Kerry Wong documented his sweep generator build:
In this post I will show you a simple sweep generator I built. It can be used to generate both the sweep waveform and synchronization signal needed for any function generator that has a VCG/VCO input. The sweep frequency can be adjusted from under a Hertz to several kHz. The sweep waveform can be switched between linear and logrithm modes and can be easily extended to accommodate other arbitrary waveforms.