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AoA: IQ samples sampling time and their correction

Ales

Hi!

There is two things confusing to me about taking IQ samples in AoA application.

1. It is clear to me that that the time between switching of antennae (TSWITCHSPACING) is dependent of sample slot and switch slot which can be 1 us or 2 us. So if we take sample slot of 1 us for example, then my understanding is that I will get one IQ value for every 2 us. That means I will have 8 IQ samples for reference period and then one IQ sample every 2 us in the interval after reference period. But after reading a couple of problems related to AoA on this forum by other engineers I saw that they are getting far more IQ samples (For example in https://devzone.nordicsemi.com/f/nordic-q-a/59322/how-to-get-right-digree-nrf52811-direction-finding/240869#240869.  They have 125 ns time period of each IQ sample). So my question is: are IQ samples taken continuously in sample slot and after the switch happen and saved to RAM (meaning more than one IQ sample in one sample period) or is taken exactly one IQ sample in one sample period and saved to RAM (so I have as many IQ samples in RAM as there are sample slots in entire CTE + samples taken in reference period)? Also this question and answers: https://devzone.nordicsemi.com/f/nordic-q-a/55626/dfe-iq-sampling-and-antenna-switching-slots-confusion contradict my understanding completely. Is sampling time independent of duration of sample slot?

2. Second question is related comparing phase information of IQ samples between antennae. For phase comparing between antennae I need phase information for each antenna in exactly the same time taken. Since one IQ sample in CTE is delayed from previous IQ sample depending of the size of sample slot and switch slot (2 us or 4 us) then I don't get samples taken at exactly the same time. So do I need to do the correction for each sample taken (except the reference one) to make it appear as they are taken at the same time (
from taken phase subtract a phase difference that RF wave would make in 2 us or 4 us period of samples taken) or is this correction done beforehand and values read from RAM appear as they would be taken at the same time?

Hope my questions are understood. Any help would be appreciated.

  • 0

    Hello, 

    My apologies for the late reply. 

    This should be covered by the nWP036 - Direction Finding white paper. Please point out what is unclear in the document, then I will need to discuss with our AoA experts.

    Thank you.

    Kind regards,
    Øyvind

  • 0 in reply to Øyvind

    I think nWP036 is pretty good enough for Q.1.

    But I guess we need more information for Q.2.

    Ales said:
    2. Second question is related comparing phase information of IQ samples between antennae. For phase comparing between antennae I need phase information for each antenna in exactly the same time taken. Since one IQ sample in CTE is delayed from previous IQ sample depending of the size of sample slot and switch slot (2 us or 4 us) then I don't get samples taken at exactly the same time. So do I need to do the correction for each sample taken (except the reference one) to make it appear as they are taken at the same time (
    from taken phase subtract a phase difference that RF wave would make in 2 us or 4 us period of samples taken) or is this correction done beforehand and values read from RAM appear as they would be taken at the same time?

    I assume RADIO peripheral output is not corrected data.

    According to ref. sample data that I had, it needs to kind of data correction process for phase shifting by time domain.

    I hope someone gives us more easy description.

  • 0 in reply to Jason Lee
    Jason Lee said:
    According to ref. sample data that I had, it needs to kind of data correction process for phase shifting by time domain.

    What ref. sample are you pointing to? 

    The IQ samples should be transmitted at the same interval for the entire sampling period, and the locator board should set antenna switching accordingly to switch between IQ. samples

    Kind regards,
    Øyvind

  • 0 in reply to Øyvind

    I pointed ref. period samples.

    And sorry for my expression. I guess it's bad choice of words.

    Jason Lee said:
    I assume RADIO peripheral output is not corrected data.

    I'm not sure IQ sample data is perfect or not. I guess it's pretty accurate.

     

    I just expect same IQ sample value in same sample point. (same ant. point)

    But it's not because time is keep changing.

    (In ref. period, every data not exactly same value.)

    But Phase diff. is pretty regular. I guess it's because sample time interval is regular.

     

    So, I think, I need to correcting phase after ref. period for calculate phase diff. of ANT. pairs.

     

    If I'm wrong, plz, tell me. (I'm not sure about this idea.)

     

    Thank you for your support. Slight smile

  • +1 in reply to Øyvind

    Hi Jason and Ales.

    Your assumption is correct. Radio peripheral takse samples one after another for antennas configured by antenna switching pattern. To evaluate angles according to method described in whitepaper nWP036 you have to apply correction to received IQ samples. You have to correct phase by angle related with the delay between first and particular sample. 

    So e.g. samples spacing is 2us

    - sample 1 -> 0 correction

    - sample 2 -> 2us correction

    - samples 3 -> 4us correction etc.

    Then all IQ samples should have values as if they were taken in the same instant.

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