Noise on PCB during 2.4GHz RF transmission

RE: Noise on PCB during 2.4GHz RF transmission

MinhDuc — Thu, 04 Apr 2024 06:13:29 GMT

Hello hmoles,

In my case, I send a continuous of 12 32-bit SPI command (16MHz SCK) every 50us (20kHz sampling rate). It takes a round ( 2us/SPI command *12 + ~1.5us/CS high * 11) = 40us.

I checked The SPI signal with oscilloscope. I didn't observe any Radio interrupt problem or unpredictable sampling.

Note that I send the RF packet of 252byte. In other word, I don't send the packet of data every 50us. I collect 252 bytes first then send it.

Bests,

RE: Noise on PCB during 2.4GHz RF transmission

hmolesworth — Wed, 03 Apr 2024 15:27:24 GMT

I was thinking more of high transfer rate with long DMA transfers to get predictably even sampling, given that the radio requires interrupt priority short transfers do not work well and lead to unpredictable sampling unless scheduled away from radio activity. Something like the STM32U535xx; For the nRF5x PWM spoofing of SCK, MOSI and /CS works in conjunction with using SPIS instead of SPIM to receive the data but the frequency is on the low side. Spoofing of SCK, MOSI and /CS with PPI-linked timers is also an option, but increases power due to the significant number of peripherals running and synchronising the timers requires care. What data rate do you achieve?

RE: Noise on PCB during 2.4GHz RF transmission

MinhDuc — Wed, 03 Apr 2024 03:32:49 GMT

Thank you for sharing.

My SPI is ok. It’s around 1.5us CS pulse high between each 32-bit SPI command.

RE: Noise on PCB during 2.4GHz RF transmission

hmolesworth — Wed, 03 Apr 2024 02:13:40 GMT

If using a battery with plenty of capacity, such as a rechargeable battery or AA cells, or external supply such as USB, then the average VDD voltage probably stays high enough for 3.3 volt operation and so use the ideal diode (small, cheap, low voltage drop 30mV). If using (say) CR2032 or CR2540 coin cells where the voltage will drop over time to (say) 2.7 volts, then a buck-boost regulator to supply the ADC with 3.3 volts becomes mandatory. If using such a buck-boost regulator to maintain 3.3 volts for the ADC, choose one which will hold the voltage output even when the input voltage sags or drops sharply. Something like a TPS63000; there are lots of options but I use this one. TPS63000 in which case this replaces the Ideal Diode.

How are you handling the 32-bit SPI transfer /CS requirement other than millions of short transfers? "The CS line must be pulsed high between every 32-bit data transfer.; ie. /CS stays high for 154 ns between words"

RE: Noise on PCB during 2.4GHz RF transmission

MinhDuc — Wed, 03 Apr 2024 01:18:12 GMT

It was very clear. I don't have experience using ideal diode for this purpose. Can you please recommend which one is better for my case? Ideal diode or separate LDO?

Appreciate your help.

RE: Noise on PCB during 2.4GHz RF transmission

hmolesworth — Tue, 02 Apr 2024 16:56:47 GMT

I would just common the ADC VDD pins and drive from a single Ideal Diode; it is not clear which VDD pin supplies which subsystem within the ADC die, often one is used for io and others for analogue but it doesn't matter here I don't think. Capacitor should be (say) 47uF 6.3V or (better) 10V or 16V rating. Normally connect EN to Vdd (ie nRF VDD) or leave floating (EN has internal pull-up). Note "It is not necessary (or recommended) to use separate “analog” and “digital” power lines."

VSTIM would perhaps be treated separately if also coming from nRF VDD.

RE: Noise on PCB during 2.4GHz RF transmission

MinhDuc — Tue, 02 Apr 2024 16:29:11 GMT

Hello hmolesworth,

Yes I think it makes sense.

In the case of ideal diode, should I use each diode for each VDD pin of external ADC?

Best,

RE: Noise on PCB during 2.4GHz RF transmission

hmolesworth — Tue, 02 Apr 2024 15:39:40 GMT

Common problem, the nRF VDD sags a lot during BLE transmission which (worse) is often asynchronous to the ADC readings. One fix is to feed the supply for the external ADC via an ideal diode, thus when the nRF VDD sags the external ADC VDD holds up with a more-or-less linear droop for the duration of the nRF VDD sag; this droop is orders of magnitude less than nRF VDD sag so does not have a big impact on ADC readings. A separate regulator which blocks reverse-drain can also be used instead of the ideal diode.

RE: Noise on PCB during 2.4GHz RF transmission

Simonr — Tue, 02 Apr 2024 12:20:07 GMT

After discussing this with one of our HW experts, this is normal behavior, as the radio pulls the voltage lower when used which thus affects the ADC.

Best regards,

Simon

RE: Noise on PCB during 2.4GHz RF transmission

Simonr — Wed, 27 Mar 2024 07:12:48 GMT

I will have to wait for some of the HW engineers to come back from Easter vacation, so someone will be able to help you Tuesday April 2nd. In the meantime, can you upload the full schematics and gerber files of your design so we can conduct a proper HW review on our end? If you'd like I can set this case to private mode so the files you upload will only be visible to yourself and Nordic engineers.

Best regards,

Simon

RE: Noise on PCB during 2.4GHz RF transmission

MinhDuc — Tue, 26 Mar 2024 07:51:57 GMT

Hi Simonr,

My ADC is Intan chip RHS2116. It is controlled by MCU via SPI communication.

Instead of connecting all VSS at top layer ground plan, I use uVia for each VSS pin for connecting it directly to the layer 2 ground plane.

In my PCB, I use a lot of via so it limit the return path. Do you think it is a possible reason?

Best regards,

RE: Noise on PCB during 2.4GHz RF transmission

Simonr — Tue, 26 Mar 2024 06:37:52 GMT

Can you share your HW files so we can do a review of schematics and the PCB you're using? Here is an image of how the ground plane and VSS pins should be connected.

Also, please share some more details on the ADC you're using. What are you measuring on it, and what is this external ADC exactly? And is it connected to the nRF52840 somehow? If so, to what pins on the nRF52840.

Best regards,

Simon

RE: Noise on PCB during 2.4GHz RF transmission

MinhDuc — Tue, 26 Mar 2024 00:02:45 GMT

Glad to hear from you Simonr. When I mentioned "ADC", it was the external ADC instead of nrf52840-ADC.

I locate that external ADC quite far from the ANT trace (around 2cm).

Best regards,

RE: Noise on PCB during 2.4GHz RF transmission

Simonr — Mon, 25 Mar 2024 14:23:12 GMT

What pin(s) are you using for ADC on your nRF52840 SoC? As you can see in the nRF52840 pin assignments section, some GPIOs are recommended to use at standard drives/low frequencies. That is because these are located near the radio, and can affect radio performance. It is also thus possible that the radio can affect pins/lines that goes close to the ANT pin/trace.

Regarding your second point, I will need to check that with one of my more "HW oriented" colleagues and get back to you. Thanks for your patience!

Best regards,

Simon