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3 / 38 page December 1990 3 Philips Semiconductors Product specification Phase-locked-loop with lock detector 74HC/HCT7046A waveforms for the PC1 loop locked at fo are shown in Fig.7. The frequency capture range (2fc) is defined as the frequency range of input signals on which the PLL will lock if it was initially out-of-lock. The frequency lock range (2fL) is defined as the frequency range of input signals on which the loop will stay locked if it was initially in lock. The capture range is smaller or equal to the lock range. With PC1, the capture range depends on the low-pass filter characteristics and can be made as large as the lock range. This configuration retains lock even with very noisy input signals. Typical behaviour of this type of phase comparator is that it can lock to input frequencies close to the harmonics of the VCO centre frequency. Phase comparator 2 (PC2) This is a positive edge-triggered phase and frequency detector. When the PLL is using this comparator, the loop is controlled by positive signal transitions and the duty factors of SIGIN and COMPIN are not important. PC2 comprises two D-type flip-flops, control-gating and a 3-state output stage. The circuit functions as an up-down counter (Fig.5) where SIGIN causes an up-count and COMPIN a down-count. The transfer function of PC2, assuming ripple (fr =fi) is suppressed, is: where VDEMOUT is the demodulator output at pin 10; VDEMOUT =VPC2OUT (via low-pass filter). V DEMOUT V CC 4 π ----------- φ SIGIN φ COMPIN – () = The phase comparator gain is: VDEMOUT is the resultant of the initial phase differences of SIGIN and COMPIN as shown in Fig.8. Typical waveforms for the PC2 loop locked at fo are shown in Fig.9. When the frequencies of SIGIN and COMPIN are equal but the phase of SIGIN leads that of COMPIN, the p-type output driver at PC2OUT is held “ON” for a time corresponding to the phase difference ( φ DEMOUT). When the phase of SIGIN lags that of COMPIN, the n-type driver is held “ON”. When the frequency of SIGIN is higher than that of COMPIN, the p-type output driver is held “ON” for most of the input signal cycle time, and for the remainder of the cycle both n and p- type drivers are “OFF” (3-state). If the SIGIN frequency is lower than the COMPIN frequency, then it is the n-type driver that is held “ON” for most of the cycle. Subsequently, the voltage at the capacitor (C2) of the low-pass filter connected to PC2OUT varies until the signal and comparator inputs are equal in both phase and frequency. At this stable point the voltage on C2 remains constant as the PC2 output is in 3-state and the VCO input at pin 9 is a high impedance. Thus, for PC2, no phase difference exists between SIGIN and COMPIN over the full frequency range of the VCO. Moreover, the power dissipation due to the low-pass filter is reduced because both p and n-type drivers are “OFF” for most of the signal input cycle. It should be noted that the PLL lock range for this type of phase comparator is equal to the capture range and is independent of K p V CC 4 π ----------- Vr ⁄ () . = the low-pass filter. With no signal present at SIGIN the VCO adjusts, via PC2, to its lowest frequency. APPLICATIONS • FM modulation and demodulation • Frequency synthesis and multiplication • Frequency discrimination • Tone decoding • Data synchronization and conditioning • Voltage-to-frequency conversion • Motor-speed control |
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