How do I vary the frequency of a signal source in LTspice?

Question

I'm trying to test the bandwidth of a common emitter amplifier for an electronics practical using LTspice. I need to vary the frequencies from 10Hz, 20Hz, 50Hz then decade by decade up to 1MHz, using the .step param function, but I do not know how to go about it.

Answer 1

If you want to get the frequency response of a circuit you can run an AC analysis instead. For that, you'll need to configure your signal source:

And the simulation command:

^{Above is just an example i.e. you are free to change the values.}

If you run the simulation and plot the output node, you'll get the frequency response (solid green: gain, dotted green: phase):

You can find the -3 dB point by putting a cursor on the frequency response plot and moving it manually, or you can ask LTspice to find it for you with .meas command:

.meas ac f find frequency when mag(V(out))=0.707

After the simulation you can open the SPICE Error Log and see the result:

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If you still want to use a voltage source with varying frequency then you can define a function for the frequency interpolation and pass it to a behavioural source:

.param start_f 1
.param stop_f 1Meg
.func fot(sim_t) { start_f + time * (stop_f - start_f) / sim_t }

The function above makes a linear interpolation between start_f and stop_f which are the start and stop frequencies, respectively. sim_t is the simulation duration which is the value you pass to .tran function normally.

Now you can use the function in a behavioural source:

The result may look a bit off: