Tuna Tin "S" (1) (Updated)

 After spending a lot of time going over my old projects from the 80s I realized that they projects were way out of date...   Making HF receivers and transmitters,  the variable capacitors and turns reducing dials proved to be so expensive.  The variable cap is maybe $25 if one can even be found, and vernier dials are selling for something like $45 each.  The question is: What is the modern way to build HF projects?

I came across an article in this month's (Dec 2022) QST titled "A Simple CW QRP Transceiver for 40 Meters" by James Forkin.  In my opinion,  the "simple" transceiver isn't all that simple,  but the article did point to an earlier article that provided a string of references to some simple projects that capture the essence of modern HF receiver and transmitter design.  In the end I zeroed in on a December 2020 QST article by Bob Fontana, AK3Y, "The Tuna Tin "S" - A Bare-Bones Synthesized QRPp Transmitter."  The transmitter basically is a  synthesized RF source driving a simple IRF512 MOSFET power stage.  

This simple project can easily been divided into to tasks: 1) Build the synthesized RF source and 2) build the power stage to drive the antenna.  The latter task has some challenges, in particular finding modern power devices for the design.  Otherwise the power stage design is a fairly straightforward task with many examples in the literature and on the web.

But at this point I want to focus on building the synthesized RF source.  This is a new technique for me, so it should be a learning experience.  The technique is very cheap,  I believe it can be built for less than $10 with a little effort.  

The basic block diagram of the RF synthesizer is shown in the figure above, which is Figure 1 from the article. In the upper right is an Si5351 synthesizer module, which produces the RF signals in response to commands from the Arduino Nano controller on the left.  A KY-040 rotary encoder is used by the operator to select the desired RF frequency by rotating the encoder knob up or down. The Nano responds to the encoder by changing the RF frequency of the Si5351 and displaying the frequency on the OLED display module.  

For my version of this RF synthesizer, I plan to use a Wemos D1 Mini clone instead of the Arduino Nano.  The D1 Mini is a very small ESP8266 based development board.  Those familiar with the 8266 know it has its own WIFI transceiver and antenna. I bought 5 of these for $16 off of Amazon and used them to build WIFI based temperature sensors to use around the house.   For this project I won't use the WIFI unit on the D1 Mini.

I ordered the other parts of the RF synthesizer from Amazon:  1) 0.96" OLED 128x64  (5 for $15),  2) Si5351 RF synthesizer module (2 for $8), and 3) KY-040 rotary encoders (5 for $9).  All told the total cost for the parts for 1 unit is about 3+3+4+2 dollars.  That's $12 per unit.  That's not bad for a wideband, rock stable RF source for transmitter and receiver projects. Beats the cost of a variable capacitor and vernier dial for sure. 

I tested the encoder last night by powering it up and observing the signals on my o-scope.  I put the parts on a protoboard and started testing the OLED.  Unfortunatly,  I'm having some problems getting the software library loaded,  so that's where I left it. 

 Update 4Dec22 13:55

Finally got the display going.  Note the "Hello world" on the display below....