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FCC conducted emissions question

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difflvl

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Hi,

I had a device that was failing with the conducted emissions test of the FCC part 15b.

It was failing in the 800khz-1.2MHz range. I do not have a clock source of that frequency on my board, nor was I able to trace where it is coming from.

My question is this what can I add to my power supply circuit to reduce noise in the 1MHz region.

I have attached my power supply schematic below. It is a 3.3 V regulator with VIN being a 6volt wall wart.

Thanks
 

pls refer the attach pdf....

do not forget to press help me button
 

    difflvl

    Points: 2
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Just place C6 100nF (X2 type) before D1. Or add new one (better)

difflvl said:
Hi,

I had a device that was failing with the conducted emissions test of the FCC part 15b.

It was failing in the 800khz-1.2MHz range. I do not have a clock source of that frequency on my board, nor was I able to trace where it is coming from.

My question is this what can I add to my power supply circuit to reduce noise in the 1MHz region.

I have attached my power supply schematic below. It is a 3.3 V regulator with VIN being a 6volt wall wart.

Thanks
 

BasePointer said:
Just place C6 100nF (X2 type) before D1. Or add new one (better)

difflvl said:
Hi,

I had a device that was failing with the conducted emissions test of the FCC part 15b.

It was failing in the 800khz-1.2MHz range. I do not have a clock source of that frequency on my board, nor was I able to trace where it is coming from.

My question is this what can I add to my power supply circuit to reduce noise in the 1MHz region.

I have attached my power supply schematic below. It is a 3.3 V regulator with VIN being a 6volt wall wart.

Thanks

6 100nf caps will help with the 1mhz frequency range? I thought those values are good for higher frequencies. At the lab I tried adding more 100nfs and it didn't really do anything.
 

- What is your input voltage?
- What is the output current that you require?
- Does the power supplier have a load when testing?

You may use higher capacitor instead of 100nF (X2). For example electrolytic one.
Can you send graphical test output here?
 

The input voltage is 6VDC from a wall wart. Output current required is ~200-450mA. Yes it has load during testing.

I don't have the test output available but I was 18dBs over the limit at ~1MHz.
 

What clock sources are there in you design?
 

Clock sources are 14.7456mhz from crystal, 28.224mhz from another crystal and 4.032mhz from an IC.
 

difflvl said:
Clock sources are 14.7456mhz from crystal, 28.224mhz from another crystal and 4.032mhz from an IC.

Did you try 100uF/25V or 1000uF/25V electrolytic instead of C6 100nF?
 

    difflvl

    Points: 2
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Hi, reading the hole topic it seems like the wall wart power supply is causing problems. I might be wrong here. Are you are testing with the wall wart power supply to get a compliance certificate for the complete system. Perhaps you can try an other power supply if the one you are using is a switching power supply. It might be market CE and all the rest but most of them are not. I'm currently evaluating power supplies from china for a product. Some of them even fail on the input voltage requirement of 85 volt...

Regards
 

If possible ( cost available ), add an emc coil at test port. It has significant improvement.

Rogerynt
 

inventor(y) said:
Hi, reading the hole topic it seems like the wall wart power supply is causing problems. I might be wrong here. Are you are testing with the wall wart power supply to get a compliance certificate for the complete system. Perhaps you can try an other power supply if the one you are using is a switching power supply. It might be market CE and all the rest but most of them are not. I'm currently evaluating power supplies from china for a product. Some of them even fail on the input voltage requirement of 85 volt...

Regards

If you have a list of a few wall warts that are good I would appreciate it.
 

difflvl, at the moment I have tried three different brands. But I can't test radiated emission. I don't have the equipment to do this. Here my customer is a bit upset about the price he has to pay to get a certificate...
Anyway, a good working and very low cost power supply can be found on: cnhitong.com
 

    difflvl

    Points: 2
    Helpful Answer Positive Rating
Hi
you can try this method also put ferrite bead in series with the input power jack to reduce the conducted emission problem
 

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