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CC430F5137: The two 2pF caps on AVCC_RF - worth installing?

John DAmours70
Prodigy 240 points
Part Number: CC430F5137

I've built many prototypes using this chip but never installed these two 2 pF caps (AVCC_RF decoupling... so small!). As required, pads are very close to the pin (opposite side of 4-layer), so they would help with something I'm sure. Consider that the entire PCB only reads a motion chip on I2C and looks at a pressure sensor on ADC, so I doubt there's much noise to start with (no LEDs or external switching). On the other hand, it's only running on a 2032, so voltage likely dips every snap of the radio. 

RF range is very important with this product, but if I only get <1% increase in range by adding these... Hmmm. These caps are on the backside giving a slight dimensional downside (cleaner if I leave off). I doubt my crude tests will show much of anything conclusive, too many variables.

Maybe I've been missing out on some nice range or increased packet quality numbers to increase probability of transmission? I can operate at ~50% loss already, so a boost would help.

Thanks!

  • 0
    TI__Guru**** 317180 points
    Decoupling is typically not to increase range but to improve signal quality. Decoupling is typically needed to avoid spurs etc. Decoupling should typically be used on the same side of the board as the pin it decouples, both to be close but also to avoid the inductance the via introduce.

    "I can operate at ~50% loss already, so a boost would help.": This indicate that you have other issues than decoupling. Go through www.ti.com/.../swra603.pdf as a start.
  • 0 in reply to TER
    Prodigy 240 points
    Good to know, (tight fit even with 402's). So my added L because of the via - not worth trying on this design then?

    Wouldn't improved quality = greater range to some degree from better noise immunity? Imagining little waves on a carrier getting distorted by noise the further they go.

    To clarify, my "50% loss" is what happens when I'm getting out of range, but still good enough for the user because of higher sampling rates is all. At ~50ft it seems lossless, but the antenna is under my foot, 5mm from skin, so not bad.
  • 0 in reply to John DAmours70
    TI__Guru**** 317180 points
    As you can see from www.ti.com/.../rf-range-estimator, range decreases when the antenna is close to ground which in effect the skin is.

    I would check the conducted output spectrum with a spectrum analyzer with and without the decoupling caps and see if you see a difference when sending CW / modulated carrier.