I'm teaching myself how to use QSPICE and I'm very new to simulators (but not so new to electronics itself).

So first, is this the appropriate forum section to ask such questions?

Second, I'm looking at very simple initial examples and see the term "transient" used as a kind of analysis.

I've always understood the term itself to mean a brief, noisy, jitter or change in voltage but it seems to be used in simulators to mean a fixed length of time with some defined voltage source (which might be DC, pulses, AC etc.).

This what Keysight say:



But in QSPICE it seems be used to represent a basic application of a source to a circuit for a defined length of time.

Is that true? am I understanding that correctly? is it like "connect the voltage source to the circuit for x mS" ?

 

hi F,
The LTSpice Help option shows this for TRAN

.TRAN -- Do a Nonlinear Transient Analysis
Perform a transient analysis. This is the most direct simulation of a circuit. It basically computes what happens when the circuit is powered up. Test signals are often applied as independent sources.

Try LTS Help. Search for TRAN...

E

Transient Analysis Options .TRAN Modifiers
UIC: Skip the D.C. operating solution and use user-specified initial conditions.

steady: Stop the simulation when steady state has been reached.

nodiscard: Don't delete the part of the transient simulation before steady state is reached.

startup: Solve the initial operating point with independent voltage and current sources turned off. Then start the transient analysis and turn these sources on in the first 20 us of the simulation.

step: Compute the step response of the circuit.

 

The more general definition of transient is short-lived. Transient analysis recalculates the currents and voltages at short intervals to approximate the behavior if the circuit over time. Perhaps that is why it is called transient analysis.

 

is this the appropriate forum section to ask such questions?

There is a forum specifically for 'EDA and simulation'.

 

For Spice "Transient" means a calculation for the circuit nodal currents and voltages over a given period, for selected source voltages.
Transient just means not static ( a DC op pnt sim will do static analysis).
Transient performs an analysis using the full non-linear device characteristics for DC, or varying pulse or AC input(s) over the specified time.
It uses incremental, short-interval digital calculations of all the node voltages and currents to approximate what a smooth analog response would look like, rather the way a digital scope takes many discrete A/D samples to display a (nearly) smooth waveform as an analog scope would show.

Alternately, AC analysis uses a linear and fixed approximation of the devices characteristics at their DC bias point to perform an an AC sweep gain/phase (Bode) response of a circuit over a selected frequency range.
The response is taken to be linear over any AC voltage range, so does not include things like saturation/clipping that the transient analysis would show.
Typically 1V is used for the AC source which then gives 0dB on the plot for a gain of 1.

 

For Spice "Transient" means a calculation for the circuit nodal currents and voltages for a given period, for selected source voltages.
Transient just means not static.
Transient performs an analysis using the full non-linear device characteristics for DC, or varying pulse or AC input(s) over the specified time.
It uses incremental, short-interval digital calculations of all the node voltages and current to approximate what a smooth analog response would look like, rather the way a digital scope takes many discrete A/D samples to display a (nearly) smooth waveform as an analog scope would show.

Alternately, AC analysis uses a linear and fixed approximation of the devices characteristics at their DC bias point to perform an an AC sweep gain/phase (Bode) response of a circuit over a selected frequency range.
The response is taken to be linear over any AC voltage range, so does not include things like saturation/clipping that the transient analysis would show.
Typically 1V is used for the AC source which then gives 0dB on the plot for a gain of 1.

Wow, it does sound pretty involved, fascinating too, thanks for explaining this.