How do you simulate voltage noise with LTSpice?

Question

Is there a way to setup a voltage supply with voltage jitter/noise? I want to experiment with filtering out noise on various voltages etc. but not sure how to configure LTSpice to create a noisy voltage supply.

Answer 1

Yes, you can inject noise using the arbitrary voltage (or current) source, then use things like the random or white function to create some noise.

Here is an example circuit (I separated the noise from the signal just to make things clearer - obviously you can combine them together in one function if you wish):

Simulation:

All the functions are detailed in the help under circuit elements -> arbitrary behavioral voltage or current sources.

Noise simulation mode

Also, just in case you were not aware, SPICE has a noise simulation mode, to quote from the help files:

.NOISE -- Perform a Noise Analysis
This is a frequency domain analysis that computes the noise due to
Johnson, shot and flicker noise. The output data is noise spectral 
density per unit square root bandwidth.

Syntax: .noise V(<out>[,<ref>]) <src> <oct, dec, lin> <Nsteps> <StartFreq> <EndFreq>

Basic example:

Simulation:

The above is rather boring as it only models the resistor noise (I stepped the resistor through various values to show how the Johnson noise increases with resistance). But it can be very useful with more complex circuits containing diodes/transistors/opamps/etc.

Answer 2

SPICE ( I can't tell you if LTSPice is a subset of normal SPICE or not) normally has the ability to model the noise that each device generates. I think your question is more about how to you measure how effective your filtering is and how much an external interfering signal may affect each node.

To do that what you need to do that is .AC analysis of the circuit.

To do a noise analysis you need to use both .ac and .noise. So noise analysis is a subset of ac analysis.

Answer 3

As you want to inject noise from the power supply, I think easiest is to put a small amplitude AC voltage source in series with the DC voltage source you already have and sweep its frequency through the range you are interested in.