APPLICATION NOTE
Obtaining Steady State of High-Q Circuits
Using Open Loop Response
The length of time it takes to perform a transient simulation on a high-Q oscillator circuit makes simulation
inefficient. The dominant time constant of the circuit, due to the Q of the crystal, means that the simulation
must run a minimum of Q cycles before the circuit reacts. And, because you must wait Q cycles, there is no
way to force an oscillator to a steady state condition. For a crystal with moderately high-Q (20,000), it can
take close to a million cycles before the oscillator reaches a steady state condition. Besides the long time
required for the simulation to run, the data file created by the simulation will be extremely large. But, it is
possible to use AC analysis to simulate high-Q circuits. These results can be used to investigate whether the
loop gain and phase are conducive to producing a stable oscillation.
