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3rd IMD to 5th IMD conversion - how?

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Full Member level 3
Feb 19, 2003
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3rd IMD 5th IMD?

Hi,my friends.
Our custom need 5th IMD spec,but we only have instrument for 3rd IMD test.Do you know how to convert 3rd IMD to 5th IMD?for passive IMD only.

it all depends

The relationship is different for different circuits. One thing that almost always is true is that the 5th spurious signals are lower than the 3rd spruious signals. This means that if your measured 3rd terms are lower in value than the 5th terms are required to be, the 5th terms will most likely be even lower.

Someone else talk to me that there is a formula for these two terms relationship.
Is this true?

it all depends

It depends upon the type of nonlinearity of your system. Different transistors (bipolar, jfets, or mos) and technologies (silicon, GaAs, hetrojunction, GeSi) have different relationships.

An ordinary spectrum analyzer and two signal generators should be able to show you the IMD products.

When a nonlinear circuit is being driven to strongly nonlinear region, there is the possibility that imd3 is lower than imd5. This phenomenon can be explained by the contribution from fifth-order imd to third-order imd.
Also, when carrying out measurement, you may check and avoid imd asymmetry.

Thanks for all!
The IMD principle of active circuits I also have known.Now my question is focusing on passive IMD,such as cable assembly's IMD.Does such IMD have an general relationship between 3rd and 5th order?

IMD 3 instrument can not measure IMD 5, you must buy for IMD 5 testing.

To my surprise, even cable have IMD?? Can anyone detail the phenomenon?? Even if it is really the case, can the IMD be neglected when comparing to that generated by active circuits.

Also, just a spectrum analyzer is enough for measuring IMDX. Why "IMD 3 instrument can not measure IMD 5"???

Why I mean IMD=intermodulation distortion, is this your definition?


One possible cause of IMD in passive component like connectors and cables is the bimetallic junction when two different metals connect adn form a low grade diode. This even happens when two pieces of metal are the same basic type (copper for instance) but have different manufacturing imperfections (chemical contaminating elements).

I remember one Prof. who described the DC offsets of relay contacts and how they had to make them from adjacent peices of one wire to reduce (but not eliminate) the DC offsets.

These IMD problems are only detected when high power RF goes through the circuit.

To make things worse, bimetalic or rusty joints in metal objects in the transmitted RF field will generate harmonics and IMD.

Our custom need 5th IMD spec,but we only have instrument for 3rd IMD test
He is said that have IMD3 instrument, not analyzer.
Instrument like that are only for measure IMD3, this is something like network analyzer with FFT processor and 3 separatly excites and receivers.
Maybe he have cable with several cabels(5 and more) in one and because he want to measure IMD5.

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