Exercise 2: Demodulation (Quadrature Detector)

Transcription

1 Analog Communications Angle Modulation and Demodulation Exercise 2: Demodulation (Quadrature Detector) EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain demodulation of an FM signal and describe the operation of a quadrature detector. You will use an oscilloscope to make signal measurements. DISCUSSION FM demodulators are referred to as discriminators or frequency detectors. A quadrature detector is one of several circuits that can demodulate FM signals. Other FM demodulator circuits include the Foster-Seeley discriminator, the ratio detector, the pulsecounting detector, and the phase-locked loop detector. All of these circuits convert FM frequency variations into the amplitude and frequency of the message signal. The frequency deviations caused by 1 khz and 2 khz message signals with equal amplitudes are shown. The X-axis is the FM frequency deviation, and the Y-axis is time. The intersection of the X and Y axes is the center frequency of the FM carrier, or 0 frequency deviation. The time span on the Y-axis is 1 ms. FACET by Lab-Volt 337

2 Angle Modulation and Demodulation Analog Communications The left plot shows the frequency deviations caused by a 1 khz sine wave message signal. The right plot shows the frequency deviations caused by a 2 khz sine wave message signal. Because the 1 khz and 2 khz message signals have equal amplitudes, the magnitudes of the frequency deviations are equal. A 1 khz message signal causes two maximum frequency deviations (one positive and one negative) per millisecond. A 2 khz message signal causes how many maximum frequency deviations per millisecond? a. 1 b. 2 c. 3 d. 4 An FM discriminator converts the magnitude and rate of the frequency deviations into the amplitude and frequency of the message signal. 338 FACET by Lab-Volt

3 Analog Communications Angle Modulation and Demodulation The FM discriminator on the circuit board is a quadrature detector. T DETECTOR, and a FILTER (see the image below). At the quadrature detector s input, the FM signal takes two paths. FACET by Lab-Volt 339

4 Angle Modulation and Demodulation Analog Communications T balanced modulator. The phase detector outputs a signal with a frequency that is twice the FM frequency and a dc voltage that varies with the phase difference between the two inputs. Because the phase difference between the phase detector s input signals varies with the FM signal s frequency deviations, the baseband output voltage changes with the amplitude and frequency of what signal? a. FM carrier signal b. message signal dc output voltage as the recovered message signal. 340 FACET by Lab-Volt

5 Analog Communications Angle Modulation and Demodulation PROCEDURE The following PROCEDURE on the quadrature detector is divided into two sections: Phase Shifter and Limiter Phase Detector and Filter Each section starts with an explanation of the signals that you will observe and the parameters that you will measure and calculate. Phase Shifter and Limiter In this PROCEDURE section, you will observe how a phase shifter changes the phase of an FM carrier signal and how a limiter reduces the amplitude of the phase-shifted signal. circuit. The FM signal takes two paths at the quadrature detector s input. FACET by Lab-Volt 341

6 Angle Modulation and Demodulation Analog Communications Because the resonant frequency (f r ) of the LC network equals the FM center frequency, it is a purely resistive impedance to the FM center frequency. 342 FACET by Lab-Volt

7 Analog Communications Angle Modulation and Demodulation However, frequencies greater or less than f r original FM signal. r of the if the frequency is lower than the tuned circuit. FACET by Lab-Volt 343

8 Angle Modulation and Demodulation Analog Communications In reference to the original FM signal, frequency deviations on each side of the FM center frequency will be a. in phase. b. c. The phase-shifted FM signal is input to the limiter. The limiter has two diodes connected from the output to ground with their polarities reversed: anodes connect to cathodes. The reversed-polarity diodes limit the output amplitude and minimize any AM modulation that the phase shifter may cause. On the VCO-LO circuit block, insert the two-post connector in the 452 khz terminals. 344 FACET by Lab-Volt

9 Analog Communications Angle Modulation and Demodulation Connect (FM) OUT on the VCO-LO circuit block to FM on the QUADRATURE DETECTOR circuit block. Connect the oscilloscope channel 1 probe to FM on the QUADRATURE DETECTOR circuit block. With the potentiometer knob on VCO-LO, adjust the unmodulated FM carrier signal at FM for 300 mv pk-pk. FACET by Lab-Volt 345

10 Angle Modulation and Demodulation Analog Communications Pull out the oscilloscope TIME VARIABLE knob, and adjust the channel 1 signal so that one Adjust the FM frequency by turning the NEGATIVE SUPPLY knob on the left side of the base unit until the waveform on channel 2 has a maximum amplitude. 346 FACET by Lab-Volt

11 Analog Communications Angle Modulation and Demodulation frequency is equal to what frequency? a. the LC network resonant frequency (f r ) b. half the frequency of the original FM signal What is the phase difference between the carrier signal on channel 1 and the PHASE Phase difference = degrees (Recall Value 1) between the input and output signals? (The signals are shown in the image below.) a. b. capacitor c. LC network d. limiter FACET by Lab-Volt 347

12 Angle Modulation and Demodulation Analog Communications Adjust the NEGATIVE SUPPLY knob on the base unit CW and then CCW to vary the FM frequency. Why did the phase difference between the input and output signals increase and decrease, while the amplitude always decreased? a. The LC network causes a phase shift when the FM frequency varies. b. LIMITER output change phase with the frequency deviations of the modulated FM signal? a. yes b. no and valleys? a. The limiter restricts the amplitude of the output signal. b. The output signal is out of phase with the input signal. 348 FACET by Lab-Volt

13 Analog Communications Angle Modulation and Demodulation amplitude of the input signal (channel 1) to about 100 mv pk-pk and then to 300 mv pk-pk by turning the potentiometer on the VCO-LO circuit block CCW, and then CW. When the input signal was reduced to 100 mv pk-pk a. b. sine wave Phase Detector and Filter signal. NOTE: Set the TIME VARIABLE to the CAL position. detector. FACET by Lab-Volt 349

14 Angle Modulation and Demodulation Analog Communications The phase detector is a balanced modulator. A balanced modulator combines FM input frequencies to produce sum and difference frequencies. Because the FM inputs have equal frequencies, the sum frequency is two times the FM frequency. If the FM center frequency is 440 khz, what is the sum frequency? Sum frequency = khz (Recall Value 1) 350 FACET by Lab-Volt

15 Analog Communications Angle Modulation and Demodulation However, when the input frequencies to a balanced modulator are equal, the difference frequency Because the message signal caused the FM frequency deviations, which are converted to phase differences, the phase detector s difference dc voltage component varies directly with the message signal. Therefore, the output of the phase detector contains the sum frequency and the message signal. FACET by Lab-Volt 351

16 Angle Modulation and Demodulation Analog Communications and passes the message signal. Connect the circuit and channel 1 and 2 oscilloscope probes as shown. Set both channels to oscilloscope signals, as shown in the graph below. 352 FACET by Lab-Volt

17 Analog Communications Angle Modulation and Demodulation Connect the channel 2 probe to the PHASE DETECTOR output. What is the PHASE DETECTOR s output signal on channel 2? a. sum frequency b. difference frequency What is the PHASE DETECTOR output difference component? a. the FM center frequency b. a dc voltage FACET by Lab-Volt 353

18 Angle Modulation and Demodulation Analog Communications What varies the PHASE DETECTOR s dc output voltage (difference component)? a. the frequency variations b. the phase variations Connect the channel 2 probe to the input of the PHASE DETECTOR. If necessary, adjust the FM frequency with the NEGATIVE SUPPLY voltage so that the phase Set the voltmeter to measure dc volts. Connect the voltmeter lead to the PHASE DETECTOR s output, and connect the common lead to ground. DETECTOR s output. V 90 = Vdc (Recall Value 2) 354 FACET by Lab-Volt

19 Analog Communications Angle Modulation and Demodulation below) by adjusting the NEGATIVE SUPPLY voltage knob CCW. DETECTOR s output. V 135 = Vdc (Recall Value 3) below) by adjusting the NEGATIVE SUPPLY voltage knob CW. DETECTOR s output. V 45 = Vdc (Recall Value 4) FACET by Lab-Volt 355

20 Angle Modulation and Demodulation Analog Communications The table shows your measured dc voltages at the PHASE DETECTOR s output. Phase difference of input signals DC output voltage Vdc (Step 8, Recall Value 3) Vdc (Step 7, Recall Value 2) Vdc (Step 9, Recall Value 4) Calculate the voltage change from a 90 V 135 voltage change: Vdc (Step 8, Recall Value 3) Vdc (Step 7, Recall Value 2) = V 45 voltage change: Vdc (Step 9, Recall Value 4) Vdc (Step 7, Recall Value 2) = a. yes b. no graph below) by adjusting the FM frequency with the NEGATIVE SUPPLY knob. You will modulate the FM carrier with a 300 mv pk-pk, 3 khz message signal. Connect the SIGNAL GENERATOR to (M) on the VCO-LO circuit block. 356 FACET by Lab-Volt

21 Analog Communications Angle Modulation and Demodulation Connect the channel 1 probe to T on VCO-LO. Adjust the SIGNAL GENERATOR for a 300 mv pk-pk, 3 khz sine wave at T on VCO-LO (channel 1). Connect the channel 1 probe to the QUADRATURE DETECTOR s input at FM. FACET by Lab-Volt 357

22 Angle Modulation and Demodulation Analog Communications Set the coupling to AC on channel 2. Set the vertical mode to ALT, and trigger on channel 1. These settings will allow you to view the modulated signal at a viewable amplitude, minus the DC offset. Compare the signals at the two inputs to the PHASE DETECTOR. Is the phase of the FM signal on channel 2 varying with respect to the signal on CH 1? a. yes b. no Connect the channel 2 probe to the PHASE DETECTOR s output to observe the sum frequency signal. vertical input and set it to the zero reference on the bottom graticule. On channel 2, is the dc level, which is the zero reference (mid-point) of the sum frequency signal, changing? a. yes b. no 358 FACET by Lab-Volt

23 Analog Communications Angle Modulation and Demodulation Connect the channel 1 probe to M at the VCO-LO circuit block to observe the message signal. Trigger on channel 2. Observe the dc variations of the PHASE DETECTOR output signal on channel 2. Compare the message signal on channel 1 with the dc variations of the PHASE DETECTOR output on channel 2. Do the dc variations of the PHASE DETECTOR output have the same frequency as the message signal? a. yes b. no FACET by Lab-Volt 359

24 Angle Modulation and Demodulation Analog Communications Does the recovered message signal on channel 2 vary with the message signal amplitude and frequency on channel 1? a. yes b. no What frequency does the FILTER remove? a. the sum frequency b. the message signal frequency 360 FACET by Lab-Volt

25 Analog Communications Angle Modulation and Demodulation CONCLUSION A quadrature detector is an FM demodulator or discriminator that converts the FM frequency variations into the message signal. The phase detector is a balanced modulator that outputs a dc voltage that varies with the phase difference of the input signals. signal. REVIEW QUESTIONS 1. What function does a discriminator or frequency detector perform? a. It converts FM frequency deviations into phase deviations. b. It limits the amplitude of an FM signal. c. It converts FM frequency deviations into the message signal. d. It removes the FM sum frequency from the recovered message signal. 2. What does the term quadrature mean? a. a frequency difference of 45 khz b. c. shifting the phase of a signal d. FACET by Lab-Volt 361

26 Angle Modulation and Demodulation Analog Communications 3. a. b. c. It converts phase deviations to frequency deviations. d. It converts phase deviations to the message signal. 4. What is the quadrature detector s phase detector? a. b. limiter c. balanced modulator d. LC network 5. What are the output signals of the quadrature detector s phase detector? a. signals equal to half and three times the FM center frequency b. the sum frequency of the inputs and the FM center frequency c. d. the sum frequency of the inputs and a dc voltage that varies with the message signal 362 FACET by Lab-Volt