Phase-Locked Loop Analog Communications Exercise 2: FM Detection With a PLL EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain how the phase detector s input frequencies affect the output signals, explain how the feedback signal to the VCO varies with the phase change between the input signals, and describe how a PLL demodulates an FM signal. You will use an oscilloscope and a multimeter to make measurements. EXERCISE DISCUSSION The PLL s phase detector is a balanced modulator that multiplies the inputs to produce sum and difference frequencies at the output. 408 FACET by Lab-Volt
Analog Communications Phase-Locked Loop The PLL output is also the feedback signal that controls f vco to match f i. When a PLL is locked, the phase detector s input frequencies (f i and f vco output contains a sum frequency, which is twice the input frequency, and a difference component, which is a dc voltage. When f i changes, what occurs? a. i and f vco. b. The phase detector s dc voltage difference component changes. c. Both of the above Every variation in f i causes a phase change with f vco, which then causes the dc voltage difference component to change. FACET by Lab-Volt 409
Phase-Locked Loop Analog Communications The dc voltage, which is the feedback to the VCO, causes f vco to change so it equals f i. When the RF signal to the PLL is an FM signal, the continuous FM frequency deviations cause continuous phase changes between f i and f vco. The FM signal s frequency deviations are a function of what signal? a. VCO b. carrier c. message Because the f i and f vco phase difference changes with the message signal, the phase detector s dc voltage difference component varies with what signal? a. message b. VCO 410 FACET by Lab-Volt
Analog Communications Phase-Locked Loop recovered message signal. The recovered message signal from the PLL is also the feedback signal to the VCO. FACET by Lab-Volt 411
Phase-Locked Loop Analog Communications The message signal feedback to the VCO varies f vco to match what signal? a. the phase detector s sum frequency b. the changing f i After capture, if the bandwidth of the FM signal stays within the PLL s lock range, the PLL recovers the message signal. However, if the bandwidth of the FM signal becomes greater than the PLL s lock range, to what frequency will f vco return? a. its free-running frequency (f o ) b. the message signal frequency PROCEDURE Connect the PHASE-LOCKED LOOP circuit block as shown. Two-post connectors should be in the 452 khz terminals on the VCO-LO circuit block and between the FILTER and AMP on the PHASE-LOCKED LOOP circuit block. 412 FACET by Lab-Volt
Analog Communications Phase-Locked Loop Connect the channel 2 probe to the VCO output on the PHASE-LOCKED LOOP circuit block. Turn the NEGATIVE SUPPLY knob on the base unit fully CCW. Adjust the potentiometer knob on the VCO-LO circuit block for a 150 mv pk-pk signal at RF. FACET by Lab-Volt 413
Phase-Locked Loop Analog Communications Set the oscilloscope vertical mode to DUAL and trigger on channel 1. Set the voltmeter to measure volts dc. Connect the voltmeter to the VCO input, and connect the voltmeter common lead to a ground terminal on the circuit board. During the next step, you will observe the RF and VCO signals on the oscilloscope and the VCO input voltage (V I ) on the voltmeter. Slowly increase (f i ) (channel 1) by turning the NEGATIVE SUPPLY knob CW. When the f vco signal starts to track f i and V I is 4.0 Vdc, stop turning the NEGATIVE SUPPLY knob CW. The signals should appear as shown. 414 FACET by Lab-Volt
Analog Communications Phase-Locked Loop On the oscilloscope screen, compare f vco and f i by overlaying the signal traces. Are the frequencies about equal? a. yes b. no Trigger the oscilloscope on channel 1. The signals should appear as shown. Is f i vco (channel 2)? a. yes b. no When the RF input frequency (f i ) equals the VCO output frequency (f vco ), what signals are in the PHASE DETECTOR s output? a. the sum frequency of the inputs and a difference component, which is a dc voltage b. f i and the sum difference frequencies of the inputs FACET by Lab-Volt 415
Phase-Locked Loop Analog Communications Connect the channel 2 probe to the PHASE DETECTOR s output. Set channel 2 to Is the frequency of the signal on channel 2 twice the frequency of the RF signal on channel 1? a. yes b. no What is the PHASE DETECTOR s output signal shown on channel 2? a. difference frequency b. sum frequency 416 FACET by Lab-Volt
Analog Communications Phase-Locked Loop Connect the channel 2 probe to the FILTER s output. Set channel 2 to dc and trigger on channel 1 (see the image below). While observing the FILTER s output on channel 2, slightly vary f i by slowly turning the NEGATIVE SUPPLY knob CCW and then CW. When f i varies, does the FILTER s dc output voltage level change? a. yes b. no What is the FILTER s output signal shown on channel 2? a. the PHASE DETECTOR s sum frequency b. the PHASE DETECTOR s difference component FACET by Lab-Volt 417
Phase-Locked Loop Analog Communications What causes the PHASE DETECTOR s dc difference component to change with a varying f i? a. an amplitude difference between f i and f vco b. a changing phase difference between f i and f vco Does the change in the FILTER s dc output voltage cause V I to change? a. yes b. no What does V I control? a. f vco b. f i Trigger on channel 1. 418 FACET by Lab-Volt
Analog Communications Phase-Locked Loop While observing f vco on channel 2 and V I on the voltmeter, vary f i by slightly turning the NEGATIVE SUPPLY knob CW and then CCW. When V I changes, does f vco change and stay equal to f i on channel 1? a. yes b. no Connect the SIGNAL GENERATOR s output to the (M) terminal on the VCO-LO circuit block. Connect the channel 1 probe to (M). Adjust the SIGNAL GENERATOR for a 150 mv pk-pk, 3 khz sine wave message signal at (M). On the PHASE-LOCKED LOOP circuit block, connect the channel 1 probe to RF at the PHASE DETECTOR input, and connect the channel 2 probe to the VCO output. Set channel FACET by Lab-Volt 419
Phase-Locked Loop Analog Communications Adjust the NEGATIVE SUPPLY knob on the base unit completely CCW. You should observe an FM signal on channel 1, as shown in the image below. During the next step, you will observe the RF and VCO signals on the oscilloscope and V I on the voltmeter. Slowly increase f i (channel 1) by turning the NEGATIVE SUPPLY knob CW. When the f vco signal starts to track f i and V I is about 4.0 Vdc, stop turning the NEGATIVE SUPPLY knob CW. The signals should appear as shown. 420 FACET by Lab-Volt
Analog Communications Phase-Locked Loop Connect the channel 1 probe to (M) on the VCO-LO circuit block, and trigger on channel 1. in the image below. What signals compose the PHASE DETECTOR output signal on channel 2? a. the sum frequency and the varying dc voltage difference component b. the VCO signal and the difference frequency What signal is the varying dc voltage difference component? a. the FM carrier signal b. the recovered message signal FACET by Lab-Volt 421
Phase-Locked Loop Analog Communications Connect the channel 2 probe to the FILTER output on the PHASE-LOCKED LOOP circuit What is the signal at the FILTER s output on channel 2? a. the recovered message signal b. the error signal c. Both of the above. Slightly vary the frequency and amplitude of the message signal from the SIGNAL GENERATOR. Do the frequency and amplitude of the recovered message signal vary with the message signal? a. yes b. no Does the message signal feedback to VCO change f vco to match the varying f i? a. yes b. no 422 FACET by Lab-Volt
Analog Communications Phase-Locked Loop Connect the channel 2 oscilloscope probe to the FILTER output. Set channel 2 to At the SIGNAL GENERATOR, increase the message signal amplitude on channel 1 to 300 mv pk-pk. Is the signal on channel 2 the recovered message signal? a. yes b. no CONCLUSION When the PLL is locked, the phase detector outputs a dc voltage difference component that changes with the phase change between the RF and VCO input signals. When the RF input is an FM signal, the phase detector s dc voltage difference component varies with the message signal contained in the FM signal. the low-frequency recovered message signal. The recovered message signal is the PLL s output, and it is also the feedback signal that controls the frequency of the VCO to match the frequency of the FM input signal. To recover the message signal, the bandwidth of the FM signal must be within the PLL s lock range. FACET by Lab-Volt 423
Phase-Locked Loop Analog Communications REVIEW QUESTIONS 1. When the PLL is locked, how is the phase detector s output affected by a phase change between the f i and f vco inputs? a. The phase of the sum frequency changes by an equivalent amount. b. The difference frequency increases. c. The dc voltage difference component changes. d. All of the above 2. When the RF input to a PLL is an FM signal, what signal causes the phase detector s dc voltage difference component to vary? a. VCO signal b. sum frequency signal c. feedback signal d. FM message signal 424 FACET by Lab-Volt
Analog Communications Phase-Locked Loop 3. When the RF input to a PLL is an FM signal, what is the PLL s output signal? a. recovered message signal b. error signal c. feedback signal d. All of the above 4. In order for a locked PLL to demodulate an FM signal, the FM signal s bandwidth must be within what range? a. capture range b. lock range c. free-running frequency ±3 khz d. audio frequency range 5. In order for a PLL to capture an FM signal, the VCO s free-running frequency must be close to what frequency? a. message signal frequency b. lock range c. FM center frequency d. product detector s sum frequency FACET by Lab-Volt 425