APPLICATION NOTE
1
In this application note, let us model some Voltage Controlled Oscillators (VCOs), such as, Dual Integrator VCO
and Controlled Reactance VCO, using PSpice. Most of the examples in the application note use the Analog
Behavioral Modeling capabilities of PSpice.
Dual Integrator VCO
An alternate approach to a behavioral model VCO is to start from a 2-integrator loop. Changing the time constant
of one or both integrators allows the frequency of oscillation to be controlled. Some form of limiting is required in
order to produce output of bounded amplitude.
A particularly elegant example can be found in Reference[1]. This sinusoidal VCO has independent control of
both frequency and amplitude. It's black-box representation reduces to:
Figure 1: Black-box representation of a sinusoidal VCO
Outputs V
d
and V
o
are in quadrature. The amplitude error term is obtained by squaring and summing the
quadrature outputs, and subtracting the amplitude-setting voltage V
a
.
Using the .FUNC command, the entire VCO can be reduced to two integrators with controlled current sources
whose expressions incorporate the multiplications, additions, and subtractions.
Following is an example to model the above VCO:
* Filanovksy VCO
.func ve(x,y,z) k*(pwr(x,2)+pwr(y,2)-pwr(z,2)
.param k=0.1, cv=10u
g1 vd 0 value {k*((-v(vo)*v(vf))+(ve(v(vo),v(vd),v(va))*v(vd)))}
c1 vd 0 {cv}
r1 vd 0 1G
g2 vo 0 value {k*v(vd)*v(vf)}
c2 vo 0 {cv}
