40m direct conversion CW receiver

this is the first receiver i ever built, using an sa602 mixer IC right at the tail end of their availability. it is designed for CW only, with a narrow audiofrequency BPF centered on 600 Hz.

• the input BPF is modelled after the QRP Labs band pass filter kit as i saw little need to reinvent the wheel there (there is a design procedure for coupled resonator filters in Radiofrequency Electronics by Hagen, but no need).
• the mixer stage is... the mixer stage. i just wung it.
• the audio BPF stage is based on the Hi Per Mite kit, but redesigned to avoid weird capacitor values and opaque hacks
• the audio output stage is a classic max gain LM386 configuration
• the VFO uses an si5351a controlled by an rpi pico

full schematic

in my actual build, i put the VFO in its own enclosure, such that i can reuse it for various other projects when i want to use a VFO without having to assemble a VFO. you can see in the photos and video that i have the VFO output connected to the mixer in the receiver with a length of coax.

i used a breakout for the si5351a. soon after building this, i had regret giving adafruit money. please use breakouts from aliexpress, or, if you have the ability to do surface mount soldering, consider putting together a bespoke breakout i designed specifically for manhattan style construction:
plug-n-play manhattan