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I've got a GPS circuit where the antenna is connected directly to the GPS receiver via a coplanar waveguide designed for an impedance of 50ohms. Unfortunately, I neglected to add the bias-t circuit that is used to provide power to the GPS antenna. Luckily, I was able to remedy this problem on the prototype by breaking the transmission line, soldering a 22pF cap across the gap and connecting the antenna side directly to a nearby 3.8V trace on the PCB. The GPS was able to get a fix in about 20s and happily tracked 9 satellites.

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I now need to incorporate the capacitor and inductor of the bias-t into the circuit (Reference Design page 150) and I was wondering what considerations I need to make in order to ensure that the transmission line remains at 50ohms?

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The capacitor and inductor values are chosen so as not to disturb the line impedance. In practice, we would aim for the capacitor in this case to be as near a short circuit (0 Ohm impedance) and the inductor as near an open circuit (infinite impedance) at the frequency of interest (~1575MHz or 1227MHz for GPS). Just use RF components (I would recommend a low ESR chip-scale ceramic cap and chip-scale wound inductor) and as a rule of thumb I'd be looking for a cap impedance <1 Ohm and inductor impedance >20x your line impedance.

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