Is the frequency difference between transmitter and receiver compensated?

RE: Is the frequency difference between transmitter and receiver compensated?

Pete W — Tue, 13 Dec 2022 13:29:12 GMT

If frequency offset is ignored, i.e. you assume the I & Q are exactly 250 kHz (or 500 kHz for 2M PHY), then the measured phase difference will be greater or less than the actual value.

RE: Is the frequency difference between transmitter and receiver compensated?

Kazi Afroza Sultana — Mon, 12 Dec 2022 12:36:16 GMT

Hello,

Frequency does not matter, the phase difference between the different antennas is taken into account. The phase difference is measured. Based on the one reference antenna, you have to measure the phase relative to that.

You can read this blog post https://blog.nordicsemi.com/getconnected/finding-your-way-with-bluetooth and the white paper https://infocenter.nordicsemi.com/index.jsp?topic=%2Fnwp_036%2FWP%2Fnwp_036%2Fintro.html to get details concept. There are two NCS samples Bluetooth: Direction finding connectionless locator — nRF Connect SDK 2.2.99 documentation (nordicsemi.com), Bluetooth: Direction finding connectionless beacon — nRF Connect SDK 2.2.99 documentation (nordicsemi.com) you can look at.

Thanks.

Best Regards,

Kazi Afroza Sultana

RE: Is the frequency difference between transmitter and receiver compensated?

Pete W — Mon, 12 Dec 2022 10:52:04 GMT

No, you will need to allow for frequency errors on your Tx and Rx. In the ideal case the CTE would appear in your Rx as a 250 kHz tone (at least for a 1M PHY), since it is generated by a run of 1's on the carrier, but if your Tx and Rx have 10 ppm crystals, for example, this could be as low as ~200 kHz or as high as 300 kHz (the limits with 10 ppm at 2.440 GHz). This is why you need to use the leading part of the CTE to estimate the rate of change of phase (i.e the frequency) before you can use it to measure the phase difference between the slots associated with your antennas. So you measure it and then use it to predict where the phase should be in the following slots.

Hope that helps.

Pete