MC1321x RF reference design question

[Nevo]

Hi, all!

I'm reviewing AppNote 3248, "Using the MX1321x Internal Transmit/Receive Switch in a Low-Cost, Single-Port, Two Layer Design."  I am not an RF engineer and I'll happily copy a tested design.

I'm curious about a feature of the design. Just before the antenna, the signal is sent through R111, which apparently is a zero ohm resistor.

Now, I'm a digital guy and anything analog is black magic to me. But I've just got to ask: what is the purpose of a zero-ohm resistor?

[rocco]

Hi, Nevo:

I can't be sure of the original designer's intent, but when I use a zero-ohm resistor, it is either as a jumper, or as a placeholder for a possible future non-zero-ohm resistor.

In this case, it looks like a jumper to select the on-board "F" antenna. To use an external antenna attached to the J100 connector, you would leave R111 out, and include a zero-ohm resistor at R112 (which is shown as "Not Mounted" on the schematic).

[Nevo]

Thanks, Rocco.  That makes perfect sense and is pretty much what I'd assumed.