digital logarithmic sweep generator - for VCOs

This circuit produces a logarithmic sweep output by digital means and has been designed for use with the voltage controlled waveform generator described in this issue (no. 68). The circuit diagram shows a 14-bit binary counter of which the clock input is connected to the sync. output of a waveform generator. The eight highest outputs of the 4020 are connected to a resistor network that converts the digital code into an equivalent DC voltage level (D/A con- verter). Consequently the DC level can range from 0 V to approximately 1/5 • Ub in 256 steps. The lower out- puts are not connected (more about this later) which means that the DC voltage level at Uout increases by one step after 128 clock pulses. This out- put can be connected to the sweep input of a voltage controlled waveform generator. The frequency supplied by the generator increases (and therefore the frequency at the sync. output) every time the control voltage increases. This means that the DC voltage and frequency inc...