TMAX for Oscillators

Multisim Help

Edition Date: February 2017
Part Number: 375482B-01
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When simulating oscillators, like 555 timer circuits, a custom value should be set for the maximum time step (TMAX) parameter. To achieve good simulation results the value of TMAX must not exceed one tenth of the expected period of oscillation. Typically, setting TMAX to one-hundredth of the expected oscillation period will achieve very good simulation accuracy without adversely compromising simulation speed.

When TMAX is not specified, Multisim estimates it using the period of the highest frequency independent voltage/current source that is present in the design. If Multisim is unable to determine a good estimate for TMAX the default value of 1e-5 s is used.

For low frequency oscillators (less than 100 Hz), such a small time step will result in unnecessarily slow simulation. And conversely, for high frequency oscillators (more than 10 kHz) a TMAX of 1e-5 will not provide the optimal simulation accuracy. For this reason, when simulating oscillators it is important to provide an appropriate value of TMAX that correctly captures the expected oscillation frequency.

Consider the following 555 timer astable oscillator:

Using the design equation below, it can be confirmed that the expected oscillation frequency is approximately 1Hz (or equivalently the oscillation period is approximately 1s).

To achieve a good balance between simulation speed and accuracy, the value of TMAX should be set to 0.01 (one-hundredth of the expected oscillation period.)


Related Information

TMAX Setting Guidelines


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