APPLICATION NOTE
3
Figure 3 – Frequency controllable Wien Bridge Oscillator
As shown in Figure 3, the Wien bridge oscillator consists of two resistors and two voltage controlled capacitors.
Each of these capacitors uses the YX subcircuit from ANL_MISC.LIB, and its own reference capacitor. In this
example 15nF capacitors are used.
The control voltage for oscillation is given by Vcontrol, which is a pulse that starts after a delay of 25ms and
moves from 1.0 volts to 1.2 volts. This changes admittance for the capacitor from 15 nF to 18 nF, which
changes the frequency of oscillation. The .IC statement causes PSpice to begin simulation with an initial
condition of 1 volt on node Ref1 to begin the oscillation. This circuit is simulated for 50ms (Run to time) with
maximum step size of 50us.
Figure 4 shows the Fourier transform of voltage V(4), which is the output of the oscillator. Using this capability,
we can easily see the transition from the first frequency to the second. The resonant frequency is given
as 1/(2π*R*C*VCOIn).The first frequency is 1/(6.28 * 10k * 15n * 1.0V) = 1kHz.The second
frequency is 1/(6.28 * 10k * 15n * 1.2V) = 0.886kHz.
In Figure 4, we can see two peaks in the plot indicating the two resonant frequencies. It can also be noted that the
period of oscillations is proportional to the control voltage VCOIn.
