Sweep Generator
The idea of developing a receiver for HF bands with submini tubes and an SSB crystal filter took me to this system. The purpose was to develope a simple tool to select crystals, check filters, and entire subsystems.
After  experimenting with analog circuits I came across a number of components that triggered the idea of a digital, computer controlled sweep generator. After looking in the Inet for ideas I discovered that I was not the first to implement this idea ("A PC controlled sweep analyzer" by Jim Rowe published in Electronis Australia September 1995). However, the use of modern component  made possible something simple and cheap (approx 100 $ in total for components).
The structure is the following:
The circuit was implemented using small SMD components. The prototype was implemented on a double face printed circuit with connectors for the digital control and output to an oscilloscope. This actually was not used because the computer display was greatly superior and it will be deleted from the final version I a planning.
Considering the small form factor I will design the printed circuit as an Arduino shield. The larger area will allow a better separation between the signal generated and the input. Currently the insulation is of the order of 50 dB.