For the generation of noise signals, the 'PULSE' - sources have an additional property, the 'rand'- Property. Setting this property to 'rand' or 'nrand' means that the output of the source is no longer a deterministic periodic pulse, it is now a pulse train where a random generator determines whether the height of the pulse is the normal height or zero instead. The 'rand' setting generates a 'return-to-zero'-signal, where the 'nrand'-setting generates a 'no-return-to-zero' - signal. When using 'nrand', the normal PULSE-source-properties 'falll-time' and 'pulse period' are not used for the determination of the pulse train.
This noisy source is available only when using the built-in SPICE3 -Kernel. The introduction of noise could alternatively be done by using a PWL-source. Unfortunately, the SPICE3 parser needs an enourmous amount of time to parse PWL sources with more than 1000 lines.