A transmitter comprises a local oscillator circuit operable to generate a reference signal, a modulator circuit operable to generate a data-carrying signal using the reference signal, and a test signal generator circuit operable to generate a test signal using the reference signal. The test signal has a first bandwidth, and a test signal insertion circuit is operable to combine the data-carrying signal and the test signal to generate a combined signal. An amount of bandwidth in the combined signal allocated to the test signal is greater than the first bandwidth such that a component of the combined signal corresponding to the test signal is bordered by whitespace. A receiver may then use the test signal to determine and correct for phase noise introduced in the transmitter.