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IWR1642: Linearity of chirp

Rustem Sultanov
Prodigy 155 points
Part Number: IWR1642

Hi everyone!
I make the radar on the IWR1642 ICs. I program the next chirp parameters (see image#1):


Frequency start: 1425452105 (76.49546177 GHz);
Idle Time:  400 (4 usec);
TX Start Time: -400 (-4 usec);
ADC Valid Start Time: 200 (2 usec);
ADC Sampling Time: 2 usec;
Ramp End Time: 6 usec;
tsr: 0.25 usec;
BWsw: 4 MHz.


Image#2 shows a downconversion chirp frequency vs time. It is seen that in the working area saw nonlinear. My application is sensitive to this. How can a saw be made more linear? Increasing the guard intervals does not correct the situation.

  • 0
    TI__Mastermind 25835 points
    Hello,

    Is this measurement done on the IWR1642BOOST EVM or on the custom hardware?
    Please provide more details about your setup? Also could you please give details of down converter used?

    Ramp duration is very short, have you tried longer duration chirps?

    Thanks and regards,
    CHETHAN KUMAR Y.B.
  • 0 in reply to CHETHAN KUMAR Y.B.
    Prodigy 155 points

    The measurement done on the custom hardware (develop on IWR1642BOOST reference design). The radar chip works from DSX321SH-40MHz crystal. Block diagram of the experiment attached:

    I try the increase duration chirps (Ramp End Time = 9 usec) and is it helps. What is the recommended minimum value for this parameter (for slope ~2-3 MHz/usec)?

    Thanks for your reply.

  • 0 in reply to Rustem Sultanov
    Prodigy 155 points

    A small addition…

    I attach pictures of “frequency-time” dependencies for a Ramp End Time = 9 usec with a spectrum analyzer filter band 10MHz and 1MHz:

     

    As well as the dependence of "frequency-power-time" for Filter Bandwidth = 10 MHz:

     

    Do I understand correctly that the PLL shift step is ~ 100 kHz? And riple on the “frequency-time” is a PLL step-locked process?

    I would appreciate your clarification.

  • 0 in reply to Rustem Sultanov
    TI__Mastermind 25835 points
    Hello,

    Crystal used is different from the reference design, We have not evaluated the DSX321SH-40MHZ to comment on the crystal performance. In the setup if the received power level to mixer is lower than it's spec limits then you might have some issue.
    Transmitter Antenna should have sufficient gain and receiver also should have sufficient gain so that mixer should operate in linear region for producing right IF signal.

    Could you please comment on the down-conversion mixer used? and gains of Tx and Receiver antennas.

    Have you tried using TI EVM and performing same measurement? Does it also show similar behavior?


    In your Chirp configuration, Slope and Ramp time both are very short;
    Typically very small slope are used with longer chirp duration for observing farther distance objects, here range resolution is traded-off with distance.
    or Faster slope with shorter chirp cycles for observing close in objects, here you would get better range resolution for shorter distance.

    You could look for Chirp database and pick a chirp configuration that would suit close to your application.
    dev.ti.com/.../node

    or you could develop your own chirp configuration based on the sensing estimator tool.
    dev.ti.com/.../

    and use that configuration and try out of box demo example shown below.
    dev.ti.com/.../node

    From there you could try to optimize you chirp configuration for the linearity requirement for your application.

    Thanks and regards,
    CHETHAN KUMAR Y.B.