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.
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