Optimum rf impedance to present to transceiver output.

[DashRiprock]

I am tying to match the nRF52840 rf output to my 50 ohm 2.45GHz pcb antenna.  What is the optimum rf impedance for best transmitter/receiver performance?  The markings on the chip are N52840 QIAAC0 1908GK. 

Thankyou,

Doug

[Martin Sivertsen]

Hello Doug,

I notice that you set the status of this case to "Open", do you need any more assistance?

I can copy some information from a previous case that asked about the output impedance of the ANT pin of the nRF52833 which has an identical radio to the nRF52840, but has a four component radio matching network instead of three:

The load impedance, seen into the matching network, should be close to 35 - j35 ohm. The impedance seen into the ANT pin doesn't change too much from 8 dBm to 0 dBm mode. This is the impedance measured into the matching network from the antenna side:

If you want to design a matching network optimal for max 0 dBm output power, you can get away with just a shunt capacitor and a series inductor. Provided you ground the shunt capacitor like in the reference layout. A 0.8 pF capacitor and 3.9 nH inductor is a good starting point. Measure the impedance seen into the matching network with the radio in TX mode and adjust for 50 ohm. (Note: Usually when measuring into the matching network, towards the radio, when the radio is in TX mode you would see an impedance closer to ~70 ohm because this gives the best operating conditions (less harmonics) from the PA.)

The additional components are needed to filter harmonics in 4 and 8 dBm mode. 

The reason for the extra inductor in the nRF52833 matching network is to have more margins for the harmonics to the radio regulatory limits in 8 dBm mode. 

Both the -833 and -840 matching networks are ok in 8 dBm mode, it's just that we wanted to attenuate the harmonics a bit more. If you only use 0 and 4 dBm, you can use the -840 matching network on both devices.

Best regards, Martin S.

[RenanSP]

[I MOVED MY QUESTIONS TO A NEW TICKET]

Hello Martin, 

I have two questions about your awesome explanation on measuring the impedance that the feed line sees towards the radio matching network while the radio is in TX mode. 

1) The two Smith charts you shown us have impedances of 94+j50 ohm and 115+j33 ohm near the carrier frequency, respectively at TX +8dBm and +0dBm. But later, you said we should expect to measure about 70 ohms. Why the difference?

2) Your Smith charts shows a discontinuity at the carrier's frequency, since the VNA (did you use a VNA?) cannot distinguish between its own weak reflected signal and the very strong radio carrier signal. During this measurement, to protect the VNA, is it typical to include a series attenuator? Any information on that would be appreciated.

I want to measure the nrf52810 in +0dBm with reference design radio matching network to verify that its impedance is near 50 ohms (or 70 ohms as you said). My VNA outputs - 9dBm during measurement, and I'm afraid that zero external attenuation will damage it, and too much external attenuation will drive the reflected signal below the noise floor.

Many thanks. 

[RenanSP]

[I MOVED MY QUESTIONS TO A NEW TICKET]

For now, to dispense with the attenuator, I programmed a harmless carrier of only -12 dBm. Hopefully, the output impedance at -12 dBm is still representative of the impedance at +0 dBm.

With the VNA looking into my radio matching network (reference layout) towards the nrf52810, it reads 130+j25.3 Ohm at the frequency closest to the carrier. That is not too far from Martin's example at +0 dBm (115+j33 Ohms).

But still, why so far from 50+j0 or 70+j0 Ohms? Thanks!