WiPy2.0: funny antenna effect

[Shaun]

I had a similar antenna issue.
My original proto PCB was designed for a LoPy and development progressed.

I then later switched over to developing with a WiPy, and I could not get the WiPy to run it would simply crash and my PC would lose connection with the WiPy WiFi AP.

Placing my finger on the antenna would allow the module to work.
I later molded a blob of clay over the antenna portion of the module to keep it stable but occasionally ran into very strange semi failed UART routines etc..

My 2nd revision PCB was designed for the WiPy and subsequently had all ground plane below the WiPy antenna removed. This PCB is running stably.

I am a little concerned that the WiPy might not pass EMC testing, as it appears to be prone to hangup when reflections from its own antenna are received.

Perhaps a Pycom representative could shed light on whether the WiPy has passed both EMI as well as EMC testing, and if so, at what field strength was it tested? (10V/m?)

[jmarcelino]

@Shaun
You can look up the WiPy's FCC certification including MPE (Maximum Permissible Exposure) test report

https://fccid.io/2AJMTWIPY2R

[Shaun]

@jmarcelino Thanks for that link.

I find reports for EMI (radiated emissions) and RF exposure, but no test report for EMC (susceptibility to external RF fields)

[Rustydusty]

Thank you for fast responses!

@Shaun
your confirmation of my observations is very helpful, indeed! Yes, I am convinced that the problem is related to to the layout of my prototype board, which seems to create a strong HF interaction with the WiPy, some kind of feedback-loop, resonance, or similar.
So, it would be nice to have some layout rules for these modules in the future.
Meanwhile, I will continue by trial and error to find a technically sound solution.

@jmarcelino
Yes, your advice related to power supply is an excellent approach, it was the first thing I have investigated. I have tested 3 different power supplies: The one of my prototype, the USB-supply from my computer, and a mains-to-USB power supply rated for 1 amp. Result: no effect.
I also added bypass capacitors between 5V supply and ground, and between 3.3V board output to ground, in various combinations, using 1uF or 10 uF placed close to the WiPy. Result: no effect.
moreover, the fact that the problem disappears when the WiPy is separated from my board by wires contradicts the power-supply-theory, it should rather make the situation worse.

Yes, I have noticed that the soldering point of my miracle wire is low ohmically connected to ground. Nevertheless, attaching the wire directly to the ground pin of WipPy has no effect.

Final remark: If WiPy required indeed a power supply capable of 500mA for running stable, it would not be suitable for IoT applications, rather for IoPP (PP=power plants) :-). Luckily, it does not.
Anyway, the power management issue will be the next topic I must get into. sleep/deep-sleep issues are not so well defined yet, as it seems to me.

And I am sorry, but I must insist: Will Pycom ever make the schematic drawings available to users? It would be very useful as long as the WiPy hardware is not in all respects a fully defined black box.

I am looking forward to the progress of the excellent WiPy project and its documentation!
Thank you again!

[Shaun]

@Rustydusty It is very important that there is no ground plane or signal tracks running under the modules chip antenna area.
It is preferred to have the chip antenna protrude beyond the edge of your application PCB

[Rustydusty]

@Shaun thank you so much for this advice!
Indeed, there are some lines running underneath this critical area and it is my suspicion that these are the responsible ones. WiPy seems to be very sensitive in this respect, the distance to the board is ca. 10 mm (due to socket), nevertheless, it interferes.

[jmarcelino]

@Rustydusty

I'm really not speculating on the power numbers. The ESP32 while running (with WiFi on but idling away) reads about 125mA on my power supply, the data sheet lists 260mA while WiFi is transmitting and if you look at the official ESP32 module:

ESP-WROOM-32 Datasheet https://www.espressif.com/sites/default/files/documentation/esp_wroom_32_datasheet_en.pdf

you'll see the "Operating current" under Recommended Operating Conditions is in fact 500mA. These are when operating from 3.3V. I expect the WiPy to be very similar.

The trick - as with anything IoT - is to keep it in deep sleep as much as possible.

One thing to try is to set WiFi(antenna=WLAN.EXT_ANT) and connect an external WiFi antenna instead (via the u.FL connector). This triggers the RF switch moving the RF path away from the internal ceramic antenna and onto the uFL connector. I wonder if that solves your problem?

It would be good for Pycom to provide a tested "reference design" (board layout and schematic) for integration and I really expect something like that for the OEM modules.

[jmarcelino]

@Shaun
Well technicaly they're both EMC subjects (emissions and immunity) but FCC only requires EMC emissions testing (immunity is required only for medical devices) so you won't find references to immunity on the FCC listing.

I guess Pycom will have passed IEC 61000-4-3 for CE compliance and has that available somewhere. I wouldn't expect it to be compatible with 10 V/m though - as you probably know that's for industrial equipment and rather difficult to achieve uncased.

Most commercial modules I know of are only rated to 3 V/m.

[Rustydusty]

@jmarcelino
yes, I fully agree with you, we must look at the average power demand a device consumes in the application to rate it "low power" or not. At the same time, we must look at the power profile for selecting the optimum supply design. This makes sufficient buffering (i.e. bypass capacitors) crucial, if the power supply is not capable of driving the required peak currents.
I am pretty sure WiPy will meet my requirements, provided I can make use of the deep sleep mode with timer controlled wake up.

@Shaun
@jmarcelino
Taking your and Shaun´s contributions together, we have a first draft of a RF design rule for WiPy:

• No ground plane and no signal tracks close to the HF/RF section of the WiPy board, preferentially have the chip antenna protrude beyond the edge of the application PCB.

• Alternatively, make use of the external WiFi antenna - and do not forget to set the software switch.

right? (I will test the external antenna approach as soon as possible)

Reference designs would indeed be very useful, in all respects (RF, power, interfacing, etc.)

Thank you both again for your contributions, it was very helpful!

[jhoguaman]

@rustydusty This is a great contribution. I have the same problem with a LoPy module, I am going to apply the RF design rule on LoPy.