Using nRF52811 for direction finding

[greentree]

nRF52811 has only one receiver. Most accurate direction finding can be made based on estimation of phase difference between two signals received in two spaced antennas. Such phase difference can be accurately estimated if we take samples in two receiving channels at the same moment. So, two receiving channels shall have the same coherent LO source and samples shall be made synchronously. Correspondingly, I would like to ask have 3 questions:

1. Can we use two nRF52811 systems with the same coherent LO source?

2. Can we synchronize IQ sampling moments in both nRF52811 systems?

3. If question 1 and 2 are negative, what AOA estimation approach shall be used with nRF52811 ?

If my questions are incorrect, I would appreciate any clarifications regarding that.

[Dmitry]

Hi Vlad,

sampling is done on demodulated signal after I/Q mixers, so initial shift of RF carrier signal by, say, 30 degrees will give phase shift in sampled sequence by 30 degrees. Accuracy should be quite high.

[greentree]

Hi Simon,

I would like to ask you a question via email. Is that possible?

Best regards,

Vlad.

[Simonr]

Hi Vlad

Due to the Easter holiday you will have to expect a delay in answers, sorry for the inconvenience!

No, you can't. If you have private questions I suggest you create a Private ticket on the forum. This way the ticket will only be visible to you and Nordic engineers, and it will be treated with confidentiality.

Best regards and Happy Easter,

Simon

[greentree]

Hi Dmitry,

Sorry for the late reply. The picture below is from here: https://web.wpi.edu/Pubs/E-project/Available/E-project-101012-211424/unrestricted/DirectionFindingPresentation.pdf . If you put real numbers in equations, you will get results I mentioned earlier. 

[robwasab]

Hi Vlad,

"IQ sampling is made over demodulated signal having frequency of 250kHz or Since signal frequency is relatively low, samples taken in different antennas can be effectively synchronized."

No. While the baseband frequency is 250 kHz, the phase shifts seen by the antennas will be preserved from the baseband's perspective. If you do the math, a phase shift at RF will equate to the same phase shift at baseband. Dmitry was alluding to this earlier. See following:

"3. If question 1 and 2 are negative, what AOA estimation approach shall be used with nRF52811 ?"

Inferring from the product specification, section 6.14.12, or page 189, and that this is part of the Bluetooth specification, the transmitter appends a "continuous tone extension" to the end of a direction finding packet. The first part of the packet is a normal BLE packet, but then after the CRC, the transmitter continues transmitting the current bit it is on, which is a CW tone. It holds this tone for a duration configured by the receiver. The duration of the tone, according to the the details in the datasheet, consist of a guard time, and several switch slots, in that order. It is during the guard time that the receiver calibrates and locks its PLL, after which, disables the PLL feedback so the LO's phase can't change once we route another RF path from another antenna into the receiver - this is what is happening during the switch slots. The various RF paths from the different antennas are fed into the receiver, and the output of the IQ modulator will be indeed output phases equal to arctan(Q/I) relative to the antenna that was activated during the guard period. 

Somebody please correct me if I am wrong.

Yes, I agree that this is not the cutting edge MUSIC algorithm. But I have yet to try their implementation to give it a thumbs up or thumbs down as to whether or not it can be used practically.