Common Mode Noise Filter Question

Wireaddict · Jan 30, 2018

One of my hobbies is low frequency radio listening & one of the challenges is the high noise level from atmospheric & man-made interference. To help alleviate the problem I have installed common mode filters at the antenna/ground leads & at the AC line between an isolation transformer & the receiver. I have a question though, which included sketch shows the connect connections? (Hopefully they will show up clearly.) Thanks for the assistance!

Harald Kapp · Jan 30, 2018

A, A and only A.

The idea of a common mode choke is that the perating current, which is differential current, creates opposing magnetic fileds within the core. These fields cancel one another such that there is (alomos) no impedance to the differential current.
Common mode (noise) currents, however create magnetic fields within each coil that add up to one strong magnetic field within the core, thus creating a high impedance to block common mode noise.

In your circuit B the rolses of common mode and differential mode currents are swapped.

Wireaddict · Jan 31, 2018

Harald Kapp said:

A, A and only A.

The idea of a common mode choke is that the perating current, which is differential current, creates opposing magnetic fileds within the core. These fields cancel one another such that there is (alomos) no impedance to the differential current.
Common mode (noise) currents, however create magnetic fields within each coil that add up to one strong magnetic field within the core, thus creating a high impedance to block common mode noise.

In your circuit B the rolses of common mode and differential mode currents are swapped.
Click to expand...

Thanks Harald, good explanation; I researched this online but it was never totally clear til now. Regards!

tedstruk · Jan 31, 2018

Its my belief that the signal to noise ratio probably won't change with induced current. But it's probably be best to use individual coils rather than tapped or double coils. I think that one coil induces a frequency and the other probably induces another. wouldn't really matter how they were hooked up. The two are definitely going to feedback and that may be where the noise is coming from. In a guitar, if you tap a single coil it makes a distortion. the type of core is going to matter too, so try to use air cores whenever your in a radio set.

AnalogKid · Jan 31, 2018

tedstruk said:

Its my belief that the signal to noise ratio probably won't change with induced current. But it's probably be best to use individual coils rather than tapped or double coils. I think that one coil induces a frequency and the other probably induces another. wouldn't really matter how they were hooked up. The two are definitely going to feedback and that may be where the noise is coming from.
Click to expand...

Your beliefs do not align with fact.

It *never* is best to use individual coils. A high-performance AC powerline common mode filter *requires* that two identical inductors be wound on a single core made of something other than air. And how they are hooked up not only matters, it is critical (see post #2). The coils do not "induce" different frequencies, and the "feedback" you mention is how relatively small core sizes can handle large line currents.

To see how the pro's do it, go through any Corcom, Schurter, or Delta power line filter catalog and study the internal schematics.

ak

Common Mode Noise Filter Question

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