After getting the 3.2" enhanced Nextion display working with my V3 uBITX, I noticed an annoying tone in the audio that went away when the Nextion was disconnected.? Tracking it down, it appeared to be getting out of the display board on the 5V power lines so I added more bypassing to the leads coming from Raduino power.? Didn't faze it.? Thinking it might be a ground loop, I added more grounding to the Raduino but that didn't help much either.
Speaking with a few people on it, the suggestion was made to power the display from an "off the Raduino board" supply but before I got a chance to build that up, Sheldon Gofberg on the uBITX Facebook group pointed me to the answer.
The tone is being generated on the Nextion board.? There is a "Display" brightness control in one of the menus that allows you to set your backlight brightness with a couple of sliders.? The way this brightness is controlled is by pulse width modulating the DC supply to the backlight LED.? It's this PWM oscillator that's causing the tone because it apparently isn't sufficiently decoupled on the board itself to keep the tone off of the DC supply line and it gets coupled back into the 5V supply on the Raduino and subsequently causes it to wind up in the uBITX audio chain and so on.
The quick fix is simply to set the display to full brightness which actually turns off the PWM and the tone goes away.
Next project is to try the external 5 volt supply independent of the the Raduino and see if that allows for brightness control again without the tone getting into everything.?
Jim Sheldon, W0EB
Jim Sheldon