Avoiding Saturation (Simple Question) - RF Cafe Forums
Post subject: Avoiding saturation (simple question)
Posted: Tue Nov 03, 2009 12:11 pm
Tue Nov 03, 2009 12:02 pm
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)?
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,
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 back-off.
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.