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Looking for help on design a OFDM wideband PA

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LNA

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Greeting!

I did search the forum there is tons of threads on PA topic but no answer for me. I hope someone could help me out.

I'm designing a OFDM 1W average PA in 500MHz~700MHz, 12.5dB backoff. The device I chose is Freescale MW6S010N, which is a 10W part. It is a hard time for me to match it to a good p1dB-- larger than 40dBm over band. However it doesn't seem the good P1dB bring me a good EVM-- the best EVM is only 32dB and system is asking 36dB. The only measure to improve it is to increase Idq to 400mA, which cause a very bad efficiency.

I don't have ADS to simulate the performance I have to spend lot of time working on the eval board. It's hard to balance many parameters (P1dB, Gain, IRL, efficiency, EVM, etc.) by experimental data. Can someone tell me how to improve P1dB/EVM without trading off efficiency too much?
 

If you back-off the power of a 10W (40 dBm) transistor with 12.5dB, means the output power will be 0.56W (27.5 dBm).

To get 1W (30dBm) linear output power for OFDM communication you need a transistor with greater linear output power than 10W.

A normal efficiency of linear PA’s for OFDM is somewhere between 20% and 25%, if there are no efficiency improvements techniques.

The PA bias filtering is very important getting good EVM.
 

Thanks for pointed the Bias decoupling. I added extra Tan caps at both side of bias and the EVM improved a lot-- close to -35dB now. The interesting thing is that although the P1dB droped to 37dBm, with 64QAM(the test PSA is not able to demod 256QAM so I have to use 64QAM) at 30dBm output the EVM is still good, -36dB.
It seems 12.5dB back off is not necessary for OFDM? or maybe something is not right with the OFDM file used to generate waveform?

The Freescale device is specified as 40dBm part but I can get slightly higher than 41dBm(before adding these decoupling caps). This is another question of mine-- Good P1dB is not necessarily translated to good EVM?
 

12.5dB back-off from P1dB should be fine for OFDM. In real practice using different OFDM modulations (e.g. 64QAM, QPSK, etc) the back-off could be slightly different, using adaptive input power, in the way to meet performances for all modulations type.

P1dB will affect the EVM. Higher the P1dB, better the EVM.

EVM is affected by both, AM/AM (amplitude compression, or P1dB) and AM/PM (phase change)
 

Maybe I didn't ask the question clearly.

I think I want to know-- why the EVM is still good when the PA is backoff for 7dB only?
 

If your EVM is fine (-36dB or 1.58%) when the back off power is 7dB from P1dB, means the peak-to-average of your signal is less than 7dB, otherwise will be compressed.

The EVM is degrading dramatically if the power or the peak modulation exceed P1dB.

For accuracy try to measure P1dB using just an unmodulated CW signal.
 

I did measure P1dB using CW. Also I measured PAR using agilent PSA, which reports 12.5dB PAR.

I double checked the measurements again--it's still -36dB EVM when PA backedoff 7dB only. Is there something I overlooked?
 

I think maybe you'd better to try this design using BLF571 which can work up to 900mhz. BLF571 is a 20W device.
 

I don't see the datasheet mentioned BLF571 700MHz performance. Can you give detailed reasons why you suggest this part? The reason I did choose it are,

it's pricy;
lower effeciency because it's a 20W part,
 

kspalla said:
Are you using Doherty configuration for final amplifier?

Added after 13 seconds:

If not use it

It's balanced design.
 

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