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IWR1443BOOST: Is it possible to improve the range resolution by adjusting ADC sampling rate & Range FFT size?

Rand SL
Prodigy 50 points
Other Parts Discussed in Thread: IWR1443

According to the example given in Fluid-level sensing using 77-GHz millimeter wave white paper (www.ti.com/.../spyy004.pdf), 
the range resolution is 3.75 cm.

I need to measure liquid level to a range accuracy of ~1 mm and max range is 5 m. 

I saw following posts which suggest to increase the Range FFT size which increase the no. of frequency bins of the FFT spectrum. 

e2e.ti.com/.../612730
e2e.ti.com/.../599055

My question is, If we reduce the ADC sampling rate by factor of 2 say 5 MSPS (it decrease the frequency bin size) and does it increase the range resolution/range accuracy by a factor of 2?

Say, if we use Range FFT size 4096 and 1 MSPS ADC sampling rate, can we achieve range resolution closer to ~1 mm without using any special algorithm?

Is there any fundamental issue/ hardware limitation of doing so?

Thanks,

  • 0
    TI__Mastermind 30405 points

    Hi Rand,

    It is important to distinguish between "Range Resolution" and "Range Accuracy" as these are two different concepts. Please refer to the following post which clarifies the distinction between the two and also provides information on high accuracy measurements.

    https://e2e.ti.com/support/sensor/mmwave_sensors/f/1023/t/618907

    In short, range resolution i.e. the minimum distance needed between two or more objects to be resolved, is a fundamental physical limit and is a function of the bandwidth swept by the ramp.

    Based on the description, What you're looking for in your applications seems to "Range Accuracy of 1mm" which may very well be achievable at 5 m for a tank level sensing application. As explained in the above post, Range accuracy which depends upon Range resolution can be better than range resolution and increasing the Range FFT size is one of the requirements to achieve the desired accuracy.

    Intuitively, the FFT granularity needs to be fine enough to measure the desired range increment and so the application may need to first get an initial coarse range measurement using the provided chirp configuration and then perform a zoom FFT on the range of interest, using additional processing in software with a higher order FFT.

    We are working on a high accuracy demo to demonstrate this capability and the above mentioned two stage approach, and it will be released on TI.com.

    Regards

    -Nitin

  • 0 in reply to Nitin Sakhuja
    Prodigy 50 points

    Hi Nitin,

    Thanks a lot for the detailed explanation. 

    Does the zoom FFT require additional DSP chip? or can it be carried out with the same hardware used in level transmitter reference design ( http://www.ti.com/tool/TIDEP-0091

    Also, can we configure the level transmitter demo software to use 4096 point FFT instead of 1024 point FFT? 

    Regards,

    Rand

  • 0 in reply to Rand SL
    TI__Mastermind 30405 points
    Hi Rand,

    The Zoom FFT can be performed on the 1443 itself using the same FFT engine (Radar accelerator). In fact, we have a 4K FFT lab available on TI resource explorer which is based on the IWR1443. The link for the 4K FFT lab is provided below and it comes with a step-by-step user guide and a CCS project which you can import into your workspace directly from the Resource explorer. Please follow the instructions provided in the user guide.

    dev.ti.com/.../

    You can change the level sense demo to use 4K FFT but you'll need to consider the impact of added processing, memory requirements, code size increase etc against the power profile of the level sense demo. You can also achieve better accuracy with the Zoom FFT approach mentioned above where the application first takes a peak from the initial 3.75cm resolution FFT, and centers a second 1K FFT about that peak.

    Regards
    -Nitin