KW2xD RF port impedance matching

[dismirlian]

Hello,

I downloaded the FRDM-KW24D512 schematics, which shows the RF signal chain:

KW24D512 differential RF port -> 1pF parallel capacitor -> Balun -> Harmonic trap -> 10pF coupling capacitor -> Antenna

I have some observations:

1) The KW24D512 datasheet (MKW22D512V.pdf) states a RF impedance of 13.7 - j18 ohms in TX mode at 2.42GHz.

2) the schematics shows "100 ohm controlled impedance from DUT to balun".

3) the balun used is Murata P/N LDB212G4005C-001, which has a balanced port impedance of 50ohms, and an unbalanced impedance of also 50ohms.

4) there is a 1pF capacitor in parallel with the RF port, which neither adapts the impedance to 100 nor 50 ohms.

So my questions are: why isn't the port impedance adapted to 50ohms before the balun? or are the PCB parasitics performing the matching? does the datasheet show the correct differential impedance of the RF port?

Thank you.

Regards,

Diego.

[jc_pacheco]

Hello Diego,

The 1:1 (50/50) was found to provide the best performance for our FRDM, TWR and USB designs which is why it was used.

Ultimately the best balun for a particular board is layout dependent, and so the design would need to be characterized to determine the best fit for a particular board.