When you first start out in the PCB layout game and know just enough to be dangerous, you simply plop down a connector, run a trace or two, and call it a hack. As you learn more about the finer points of inconveniencing electrons, dipping toes into the waters of higher performance, little details like via size, count, ground plane cutouts, and all that jazz start to matter, and it’s very easy to get yourself in quite a pickle trying to decide what is needed to just exceed the specifications (or worse, how to make it ‘the best.’) Connector terminations are one of those things that get overlooked until the MHz become GHz. Luckily for us, [Rob Ruark] is on hand to give us a leg-up on how to get decent performance from edge-launch SMA connections for RF applications. These principles should also hold up for high-speed digital connections, so it’s not just an analog game.
Return loss vs frequency for various test coupons
Everything that happens to those unfortunate electrons along their travels will affect performance in a subtle way, but the higher the frequency components of a signal, the worse it gets. An SMA edge connector may be designed to present a characteristic impedance of 50Ω typically, but that is to the end of the connection pins. Once it’s soldered down, there is a discontinuity unless precautions are taken. Even the transition from the pad to the signal trace can push a system out of spec, but what about the stack-up? What about the ground plane under the pad?
Simulation vs measurements
The first part of the job is to lock down your PCB process ..
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