What's the purpose of this capacitor here? I certainly don't think it's for decoupling but maybe bypassing? I presume we don't want any of the amplified RF which might have sneaked by the inductor getting back into the GNSS module.

  • Antenna is tuned for GPS / GLONASS at 1590 MHz.
  • Maximum input power at RF_IN will be 15dBm.
  • VCC_RF typical current rating from datasheet is 50 mA (Max 100 mA). - I dont have specifications from Active antenna yet.
  • VCC_RF typical is about 3.2 volts for this application.
  • The inductor I'm using is the recommended (in hardware integration manual provided by supplier) 27nH (LQG15HS27NJ02) with impedance at GPS freq > 500 ohms.
  • There is no recommended value for the capacitor however.

*Note: The recommended 10 ohm R bias resistor is relatively beefy at 1/4 watt.

What's the best way to determine it's capacitance / parameters?

enter image description here

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    \$\begingroup\$ Looks like a filter cap... L and C are a low pass for the power going into the active antenna. \$\endgroup\$ – George Herold Mar 15 '16 at 14:59
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    \$\begingroup\$ "The inductor I'm using is the recommended 27nH" recommended where? If that comes from a datasheet, then it would be good to link to it. The capacitor is there to form a low pass filter together with the inductor, and it is there to keep RF away from Vcc. \$\endgroup\$ – JRE Mar 15 '16 at 14:59
  • \$\begingroup\$ @JRE. Thank you. I updated question indicating source of recommended inductor. \$\endgroup\$ – Samee87 Mar 15 '16 at 15:04

The block on the right looks like it might need a dc bias on the RF input to make it work. This dc bias ultimately comes from the VCX pin via FB (ferrite bead) and the first decoupling capacitor then thru a 10 ohm resistor and the 2nd decoupler that you have highlighted.

Given the frequencies you are using (and my suspicion that it's a decoupler) then anything from 100 pf to 10n is probably going to work but, for the higher end of the range check that the self resonant frequency of the capacitor isn't so low that it might cause a resonance problem at 1590 MHz.

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    \$\begingroup\$ It's more likely powering the antenna than biassing RF In. But I think you're rspot on to warn about the self-resonant frequency. \$\endgroup\$ – Brian Drummond Mar 15 '16 at 15:04
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    \$\begingroup\$ @BrianDrummond aha I see what you mean!! \$\endgroup\$ – Andy aka Mar 15 '16 at 15:05
  • \$\begingroup\$ @Andy I agree with you but this GNSS receiver has an internal DC block on RF_IN that's not listed in this diagram. \$\endgroup\$ – Samee87 Mar 15 '16 at 15:05
  • \$\begingroup\$ OK so as per what Brian says it's just for phantom powering the active antenna. \$\endgroup\$ – Andy aka Mar 15 '16 at 15:07
  • \$\begingroup\$ X5R 1N 10V - straight from ublox's hardware integration manual (which is where the diagram in this question is from). u-blox.com/sites/default/files/products/documents/… \$\endgroup\$ – Jon Mar 15 '16 at 15:13

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