nrf52832 impedance matching with VNA

[bstarbuck3]

Hello,

After reading many things, I would like someone to confirm or correct my understanding.  I believe this will be great clarity for others as well.

The nrf52 requires matching components right after the antenna pin (a capacitor and inductor).

Q1) Should this be 0.8 pF and 3.9 nH or 1.0 pF and 3.3 nH?

After this, a 50 ohm rf trace with a correctly computed width follows up to my chip antenna.

Q2) At the end of this trace and just before the antenna, should be a pi matching circuit?

Finally, I need to evaluate this design with a VNA for impedance matching.

Q3) Where exactly do I measure with the VNA?  Just after the nrf52 pin but before the nrf52 capacitor and inductor matching components?  Or just after the rf trace but before the pi matching?  Or both?

The PCB and traces are quite small and I don't see how I could possibly measure this. 

Q4) Should I make another PCB just for measuring this consisting just of everything coming out of the antenna pin on the nrf52? Or just everything from pi matching and antenna.

Check my diagram to better understand my questions.  Thank you!

Any clear answers and pictures would be greatly appreciated.

[Jared]

Hi,

Q1) Should this be 0.8 pF and 3.9 nH or 1.0 pF and 3.3 nH?

It depends on the package. For QFN it's the former, for WLCSP it's the latter.

Q2) At the end of this trace and just before the antenna, should be a pi matching circuit?

The kind of the matching network depends on the antenna and what the antenna producers recommend. But in most cases we suggest a Pi network. Yes, the placement is correct.

Finally, I need to evaluate this design with a VNA for impedance matching.

Q3) Where exactly do I measure with the VNA?  Just after the nrf52 pin but before the nrf52 capacitor and inductor matching components?  Or just after the rf trace but before the pi matching?  Or both?

The PCB and traces are quite small and I don't see how I could possibly measure this. 

See this thread. 

Q4) Should I make another PCB just for measuring this consisting just of everything coming out of the antenna pin on the nrf52? Or just everything from pi matching and antenna.

I didn't quite understand this question. The PCB that you're tuning should be the final prototype that contains everything that is going to be included on the board in addition to the enclosure.

best regards

Jared 

[bstarbuck3]

Hi Jared, thank you for your quick response.  I will restate Q3 and Q4 more concisely.  On the final prototype PCB, to verify and measure the impedance, do I solder the VNA probe at the point that I want to know the impedance and then also solder the other end to the PCB ground plane?

[Jared]

Yes that should be correct.