RG-174 will add about 32 pF/foot of capacitance to your circuit, unless it is well matched, (50 Ohms in and out)
Bill WA9PWR drmail377 wrote: > 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 >>> > > > > ------------------------------------ > > Yahoo! Groups Links > > > > > > > >
