I am interested in adding a more complex digital signal into my spice simulation. Is there a tutorial which covers this?

There are a handful of different ways to introduce a complicated digital signal into a SPICE simulation. I think the xspice interface is the easiest. There are some examples in the ngspice source; see, for example, from ngspice-36,

examples/xspice/d_source/

. The netlist is

d_source.cir

, the device model creating the digital input is the xspice

d_source

primitive, and the digital stimulus is read from a text file. The example looks like it is all xspice digital, which means that if you want to connect those devices to the input of a circuit with analog components like transistors, you would need to use the xspice

dac_bridge

primitive between the

d_source

and the rest of the circuit.