• Preselector

    Trevor R.H. Clarke06/21/2020 at 20:46 0 comments

    I designed and build the preselector this weekend. It's a 3rd order Bessel response. The schematic and Qucs simulation results are below.

    Preselector Schematic
    Simulation Results

    The plots show a response from about 51 MHz to 55 MHz, a bit high for a weak signal 6m radio. I've found that stray capacitance and such usually pull the response lower when I actually implement a filter on an FR4 based prototype board so I went ahead and used the values as above.

    I used SMD parts with C0G capacitors and standard value inductors. I happened to have the caps around otherwise I would have used something more temperature stable, we'll see how these do.

    Initially, the center frequency was around 50 MHz with an upper range (-3 dB) of 52 MHz. This was a little low but usable. I removed 100 pF shunt capacitor to try and pull the frequency up a bit. The new center is at 50.26 MHz with a -3 dB range or 47.5 MHz to 53.1 MHz which is a bit better. Since this will be a radio for weak signal work in the lower part of the band I only need reasonably passage through about 51.6 MHz or so. While I could have dialed it in a bit more I didn't feel like looking for other capacitor values in my junk bin so this will do. The blue traces are the original plots and the yellow-green traces are with the 100 pF cap removed.

  • Some back of the envelope design

    Trevor R.H. Clarke02/26/2020 at 19:46 0 comments

    Here's my initial take on an overall design.

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  • Initial design requirements

    Trevor R.H. Clarke02/26/2020 at 14:41 0 comments

    Here's my initial specs although it might change:

    • SSB voice and data, probably CW.
    • Headphone/line out as well as a built-in speaker
    • Mic in (probably an electret only) and line in/PTT line for sound card use
    • Basic built-in CW "straight key" for basic morse and transmission for tuning
    • Support for an external keyer
    • Superhet, most likely single conversion but if image rejection is a problem I'll add another IF
    • QRP level output (5W) and the capability to feed an external amp with pre-PA power (such as this)
    • Basic controls on a simple LED/LCD display with tuner, power level, CW configuration, some memories, etc.)
    • AGC and S-Meter
    • Ideally a tunable IF filter, or at least a couple of different options