nRF52840 and Pin Limiter Diodes on the Bluetooth Antenna Line

[hendrikB]

Hi there.

Does the nRF52840 require any protection on the Bluetooth Antenna Line needed to block large unwanted signals when fitted on a Modem that also incorporates GSM and / or 4G LTE?

We have a 2G / 4G LTE Modem design that is fitted with an nRF52840. A shunt PIN Limiter Diode (SMP1330-040LF) is placed on the Bluetooth antenna line just before connecting to the ANT pin. 

This design was done by one of my predecessors before I joined the company and the original design notes and files are long gone.

Will it be safe to simply remove this PIN Limiter Diode? Do you have any design recommendations for applications that also run 2G / 4G, or is protection not needed.

Thanks a lot

Hendrik

[Simonr]

Hi again

The main purpose of the band pass filter on the BLE radio is to reduce blocking (of the BLE radio) when it is in RX and LTE is transmitting. The nRF52 series can tolerate 10dBm on the ANT pin without any problems, so a PIN diode is not needed for that purpose.

However, in any case you should avoid transmitting with both radios at the same time, as it will greatly increase the necessary production testing and cost on your end.

The LTE harmonics are more likely to cause trouble as they are on frequencies close to the 2.4GHz band. Thus on designs like the Thingy:91X, we have an additional RF filter and an ESD diode to reduce this. Physical distance between the antennas will also help.

Best regards,

Simon

[hendrikB]

Hi Simon

Thanks for the update and info.

Kind Regards

Hendrik