PreenFM > preenfm2 and preenfm3

pfm2 with yellow OLED : high frequency solution

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Situation : Yellow OLED display have a big inductor that sends a 13.5Khz sin wave in the preenfm2 audio path.

Yellow OLED solution

I spent some time trying to find a solution to this problem, and i have somthing really efficient.
Look at the After and Before screenshot.

If you have a PCB R5d (or +) : solder a 22ohm at the "oled" position and go to step 4 directly... DON't do step 1,2,3.

If you have a PCB R5c (or -) : step 1,2 and 3 is for you.

The idea is to put a resistor 22 (ohm) between the LCD and the preenfm2 5V.
As there's no direct possibility, we'll use the Led+ pins (pin 15 of the LCD) that is not used with OLED display and where we can insert a resistor.

Step 1 : put a 22 ohm in the 68 ohm slot... this resistor is between the 5v and the pin 15 of the display.
Step 2 : cut the pin 2 of the OLED so that it's disconnected from the preenfm2 5V.
Step 3 : Link the pin 2 and 15 on the Display. Now the Oled is powered throught the 22 ohm resistor and get around 4v.
Step 1 to 3 makes the parasit frequency much more sensitive to shield. I think it removes almost totally the electronic noise but the magnetic noise is still there.

Step 4 : build a shield with many alumnium sheets that you use for food. you may have that in your kitchen.
The more you put the more efficient it is.
Take a 22cm x 22cm, fold 1 time then 2 times horizontaly, same thing vertically. You get 16 layers.
Do that with 4 sheets to get at least 64 layers.
Put that in a plastic bag (antistatic may not hurt) and add some tape. Be sure no aluminium will touch the contact.

Step 5 : put your shield under the screen.

Step 6 : Let me know :)


Details screenshots....

Oh nice Xavier! Gotta try it out soon thanks for taking your time looking into this :)

Done and done, 64 layers of alu foil, wrapped in antistatic bag twice and then sticky tape (not sure if antistatic bag can damage something, just to be sure).

High pitch noise is drastically reduced just by adding this shield. (sadly, I didn't try to make a shield as proposed by clément, I suppose I was too lazy to cut and solder perfboards :) )

I didn't hear any difference with or without 22 ohm resistor, but I was only using my ears for testing, as I don't have oscilloscope... After soldering, I used soundcard oscilloscope (didn't come to my mind earlier), and peak is much lower with shield.

I am thinking about making (some day) shield out of thin steel sheet, like on RF tuners, this could maybe lower magnetic noise...

Thanks very much,

The 22 ohms resistor alone does not change anything.

You have to cut the pin 2 of the OLED header then link pins 2 and 15... See the red wire on the picture above and the missing pin between the OLED and the preenfm PCB.
I remove the whole pin but simply cutting it on the OLED side will work.

Then yes it's really worth it...


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