Clarification on 14-bit POA Conversion and Discrepancy from First Path Phase in PDOA Mode 3

jinew · May 15, 2025, 3:58pm

I read your post, and based on that, I added logic to check the sign using the 14th bit (MSB) and apply appropriate adjustments. I also included the division by (2**11) as mentioned.

After that, I calculated the phase of the sample at the First Path Index (FP_INDEX) and compared it, only to find that there’s a noticeable discrepancy. I also computed the PDOA between STS_1 and STS_2 and observed that the result differed somewhat from the (PDOA / 2**11) value provided in the diagnostics.

I suspect this is because FP_INDEX refers to samples at 1 ns resolution, rather than the finer 1/64 ns resolution. As a result, the derived phase may be slightly inaccurate. I believe that if we compensate for the fractional part of the FP_INDEX as a phase shift, the result might align more closely with the diagnostics value. What do you think about this approach?

This is my poa_uint16_to_radian function in my python project:

def poa_uint16_to_radian(poa_uint16):
    """
    Convert a 14-bit signed POA value (stored in uint16) to radians.
    POA format: signed 14-bit fixed-point (rad × 2^11)

    Parameters:
        poa_uint16 (int): Raw POA value (0–16383 range assumed)

    Returns:
        float: Phase in radians (range: -π to +π)
    """

    # 1. Extract 14-bit value
    poa_14bit = poa_uint16 & 0x3FFF

    # 2. Convert to signed 14-bit integer
    if poa_14bit >= 8192:
        poa_signed = poa_14bit - 16384  # Two's complement for negative values
    else:
        poa_signed = poa_14bit

    # 3. Convert fixed-point value to radians
    phase_rad = poa_signed / 2048.0  # Since value is in rad × 2^11

    return phase_rad

And this is the result of two samples.
Sample 1:

[sts_1_fp_sample, sts_2_fp_sample]
2.826986177688784 2.997290328233547
[diag_stsPOA, diag_sts2POA]
2.501953125 2.666015625

[sts_1_fp_sample - sts_2_fp_sample]
-0.1703041505447631
[diag_stsPOA - diag_sts2POA]
-0.1640625

[diag_pdoa]
-0.18603515625

Sample 2:

[sts_1_fp_sample, sts_2_fp_sample]
-1.3774794636095957 -1.5226034746003427
[diag_stsPOA, diag_sts2POA]
0.705078125 0.5537109375

[sts_1_fp_sample - sts_2_fp_sample]
0.145124010990747
[diag_stsPOA - diag_sts2POA]
0.1513671875

[diag_pdoa]
0.17333984375

I’m conducting the measurements in a clear LOS environment with a 50 cm distance, but the values still vary each time. What do you think could be the reason for this?

Additionally, the results of 1000 samples’ PDOA is:

Thanks!