On Tue, Jul 15, 2014 at 2:24 PM, Sean Breheny wrote: > If you do this with relays, I believe it would work but you want to make > sure you get low capacitance relays, like RF-switching relays. Otherwise, > they might still allow signal propagation in some cases when open (or for > the DPDT case, they might allow crosstalk between the two selectable > signals). > > I ran into this once when I made a remote antenna tuner (for receive only= ). > It used a binary sequence of capacitors (e.g., 1024 pF, 512 pF, 256 pF, > etc.) to synthesize the required capacitance to match the inductive > antenna. The purpose was both for pre-selection and matching to minimize > coax cable losses and the degree of effect which the cable had on the > tuning of the antenna.. When I tried to do it with regular telecoms relay= s, > they had something like 20pF of capacitance across the contacts when open= .. > This did not greatly affect the larger values but down in the 10pF range, > it made a huge difference. I ended up using mostly telecom relays but one > (more expensive) RF relay to isolate a bank of the larger capacitors+rela= ys > when small capacitance values were commanded. It worked well! Good advice, thank you! Josh --=20 A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .