1:800 Oscillator

Oscillators are ten a penny, but this one has something special. Its frequency can be adjusted over a range of 800:1, it is voltage controlled, and it switches off automatically if the control voltage is less than approximately 0.6 V. As can be seen from the chart, the characteristic curve f = f(Ue) is approximately logarithmic. If the input voltage is less than 0.7 V, T1 and T3 are cut off. The capacitor then charges via the 10-kW resistor. The combination of the capacitor, the two Schmitt triggers and T2 form the actual oscillator circuit. However, T2 cannot discharge the capacitor, because T3 is cut off.

 

In this state, a low level is present at A1 and a high level is present at A2. If the input voltage is increased, T3 starts conducting. This allows the capacitor to be discharged via T2, and the circuit starts to oscillate. If Ue is further increased, the capacitor receives an additional charging current via T1 and the l00-Ω resistor. That causes the oscillator frequency to increase. In situations where the duty cycle of the output signal is not important (such as when the circuit is used as a clock generator), this circuit can be used as a voltage-controlled oscillator (VCO) with a large frequency range and shutdown capability.