I think that the induced magnetic field will not cause an EMI problem, because the coaxial cable will not couple the EM perturbations. Nonetheless, you must be careful regards the data acquisition board, because it might be susceptible to the CW magnetic field in the proximity, and some currents might be induced in the PCB traces. I suggest you to orientate the Data acquisition system to minimize the possible inductive couplings. Be alert to any abnormal behaviors of your system and if you note any EMI problem, consider other alternatives to mitigate it, such as shielding and filtering techniques.

Dear all,

Thank you very much for your answers. For the time being, , we wish to mount a set of microphones in an anechoic chamber which hasn't been built yet. It is said that the RG-174/U cable is very robust against external EMI. But how effective is the shielding? For example, is the isolation at least 50 dB/m for the range 200Hz-100kHz?

What test do you suggest to perform for the measurement of external EMI over the coaxial cables? Qualitative results are needed, just to know the order of magnitude of noise in the coaxial cable produced by external EMI.

Thanks!

Hi, to answer to you question, you must know that coaxial cable is not able to protect against low frequency EM disturbance, whatever its kind (magnetic/electric). From your description of experimental setup, the main disturbance sources are magnetics. A better way to limit the coupling with your microphone cable is to use a twisted cable pair. The usage of electric shielding has not any effect on magnetic coupling. If this is not enough to cancel disturbance on microphone signal, try to setup a differentiel configuration for your microphone cable.

I agree with A. Coustou: Magnetic influences cannot be shielded by wires or nonmagnetic metal like copper shielding.

The problem arises from the Faraday law saying that you may induce a voltage which is in a simple model proportional to the cross section area between he inner wire of your cable and the shielding with respect to the magnetic field. So in this case you have two of that areas - viewing from the inner wire and imaging that the magnetic field comes from top the surface left of the wire works against the one right hand. The difference of induced voltage is your EMI voltage. So in a homogeneous field there is no EMI. Unfortunately the field is in regular cases not homogeneous which results into EMI.

Now coming to twisted pair: Here my arguments apply as well but due to twisting the EMI cancels out between the twists and the resulting EMI is much more reduced. Therefore a normal twisted pair cable is favorable with respect to magnetic field interference. Using shielded twisted pair you can reduce the electric interference at the same time.

This results into a recommendation for shielded twisted pair cables as used for Ethernet connections (CAT 5 will do a good job). Moreover one cable can connect 4 microphones to your acquisition board.

Be careful with your connections:

You must match the impedance of your microphone to that of the cable and the one of the cable to the input of your amplifier, otherwise you loose a lot of signal being worser than having some EMI. The impedance matching can be done by simple transformers. I have no clue about the impedance of a twisted pair.

Can anybody comment on that?

Hello. Thank you very much for these new comments, which are very helpful for me. I have seen a coaxial cable which has 2 twisted pairs. Its name is Belden 1192A (the datasheet is here: http://www.belden.com/techdatas/english/1192A.pdf), but its nominal impedance is 40 ohm and uses XLR connector. We need a 50-ohm cable that uses BNC connector. Is there any 50 ohm cable with 2 twisted pairs and BNC connector?

Thanks!

PS: I 've recently found this guide in IEEE Xplore, which seems to be useful:

IEEE Guide on Shielding Practice for Low Voltage Cables

Just curious: what kind of microphones are you using? Microphones in professional sound systems are almost without exception low-impedance differential (balanced), using shielded twisted-pair cables and three-pin XLR connectors. Note that the standard impedance in low-level audio is around 600 ohms (if I remember correctly). The characteristic impedance of the transmission line (in the RF sense) does not matter that much (as long as the capacitive loading due to the line on the input is not excessive as to cause excessive attenuation of the high-end of the audio signal).

If you are using an unbalanced input on your acquisition system, use a differential line with an audio balun at the input. This reduces the possibility of interference, hum and ground loops.

Cheers!

It was really nice reading the above all. We have also experienced EMI effects at low frequencies, which we have been able to reduce by filtering, impedance matching and providing suitable shielding up to 20 dB.

Dear all,

Thank you very much for your help. I finally found a conditioning amplifier for improving the signal-to-noise ratio. But an alternative is to use a balanced line as George Slade suggested.

@George Slade: I am planning to use 12 1/4" microphones for the measurement of acoustic signals in the range 200 Hz - 100 kHz, for a test jet rig facility. Each microphone has microdot connector, and a microdot to BNC 3-meter length 50 ohm cable.

To conclude, the system you suggested would consist of:

- Microphones,

- 50 ohm BNC cables,

- Baluns with impedances Zun and Zbal (for unbalanced and balanced connections respectively),

- Low impedance differential connection with shielded twisted-pair cables and three-pin XLR connectors (40 ohm according to Belden 1192A datasheet),

- Transmision line,

- Balun (at the receiver),

In case the balun doesn't have Zun=50 ohm and Zbal=40 ohm, I will include transformers between the baluns and the corresponding cables for impedance matching.

Thank you very much, your comments were very helpful!

If you don't want to use baluns, you can use wideband differential op amps (e.g. the Analog Devices AD8138 or similar) to do single-ended to differential conversion and back again. The advantage of this is that you avoid the use of magnetic components (transformers), can add gain right at the microphone (helps S/N ratio) and can make the source and load impedances anything you want by designing the appropriate resistive network.

It is very late comment for you. We're in gas-discharge plasma lab and there should be huge intense EMI in form of radiation from discharge. When I use typical cheap BNC cable huge signal fluctuation is observed. But with using expensive double-shielded BNC cable (shielding layers are two)  Signal become clean. Usually RF cable shield is considered to be very stable in potential and really 'shield' for noise. But if noise is intense enough so shield has potential fluctuation, huge noise can be added to the signal. In this cage, shielding is not 'shield' anymore.

Thank you very much for your comment, Prof. Donggyu Jang. We've seen this pair if double-shield cables called starquad, which minimizes the EMI noise. The residual noise passing through the shield is compensated since it is detected equally by each cable of each pair:

http://www.belden.com/blog/broadcastav/How-Starquad-Works.cfm

It may be more useful for your needs.

Regards

http://www.belden.com/blog/broadcastav/How-Starquad-Works.cfm