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RTOS/AWR1642BOOST: What defines my sample rate?

Samuel Miller99
Expert 1005 points
Part Number: AWR1642BOOST

Tool/software: TI-RTOS

I am trying to configure my own chirp profile, but I am confused on some of the parameters. I am able to configure the number of ADC samples per chirp with this parameter:

rlUInt16_t  numAdcSamples
 

Number of ADC samples to capture in a chirp for each RX 
Valid range: 64 to MAX_NUM_SAMPLES
Where MAX_NUM_SAMPLES is such that all the enabled RX channels' data fits 
into 16 kB memory, with each sample consuming 2 bytes for real ADC output 
case and 4 bytes for complex 1x and complex 2x ADC output cases 
For example:
4 RX, Complex ADC output -> MAX_NUM_SAMPLES = 1024
4 RX, Real ADC output -> MAX_NUM_SAMPLES = 2048
2 RX, Complex ADC output -> MAX_NUM_SAMPLES = 2048
2 RX, Real ADC output -> MAX_NUM_SAMPLES = 4096 

But I'm also able to alter the sample rate and ADC sampling time with the following three variables:

rlUInt16_t  digOutSampleRate
  ADC Sampling rate for each profile is encoded in 
2 bytes (16 bit unsigned number)
1 LSB = 1 ksps 
Valid range 2000 to 37500 
rlUInt32_t  rampEndTime
  End of ramp time relative to the knee of the ramp
1 LSB = 10 ns
Valid range: 0 to 500000 
rlUInt32_t  adcStartTimeConst
  Time of starting of ADC capture relative to the knee of the ramp
1 LSB = 10 ns 
Valid range: 0 to 4095

If my adcStartTimeConst is 0 and my rampEndTime is 10 then that means there is 10 nanoseconds of sample time. If my sample rate is 8000 kHz then I should have 80 samples per chirp. What happens if, in this case, I set my ADC samples per chirp to 100? Would the ADC sample 80 samples per chirp in accordance with its programmed frequency, or would it increase its frequency to fulfill the maximum number of ADC chirps that I specified - 100?

  • 0
    TI__Mastermind 21295 points
    Hello Samuel,

    The rampEndTime corresponds to the length of the transmitted frequency ramp. It does not control the receiver/ADC. The receiver/ADC sampling is controlled by the other parameters you listed which define the ADC start time, sampling rate, and number of samples.

    You can look at the following application note to find more details about chirp programming.
    www.ti.com/.../swra553.pdf

    There is also a figure and some chirp description in the mmWave Sensing Estimator tool.
    dev.ti.com/mmWaveSensingEstimator

    Please reply if you have more questions or mark the thread as answered if your question is resolved.

    Regards,
    John
  • 0 in reply to JohnH
    Expert 1005 points
    I agree that the rampEndTime corresponds to the length of the transmitted frequency ramp, but this affects the length (in time) of the chirp that is received. If I define an ADC sample rate that is too slow and incapable of sampling my desired number of ADC samples per chirp, then which of my definitions does the ADC honor? Does it honor my sample rate, which means less ADC samples than desired, OR does it honor my specified number of ADC samples per chirp, which means that it ignores my sample rate specification (because it would need to sample faster in order to fulfill my # of ADC sample requirement)

    I tried my best to describe the problem thoroughly, but let me know if you are not on the same page. Thanks for your help!
  • 0 in reply to Samuel Miller99
    TI__Mastermind 37500 points

    Example_RCS_LinkBudgetCalc_a.xlsxHello,

    If you use the FMCW chirp equations to determine the target distance, we detect 2x target distance.

    Time of flight is 3x10^8meters/sec.   The Synthesizer slope, and DFE out rate define the IF frequency proportional to your distance.

    As John described the Sensing Estimator tool is used to compute this from you system requirements.  SWRA553 also describes the radar equation and some required settings.  

    Your system questions, what DFE out rate, what Synthesizer slope, what ADC samples per chirp are tied together.  

    a) Start with using Sensing Estimator

    b) take the Sensing Estimator settings and put them into the attached spreadsheet for learning what IF frequency you can measure.

       <= .9* DFE outrate

       >= 200khz Frequency for HPF1, HPF2 filtering

    c) if you save the OOB demo configuration, you can override the FMCW settings with the spreadsheet, or sensing estimator - however the idle

    time needs to match the OOB demo to allow sending UART data over USB

    d) there are other TIDEP designs, and Industrial Toolbox labs, experiments that can help guide your engineering

    Regards,

    Joe Quintal