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47khz noise on top of the DC supplies of the SMPS used in R&S FSEM

hafrse

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Hello,

I have a 47 Khz noise in the DC supplies of the analyzer ( R&S FSEM) , this noise was detected accidentally when I have replaced the LCD screen to a newer type where interference can be seen visible on low contrast images in this LCD panel, using an external DC supply , panel works perfekt!



The supply I am using is +15V , attached is the trace taken from the +15V at 10us. Any clues how to filter that noise out ? thanks

20200730_102127.jpg
 

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barry

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Whaaaaat? Your attachment is a picture of DIMM card. NOT USEFUL!!

But from your description, the problem is your power supply, or how your system is grounded. No way to tell.
 

hafrse

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Sorry for that image,I could not edit my post or delete the image. Isn't possible to edit the post ? after a period of time when I created the post?

Yes, I suspect the power supply, where should I look? caps with high ESR ? the analyzer works fine, no spurs on spectrum trace or any other artifiacts except some times it hangs (operating system hangs and I need to restart the instrument) if I navigate quickly through the menus especially when stepping the input step attenuator.
--- Updated ---

I can edit my last post which I posted just now but not the first one , it seems this forum does not permit to edit the post after a specific period of tid :(


Image for a similar power supply taken from another user + block diagram, I do not have the schematics

2.jpgps.jpg
 
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BradtheRad

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Slight noise around 50 kHz appears on my homemade power supply. It must come through house voltage. Cause unknown. The 3300 uF smoothing capacitor does not eliminate it.

This simple LC filter might do the job for you. An inductor and capacitor arranged as a second-order low-pass filter. My simulation has LC values that create a 13 kHz resonant frequency, effectively absorbing 47kHz.

I suppose you already have a capacitor installed near the module. Thus it should be sufficient if you just add a 1-2 mH choke on the supply wire, based on how much current goes to your load. It's not crucial to create a 47 kHz resonant frequency.

inductor and capacitor 2nd order filter reduce 47kHz noise.png
 

    hafrse

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hafrse

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Thanks, I will try it and see if it helps :)
 

treez

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yes, BradTheRad's filter will sort it.
Its just switching frequency ripple.
The sudden rise is due to the ESL of the output caps......then there a ramp-down, which is the secondary rectifier current going down...then a smaller sudden drop due to the sudden disappearance of current in the ESR....then the discharge of the cap....then it all starts over.
 

    hafrse

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hafrse

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May be the problem in the power supply itself, caps with high ESR?
 

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