Amateur Radio

SSB Transceiver Project

What always fascinated me most about amateur radio was the practical side. That's why, shortly after passing my HB3 exam, I decided to upgrade to HB9 so I could really start building things. My final-year project (Matura thesis) gave me the chance to put theory into practice and build a radio myself. This turned out to be much more ambitious than expected. In December 2021 I started thinking about how to expand my still very limited, theoretical knowledge and finally build some hardware. Over time it became clear that theory and practice didn't line up at all. Drawing the circuits was done quickly, but what was missing from the "circuit law" were concrete calculations. So I searched the internet and quickly noticed that everyone says something different, but nobody really explains everything from the ground up. But with some help from the HB9CL club, after a while the first attempts succeeded and everything started to come together.

After several attempts and setbacks, the microphone input and mixer became functional. But especially the oscillator was still stubborn. The signal was accompanied by countless spurious peaks. It turned out I had forgotten the coupling capacitor between the Colpitts crystal oscillator and the buffer transistor. This caused a DC current to flow to ground through the crystal, creating the distortions. I also left out impedance matching between the individual components for simplicity. In hindsight, this showed up, among other things, as too little power for effective radiation. It didn't help that I wasn't using any IF amplifiers. These low power levels also caused problems in the double balanced mixer I used for modulation. Due to their relatively high forward voltage of 0.7 volts, the diodes there need stronger drive, otherwise they don't switch correctly and create further distortion. This was especially problematic on the receive side, since I only amplified after demodulation there. In hindsight, a design with an IF amplifier before and after the crystal ladder filter would have been much more effective. I first tried to calculate this filter using a website, which didn't work. So I decided to simply use 68pF for all capacitors. That worked right away, but gave a somewhat too narrow bandwidth. However, this could easily be adjusted by increasing the capacitor values.

I plan to implement these and many other improvements over the coming time, hopefully soon being able to make my first successful QSO with my own transceiver. Through this final-year project I really realized everything we amateur radio operators are allowed to do, and especially everything we're capable of, and I'm looking forward to all the projects still ahead of me. There's certainly no shortage of possibilities and ideas. I can only recommend that everyone explore this side of our hobby and build a kit, or even develop one themselves. It offers completely new insights, is an important part of what we can do, and shouldn't be forgotten. So I wish everyone happy building and operating, and I'm looking forward to another year full of QSOs.

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