This invention relates generally to electronic circuitry and more particularly to electronic circuitry adapted for use in voltage-to-frequency converters.
As is known in the art, voltage-to-frequency converters have been used for many years in a variety of applications. One type of voltage-to-frequency converter produces a series of pulses of fixed time duration and height at a rate proportional to the level of an input voltage. In one such type of converter an input current proportional to the level of the input voltage being converted is fed to the input summing mode of an integrator, causing the output of the converter to ramp, say negatively. When the output of the integrator reaches a trigger threshold level a one shot (monostable) multivibrator is activated to couple a constant current source to the input summing node of the integrator for a fixed period of time, thereby causing the integrator output voltage to ramp, positively, at a rate proportional to the difference between the level of the input current and the level of the current supplied by the constant current source. In the steady state mode the one shot multivibrator is triggered at a rate proportional to the level of the input current and hence to the level of the input voltage. The output of the one shot multivibrator is coupled to output circuitry for producing a series of pulses fixed in time duration and amplitude having a rate proportional to the level of the input voltage.
In order for the converter to operate accurately over a relatively wide bandwith, say in the order of 100 KHz or greater, it is necessary that the one shot multivibrator have relatively rapid switching characteristics. Further, while one shot multivibrators have been designed using transistor-transistor-logic (TTL) circuitry, transistors used in such circuitry are placed in a saturation condition when conducting and a relatively long time delay results when such transistors are driven or switched to a nonconducting state thereby lengthening the time duration of the current produced by the current source. At relatively high switching rates this relatively long delay becomes a significantly greater percentage of the time duration of the current pulse thereby reducing the effective operating bandwidth of the converter. Still further, the time delay in such TTL circuitry changes with temperature and hence at relatively high frequencies relatively large inaccuracies may result thereby limiting the effective operating bandwidth of the converter.