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Calculate load capacitors with Flex-Rigid 4 layers PCB

Patryk Lamprecht

What value should I use to calculate load capacitors with 4 layers?

Using formula:
C1 = C2 = 2CL - CPCB - CPIN

I found that for nRF52832 I should take that CPCB + CPIN is approximately 4 pF but it was for 2 layers, 1.5mm, FR4 PCB.

I have 0.8mm, 4 layers, FR4 + polyamide (Flex-rigid PCB).

Is some formula to calculate approximately CPCB + CPIN value? 

What value should I take?

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  • 0

    Hi,

     

    Cpcb is usually between 0 and 1 pF. You need to measure it to make sure that you've gotten it correct, so you might just as well tune it as the same time as you're measuring it. This is measured by transmitting a TX carrier on a given channel and measuring the center frequency with a spectrum analyzer. The crystal offset will then be proportional with the RF frequency offset, resoldering larger/smaller C1 and C2 will then lower/increase the TX frequency.

     

    Best regards,
    Andreas

Reply
  • 0

    Hi,

     

    Cpcb is usually between 0 and 1 pF. You need to measure it to make sure that you've gotten it correct, so you might just as well tune it as the same time as you're measuring it. This is measured by transmitting a TX carrier on a given channel and measuring the center frequency with a spectrum analyzer. The crystal offset will then be proportional with the RF frequency offset, resoldering larger/smaller C1 and C2 will then lower/increase the TX frequency.

     

    Best regards,
    Andreas

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