Problem with I and Q Samples During the Reference Period of CTE Packets

Question

Hi 
 
 As described in this white paper: https://infocenter.nordicsemi.com/pdf/nwp_036.pdf during the reference period of CTE signals, 8 samples are taken. These samples contain both "I" and "Q" elements of signal which has different phases relatively. 
 I have captured the reported "I" and "Q" samples using the samples provided by SDK: 
 
 https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/bluetooth/direction_finding_connectionless_tx/README.html 
 https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/bluetooth/direction_finding_connectionless_rx/README.html 
 
 For the reference period, I can not see any phase difference between "I" and "Q" elements (while it should be different). I have visualized the data and you can see the result in the below images: 
 IQ samples with 2us switching slots and using PCB antenna: 
 
 IQ samples with 1us switching slots and using PCB antenna: 
 
 IQ samples with 1us switching slots and using array antenna:

As you can see, in all conditions, the reference period has the similar behavior. I have plotted a third party product which uses the CTE packets and you can see the visualized data here, 
 
 Obviously the "I" and "Q" samples are in different phases during the reference period. What is wrong in the above nRF examples? 
 
 Regards, 
 Saleh

Piotr · Accepted Answer

Hello, 
 I've taken a look into the issue. I've made some small custom change to enable oversampling of received CTE. Samples spacing I've used is 125ns instead of 1us that is BT Core spec ref. period spacing. 
 Pay attention that samples were collected before preparation to transport through HCI layer. They have different amplitude than what you would observe in application layer. 
 Have a look on the plot below. 
 
 I and Q lines are samples collected every 125ns. Selected I and selected Q are samples you would observe in application (selected every 1us) 
 As you can clearly see, the selected I and Q look like there is no PI/2 phase shift between actual I and Q. In reality the shift is there but undersampling makes is not visible.

AllenWang · Answer

Hello, Saleh: 
 have you got this question resolved. I met the same issue as you. hope get kindly guidance from you 
 
 thanks and best wishes

Problem with I and Q Samples During the Reference Period of CTE Packets

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