A signal source providing a swept-frequency output (see timebase) that tracks the internal swept-frequency local-oscillator signal in a spectrum analyser, enabling frequency-response measurements to be made on two-port circuits using the spectrum analyser. The tracking generator is driven by the same timebase as the local oscillator so that the frequency sweeps are synchronized, and the output frequency of the tracking generator is offset from the local oscillator by the value of the first intermediate frequency, thus producing the desired input frequency.