Hi Pete, 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.
www.datafilehost.com/download-6b52d2cd.html The link to the YU1LM BPF mentioned by Javier is not really a switched BPF, it is a variable pre-selector that uses switches to select various combinations of reactive elements for covering different parts of the full HF spectrum. This is an interesting design, especially for general-coverage. I've simulated the YU1LM design and sourced the components. But the large air-variable cap is an issue, and finding polyvaricons with such large values isn't possible. In addition, I would consider wideband 4:1 transformers at the input and output of the YU1LM design to improve Q, although the design as-is results in selectivity similar to the Softrock fixed frequency input filters. Alas, there is a problem with using the YU1LM pre-selector with Softrock. The Softrock input transformer introduces a reactive element that screws up the response of the YU1LM pre-selector. An isolating amplifier would help, but that seems like heresey in a design like the softrock. In the end, the input transformer makes it very difficult to use the Softrock as a general coverage receiver without using fixed switchable BPF's like the one available at www.wb6dhw.com. In the end, I would like to see if someone can design a way to broadband the Softrock input for general-coverage. Yes without input filtering it is possible to recieve harmonically related interference if it exists at sufficient level, and even receive signals at the wrong frequency (but this can be checked by listening at double and/or triple the receive frequency). But having a "no filter" board to swtich in along with the ham-band BPF boards would at least allow some form of general-coverage and the ability to listen to short-wave broadcasts, certainly worthy of a position on the BPF switch! I've looked at this too - but again, without a pre-amplifier, the input transformer's inductance rears it's ugly head when directly connected to an antenna impedance. 73's, David --- In [email protected], "Javier Albinarrate" <[EMAIL PROTECTED]> wrote: > > Hi Pete, > > I think you have to switch both, input and output, since if the > other inputs are not switched off, they will be seen as a paralel impedance, > thus, altering the behaviour. Although, I wonder how much would that really > affect. > Here you have an interesting design from YU1LM, mainly places the > switches in different way. > http://yu1lm.qrpradio.com/bp%20yu1lm.htm > > Regards! > > ----- Original Message ----- > From: "Pete Smith" <[EMAIL PROTECTED]> > Sent: Tuesday, May 13, 2008 4:46 PM > Subject: Bandswitching BPF modules - softrock Lite+xtall 8.3 > > > > Is there any particular good/best way to switch the bandpass filter > > modules? I am working out a design for a bandswitched SoftRock Lite+Xtall > > and the 5-pole BPF modules are a bit of a challenge. Do I need to switch > > all 5 leads, or would it suffice to switch the three on the output side, > > while paralleling the antenna input and return ports of all 4 modules. > > > > 73, Pete N4ZR >
