Alex wanted to see frequency impact on different components. First of all he built a voltage controlled oscillator using couple of op amps. But he wanted something simpler and more digital. For next step he took Attiny25 microcontroller which comes with internal PLL for fast peripheral clock. It enables to generate high frequencies that drive MCU peripherals like timers while system clock remains intact.
Using timer compare on match method he was able to generate 32MHz clock signal on pin. Since the idea was to have voltage controlled oscillator, he added potentiometer to ADC input. By reading its value microcontroller determines the frequency. By underclocking or overclocking MCU frequency ranges from like 128kHz to 32MHz in fairly large steps. Alex made few interesting experiments on how noise is picked up on breadboard depending on wire placing. This gives clear idea how this impacts the results if you are designing sensitive projects. Anyway this experiment is a great tutorial on using PLL as clock source for peripherals on Attiny25.