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Generation of Voltage Controlled Signal for Phase Locked Loop

3.3 Generation of Voltage Controlled Signal

3.3.1 Amplifier

A differential amplifier and low pass filter convert the phase detectors' signals to a variable voltage control signal for the VCO current source. The differential amplifier increases the phase lock loop signal with a gain of -1.8. While the low pass filter integrates this amplified signal causing a variable output at was calculated from 3.05V to 6.00V. A schematic of the amplifier and filter is shown in Figure 3.3.1.

Figure 3.3.1 Differential amplifier and low pass filter schematic.

Av ≈ -R12 / (R13 + R14) = -1.8

Output to VCO = VCC - R12 / DCBias

High = 6V - 3.6k / 0A = 6V

Low = 6V - 3.6k / 802μA = 3.05V

3.3.2 Implementation of Low Pass Filter

A low pass filter is used to perform an integrator function at the free running frequency of the phase lock loop, 10KHz. The LM565 has an internal resistor, R12, of 3.6KΩ for the use of the low pass filter. We choose to use a 0.1μF capacitor with resistor R12 because it provided a breakpoint frequency at 442Hz, giving us a wide frequency integration range, and availability of this value of capacitance. The following is the transfer function and bode plot for the low pass filter.

fc = 1 / (2Π R12 C2) = 442Hz

Figure 3.3.2 Bode plot of the transfer function for the low pass filter.

3.3.3 Output Results of the Amplifier and Filter

The input of the amplifier and filter provides a variable voltage from a measured value of 3.48V to 5.74V to the VCO depending on the signal that comes from the phase detector.

When the input signal to the phase detector is in phase with the VCO, the output voltage if the amplifier and filter will be 3.48V. When the input is 180° out of phase with the frequency of the VCO the voltage of the amplifier and filter will be 5.74V. The calculated results shown in Table 3.3.2 are based on ideal situations.

Table 3.3.1 Output to VCO and phase relationship results.

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