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PGA460: Strange triggering behavior near SR to LR gain transition

Karl Grueschow
Prodigy 90 points
Part Number: PGA460

Hello,

I've been noticing some strange behavior with respect to object detection when that object is near the transition from SR to LR gains. In the pic below, you can see that there is definitely an object detected by the echo data dump at about 0.6m, but the system is only recording measurement to the second peak at 1.160m and completely ignoring the object at 0.6m. The object at 0.6m is a soft, round object of 6cm cross section. The distances do not change if I disable the echo dump in the GUI and just pull the UMR. The "object" at 1.16m is presumably the base of the vertical target at 0.6m. Lowering the threshold in that region has no effect. I assume this has something to do with datasheet 7.3.4.5, but it seems it should be more likely to detect that object with my gain settings (SR = 2, LR = 8) given there was no manual adjustment made to thresholds at that point. Or is the adjustment shown in figure 9 applied internally? 

I've noticed this a lot- where the echo dump will either not set a UMR where it the GUI shows it crossing a threshold, or it will set a UMR for somewhere the GUI shows the echo data nowhere near crossing the threshold. 

Any thoughts? Thanks,

Karl

  • 0
    TI__Mastermind 27950 points
    Hi Karl,

    A few questions:
    1) Is the 0.6m echo ignored even when both the SR and LR digital gains are the same?
    2) What is the value of your Threshold Deglitch Time on the General page's Receive block? If it is not 0us, can you set it to 0us, and retry the 0.6m measurement?
    3) Is the 0.6m echo consistently ignored, or is it occasionally captured when the burst/listen command is looped?

    As you've pointed out in section "7.3.4.5 Digital Gain" of the datasheet, when the LR gain is applied starting from thr point 9, threshold point 8 is automatically scaled internally. Excerpt for datasheet Figure 4:
    "If the LR gain is different than the SR gain at threshold level point 8, the threshold level 8 is multiplied by the ratio between the LR gain and SR gain (DIG_GAIN_LR/DIG_GAIN_SR) 1 μs after the end of the SR threshold level 9 point."

    I can add a visual to the GUI to visually show the internally scaled change to point 8's slope in the next revision update to better indicate where the threshold would be triggered. In the meanwhile, try decreasing the level of point 8 so the threshold level profile is more uniform across the noise floor.
  • 0 in reply to Akeem Whitehead
    Prodigy 90 points
    Akeem,

    1. It does seem to pick it up when I set the gains the same, though not reliably because the signal is too weak at that point.
    2. Deglitch was set to 0us
    3. It was consistently ignored, even while looping.

    It looks like it is in fact the transition to the higher gain level. Decreasing the level at point 8 does seem to help and I've gotten it to work a little better. I'll try to get it working with equal SR and LR gain values.

    Thanks.