nRF52805 antenna matching

[Alessandro Vitale]

I need your help to design the radio matching network on device nRF52805.

Here the reference design:
infocenter.nordicsemi.com/index.jsp
In this reference design L1, L2 and C3 are used to match the output impedance of the ANT pin #E1 that it is not 50 Ohm.

Can you please let me know what is the impedance that the device wants to see on ANT pin #E1 (from device to output) ? I read some where that it is 37 - j20 ohm

Thank you

[ketiljo]

It's not a good idea to match the radio to the antenna directly. You will most probably not have enough attenuation of the harmonics there are serious risks of failing radio qualification testing. 

The reference design takes care of both the impedance transformation and low pass filtering. You can measure S22 when the radio is in TX mode. It's expected to see ~50 ohm into the matching network from the antenna side. 

The radio is expecting to see 37 - j20 ohm

[Alessandro Vitale]

It is clear that close to the antenna I have to place another matching network, maybe pi-network.

What is not clear is why in the reference design there are:  L1 = 2.2nH , L2 = 3.3nH and C3 = 1.2pF

With these values I get 58.6-j16.2  @ 2440 MHz:

IN order to obtain 37-j20 I have to change those values: L1= 1.8nH, L2= 3.3nH and C3 = 1.5 pF

[ketiljo]

You need to simulate with non-ideal component in order to get correct results. There are parasitic effects that will have impact. Also the layout will have some impact. Ideal components in simple tools like this will not be usable at this frequency.

[Alessandro Vitale]

Got it, this means that the values in the reference design are based on that specific layout, this is why there is a difference of 0.4nH on L1 and 0.3pF on C3.

I will do a more accurate simulation with Ansys HFSS.

Thank you

[ketiljo]

Most probably, yes.