How do I change radio frequency in the eddystone ble example?

[SGC]

Im developing a BLE beacon based on the eddystone example provided for the NRF52832. I've found that in softdevice applications, the radio frequency can not be changed directily on the RADIO register, it needs to be changed by means of the softdevice API. But I dont find how can I do that.

Additonaly, I dont see any reference for the radio frequency or the channel selection on the Eddystone code example Im using.

Hope someone can help me. Thanks.

[ketiljo]

You can't really change the frequency of a BLE beacon. It will and shall use the three advertisement channels. Only the softdevice (BLE protocol stack) can change the frequency.

[SGC]

Thanks for your reply.

So, I understand, that is possible to change between the three advertisement channels for beacons (37, 38 and 39). Im I right?

If it is possible to force the use of one channel or another. Could you kindly tell me how can these channels be selected in the software? As I said, Im using the eddystone example provided with the softdevice sdk.

Thanks.

[ketiljo]

The softdevice have an option to advertice on only one channel, but doing this may cause the Eddystone example to misbehave. Normal advertising of devices and beacons will happen on all three channels consecutively.

[SGC]

Thanks for you comments.

I wanted to change the frequency/channels because there are some phones that seems to not receive the frames. To be honest, just one phone has give us problems (Samsung A10), we've tested more than 10 different phones and all of them seems to work fine.

We've developed a custom hardware based on the reference desing. The thing is that, with the same software, the phone detects the NRF52-DK development board and not our custom hardware. As I said, we have made a copy of the reference desing.

The only thing we didnt copy is the antenna desing, we have tested with a PCB based antena and with a ceramic one. Neither of them works with the Samsung A10 phone. So we thought that could be a problem with the tx frequency.

Could you give us any input on this problems? Hope you could help us.

Thanks.

[ketiljo]

That must be caused by something else. All phones should be able to use all advertisement channels. The antenna not contribute to what you see either. 

What's the CL value of the 32 MHz crystal and the load caps you are using? Do you have a 32 kHz crystal?

[SGC]

Thank for answering.

Yes, Im using a 32MHz crystal. The CL of the crystal is 12pF and Im using 12pF load caps as well.

The crystal Im using is this: https://www.digikey.com/product-detail/en/epson/TSX-3225-32.0000MF10Z-W6/SER4378CT-ND/7932362

[ketiljo]

The CL value of the crystal is not the same as the value of the load caps. The crystal will see both capacitors in series so the capacitance will be half of the value of the load caps, provided they are equal. In addition you have the pin capacitance of XC1 and XC2 in parallel. The equation then becomes: 

Ccap = CL * C - Cpin

Cpin = 4 pF and CL = 12 pH, so you must use use 

Ccap = 12 pF *2 - 4 pF = 20 pF 

Round down to 18 pF as this is a standard value.

If you load the crystal incorrectly, you will pull the frequency of the crystal. Some radio front end are more sensitive to frequency error that other, so this is why you see problems on only some phone models.