Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for detecting characteristics of an input signal. One aspect includes a first finite input response (FIR) filter, a second FIR filter, and a controller coupled with the first and second FIR filters. The first FIR filter receives an input signal and a first reference signal. The first FIR filter filters the first reference signal to generate a first sinusoidal signal and mixes the first sinusoidal signal and the input signal to generate a first mixed signal. The second FIR filter receives the input signal and a second reference signal. The second FIR filter filters the second reference signal to generate a second sinusoidal signal and mixes the second sinusoidal signal and the input signal to generate a second mixed signal. The controller determines characteristics of the input signal based on the first and second mixed signals.