Radiated immunity test problem in home healthcare product

[ray]

We are making ble IOT equipment and currently trying to get medical device certification.

I’ve done radiated immunity test for our devise based on used the nRF52840.

The hardware has to withstand an electric field strenght of 10 V/m, however our device is only certificated for 3 V/m.

The problematic frequencies are 2.19 ~ 2.26GHz.

The connection between device and smart phone was completely interrupted and stop pairing .

We checked the post about 3 V/m immunity test with nRF52840DK, 10 V/m was not found.

Has anyone advise on how to solve this problem? Is the nRF52840 designed to resist electric fields of 10 V/m?

Thanks in advance.

[ketiljo]

Yes, this is to be expected. The device is designed for 3 V/m. For higher field strength, you need a band pass filter after the antenna. Possibly also a shielded box around the nRF52840 and the RF section. 

[ray]

Thank you for quick response.
We tried a shielded box around the nRF52840 and the RF section but still getting disconnected.
We will try a band pass filter after the antenna.
Could you advise us which a band pass filter to use?
and in the code something that I can try such as changing BLE config?

Additionally has any chip designed for 10V/m in nRF52 series?

Thanks in advance

[ketiljo]

The energy from the disturber come in via the antenna. So you will always need a band pass filter and maybe the shielded box, not the other way around. There are several filters that will do this fine. You need >15 dB attenuation at the closest frequency, Something like this https://eu.mouser.com/new/qorvo/qorvo-885033-signal-conditioning/ Note that the insertion loss is higher when the attenuation is higher 

All nRF devices are designed for 3 V/m as this is the standard for consumer electronics and it's relatively simple to increase this with a filter. 

There's nothing  FW related that can improve this.

[ray]

Thank you for reply.

We checked recommended parts and a little confused.
The chip have one pair input and output pin.
It is understood that it operates in one direction, and it is questionable whether Rx/Tx can be used.

Will be trying to design the circuit as below.

Is there any problem with ble communication even if I use it like this?

Thanks in advance.

[ketiljo]

It's bi-directional so it doesn't matter what way you connect it. 

The nRF52840 matching network have 3 components depending on what version of the schematic you use. Place the filter between this and the antenna tuning components. 

The schematic is mostly correct, you just need a shunt capacitor between L3 and the filter.

[wdabkus]

I would like to know if the problem raised in this post is still valid.
I am currently designing a medical device that will also be exposed to 10V/m exposures during EMC testing, and I am wondering if the added SAW filter is necessary.

In the post 4 month ago: devzone.nordicsemi.com/.../immunity-issue-2-3ghz

it was reported that in revision 3 of nrf52840 the problem of immunity to 10V/m has been eliminated.

IN-141 rev 1.1 description for 3 rev nrf52840 and with IN-149 (for 2 rev nrf 52833)

"Improved wide band blocking:
The blocking performance improvement will be in the order of 10-15 dB for frequency offsets of 50-300 MHz from the carrier
frequency compared to previous nRF52840 revisions. "

Can I ask you to confirm this information and whether this means that it is safe to use in healthcare devices nrf52840 rev. 3 or nrf52833 rev. 2, without additional modifications to the radio path (we want to use precertified modules)

Thanks! 

[ray]

We solved the problem by using a bandpass filter after the antenna. 
Thank you.