Experiment No. 3 Pre-Lab Phase Locked Loops and Frequency Modulation The Pre-Labs are informational and although they follow the procedures in the experiment, they are to be completed outside of the laboratory. There are questions given in the pre-lab. Your answers are to be submitted to your lab instructor before the experimental procedure is performed. Introduction A phase locked loop is a control system that allows the frequency of a voltage-controlled oscillator (VCO) to be synchronized to a reference oscillator. When the system is locked the VCO frequency will follow changes in the reference oscillator frequency and there will be a constant phase difference between the oscillators. By definition two signals having a constant phase difference have the exact same frequency. When reference frequency is varied it first causes a phase change between the oscillators. A phase detector (also called phase comparator) within the control loop will sense the phase change and feed a voltage to the VCO input to change its frequency. The phase difference between the two signals will adjust to a new value for each reference frequency value. When the two oscillators are synchronized and their frequencies are the same the system is said to be locked. When the phase difference exceeds the limits of the phase detector (±180º) the system loses lock and the oscillators are no longer synchronized. An integrated circuit (IC) phase locked loop (PLL) will be tested to determine capture and lock in ranges. The frequency deviation of an FM signal produced by the function generator will be measured and its spectrum will be observed as the frequency deviation and modulating frequency are varied. The PLL will also be used to demodulate the FM signal. Part A: Phase Locked Loop Operation Objective: To investigate the operation of a phase locked loop and measure the capture and lock in range 1
The circuit shown in the figure will be built and tested. The physical blocks in the diagram are interior to the IC and are shown only to clarify the function of the system, and it will only be necessary to connect the resistors, capacitors and the power supply exterior to those pins. A 100kΩ potentiometer will be used for R1 and C1 will be 250pF. The power supply voltage will be set to +10V. Without a jumper wire connected between pins 3 and 4, the value of R1 will be adjusted to produce a free running VCO frequency of 50 khz. The output of the phase comparator (phase detector) is a voltage proportional to the phase difference between the input signal frequency and the VCO frequency. The low pass filter removes high frequency variations of the phase comparator inputs so that only the phase variations are applied to the VCO. It also determines the capture range and provides stability for the system. The function generator will be connected to the input terminal and set for a 50 khz sinusoidal waveform. A jumper wire will be connected between pins 3 and 4 to close the loop and allow the VCO frequency to be controlled by the function generator. You should see a 50 khz square wave at pin 3 (or 4) that is phase locked to the function generator signal. To prove this you will observe the function generator and the VCO signals at the same time using both oscilloscope channels. The phase difference between the waveforms will be determined by triggering on the positive leading edge of the square wave and measuring the time difference between the positive leading edge of the square wave and the positive zero crossing of the sinewave. The phase difference in degrees is the time difference divided by the period of the waveforms (or multiplied by the frequency) and multiplied by 360 degrees. 2
Q1. In the figure below, determine the phase difference between the waveforms. The function generator frequency will be varied a small amount. The phase will change but the waveforms should remain synchronized. The phase change produces a voltage that adjusts the VCO frequency to maintain phase lock. (Footnote #1) The hold-in range (also called lock range) is the range of frequencies at which the PLL can maintain lock. The capture range is the range of frequencies at which the PLL can regain lock after lock is lost. The function generator frequency will be increased until the system loses lock and the VCO will be free running. This frequency will determine the upper boundary of the hold-in range. The frequency will be lowered to regain lock and this frequency will determine the upper frequency of capture. The function generator frequency will then be decreased until the system loses lock in order to determine the lower boundary of the hold-in range. The frequency will then be increased to regain lock and determine the lower capture frequency. The hold-in range (2fL) is determined by subtracting the lower limit of hold-in frequency from the upper limit, and the capture range (2fc) is determined by subtracting the lower limit of capture frequency from the upper limit. You will find that the capture range is significantly smaller than the hold-in range. In other words it is more difficult for a PLL to achieve lock than it is to maintain lock. The theoretical hold-in range using the configuration given in the circuit diagram is equal to the center frequency of 50 khz. This means the system should be able to remain locked to twice the center frequency and theoretically remain locked to near dc. The theoretical capture range given in the CD4046B data sheets is, 2fc = (1/π)(2πfL/R3C2) 1/2 3
Q2. Using the values given for R3 and C2 in the experiment, calculate the capture range that you should expect in the experiment. Q3. What causes the system to lose lock? Q4. How could a wider hold-in or capture range be obtained? (Footnote #2) Part B: Frequency Modulation Objective: To measure the frequency deviation and frequency spectrum of an FM signal for different modulation conditions The function generator will be set to produce a 50 khz sinewave with amplitude of 200 mv p-p, and the FM function will be selected by pressing Shift FM. You will see FM displayed on the front panel readout. The default modulation waveform is sine wave. The modulating frequency will be set to 8 khz using Shift Freq and the peak frequency deviation will be set to 30 khz using Shift Level. The function generator will be connected to the oscilloscope in order to observe the FM waveform. You will be able to observe the period of the signal getting larger and smaller at the modulating frequency rate. This is indicative of frequency modulation. You will be able to vary the modulation amplitude and frequency slightly and observe the changes in the FM waveform. 4
You will set the modulation frequency to 2 khz and the deviation to 5 khz and the oscilloscope will be set to the FFT mode. The spectrum of the FM waveform will be observed. You will identify the 50 khz carrier peak, which should have several sideband peaks around it, and center it on the screen, and use the Zoom function to spread the spectrum for better observation of the sidebands. You will verify that there are about the same number of sidebands and that they are at about the same relative amplitudes as predicted by theory based on the Bessel function coefficients. You will then change the frequency deviation to 100 Hz and observe that the spectrum changed and calculate the modulation index. You should see a spectrum not unlike an AM spectrum. Q5. What is the name given to this type of FM signal? You will experiment with other frequency deviations and also vary the modulation frequency to see the effects on the spectrum. You will also exit the FFT mode and observe the FM waveform as you vary the modulation frequency and deviation. An interesting picture should appears when you set the modulation frequency below about 10 Hz since your eyes can easily follow the frequency variations. This is also interesting to view in the FFT mode. Part C: FM Demodulation Objective: To observe the 565 PLL as an FM demodulator The modulation frequency will be readjusted to 500 Hz and the deviation to 5 khz. The function generator will be connected to the input of the PLL circuit and you will observe the output. You will see the 500 Hz demodulated sine wave at the PLL output. The signal variation will be small compared to the DC level so you will need to AC coupling the output. If high frequency noise is present you can add an RC low pass filter (10kΩ and 5nF are suggested). The PLL must be locked throughout the frequency variation of the input for proper demodulation. You may need to adjust R1 if you are loosing lock. Note that to maintain lock, the PLL must change phase as the input frequency changes. The output voltage is the amplified phase detector output and therefore changes with the input frequency. You will vary the modulation frequency and frequency deviation of the function generator and observe the changes in the PLL output. You should see the output signal vary in frequency and amplitude according to the generator frequency and deviation changes. (Footnote #3) Q6. What would happen to the demodulated signal if the PLL became unlocked? 5
Footnotes: 1. The PLL is able to attain frequency synchronization between the input frequency and the VCO by keeping the phase difference between them constant. With a constant phase difference there can be no frequency difference. When the system is locked, input frequency changes cause the phase detector output voltage to adjust the VCO frequency to match. The phase relationship between the signals will be different but the phase will not vary. Any practical phase detector only responds over a maximum range of ±180 degrees or ±90 degrees in the case of the CD4046B. When this limit is exceeded the system will lose lock. When the system loses lock the phase will be varying at a frequency equal to the difference between the function generator and VCO frequencies. 2. The VCO can be made more sensitive to phase changes if an amplifier were placed in the loop. This would allow a smaller phase change to adjust the VCO frequency. The hold in range (also called the lock range) for the CD4046B with the configuration used, is the center frequency of the VCO or 50kHz. The loop gain, which depends on the sensitivities of the phase comparator and the VCO, could be increased if we could add a voltage amplifier between the phase comparator and VCO. However, there are limitations to extending the lock range. Generally as we increase the gain of a control system, the system tends to be more unstable and unable to maintain control. 3. Since to maintain phase lock the phase detector output voltage must change in direct proportion to the input frequency, the output voltage will be a measure of the input frequency as long as the loop remains locked. Therefore, the output represents the demodulated FM signal. There are many other types of FM demodulators, most of which are described in your textbook. The PLL is one of the best at demodulating signals that are noisy, because it does not respond to amplitude changes in the input. 6