STM32F769I-DISCO noise

[benny]

Posted on June 19, 2016 at 01:46

Hi,

I want to ask if someone can confirm that the STM32F769I-DISCO has a high-pitched

https://www.dict.cc/englisch-deutsch/buzzing.html

https://www.dict.cc/englisch-deutsch/sound.html

. I think it's from the display unit. So this is normal or should i complain my board? Kind regards Nyix #worst-forum-ever Note: this post was migrated and contained many threaded conversations, some content may be missing.

[AvaTar]

Posted on February 23, 2017 at 11:36

Even with 'degraded' high-frequency hearing, one can still hear the lower harmonics.

I guess this 18kHz sound is not the PWM frequency, but one of those harmonics.

[benny]

Posted on February 23, 2017 at 11:53

I'm not an audio expert but as far as I know there are now harmonics under the base frequency which is for example 26.562 kHz (base frequency means the PWM frequency not the internal clock). J B's problem is exactly the PWM frequency which is coupling over the GND. He measured 8 kHz and this matches perfectly to the default PWM frequency. The internal clock is not coupling back to the DAC. Only the PWM signal which is going to the LCD-backlight-driver makes problems. So in theory if he uses the 26.562 kHz there should be a coupling too, but you can't hear it. Normally you should be able to send the signal through a low pass filter to decrease the noise or just as mentioned set the brightness to 100% in hard- or software to completely avoid the problem.

BTW: Did you read his question about the glue? This would be interesting for me too. What kind of glue (specific product would be fine) and what about the heat dissipation?

Kind regards

Nyix

[AvaTar]

Posted on February 23, 2017 at 12:26

Well, mathematically yes. Harmonics are multiples

However, there are two things to consider. First, a vibratory system, like the mechanical item 'coil', can be excited by fractions or multiples of it's own resonance frequency. Second, there are possible down-mixing effects between PWM pulse length and base frequency. That would result in a backlight level dependent noise.

I don't have a F769-Disco, so just theorizing.

BTW: Did you read his question about the glue? This would be interesting for me too. What kind of glue (specific product would be fine) and what about the heat dissipation?

Ahhh no, probably missed that.

Glue or lacquer would reduce the mechanically possible oscillation amplitude to zero.

But heat dissipation might be a problem. I'm no hardware designer, so I can't recommend a specific brand. But the stuff used to isolate electronics for all-terrain/all-temperature operation (car electronics etc.) should work. All my company's products are isolated that way, most include switching power supplies.

[waclawek.jan]

Posted on February 23, 2017 at 12:33

Isn't the coil in question replaceable for a part from reputable source?

I understand there are gotchas with magnetics.

JW