Wideband Bias Tee for RF Testing
A useful RF test accessory is a bias tee, which allows you to inject DC current into a device under test while isolating the test equipment from that DC bias. I have a nice design for which there is a full writeup: Bias_Tee_Design_V2R.pdf. It’s usable from kHz to 2 GHz with low loss, high DC isolation, and a good match, for about $20 worth of parts. Here is the LTSpice simulation file. Note that this is NOT a high-power bias tee that you might use with a transmitter to remote-control antenna switches and such.
April 2016 Update. I’ve gotten quite a few comments on this design, including emails from no less than Dr. Jim Andrews, KH6HTV, who founded Picosecond Pulse Labs and is the original designer of this circuit. He’s one of my all-time favorite engineers. Also, several other guys have done further optimization and alternative board layouts. An excellent upgrade you might consider is to use some of the new conical or spiral RF/microwave inductors that have minimal self-resonance issues. An example is the Coilcraft BCR series. Amazing parts. Your board layout will be the limiting factor in ultimate bandwidth.