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AFE58JD48: VCAT setting cause interference on CW mode receive signal

Nate Wang
Prodigy 170 points
Part Number: AFE58JD48

Hi Sir,

in the figure below, blue arrow indicates the signal path of CW mode, while the red rectangle indicates the VCAT stage

to my understanding, VCAT is not within the signal path of CW mode, however, I found the setting of VCAT (ie, the TGC(time-gain compensation) setting ) causes interference on the receive signal of CW mode.

so before performing CW mode operation, in our current design, we need to set all the VCAT setting along image depth to the highest gain(or the lowest gain, the point is not to change VCAT gain during CW mode operation) to ensure no interference on CW mode signal, it took us a lot of time to discover this and is really strange to me.

On the above figure, I do notice that there is an arrow connected from VCAT to the CW mixer, but I do not understand why CW mixer needs the output from VCAT ? could someone give me an explanation about this ?

Thanks.

  • 0
    TI__Expert 5915 points

    Hi,

    VCAT is not within the signal path of CW mode and the arrow in the image is incorrect . We typically recommend customers to power down the VCAT and PGA block (PDWN_VCA_PGA) to save power .

    VCAT is at the output of LNA and when you change the attenuation, LNA output load changes . This usually does not degrade the performance . Some change in performance is expected . What is the interference you are seeing in the system when in CW mode ? Can you explain in detail ?

  • 0 in reply to Sachin H K
    Prodigy 170 points

    Hi Sir,

    Below is the CW mode data acquired by ADC(I or Q channel), the interference is indicated with a green circle

    In our CW mode design, we acquire 4000 samples per time period, and within each time period we adjust TGC gain(along the image depth) once, the timing of interference matches the timing of adjusting TGC gain, and if we set all the TGC gain to zero(TGC gain is not required in CW mode) the interference is gone.

    We just tried to power down VCA/PGA through register "PDWN_VCA_PGA" in CW mode, and the interference also can be eliminated by this way, thanks for your support.