delayltspice
I want to change the transmission delay-time Td (see picture) of a tline-component in LTspice as a function of time during simulation.
From my understanding, I cannot do that using the .step command, since that would generate individual simulations, each with a distinct value of the delay-variable {Td}.
Is the delay a variable that can be changed in time using a transient LTspice simulation?
You can do this by using a little trick: the .param statement in Spice only works with numbers, but Spice has a way to rename models with numerical names, the ako (Also Known As) option. IIRC, this may not be documented in LTSpice docs, but it does exactly what you want. See ltwiki:AKO Aliases (A Kind Of) for a little more information
To sweep models, simply use .model n ako:<name> to rename each model you want to sweep, and then use a .step param MParam list 1 2 .. n to sweep through those models. See below for an example, where I sweep through three simple diode models plus a "real" diode model.
LT spice should be consistent from run to run. Thermal noise is not simulated in a normal transient simulation. If your seeing differences from run to run they could most likely be explained from a numerical solver standpoint and differences on rounding, which some circuits can be quite sensitive to.
Another explanation may be that the models you are using have b-sources with a .rand statement in it, but this is unlikely.
If you are using a .rand statement, there isn't a way to set the seed (documented anywhere that I know of). If you need a .rand section to be repeated from run to run, use a PWL source, record the data as a .wav or PWL file and use that to make the runs consistent.
Best Answer
tlineandltlinemodels have fixed delay, i.e. they cannot be a function of time. If you need such a delay, you can only use a behavioural source with the builtin functionV=delay(V(in), f(time))(forBVsource, for ex.), wheref(time)is a function defined previously with a.funcstatement, or made up ad hoc (e.g.V=delay(V(in), V(control))).You should know that, while you can use that, in both
.ACand.TRANanalyses, in the latter it may suffer from minor (seemingly erratical) drifts, depending on the configuration of your schematic. One cause may be the numerical accuracy (dynamic range) which, unfortunately, affects the behavioural sources, but I can't point out exactly what/who/where, I'm afraid that's up to you to find out.Alternatively, you can build your own LC delay line built with the behavioural inductors (
Flux=f(x)) and capacitors (Q=f(x)), but I, personally, would recommend avoiding. Your choice.