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New Project: QRP Transceiver

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:

Github Gist

The partially assembled VFO

The partially complete VFO/main board.

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 RF mixer board

The completed RF mixer board.

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.



Update: 2020-09-16

I did not finish this project before leaving for college. Progress so far has been success in creating a crude receiver, which has some extremely high noise from the OLED display.

During winter break I hope to finish it, and bring it for bicycle-portable use during the spring semester. I'll power it on ~8v from two 18650 batteries in series.



Update: 2020-10-14

My plan currently is to complete a mk.1 schematic before going home from college. After returning, I will build a prototype, test, and refine. The final product will hopefully be much nicer on a custom PCB.

I have put together a to-do list for this project:

I hope to have this mk.1 plan done by November 25 and the first prototype built by December 5. After testing and tweaking, I will finalize the design and build mk.2 on a custom-designed PCB.


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