My current arduino project, has been to build an affordable and simple transceiver for 4m.
It is not hardcore design, but my attempt to have fun with hamradio and Arduino
Previously, based on the same concept, I have built a 10-band transceiver, 160 to 10m.
My goal has been to build it from components I already had in storage, without having to buy various expensive components.
It is built out of standard components, and for that reason, of course, it cannot be compared with more advanced radios such as UBITX.
I use an Arduino Uno because I have the most of them, but a Nano should also be able to handle it.
The software is in my case from JA2GQP, "si5351a_TFT_Ver3.1", where I have modified his layout around the arduino, and the connection
of this, including removing the mode switch, adding the PTT out, which via a BC547 on my mixer board to pulls the necessary relays.
The original can be found on his blog of the same name, where there are also several exciting projects.
I have modified the software itself by adding a CLK1, in order to get a BFO signal of 8Mhz. The 8 Mhz is due to my crystal filter
in the mixer, 4 x 8Mhz crystals. Other crystals can be used if you just remember to change the BFO frequency in the ino file.
The mixer I have made is very scratchy, and I have then added separate amplification for mike and speaker as well as RX in and TX out.
I have chosen this solution for 2 reasons, partly so that the mixer print could be in the cabinet, partly to be able to experiment with input and output separate.
The files for this project cand be found here: 4m