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 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.
hope this helps!