HCT7046A

The 74HC/HCT7046 are high-speed Si-gate CMOS devices and are specified in compliance with JEDEC standard no. 7.

The 74HC/HCT7046 are phase-locked-loop circuits that comprise a linear voltage-controlled oscillator (VCO) and two different phase comparators (PC1 and PC2) with a common signal input amplifier and a common comparator input. A lock detector is provided and this gives a HIGH level at pin 1 (LD) when the PLL is locked. The lock detector capacitor must be connected between pin 15 (CLD) and pin 8 (GND). The value of the CLD capacitor can be determined, using information supplied in Fig.32. The input signal can be directly coupled to large voltage signals, or indirectly coupled (with a series capacitor) to small voltage signals. A self-bias input circuit keeps small voltage signals within the linear region of the input amplifiers. With a passive low-pass filter, the "7046" forms a second-order loop PLL. The excellent VCO linearity is achieved by the use of linear op-amp techniques.

The 74HC/HCT7046 VCO requires one external capacitor C1 (between C1A and C1B) and one external resistor R1 (between R1 and GND) or two external resistors R1 and R2 (between R1 and GND, and R2 and GND). Resistor R1 and capacitor C1 determine the frequency range of the VCO. Resistor R2 enables the VCO to have a frequency offset if required.

The high input impedance of the 74HC/HCT7046 VCO simplifies the design of low-pass filters by giving the designer a wide choice of resistor/capacitor ranges. In order not to load the low-pass filter, a demodulator output of the VCO input voltage is provided at pin 10 (DEMOUT). In contrast to conventional techniques where the DEMOUT voltage is one threshold voltage lower than the VCO input voltage, here the DEMOUT voltage equals that of the VCO input. If DEMOUT is used, a load resistor (RS) should be connected from DEMOUT to GND; if unused, DEMOUT should be left open. The 74HC/HCT7046 VCO output (VCOOUT) can be connected directly to the comparator input (COMPIN), or connected via a frequency-divider.

The 74HC/HCT7046 VCO output signal has a duty factor of 50% (maximum expected deviation 1%), if the VCO input is held at a constant DC level. A LOW level at the inhibit input (INH) enables the VCO and demodulator, while a HIGH level turns both off to minimize standby power consumption.