My longish term goal is to build the LBS transceiver described by N6QW and KK6FUT (who I see is now AI6YR) and I am methodically working my way through that project.
However, I am not above taking a shortcut for instant gratification along the way. Remember the 80-meter Sudden Receiver variant I built a few weeks ago? Well, not much use for it in my current location. It occurred to me that I could gut the 80-meter parts of it and use the NE602/LM386 core as the rest of a very minimal direct conversion receiver for a more useful band, say 20-meters. So that’s what I did and long story short, it worked.
So, from the front end of the Sudden Receiver, I plucked off the parallel coil and variable capacitor arrangement that comprised a tuned 80m tank circuit. That was replaced with a 20-meter band pass filter based on the doubly-tuned BPF modules designed by QRP Labs.
I used the recently built VFO-in-a-box to peak the variable capacitors on the filter around a center frequency of 14.125 Mhz. I ran the VFO output through the filter and terminated with 50 ohm dummy load and watched the signal level on my oscilloscope.
The VFO-in-a-box also had a starring role as the source of local oscillator. I took out the VXO circuit and just routed the DDS output to single-ended into the NE602. I figured it worked in the Forty-Niner mod described in QST, and that the same circuit would serve here.
Tuning across 20 meters, I heard stations SV1ME (~7000km) and YB71RI/5 (~6500 km) calling CQ down in the morse code end of the band, and ZS1SA (only 3300 km) calling on SSB. These signals were clearly intelligible, but somewhat diluted in the wide open band pass of the receiver. Adding a 700hz hi-per-mite audio filter really helped the CW signals stand out. I wouldn’t have thought that adding selectivity so far down the chain would have been so helpful, but I suppose it’s all a matter of digging the signal out of the noise.
When it comes to building the LBS, I am aiming for 20-meters, since there is very little activity on 40 meters in my location. I will end up using the same bandpass filter on the front end. In the original LBS project, the BPF was based on shielded transformers, but they are no longer available.
Like the BPF, the VFO will carry over to the LBS project. At least as I integrate in these components with the RF amplifier, diode ring mixer, and audio amplifier of the LBS, I know that these building blocks work.