At 03:31 AM 5/14/2008, you wrote: >I thought about switching the BPF's and adding a rotary hex encoded >switch instead of the DIP switch to change the Si570 frequency. Below >is a link where you can download a .pdf that shows a BPF switch >schematic, a data sheet for a plausible rotary BPF switch, and a price >quotation for the switch. I would connect the switch to the BPF's with >RG-174 (or similar) coax. Yes you do need to switch both the input and >the output of each filter. The balanced nature of the input >transformer's secondary requires you to switch both secondary >windings, the center-tap can be common.
Thanks for the input on this David - very useful. I hope to use a single switch to control both the BPFs and the control inputs to the SI570. I also hope not to have to reprogram the PIC on the Xtall board, which means that I need a 16-line encoder. I have found encoders that do 8 lines each and can be cascaded (74LS348 or equiv.), so that looks pretty viable. My thought is to use a bank of 3pst small-signal relays to switch the appropriate BPF into the circuit, using the relay coils (with steering diodes) to function as pull-up resistors for the inputs to the encoders. That'll let me massively shorten the RF signal lines as compared with a rotary switch. Do you see any issues with this approach?
