In Embodiment 2, the subject matter of Embodiment 1 optionally includes wherein the comb error measurement circuitry includes: mixer circuitry configured to mix the carrier envelope offset out of the first signal using the first and second signal, and to provide the beat tone and the carrier envelope offset; beat tone digital phase detector circuitry configured to receive the beat tone, to use quadrature demodulation to detect a phase difference between a reference signal and the beat tone and a reference signal generated by a local oscillator, and to provide the beat tone error; carrier envelope offset digital phase detector circuitry configured to receive the carrier envelope offset, to use quadrature demodulation to detect a phase difference between the carrier envelope offset and the reference signal generated by the local oscillator, and to provide the carrier envelope offset error.

In Embodiment 3, the subject matter of Embodiment 2 optionally includes wherein the beat tone digital phase detector circuitry includes: a mixing circuit to mix the beat tone with a first reference signal and second reference signal generated from the local oscillator to produce a first mixed signal and a second mixed signal; an analog-to-digital converter to sample the first mixed signal and the second mixed signal to provide a first mixed digital signal and a second mixed digital signal; and arctangent 2 circuitry configured to receive the first and second mixed digital signals, to perform an arctangent 2 operation on the first and second mixed signals, and to provide the result of the arctangent 2 operation as the beat tone error.