An I-component mixer 211 mixes the input signal 210 with a reference signal generated from a local oscillator 215. The I-component mixed signal is then provided to sine circuitry 212. The sine circuitry 212 multiplies a sine function with the I-component signal. After the sine circuitry 212, the signal may be sampled by an analog-to-digital converter 213, to create a digitized I-component signal. The digitized I-component signal then goes to an arctangent 2 block 214, explained further below.

A Q-component mixer 217 mixes the input signal 210 with a ninety-degree shifted reference signal generated from a local oscillator 215 and a phase shifter 216. The Q-component signal is provided to cosine circuitry 218. The cosine circuitry multiplies the Q-component signal by a cosine function. After the cosine circuitry 218, the Q-component signal is sampled by an analog-to-digital converter 219, to produce a digitized Q-component signal. The digitized Q-component signal is provided to the arctangent 2 circuitry 214.

The arctangent 2 circuitry 214 is configured to receive the digitized I-component and Q-component signals. The arctangent 2 circuitry 214 performs an arctangent 2 function on the I-component and Q-component signals. The arctangent 2 circuitry 214 provides a phase difference 220 based on the I-component and Q-component signals, which is an error measurement. The phase difference 220 may be the beat tone error and/or the carrier envelope offset error from FIG. 1. The arctangent 2 function is an arctangent based on two inputs. The arctangent 2 function provides an angle (in radians) between the x-axis and a point defined by the two inputs.