New Project: QRP Transceiver (Update #1, February 21, 2020)

Recently I’ve taken up a new project – I am building a QRP transceiver for 40m, CW only. The intention is to have a small radio, 5 watts or so, which I can carry almost anywhere and operate easily. Also, it must be completely scratch built by me. I decided on 40m as I already have a 4SQRP Hilltopper for 20m, as well as a QCX also for 20m.

I decided on a design using an Arduino Nano(well, a clone) as the heart of it along with an Si5351 for the VFO. This section I completed first on a breadboard, and then soldered down. I wrote all of the code for this from scratch. The Si5351 produces an output between 7MHz and 7.3MHz, controlled by a rotary encoder with a variable step(10-1000Hz). The Arduino also controls TX/RX switching with the key input, and an OLED display for the frequency readout. The code for this can be found here: https://gist.github.com/photoaram999/cc16043057b7f42aa06df049c98f06ee

The Arduino and Si5351 attached to what will be the main board for the radio

Having decided to get the receiver(direct conversion) working first, the next step was to build a mixer. I decided on a double-balanced diode ring mixer because it would provide a good level of sensitivity while also being easy to make myself. I used BAT41 diodes and FT-37-43 cores for the transformers, with 14 windings per side(or 7+7 for the tapped ones). I don’t know much about mixers, so I had to do extensive testing to get it to work. Using one of the other clock outputs on the Si5351 and an oscilloscope, I was able to get the proper output from it. I determined that I would need some filtering on it before the AF amplification stage. A 0.1uF capacitor across the output seemed to eliminate the high frequencies. It’d probably be best to make it a real low-pass filter with a choke on the line as well, though.

The mixer with both inputs and the output connected

Next, I started working on the AF amplification section. This is pretty simple, and I chose an LM386 for it. The next update will contain more information on this, as it is still in progress

After completing the AF stage, I will work on a bandpass filter, and then a TX amplifier, and TX/RX switching. I’d also like to add a built-in iambic keyer using an ATTiny.

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