MX105A

The MX105A implements a frequency detector with a phase locked loop (PLL) and a lock detector. The voltage controlled oscillator (VCO) center frequency, detection bandwidth, loop filter, and detect filter are all independently controlled by external components.

The MX105A provides a pair of pseudo-sinewave multipliers for splitting the input signal into approximately orthogonal
components. These multipliers are implemented with commutating filters (cyclically sampling filters) which translate an in band AC input signal to DC. The commutating loop filter is used as the phase detector of the PLL while the commutating detect filter provides for lock detection. Each pseudo-sinewave has a cyclic form (1 1 0 -1 -1 0) to eliminate low order harmonic responses. The loop filter produces an error signal, which when applied to the VCO input allows frequency locking. A limiter between the loop filter output and the VCO input provides tunable control of the detection andwidth (BW). Once lock is achieved the detect filter produces a DC value proportional to the input tone amplitude. An internally generated reference is compared to the detect filter output to determine whether the PLL is locked to an input tone. Once lock is determined the internal reference is reduced by 50% to minimize output chatter with marginal input signals.

The sampling clocks of the detect filter lag those of the loop filter by 60°. To improve performance, a capacitor (C4) can be used to phase shift the input to the loop filter by 30°. This shifts all sampling clocks an additional 30° relative to the input tone to phase align the detect filter sampling clocks with the amplitude peaks of the input tone.