The purpose of this lab is to use a phased-locked loop circuit to implement a simple demodulator for phase modulated signals.

Voltage-Controlled Oscillator

Phased-locked loops are available in integrated-circuit packages which contain a phase-detector and a VCO. The PLL we will use in the lab is the popular 74HC4046. The VCO frequency range for the 74HC4046 is determined by two external resistors and an external capacitor. The discussion on the 74HC4046 in your HCMOS data book shows how to select values for these components.

• Design the VCO part of the PLL to have a frequency range of 10 kHz centered around your carrier frequency (use 50kHz if you haven't built your oscillator).
• Build the VCO portion of the PLL. Verify that the with 0V in, the VCO frequency is at its minimum frequency, and with 5V in, the frequency is at its maximum. You may have to fiddle with the values of R1 and R2 to get good results.
  

Phased-Locked Loop

To use the PLL as a demodulator, we need to finish the PLL by adding the phase detector and low-pass filter. The 74HC4046 has three different phase detectors, as discussed in the data book. For this experiment, we will use the Type~2 detector. We will also use a simple RC filter as the low-pass filter, as discussed in class.

• Add the low-pass filter to the circuit. As discussed in class, choose your components so that the lowpass cutoff is a decade below the lowest frequency in the transmitted signal (e.g. use a 5 Hz cutoff if you expect the lowest frequency in the modulation signal to be 50 Hz).
• Add the phase detector. Connect the output of the VCO to the Comparator In pin of the 74HC4046. Set up your function generator to generate a square wave from 0~V to 5~V. Connect the output of the function generator to the Signal In pin of the 74HC4046. Verify that the PLL works; i.e., verify that the output frequency of the VCO is the same as the input frequency from the function generator for frequencies near your carrier frequency. What are the maximum and minimum frequencies the PLL can lock onto?
• What is the phase difference between the two signals going into the phase comparator for different input frequencies?
• What is the (average) input voltage to the VCO for different input frequencies?
• Sketch and explain the output of the Phase Comparator II Out pin.

PM demodulation

• Connect the signal generator to your phase modulation circuit to your phase modulation circuit from the previous lab. Set the generator to produce a 1 kHz sinusoid with an offset and amplitude which will produce maximal phase shifts.
• Construct a comparator which will produce a 0-5V square wave from the output of the phase modulator. Then connect the modulator to the comparator input and the comparator output to the PLL.
  • Observe the output of the phase comparator. How is it related to the output of the signal generator?
  • How can the original signal (the signal generator output) be recovered? Construct a circuit to do this and test it.
  • Observe the output of the lowpass filter in the PLL. Is it stable? Why or why not?