between them.

Any idea???? Please.


 
   
 
IR
 Post subject: Re: Better driver, worse system performance???
Posted: Tue Jun 08, 2010 3:06 pm 
 
Site Admin
 

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

I think that the problem is not within the driver.

The new driver which you tested proves that point.

If the problem is linearity - which seems to be based on the description you showed, then the problem lies within the balanced amplifier.

The significant stage for cascaded IP3 calculation is the last stage of the Tx chain.

(Same as the first stage for cascaded NF calculation).

Try to focus on the balanced amplifier and optimize it.

In many data sheets of power transistors, there are measured results for different signals.

--> Have you looked on what are the expected performance of the transistor (within the balanced amplifier) for the type of signal you are using (i.e. modulation type, filter applied, clipping etc).

It might be the case that you need to consider a final stage with more output power.

_________________

Best regards,

- IR


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