37883

The MM74HC4046 is a low power phase lock loop utilizing advanced silicon-gate CMOS technology to obtain high frequency operation both in the phase comparator and VCO sections. This device contains a low power linear voltage controlled oscillator (VCO), a source follower, and three phase comparators. The three phase comparators have a common signal input and a common comparator input. The signal input has a self biasing amplifier allowing signals to be either capacitively coupled to the phase comparators with a small signal or directly coupled with standard input logic levels. This device is similar to the CD4046 except that the Zener diode of the metal gate CMOS device has been replaced with a third phase comparator.

Phase Comparator I is an exclusive OR (XOR) gate. It provides a digital error signal that maintains a 90 phase shift between the VCO's center frequency and the input signal (50% duty cycle input waveforms). This phase detector is more susceptible to locking onto harmonics of the input frequency than phase comparator I, but provides better noise rejection.

Phase comparator III is an SR flip-flop gate. It can be used to provide the phase comparator functions and is similar to the first comparator in performance.

Phase comparator II is an edge sensitive digital sequential network. Two signal outputs are provided, a comparator output and a phase pulse output. The comparator output is a 3-STATE output that provides a signal that locks the VCO output signal to the input signal with 0 phase shift between them. This comparator is more susceptible to noise throwing the loop out of lock, but is less likely to lock onto harmonics than the other two comparators.

In a typical application any one of the three comparators feed an external filter network which in turn feeds the VCO input. This input is a very high impedance CMOS input which also drives the source follower. The VCO's operating frequency is set by three external components connected to the C1A, C1B, R1 and R2 pins. An inhibit pin is provided to disable the VCO and the source follower, providing a method of putting the IC in a low power state.

The source follower is a MOS transistor whose gate is connected to the VCO input and whose drain connects the Demodulator output. This output normally is used by tying a resistor from pin 10 to ground, and provides a means of looking at the VCO input without loading down modifying the characteristics of the PLL filter.