The xDot Developer Guide has two contradictory reference circuits for antenna matching at 915 MHz.
The first circuit, from the Antenna Matching Network section on page 33, shows a pi network made up of R30, L1, C34, C23, and C24. R30, L1, and C24 are labeled “OMIT”. There’s also C26, which is connected from the antenna to ground, forming another pi network. This network consists of a 7.5 pF capacitor on the left leg, a 6.8 pF capacitor in the middle, and an 82 pF capacitor on the right leg.
The second circuit, from the developer board schematics on page 52, shows R30 as a zero-ohm resistor shorting out L1 and C24. This results in three parallel capacitors with a combined capacitance of 102 pF, since C23 and C24 are 10 pF in this version.
What is the actual matching network we need? Our current board has just an 82 pF capacitor connected between ANT1 and ground, which passes FCC testing with a U.FL antenna. Is this okay? We’re reusing the design on a new board and I want to make sure there aren’t any hidden problems.
Thanks,
-Adam