I'm trying to make my own board for a CC1101 transceiver that will operate at 915 MHz. To do that, I'm just copying the reference design layout from the
eZ430-Chronos-915 RF Access Point that uses a CC1101 with a MCU to be used as a dongle. This is the reference design PCB, I could only find it in PDF format as an image:
(Inside the black square are the 0.25 mm tracks and inside the red square is the 0.4 mm track)
And this is the schematic section corresponding to the CC1101:
By measuring the image I got that the tracks coming out from the
RF_P pins are 0.25 mm wide, this is on a two-layer FR4 board with a thickness of 0.8mm, so that will be around 100 Ohm impedance for the tracks which is right because it's a differential signal AFAIK (to maintain 50 Ohm impedance, correct me if I'm wrong please).
These two tracks go to a Balun (this one) and the antenna connection is made on pin 1 of the balun. This is the typical layout:
The balun says the impedance of the track going to pin 1 must be 50 Ohms. I measured that the track that goes out from pin 1 is 0.4 mm wide, with a two-layer, FR4 board with a thickness of 0.8 mm that is an impedance of around 88 Ohms, not even close to 50 Ohms! To get a 50 Ohm impedance the track should be 1.5 mm wide.
What am I missing? I know this design works because I have this board and it works as expected. I was planning to put a 50 Ohm SMA connector in the end, should it then be 50 Ohm or something closer to 88 Ohm?
All the pictures are taken from here.