ESP32-WROOM-32D EN55014 FCC Part 15 B

marko.brelak
Posts: 19
Joined: Wed Apr 10, 2019 1:26 pm

ESP32-WROOM-32D EN55014 FCC Part 15 B

Postby marko.brelak » Mon Jun 29, 2020 8:13 pm

Dear colleagues,

I am working on a commercial device that has the ESP32-WROOM-32D module inside. So far, in terms of the functionalities and performance, the module kick ass :) Nothing to complain about. But there is a problem when it comes to the EMC testing, and how I see it now, I have a couple of 80 MHz harmonics, and one (319.63 MHz) gives me a headache.

EMC.JPG
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I have read in the datasheets and manuals for the ESP32 that U0TXD needs around 499 Ohms (470 Ohms) in series in order to suppress the harmonics. Which seems to be quite correct, if you need a UART at all.
Link: https://www.espressif.com/sites/default ... nes_en.pdf
Section: 2.1.8 UART

In my case, I don't need UART and especially not during the EMC testing, and unfortunately, I don't have the opportunity to put the 499 Ohms resistor in series (I would put the 499 Ohms resistor to the GND) so I need to think about another approach. By the way, even though I don't need UART, still we have some debug messages there on 115200 baud rate, on the line which is pulled up to 3.3V via 10K resistor, on the line which is around 25mm long.
How does this sound to you? -> I would configure UART0 pins to GPIO, Outputs, and put them to logical 0 (LOW). That way, using low-side internal MOSFETs, the "harmonics" would go directly to the GND plane instead of radiating to the U0TXD line. Do you feel that I could reduce that 1.1dB of overshoot?

Many thanks to all who read this!

Regards
Marko

ESP_Sprite
Posts: 3621
Joined: Thu Nov 26, 2015 4:08 am

Re: ESP32-WROOM-32D EN55014 FCC Part 15 B

Postby ESP_Sprite » Tue Jun 30, 2020 7:13 am

Sounds like something that's at least worth trying.

marko.brelak
Posts: 19
Joined: Wed Apr 10, 2019 1:26 pm

Re: ESP32-WROOM-32D EN55014 FCC Part 15 B

Postby marko.brelak » Wed Jul 01, 2020 7:30 am

From your experience, do you know where can I expect that 80MHz harmonics to radiate from, which pins?

Regards
Marko

ESP_Sprite
Posts: 3621
Joined: Thu Nov 26, 2015 4:08 am

Re: ESP32-WROOM-32D EN55014 FCC Part 15 B

Postby ESP_Sprite » Wed Jul 01, 2020 12:03 pm

I don't have any experience myself there, but I think it was only on the Tx pin of UART0. I'm not sure if the noise follows the pin that Tx is connected to, or if it's something that is specific to that chip pad or not, though. I can ask around, but it's probably quicker to just run some experiments if you already have the gear there.

marko.brelak
Posts: 19
Joined: Wed Apr 10, 2019 1:26 pm

Re: ESP32-WROOM-32D EN55014 FCC Part 15 B

Postby marko.brelak » Wed Jul 01, 2020 1:36 pm

I have read that the main cause of the mess is the TXD0 because of its location and the near proximity to the 40MHz crystal. I guess that it is due to the way of the in-module routing. And in addition to this, I think that the problem is in a specific pad since it's a non-muxible interface (UART0).
I should get the EMC pre-compliance testing equipment soon, in the next couple of days probably, but until then we need to try our luck with this approach since we don't have thank gear.
Please if you are able, ask your colleagues or friends if they had a similar problem. In the meantime, I will share my results.

ESP_Sprite
Posts: 3621
Joined: Thu Nov 26, 2015 4:08 am

Re: ESP32-WROOM-32D EN55014 FCC Part 15 B

Postby ESP_Sprite » Thu Jul 02, 2020 2:00 pm

I asked around, it looks like the 80MHz actually is generated when the UART is outputting something. According to my colleague, disabling the TxD0 output and not connecting anything there should get rid of the harmonics. Not sure if it's still applicable to your situation, but he also mentioned that adding some decoupling of the 3.3V supply as indicated in the picture can help getting rid of 80MHz harmonics.
sch.png
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