![]() Crystal oscillators in particular can take many hundreds of thousands of times the oscillator period to start up and reach a stable state. If there are no noise sources in the circuit (the default state for all components unless otherwise specified such as resistors defined to have noise contributions) then there is nothing to nudge the circuit away from equilibrium and so it may never start oscillating.Īlthough in most cases such oscillators will eventually start up due to the ‘hidden’ noise source which is simply due to the mathematical noise generated by the finite resolution and rounding errors of the calculations carried out in running a simulation, this can take a very long time compared to the time taken to run the oscillator in a stable oscillatory state for a few cycles. ![]() The initial state of oscillators based on a tuned circuit such as phase shift, Wien Bridge and Crystal Oscillators will be defined by their DC bias conditions. It will then either continue in that steady non-oscillating state or will slowly drift away from the steady DC state and oscillations will build up. At t=0 the circuit will then start from those initial DC conditions. Consequently, the voltages across all capacitors and the currents through all inductors in the circuit will have reached their DC steady state values prior to the simulation starting at t=0.Įven if the circuit is an oscillator, prior to t=0 it will have been assumed to be in a stable non-oscillating steady state. Therefore the voltages in a circuit, such as bjt base bias potential dividers and so on, will have been set to DC steady state values for all t<0. Similarly, a COS source with a 0V DC offset would be treated by the simulator as having been at +1V DC for all time prior to t=0. Similarly, a SINE source with a -1V DC offset or a PULSE source with an initial level of -1V are treated by the simulator as having been at -1V DC for all time prior to t=0. So for example, a DC power rail that is set to 9V is treated by the simulator as having been applied to the circuit at 9V for all time prior to t=0. IC statements included in a simulation and the Startup and UIC modifiers are not used, then the initial DC conditions of a circuit are completely defined by the initial levels or DC offsets of any sources present in the circuit and these are treated as having been present at those levels for all time prior to t=0. Whatever DC conditions are applied to the simulation at t=0 are considered to have been present for all time t<0.Įven though it is possible to start plotting the results of a time domain simulation at some time after t=0, the simulation itself always starts at t=0.Įven though an AC Analysis has no time duration associated with it, the DC operating point is calculated at t=0. There are several important points to understand about what happens at the start of a simulation. Some background and basic start-up techniques ![]() ![]() Initial conditions and starting up circuits Using the ‘OFF’ option to help start-up.Replacing ideal and Thevenin voltage sources with band-limited Norton Sources to help start-up.Setting the initial current through an inductor.Setting the initial voltage on a capacitor.Using a current source to set an initial current through an inductor.ic spice directive to set an initial voltage condition on a net Setting initial voltages on nets and currents through components.Some background and basic start-up techniques.Initial conditions and starting up circuits.
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