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Sunday, December 1, 2013

11 MHz IF for new BITX 20/40 Dual Bander? Also: Cabinetry and Socketry

I am gathering parts and ideas for a BITX dual bander (20 and 40 meters).  I know Farhan used a 10 MHz filter for his "Simple SSB Transceiver."  But I was thinking of going a bit higher, to 11 MHz. This would allow me to run the VFO from 3.175 to 3.355 for 20 meters, and 3.695 to 3.875 for 40 meters. I'm hoping that I can do this with one single VFO (Farhan used two VFOs), perhaps with a reed relay switching in some additional capacitance for the other band. I'll also follow Farhan's lead and switch the Low-Pass and Band-Pass filters with DPDT relays.

I set up a simple spread sheet and looked at the VFO harmonics to see if any fell within the desired tuning ranges.  That looks OK. I have not looked at mixing products between VFO and BFO.  What do you folks think?   Would the 11 MHz IF for these bands work?  Or are there evil birdies lurking in my future?

I've gone ahead and bought another wood box for the new rig (I didn't even have to suffer through a second visit to the crafts store -- they are available on Amazon).  I also got a roll of copper sheeting at Home Depot.  This time I will prepare the box first, lining the inside with copper and preparing all the "socketry" (George Dobbs' word) before putting the PC board in.
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  1. Haven't tried this but have you thought of using a 10.7 MHz IF? Using crystals salvaged from old filters from VHF FM transceivers. A 10.7 MHz IF would allow coverage of two bands with a single 3.3 - 3.7 MHz VFO. There may however be a birdie due to VFO harmonics but the arrangement would remove the need for VFO switching. 73, Peter VK3YE

  2. I thought about that Peter. In fact, that was the solution to the simultaneous equations asking what IF and VFO freq could be used for 40 and 20. But I think those birdies would be fierce it might be better just to switch in some additional reactance to move the VFO freq. Thanks and 73 Bill

  3. Hey Bill - how about getting "fancy" and using diodes to switch in the extra capacitance instead of a reed relay.

  4. Here are some vids to help:

    Basics of PIN diodes for RF switching:

    and basics of using diodes as switches (general):


  5. Hola Bill,

    De acuerdo con Alan W2AEW. Iba a sugerirte ahorrarte el relé y usar conmutación con un diodo o un transistor. Por ejemplo, el RIT del Optimized QRP Transceiver (un clásico). Un abrazo, Liam y Felices Fiestas que se aproximan...

    de Juanjo EC5ACA

  6. Hi Bill,

    must read:

    best 73


  7. Dear Bill,

    Why don't you get really fancy and not worry about switching a VFO and use a DDS for the oscillator with a digital readout?

    I bet Billy can help you do it with a Ardunio or a PICAXE for a processor. They do work nicely as VFO's and HFO's for Amateur Radio Frequencies and almost as cheap as Crystals. Then Later on if you want to change frequecies, you just change the tuning of the DDS. (Just my humble opinion.

    73 Dave WA5DJJ in New Mexico.

  8. The idea of getting crystals from 10.7MHz filters is interesting, but is it practical?

    I've never taken a crystal filter apart, but what's inside? In the fifties there were articles about modifying Motorola Permakay filters, but the first step was to get rid of the epoxy or whatever that filled the can. And if the 10.7MHz filters are recent enough, are they likely to have crystals in holders inside?

    For that matter, I have assumed that if a filter is old enough to use standalone crystals, then the circuit would not be a ladder filter (ie crystals on the same frequency) but sets of crystals spaced enough apart to provide the bandwidth. So if it's a simple filter, you'd get only a few crystals on the same frequency, if that.

    Considering it is a common IF frequency, one might think 10.7Mhz crystals had become commodity crystals, but when I've looked, they haven't been part of the off the shelf selection.

    10.240MHz is common, converting 10.7MHz to 455KHz, and also common in synthesizers in CB sets. Older cordless phones used them a lot, though by now the older cordless phones seem to have disappeared from the garage sales. I'm not doing the math to see if 10.240MHz would be useful.

    I thought I'd found something really great a few months ago, an SSB CB set. I've long thought that sort of thing would be useful, if found cheap, and this one was only five dollars. But it seems that after a certain point, the filters were not so great, so they are relatively wide filters, used for AM and SSB. Probably still useful for VHF worked (or just run the CB set into a transverter), but not a useful source of 10.7MHz SSB filters like I'd originally thought.

    Michael VE2BVW

  9. Thanks for all the comments.
    Re the DDS. As Farhan has said, this is kind of a religious thing for me. I have to keep everything analog, discrete, HDR.
    On the 10.7 Mhz rocks, I think I saw some on Mouser or Digikey. Maybe 10.73 (!) but for 20/40 the harmonics from the VFO would be bad birdies.
    On diode switching... Good idea Alan and Juanjo!
    Henk -- thanks for the IF freq paper. 73 to all

  10. Spur Calculators:

    Hittite Microwave has a powerful online spurious calculator. From their homepage [hittite(dot)com] look under Engineering Tools. Be sure to enable Java in your browser. Also click the Help button in the calculator and from the Help pop-up page click to watch the 8-minute tutorial.

    In the Hittite tool it is not apparent, but you can click the units under the entry boxes and select KHz, MHz, GHz, etc.

    A couple of free RF tools downloadable (free) programs have spurious calculators:

    RFSim99 (Dated but OK)

    AppCAD (HP/Agilent)

    Google knows where to find these programs.

    The Microwaves101(dot)com site has a downloadable Excel Spurious Calculator spreadsheet. Nice, lots of help embedded in the spreadsheet. It works in MSOffice 2000 so it should work in LibreOffice/OpenOffce as well.

    leleivre(dot)com/rf_mixerspur(dot)html this is a simple online spur calculator, have a look at the other RF Tools available while you're there.

    I'm sure there are more spur calculators/simulators out there (anyone else?)

    Best 73's, David WB4ONA


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