APPLICATION NOTE
4
Example Circuit
Figure 2 shows a 100 kHz crystal oscillator circuit with a Q of 20,000. To analyze this circuit, the
LOOPGAIN subcircuit is inserted into the feedback path. The A node is connected to the output side of
the feedback path and the B node is connected where the feedback is summed into the circuit. If the
subcircuit is inserted backwards, the results will show a loss, or be upside down. The AC analysis is set
up to sweep the circuit from 99.9 kHz to 100.1 kHz using a linear sweep of 100 points.
Figure 2: 100 kHz crystal oscillator circuit
After the analysis is complete, the results may be viewed using Probe. The following Probe macros can
be defined:
par(a,b)=(((a)*(b))/((a)+(b)))
Ti = ((I(X_X2.V1)@1)/I(X_X2.V2)@1)
Tv = (V(A)@2/V(B)@2)
T=(par(Ti+1,Tv+1)-1)
The macro par(a,b) defines the parallel combination of the arguments a and b.
Example Circuit's Simulation Results
The simulation results (Figure 3) for the oscillator circuit show the peak gain and a 180 degree phase shift
at the resonant frequency of the crystal.
