2
\$\begingroup\$

I am new to designing for RF and have a question regarding the design of the matching network for a GPS module I am working on.

The datasheet of the chip antenna ANT161575ST-1202A1 recommends a frequency tuning inductor of 1.2nH

enter image description here

and the application notes recommend a range from 0.5nH to 4.3nH, with a sweet spot seeming to be around 2nH.

enter image description here

However, the TESEO design guide recommends a 5.6nH inductor, and also recommends the two 120pF capacitors, which I have circled in red below. I assume these are for blocking DC signals.

enter image description here

I don't know how to calculate these values myself and so I am at the mercy of two competing datasheet recommendations with no idea what is best. Any help you can give to make a good decision here would be much appreciated.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Please clarify: are you using the LNA and SAW filter in your design? Or are you connecting directly from the chip antenna to the GPS module? If you're connecting directly, you don't need DC blocking, and the inductor recommendations given with the ANT16157 are appropriate. \$\endgroup\$
    – Mark Leavitt
    Apr 21, 2022 at 16:49
  • 1
    \$\begingroup\$ Yes I am using the LNA and SAW filter recommended on the schematic provided in the TESEO design guide, because I am new to this and have no idea what is required or not. I have basically copied the circuit from this schematic, and am just left with the question of which inductor value to use. \$\endgroup\$
    – humanistscience
    Apr 21, 2022 at 17:55

1 Answer 1

Reset to default
3
\$\begingroup\$

Since you're using the LNA and SAW filter, you need to include the series capacitors, but they don't play a role in impedance matching. Why? The reactance of a 120 pf capacitor at 1.5 GHz is less than 1 ohm, not of significance in a 50 ohm antenna/transmission line system.

The inductor should be the one recommended in the chip manufacturer's application note. Its purpose is to cancel out the capacitive reactance of the antenna, at frequencies lower than the resonant frequency of the antenna. Unless you have a VNA and can select a value experimentally once your board is built, you're best off choosing the mid-range value of 2 nH. Your actual VSWR may not be zero, but for a receiver circuit you'll see very little degradation in performance. Minimizing VSWR is more important in a transmitting circuit, where high reflected power could damage a power amplifier.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you. I really appreciate all the help you have given me on these RF questions. I have asked another question here about isolating the RF GND. If you could leave your thoughts on the matter I would really appreciate it. electronics.stackexchange.com/questions/616754/… \$\endgroup\$
    – humanistscience
    Apr 21, 2022 at 19:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.