--- In [email protected], Pete Smith <[EMAIL PROTECTED]> wrote: > > 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? > I thought about using a single switch with some sort of combinational logic to drive the Si570 frequency change. But then I realized there are only four BPF's and nine bands. So to control the frequency plus filter switching you you would need a 4P9T rotary switch whereas switching the BPF's alone requires a 3P4T switch. I can't find a rotary switch larger than 3P4T.
There may be another advantage to switching the frequency and BPF separately. Five of the Ham bands are harmonically related. Therefore, you would be able to switch to another band to see if you're listening to a harmonic, independantly of the BPF. This might even be a necessity if you were to remove the BPF entirely. Lastly, hex encoded rotary switches are inexpensive and easy to come by. You need only four lines when replacing the DIP switch. I briefly looked around for DIP relays that had enough poles. What I could find had lousy cross-talk specifications. You should look at shielded relays. Solid state analog multiplexers might be a better alternative. I don't know - haven't really researched that approach yet. Bill raises a good point about coax introducing capacitance if un-matched. But I envision connections that are no more than an inch or so. I've asked Bill if he can suggest a better alternative. 73's David
