How to Improve the IM3 - RF Cafe Forums
Post subject: how to improve the IM3 Posted: Wed Jun
10, 2009 2:57 am
Joined: Sun Jun 07, 2009
I am designing an amplifier
The circuit and picture
the Mosfets are two MRF9045.
but the amplifier's IM3=-18dBc(at)400Mhz(at)25W(CW)
How can I improve the IM3?
Post subject: Re: how to improve the IM3Posted:
Sun Jun 14, 2009 11:40 pm
Joined: Sun Jan
11, 2004 4:44 pm
That's a nice
project you have going there! I'm not the world's foremost expert
on amplifier design, so I'm just shooting from the hip here. Of
course one way to increase IP3 performance is to increase the current,
but I assume you've already considered that. The next thing is to
look for lossy components that are wasting the current you have
available. I cringe just a bit at those coax transmission lines
having such small radius bends when passing through the ferrites.
Have you confirmed that they are not causing reflection problems?
That could be a huge source of performance loss if they have really
bad S11 or S21 because of that radius. Just a though - you've probably
already thought of that too, but just figured I'd throw it out there.
Like I said, nice job!
subject: Re: how to improve the IM3Posted: Mon Jun 15, 2009 9:40
Joined: Wed Apr 08, 2009 11:07 am
That is the subject of very many recent research papers!
Obviously, the bias point of the devices is critical. You can
try varying the bias to get some improvement.
is important, so try varying that. 2nd, and even 3rd harmonic match
canbe important also, although your broadbandwidth will make that
Your choice of devices matters, try a different
device. Some are more linear than others.
Then there are
a raft of more complex methods, such as Doherty amplifiers, feedforward
amplifiers, or predistorters. I would be tempted to try a predistorer
(you can make one up using some diodes and a transformer).
Post subject: Re: how to improve the IM3Posted: Wed Jul 29,
2009 11:22 am
Joined: Wed Jul 29, 2009
i am susan here, i am
interested in this post, but i too have the problem to improve the
im3 so please any let me know about my problem.
Post subject: Re: how to improve the IM3Posted: Thu Aug 06,
2009 2:09 pm
Joined: Wed Apr 08, 2009
I think the problem is hat
you are having a small P1dB.. In the P1dB, the IM3 is -30dBc.. and
you are having this just at 30W, where ideally you should have it
at 90W. This cold be for many reasons, but perhaps the most important
ones are: bias pioint and output matching.
First, i´ll try to
find the best bias point for the implemented matching, if this doesn´t
solve the problem, then i think you should review the output matching.
Are you sure you are matching the output for the highest P1db?