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AWR2243: AWR2243

Vladimir Kolinko1
Prodigy 240 points
Part Number: AWR2243

Please provide more details regarding the need for frequency calibration when using cascaded radar designs.

1) TI user guides suggest performing frequency calibration of the multi-chip radar systems due to imperfectly consistent transmit frequency and the frequency chirp slope.  The correction is to be applied to the time domain/ADC data prior to phase calibration and beam forming. The correction equation requires experimental evaluation of the frequency and chirp rate variations, possibly separate for different MMW operation frequencies.

2) It is not clear what can be the origin of the TX frequency difference and the chirp rate since all cascaded chips are driven by 20GHz master chirps and x4 multipliers.  Is it because the MMW frequencies are not simple harmonics and there are some frequency control loops involved in each chip?  That would explain some frequency difference but not the slope, although it can slightly vary over duration of the chirp. Please clarify, since e.g TI Signal Processing User Guide_4chipCascade doc Fig.14 shows experimental data confirming that frequency variations are real.

3) Please describe some details regarding frequency variation measurements among VR channels.  Does it require transmission of single tones and external MMW spectrum analyzer?  The above mentioned user guide shows FFT in the range domain, but that would require transmitting chirping signals and is related to the chirp rate rather than a single frequency as suggested by the Fig.14 description. In addition, some variations in the peak location could be attributed to the range difference between target and the VR channel location if the target is too close. Please clarify. A tutorial would be helpful.

4) For the same reason as above it is not clear how the chirp rate can be different among different chips. An explanation and a guide how to estimate the rate variations would be helpful.

5) It also appears that TI team recommends to generate multiple calibration matrices for different beam angles in order to improve beam quality. Is it the case?

Thank you in advance.

  • 0
    TI__Guru 69960 points

    Hi,

    Let me forward this to right expert to assist you with above queries.

    Please wait for 2 days for the response.

    .

    Regards,

    Jitendra

  • 0 in reply to Jitendra Gupta
    Prodigy 240 points

    Hi Jitendra,  I have not heard yet from TI team regarding my AWR2243 Cascade frequency calibration questions posted above. Please remind them since it is important to understand why this calibration is required and get suggestions on how to collect test data and implement this if indeed necessary.

  • 0 in reply to Vladimir Kolinko1
    TI__Mastermind 20605 points

    Hi Vladimir,

    Appreciate your patience! Please find my responses in blue:

    1) TI user guides suggest performing frequency calibration of the multi-chip radar systems due to imperfectly consistent transmit frequency and the frequency chirp slope.  The correction is to be applied to the time domain/ADC data prior to phase calibration and beam forming. The correction equation requires experimental evaluation of the frequency and chirp rate variations, possibly separate for different MMW operation frequencies.
    Yes. Frequency calibration will be required in a cascade system.

    2) It is not clear what can be the origin of the TX frequency difference and the chirp rate since all cascaded chips are driven by 20GHz master chirps and x4 multipliers.  Is it because the MMW frequencies are not simple harmonics and there are some frequency control loops involved in each chip?  That would explain some frequency difference but not the slope, although it can slightly vary over duration of the chirp. Please clarify, since e.g TI Signal Processing User Guide_4chipCascade doc Fig.14 shows experimental data confirming that frequency variations are real.
    Here are reasons why there could be a change in Tx frequency:

    • Change in process between chips.
    • Difference in temperature between the chips
    • Over time wear and tear of the chip.

    All these factors contribute to a slight difference in the Tx frequency out and hence, a calibration is a must.

    3) Please describe some details regarding frequency variation measurements among VR channels.  Does it require transmission of single tones and external MMW spectrum analyzer?  The above-mentioned user guide shows FFT in the range domain, but that would require transmitting chirping signals and is related to the chirp rate rather than a single frequency as suggested by the Fig.14 description. In addition, some variations in the peak location could be attributed to the range difference between target and the VR channel location if the target is too close. Please clarify. A tutorial would be helpful.

    4) For the same reason as above, it is not clear how the chirp rate can be different among different chips. An explanation and a guide how to estimate the rate variations would be helpful.

    For questions 3) and 4), please refer to the following document:
    Cascade Coherency and Phase Shifter Calibration Application Note

    5) It also appears that TI team recommends generating multiple calibration matrices for different beam angles in order to improve beam quality. Is it the case?
    It would make sense as there typically would be angle dependent phase deviations and calibration matrices for each beam angle could improve the overall accuracy as per my understanding. If possible, can you please point me to the literature that refers to this?

    Regards,

    Kaushik

  • 0 in reply to Kaushik Gowda
    Prodigy 240 points

    Hi Kaushik,

    I appreciate your comments but I don't see how one time calibration can help maintain frequency consistency of the cascade system.  Since drift takes place over time it does not matter if the frequency is similar at some moment, it will change anyway. So what is the point of calibration?

    In my questions I have argued that all cascade chips are driven by the same 20GHz master source and then frequency multiplied by a factor of four which should be exact number.  As a result, all chips should have the same ramp rate. The only possible difference is non-matching delay in the chips, feedlines of the 20GHz signals, MMW lines connecting to the antennas and signal propagation in the antennas. But all of these can be calibrated out by the phase correction which is important calibration step before beam forming. 

    The difference in the chirp starting frequency due to latency should only affect phase as long the chirp rate is the same in all chips.

    I initially though that what TI referred to as frequency calibration was intended to equalize group delay in the channels rather than phase, but it is not how it is done according to the TI guide where a linear phase shift applied to the time domain data in order to move the signal frequency.

    Our radars require maximum sensitivity due to long operation range and very small RCS of the targets.  We observe small chirp to chirp phase variations which causes issues with keeping very low clutter level after background subtraction. Apparently, we operate near best possible performance of the chips but if there is a way to improve TX/Rx signal consistency over several seconds signal integration time it could be very helpful. Any ideas?

    Thank you.

  • 0 in reply to Vladimir Kolinko1
    TI__Mastermind 20605 points

    Hi Vladimir,

    Vladimir Kolinko1 said:
    So what is the point of calibration?

    Calibration is meant to address the constant delta that needs to be corrected and also takes care of the correction with respect to temperature variation form the LUT. As far as drift over time is considered, we have performed reliability tests on these and the impact should not be very significant.

    Vladimir Kolinko1 said:
    We observe small chirp to chirp phase variations which causes issues with keeping very low clutter level after background subtraction.

    The phase difference you see from chirp to chirp is also going to be within similar limits for a given sample or system. Hence, this is something that can be addressed by calibration. Do you have any plots regarding the impact this phase variation in your doppler spectrum?

    Regards,

    Kaushik