Design Ideas: July 6, 1995

Behavioral Spice model emulates VCO

Dr Bashir Al-Hashimi,
Staffordshire University, Stafford, UK

A behavioral model treats a circuit as a black box, which you can describe using an equation or table. Listing 1 is a Microsim PSpice behavioral model that simulates the operation of a voltage-controlled oscillator (VCO). The output signal of a VCO is V_O=Aβsin(2ãf_Ot), where A is the amplitude, t is the time variable, and f_O is the output frequency. This frequency is f_O=f_C+KV, where f_C is the center frequency, K is the sensitivity of the VCO in Hz/V, and V is the control-input signal.

Listing 1--VCO behavioral model
.SUBCKT VCO 1 2 PARAMS:A=1, Fc=1, K=1 E 2 0 VALUE={ASIN(6.283TIME(Fc+KV(1,0))) .ENDS VCO

Listing 1--VCO behavioral model

.SUBCKT VCO 1 2 PARAMS:A=1, Fc=1, K=1

E 2 0 VALUE={A*SIN(6.283*TIME*(Fc+K*V(1,0)))

.ENDS VCO

The model expresses these equations using a VALUE description of a voltage-controlled voltage source. Note that the parameter TIME in the model represents the PSpice internal sweep variable used in transient analysis. To simplify use of the model, describe it as a parameterized subcircuit called VCO, whose input is at node 1 and whose output is at node 2. Describe the model function using the three parameters A, f_C, and K. You must give the subcircuit parameters initial values to satisfy the .SUBCKT statement requirement. In this example, I arbitrarily set them to one; you'll change them to the required values when the program calls the subcircuit.

To use the model to simulate a VCO circuit, specify the parameters A, f_C, and K. For example, if A=2V, f_C=10 kHz, K=5 kHz/V, and the input control voltage ranges from +1 to 1V, the output frequency changes from 15 to 5 kHz, as Fig 1 shows. Listing 2 gives the PSpice netlist of the VCO circuit. (DI ##1726)

Listing 2--VCO-circuit PSpice netlist
.LIB C:\PSPICE\MODELS ;VCO model location V 1 0 PULSE(-1 1 0 10ns 10ns 0.5ms 1ms) ;control input signal .TRAN 0.01m 2ms 0m 0.01m ;transient analysis range X 1 2 VCO PARAMS:A=1, Fc=10K,K=5K ;call VCO subcircuit .PROBE V(1),V(2) ;graphic input & output signals .END ;end of netlist

Listing 2--VCO-circuit PSpice netlist

.LIB C:\PSPICE\MODELS ;VCO model location

V 1 0 PULSE(-1 1 0 10ns 10ns 0.5ms 1ms) ;control input signal

.TRAN 0.01m 2ms 0m 0.01m ;transient analysis range

X 1 2 VCO PARAMS:A=1, Fc=10K,K=5K ;call VCO subcircuit

.PROBE V(1),V(2) ;graphic input & output signals

.END ;end of netlist