[Jack] tipped us about a Crossed Bananas Display (CBD) he just designed. A CBD is a tuning aid for frequency-shift keyed (FSK) modes and is basically an oscilloscope in X-Y mode. At one time, radioteletype operators used binary FSK to transmit text over radio waves. In this scheme, the “1” is called the mark frequency and the “0” is called the space frequency. If both frequencies were perfectly tuned (correct phase) the resulting display would look like the one shown above, explaining the origin of the “crossed banana” name.
The build is based on an ATmega328 and a 1.8″ ST7735R display which has a 128×160 resolution. The MC33204PG operational amplifier is used in conjunction with a potentiometer to scale the input in the microcontroller ADC’s range. Another potentiometer sets the refresh rate of the graph. The whole project is enclosed in a painted cast-aluminium bud box and all the sources for this project can be found here.