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IWR1642: Two high-pass filters

c64
Intellectual 480 points
Part Number: IWR1642

Hi,

This is probably a stupid question. Figure 6-3 on page 63 in the IWR1642 data sheet depicts the receiver subsystem, where a high-pass filter is followed by a buffer/amp and a low-pass filter before being sampled by the ADC. Then why are the filters associated with the two cutoff frequences you can choose both called high-pass filters? 

Thanks!

  • 0
    TI__Mastermind 25835 points
    Hello,

    The first HPF filter helps in improving the isolation between the Tx and Rx as the beat frequency would fall into close to DC, Hence it attenuates and helps in improving the linearity of the receiver. The low pass filter band limit the IF frequency to ~5 MHz improving the noise figure before digitizing it by the ADC. ADC also has the programmable digital low pass filter which improves transition band.

    For detail you could refer to "Using a complex-baseband architecture in FMCW radar systems"
    www.ti.com/.../spyy007.pdf

    Thanks and regards,
    CHETHAN KUMAR Y.B.
  • 0 in reply to CHETHAN KUMAR Y.B.
    Intellectual 480 points
    Hi,

    Well yes, that makes perfect sense. I was more wondering about the naming that TI does. For example on page 36 in the IWR1642 spec sheet it says in the footnote " ... with two independently configurable first-order high-pass corner frequencies", and then they're named HPF1 and HPF2. Why aren't they called HPF and LPF instead, for example? I just haven't seen it that way before, since a bandpass filter consists of an HPF part and an LPF part. It's just a curiosity thing.
  • 0 in reply to c64
    TI__Mastermind 25835 points
    Hello,

    Oh, ok, Understood. HPF itself has two stages HPF1 and HPF2 apart from the LPF. HPF is implemented in Analog domain, hence from the single stage it's difficult to get the sufficient attenuation between the Tx and Rx which falls very close to DC, Hence to get sufficient isolation two stages of HPF filters are cascaded with customer programmable filter options. Hence you would notice there are two HPF filters and a LPF filter on every receiver chain. Apologies if this naming convention causes some confusion.

    Thanks and regards,
    CHETHAN KUMAR Y.B.
  • 0 in reply to CHETHAN KUMAR Y.B.
    Intellectual 480 points

    Ah I see, thank you very much for clarifying. While on the note, could you please direct me to where I can find information about the low-pass filter? I seem unable to find it.

    Thanks!

  • 0 in reply to c64
    TI__Mastermind 25835 points
    Hello,

    You could refer to below E2E response for the low-pass filter.
    e2e.ti.com/.../2745708

    Thanks and regards,
    CHETHAN KUMAR Y.B.
  • 0 in reply to CHETHAN KUMAR Y.B.
    Intellectual 480 points

    Thanks! That was very helpful.