Electronics > RF, Microwave, Ham Radio
Conducted immunity test, coupling path?
KlausKragelund:
Hi
I have a product that has failed conducted immunity tests at 30+ MHz
The product tested it in metal enclosure, and using M12 shield cables (so 360 degrees shield towards the product).
The cable that has failed has differential output, and in the test setup, a unit (AE) is measuring this signal, also with differential input.
The test is conducted by using a 50ohm generator, a 6dB attenuator and a 100 ohms series resistor that feeds current into the shield of the cable (with a clip on the cable, insulation removed). See setup figure attached.
I have reproduced the setup in my own lab, and measured that I can indeed measure a differential voltage on the AE unit.
The signal pair in the cable would ideally be 100% symmetrical, so no voltage should occur, but in practice it seems a signal pair would normally have about 5% mismatch.
I have tried with a cable with double shield, but it does not seem to make a big change.
I have also tried to measure the shield resistance over frequency, but are not seeing big changes in between the cables. Seems that most of the current injected into the cable is due to CM capacitive coupling from the cable to the surroundings.
I have also tried to measure just the cable coupling, from current into the shield to a voltage on the cable pair with a differential probe. Sort of like a poor mans transfer impedance test.
Anyone here have experience with this test and how the current is coupled into the signal pairs?
selcuk:
The definition in chapter 12.3.4 (Common-to-Differential-Mode Conversion) of the below book is similar to your description:
https://link.springer.com/chapter/10.1007/978-3-031-14186-7_12
I didn't have a similar issue before, but what are the protection components against common mode and differential mode noises at the termination of the cable on your board?
mtwieg:
--- Quote from: KlausKragelund on November 03, 2024, 01:52:34 am ---Hi
I have a product that has failed conducted immunity tests at 30+ MHz
The product tested it in metal enclosure, and using M12 shield cables (so 360 degrees shield towards the product).
The cable that has failed has differential output, and in the test setup, a unit (AE) is measuring this signal, also with differential input.
The test is conducted by using a 50ohm generator, a 6dB attenuator and a 100 ohms series resistor that feeds current into the shield of the cable (with a clip on the cable, insulation removed). See setup figure attached.
I have reproduced the setup in my own lab, and measured that I can indeed measure a differential voltage on the AE unit.
--- End quote ---
If so then that means the "Decoupling device" fitted between the direct injection point and the AE apparently isn't doing a good job.
Is that the only issue, or is there some other specific reason you "failed" the test. Keep in mind the pass/fail criteria for these tests are usually tailored for the specific EUT, so we have no way of knowing what "fail" means. But it's usually based on what happens to the EUT, not the AE.
KlausKragelund:
--- Quote from: selcuk on November 03, 2024, 04:09:15 pm ---The definition in chapter 12.3.4 (Common-to-Differential-Mode Conversion) of the below book is similar to your description:
https://link.springer.com/chapter/10.1007/978-3-031-14186-7_12
I didn't have a similar issue before, but what are the protection components against common mode and differential mode noises at the termination of the cable on your board?
--- End quote ---
During the test, the EUT failed, due to too large deviations seen on the signal at the AE (recording unit). I repeated the test, even just removing the wires from the AE, just lets them hanging. And I can measure a voltage on the pairs of the cable when left both floating and connected to the AE.
So to make it even simpler, I am testing only the cable, injecting a current into the shield and measuring the voltage on the signal pairs. I see a significant signal, so driving the shield with 7Vrms at 25MHz, I see about 100mVrms on the signal pairs. So some coupling from the shield to the pairs is happening. Capacitive and/or magenetic coupling
KlausKragelund:
--- Quote from: mtwieg on November 03, 2024, 06:22:23 pm ---
--- Quote from: KlausKragelund on November 03, 2024, 01:52:34 am ---Hi
I have a product that has failed conducted immunity tests at 30+ MHz
The product tested it in metal enclosure, and using M12 shield cables (so 360 degrees shield towards the product).
The cable that has failed has differential output, and in the test setup, a unit (AE) is measuring this signal, also with differential input.
The test is conducted by using a 50ohm generator, a 6dB attenuator and a 100 ohms series resistor that feeds current into the shield of the cable (with a clip on the cable, insulation removed). See setup figure attached.
I have reproduced the setup in my own lab, and measured that I can indeed measure a differential voltage on the AE unit.
--- End quote ---
If so then that means the "Decoupling device" fitted between the direct injection point and the AE apparently isn't doing a good job.
Is that the only issue, or is there some other specific reason you "failed" the test. Keep in mind the pass/fail criteria for these tests are usually tailored for the specific EUT, so we have no way of knowing what "fail" means. But it's usually based on what happens to the EUT, not the AE.
--- End quote ---
The test failed since the signal at the AE end deviated by 1% as defined in our spec for the max allowed deviation.
In the standard the decoupling device is defined as in the attached picture. It should create a defined impedance towards the AE. Still, since the resistance injected ti 150ohms, that decoupling impedance needs to be high to matter, and fi you wind as described, a big inductor can have parasitic capacitance, so it might not be all that good.
I am suspecting that we are in fact just testing the cables and the immunity of the AE instead of the EUT. The separate tests of cables alone seems to support that claim
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