In thinking about oscilloscope sections (see
So there's the classic diode switch, ala fast sampler stuff. Foward bias FWB with CCSs and the AC terminals become connected (within capacity of the CCS). Reverse and it opens up. Easy enough. So put in two (that's eight diodes), and alternate their current sources for the SPDT action. And to do that, I can use diode gates (counting 16 diodes...) and a complementary square wave, which can easily be made with reasonable rise time (down to 30ns or so from mere 2N3904s). And with just a pF or so between the diodes, isolation won't even be too bad up at 20MHz (figuring a line impedance of 50 ohms I think). Takes an awful lot of diodes though, and I wonder about linearity -- it's that first 0.5V that needs to be beautiful.
JFETs are classic switches. Not usually too conductive, so I'm thinking moderate impedance (500 ohms-ish?) at which point Cds would be worth making a "T" switch. And that's fine, the ground part is switched inverse to the signal parts, and there's two of them (a tee for each input) that are switched inversely, so I need the complement anyway. Switching hooks up about the same as for the diodes, maybe even using current sources (that's six of them...) to supply the gates for faster response (a current sink on the output compensates for the gate bias). MOSFETs are just as classic, but transistors with substrate taken out to a pin are harder to find.
Or there's the BJT switch. Little known, and rarely used (I only know of one "invertible" transistor, the 2SC2878, of which I have two). But surprisingly effective, having MOSFET-grade "Rce(on)" specs (C2878 says about 5 ohms at Ib = 10mA), low enough to couple 50 ohm lines. Capacitance is a bit high though, not really giving enough isolation at 20MHz, even at 50 ohms. Maybe some smaller transistors would work here (C2878 is some 400mA Ice(max)).
Tim (damn googlegroups...)