nrf51822 advertising packet not detected

[Alessandro Fato]

Hi,

I would like to ask your support for a very unusual problem that I'm facing with a project based on nrf51822.

The board under test is based on nrf51822, SDK12.1, softdevice S130. The nordic example used for testing is the Nordic HRM.

The advertising works perfectly with 5 different smartphones (different brands), with 2 laptops (internal BT chip) and is always recognised by the nordic sniffer.

Two tablets, an Asus zenpad 10 and a kindle fire HD do not identify any advertising packet. This is true both using android bluetooth discovery and the nrf-connect application (same version and settings on all mobile devices).

On the other side, the same two tablets are able to discover the advertising packets both on a PCA10028 and on a PCA10031 (using the same code and softdevice hex).

The packets recorded by the nordic sniffer are identical (except for the BLE address). Even setting explicitly on the board under test the BLE address to one of the eval board didn't resolve the problem.

Could you help me in understanding the source of the problem and how to address it?

[Petter Myhre]

Strange. What RSSI do you get from PCA10028 on all devices? And what RSSI do you get from you custom board on all devices (that are able to receive)?

[Alessandro Fato]

Hi Petter, the RSSI is similar and in general is around -60dbm and -70dbm. Today I went to a big tech store to try different tablets and our board was discovered by 8 out of 10 tablets. Any ideas on which could be the problem? Furthermore, the tablets that didn't discovered our board were capable to discover advertising devices with RSSI down to -90 dBm.

[FormerMember]

My suspicion is that the problem is related to an offset of the carrier frequency: The carrier frequency output from the radio is heavily dependent on the value of the load capacitors used with the 16 MHz crystal.

On your custom board:

• Which 16 MHz crystal do you use?
• What is the load capacitance of that crystal?
• What is the value of the load capacitors used? Is their value calculated according to the formula in the "Crystal and load capacitors" section in this tutorial?

Do you have access to a spectrum analyzer? If so, could you use the radio_test example in the SDK to check the offset of the carrier frequency?

[jemajuca]

Alessandro, have you found the reason to that behaviour? Some similar cases here with smartphones.

[Alessandro Fato]

Hi jemajuca, reviewing the schematics we identified different issues both with the matching network of the antenna and the load capacitors. We still didn't try to fix the capacitors but we planned to do some tests during this weekend. Did you try to check the load capacitors and the antenna matching circuit?