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James Bond Community Member Offline Posts: 82	IM3 problem in the active low pass filter Sep 02^nd, 2008, 12:24pm Hi, guys Does anybody know how to reduce IM3 in a nth order active low pass filter? I guess one has to start from the designing a low IM3 opamp, right ? The only way I am realizing is to increase the signal swing, can you suggest any tips ? BR James
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buddypoor

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Re: IM3 problem in the active low pass filter
Reply #1 - Sep 3^rd, 2008, 1:24am

James Bond wrote on Sep 2^nd, 2008, 12:24pm:

The only way I am realizing is to increase the signal swing

Increase ???

For my opinion, the linearity of a filter circuit depends strongly on the chosen topology.
Therefore the question: What is the structure of your filter - cascade or compact/direct realization ? Which topology ? What kind of amplifier - opamp or OTA ?

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LvW (buddypoor: In memory of the great late Buddy Rich)

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James Bond Community Member Offline Posts: 82	Re: IM3 problem in the active low pass filter Reply #2 - Sep 3^rd, 2008, 7:55am Hi, Forgive me I didn't explain my situation clearly. The situation is that the topology(leapfrog) is fixed , filter order is fixed, schematic is fixed. The only thing I can do is to optimize opamp I think . As I mentioned above, try to give more headroom so that it can handel larger input signal. BR James
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buddypoor

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Re: IM3 problem in the active low pass filter
Reply #3 - Sep 3^rd, 2008, 8:22am

Yes, if everything is fixed - except the opamp type - then, indeed, the opamp seems to be the only part to be "optimized" - whatever this means. May I remember you, that - in addition to more or less classical sources of non-linearity - sometimes slew induced distortions exist.
That means, you could try to use opamps with a higher slew rate.
I suppose, you have already checked the different signal levels the various opamps have to accept (saturation effects).
That�s all I can say at the moment.
Good luck.

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LvW (buddypoor: In memory of the great late Buddy Rich)

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cgt

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Re: IM3 problem in the active low pass filter
Reply #4 - Apr 3^rd, 2009, 6:15am

Hi,
One common problem is that the OP amps output stages are running out of current. This will set the compression point and hence the linearity to some degree depending on loop gain.
In RFIC RX designs this normally happens for large interferers in the first stages of the filter. For the wanted channel this happens for the last stages. I assume that there is (distributed) gain in the filter.
Please remember that the out of channel interferers do "see" a quite low impedance wich requires higher current than the wanted channel (in the first stages).
/CG

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aaron_do Senior Fellow Offline Posts: 1398	Re: IM3 problem in the active low pass filter Reply #5 - Apr 3^rd, 2009, 6:31pm Hi, I guess i must be missing something...can't you just increase the loop gain of the op-amps? cheers, Aaron EDIT...just noticed how old the original post is...
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loose-electron

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Re: IM3 problem in the active low pass filter
Reply #6 - Apr 5^th, 2009, 4:56pm

Two things needed:

1. Examination of the worst case (largest) signal amplitudes at the outputs of all active amplifiers. Need to stay well within the limitations of the amplifier going into compression. FWIW an OTA will give wier amplitude swings (AB output type OTA)

2. Get the open loop gain of the amplifiers as high as possible and maintain the gain out to a BW that is a decade (or more) beyond you passband.

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Jerry Twomey
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