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Best way to shield already fitted cable

Winsu

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Hi all,

I need something that I can use to shield a cable that seems to be affected by EMC noise. The cables are already fitted so I would need something that can cover the whole length of the cables. Anyone has an idea?.


Thanks,
Winsu.
 

srizbf

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you can try with a foil wrapped around the cable and grounding it.
 

c_mitra

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I need something that I can use to shield a cable that seems to be affected by EMC noise.
It is better to tame the noise at the source: please do some digging and try to identify the source of the noise and try to shield it at the source.

Once you identify the source, you may try to find the root cause; you may not be able to eliminate the noise but you can certainly tame it.
 

Winsu

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It is a susceptibility test and the product starts to be affected by the RF at 80Mhz and 300Mhz.We are not sure but we think the cables inside the luminaire are being affected by the RF, though the flickering is visible. We think that the PWM cables that are inside the lamp are picking noise and affecting the behaviour of the external PWM tool. I don't know what type of filter would stoip that from happening, that is why I think we could fix it by covering the PWM cables.
 

c_mitra

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So the long cables are carrying the PWM signal to the lamp that is fitted at a distance? I understand now.

Do you have some idea about the PWM frequency? RF at 80 MHz and/or 300 MHz are not difficult to filter. If the PWM is not close. The RF carrier is modulated with perhaps the power line frequency: could you please check with a LDR or a photodiode (a scope is needed) the flicker frequency?

Is the PWM signal is connected to a high impedance driver? Some resistor capacitor may be needed at the driver end.
 

Winsu

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The PWM frequency is 1Khz and yes there long cables leading to a terminal connector which is at the input stage of the lamp.

Unfortunately it is not possible to measure the flickering frequency because It is in a room where the antenna is radiating, so it is not a good idea for anyone to get in there and test it.

We know the amount of lumens decrease so we think that the duty cycle is affected.

We put a cnd filter ( it doesn't allow the signal to come out but nothing to coming in) right after the external PWM tool and the problem is strangely translated to 880Mhz.

We also put a PWM tool inside the lamp and run a test and it was a pass. It suggests that the lamp is putting some noise in somehow all the way back to the PWM tool...…, We think that the noise can be couple in the PWM wires or LED loom and go back to the PWM disturbing its behaviour...

We have already tried to put capacitor at the driver end and it did have any effect.
 

c_mitra

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The PWM frequency is 1Khz and yes there long cables leading to a terminal connector which is at the input stage of the lamp.
I suspect that somehow some noise is feeding back the PWM generating unit. Bulk of the electronics is there.

I guess you can measure the flicker freq just by using a photodiode connected to a freq counter in AC coupled mode.

Try changing the PWM freq and see the effect. Say 2K or 3K Hz - also try to shield the PWM unit.
 

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