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how to improve the IM3 - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: how to improve the IM3 Posted: Wed Jun 10,
2009 2:57 am
Joined: Sun Jun 07, 2009 9:14
I am designing an amplifier
The circuit and picture as following:
the Mosfets are two MRF9045.
the amplifier's IM3=-26dBc(at)270Mhz(at)25W(CW)
but the amplifier's IM3=-18dBc(at)400Mhz(at)25W(CW)
I improve the IM3?
Post subject: Re: how to improve the IM3Posted: Sun Jun 14, 2009
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!
Re: how to improve the IM3Posted: Mon Jun 15, 2009 9:40 am
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
Output match is important, so try varying that.
2nd, and even 3rd harmonic match canbe important also, although your
broadbandwidth will make that interesting.
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).
Maguffin Microwave Consulting
Re: how to improve the IM3Posted: Wed Jul 29, 2009 11:22 am
Joined: Wed Jul 29, 2009 11:11 am
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
Wed Apr 08, 2009 7:53 am
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.
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?