Frequency multiplication can also be attained by locking the VCO output to the "N"th harmonic of the reference signal. Instead of a simple phase detector, the design uses a harmonic mixer (sampling mixer). The harmonic mixer turns the reference signal into an impulse train that is rich in harmonics. The VCO output is coarse tuned to be close to one of those harmonics. Consequently, the desired harmonic mixer output (representing the difference between the "N" harmonic and the VCO output) falls within the loop filter passband.