Since noise figure is defined to be the ratio of the signal-to-noise at the input, to the signal-to-noise at the output, clearly you must have some noise at the input of the circuit.

Spectre implements this by looking at the contribution of the inpit noise source at the output of the circuit, and dividing this into the total noise at the output of the circuit (less the noise in the load). The exact definition depends on whether it is SSB, DSB, or IEEE SSB noise figure, but broadly speaking they are similar. Essentially in the formulation used, the signal cancels out, so you only need to look at the noise contributions.

Using a vsin won't give you any noise input, and so you can't then measure the noise contribution from the input (also, the signal-to-noise at the input would be infinite). A port component has a built-in resistor and so generates thermal noise at (by default) 290K.

So the answer is no. You need a port.

Andrew.