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Electrical Noise Generator

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kotshe

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Hi All,


I need your help , I'm using RS 485 communication . and I would like to test my circuit communication resistance against electrical & Electromagnetic noise ,

Could you please advise me how can I test that . or can I buy low cost machine as noise generator. or anyother low cost way

Thanks A lot
 

Its a long time since I used RS 485, but I seem to remember thats its balanced with separate send and receive lines (4 wires). So there are two types of interference, differential and common mode. Common mode is easy, just feed a signal generator via a resistor into each Rx line. Differential mode is difficult, you have to put anti-phase voltages into each Rx line so a selection of transformers have to be used with the signal generator so a wide frequency range can be tested.
Frank
 

A EMC standard compliant test would mainly refer to IEC 61000-4-4 burst specification. But I don't see an easy way to reproduce it with DIY methods. The test involves a burst of high voltage pulses, of e.g. 1 kV magnitude, standard 5n/50 ns shape, repetition rate up to 1 MHz. For peripheral interfaces, the standard specifies capacitive coupling to the cable screen respectively outside of unscreened cable.
 

Is there any particular spec you are testing to? Is there a particilar environment you want to test (such as near spark wires of a gas engine)? chuckey is right about testing both common mode & differential, though 422 is 4 wires, 485 is only 2 wires.

For one application I have I do a quick & dirty test by running the 485 through 4 wire 1000 foot telephone cable, and run a small 110VAC brush motor on the other two wires. I crank up the motor voltage with a variac and if I reach 150VAC with few 485 errors I pass it. Its not really quantifiable but it works for my application.
 
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though 422 is 4 wires, 485 is only 2 wires
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According to common knowlegde, but not strictly according to the EIA-485/RS-485 standard. There's also a 4-wire RS-485 configuration. See: RS-485 - Wikipedia, the free encyclopedia

I appreciate your "burst" test setup. It's the necessary supplement to my statement that standard compliant tests can't be easily reproduced. In fact, you can perform similar tests. The problem is of course to scale the noise level.
 

I have re-read your question and find that I still do not know exactly what your problem is. You could be developing and testing a new 485 detector chip, or developing a new type of cable. Or more realistically building some kit to be used somewhere. So the best place to test it is where its going to be used. As the output of the 485 chip will be digital if you do loose or gain a bit now and then, how crucial this is to you, depends on your error detection and correction.
Frank
 

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