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Avoiding saturation (simple question) - RF Cafe Forums
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Post subject: Avoiding saturation (simple question) Posted:
Tue Nov 03, 2009 12:11 pm
Joined: Tue Nov
03, 2009 12:02 pm
I'm new to this,
so please bear with a simple question. I want to run an RF signal through
the linear range of a power amplifier, but at as high a power as possible.
It's a complicated signal (sum of FM signals), which is why I want to
keep it in the linear part of the transfer curve. Say the PAR is 9 dB.
How far away from the P1dB point do I have to back away to completely
avoid saturation? Someone was telling me that since a pure sine wave
has a 3 dB PAR, you only need to back off (in this case) 6 dB from the
P1dB point. Is that true? If so, does this hold in general: if your
signal's PAR is x dB, you will avoid saturation at P1dB - (x-3)?
Thanks for any help you can provide (and please don't hesitate to
ask for more details -- I just don't know what you need to know).
Post subject: Re: Avoiding saturation (simple
question)Posted: Wed Nov 04, 2009 4:50 pm
Joined: Mon Jun 27, 2005 2:02 pm
First, the values that were told you about sine wave
are true, because for sine wave the RMS values for voltage and current
are 0.707... (1/v2) and if you multiply current and voltage then you
will get 0.5 for average power which is then 3dB of the peak power.
The back-off which yields better linearity always contradicts
with the requirement for efficiency. If your PA does not have restirctions
on the efficiency (No problem of over heating, limited supply voltage
source etc), then a reasonable value for backing off would be 6dB away
from saturation level, that means let's say: if your P1dB=+50dBm, Psat=+52dBm,
PAR= 9dB, then your peak output power should be not more than +46dBm,
therefore you keep a margin from saturation and from 1dB compression
point. If you have non-linear large-signal models for your power devices,
then you can simulate the operation of the device under such power conditions
and see the spectral growth of the 3rd, 5th...harmonics. By this you
can consider if this margin is enough for your design or you need additional
If efficiency is a requirement for your design, then
you need to reduce the back-off to minimum. In this case the peak output
power should be +50dBm.
I hope this helps!