Exercise 3-3 Differential Encoding EXERCISE OBJECTIVE When you have completed this exercise, you will e familiar with the technique of differential encoding used with QPSK digital modulation. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Phase amiguity Differential encoding Differential QPSK (DQPSK) DISCUSSION Phase amiguity In the receiver of any PSK communication system, the demodulator demodulates the received signal y comparing the instantaneous phase of this signal with recovered copies of the sinusoidal I- and Q-channel carrier signals. It does this once per symol interval in order to determine which symol is eing transmitted. Since the PSK is a suppressed-carrier signal, there is no stale carrier present in the received signal. For demodulation to occur, a carrier must e recovered (regenerated) y the receiver. The receiver does this y locking a local oscillator onto one of the phases present in the received signal. This oscillator then generates a stale sinusoidal waveform that serves as the I-carrier. The Q-carrier is generated y phase shifting the I-carrier y 90. In order to accurately demodulate the signal, the recovered I-carrier should have the same phase, relative to the symols, as the original I-carrier in the transmitter. However, there are n different phases present in an n-psk signal and there is no way for the receiver to know which phase is the correct one to use. As a result, the local oscillator in the receiver has one chance out of n to lock onto the correct phase. If it does, the data will e recovered correctly. If it locks onto one of the other phases, however, the recovered data to e erroneous. This phenomenon is called phase amiguity. One common correction scheme for phase amiguity is to transmit a known sequence as a data preamle. The demodulator either corrects the phase of the local oscillator or changes the recovered data until the preamle is decoded correctly. Another method for overcoming phase amiguity is differential encoding. Instead of using the data to set the asolute phase of the PSK signal at the modulator, the data is used to change the phase y a specific amount. The Festo Didactic 86311-00 249
Ex. 3-3 Differential Encoding Discussion demodulator then detects the differences in successive phases, rather than the asolute phase. Differential encoding Differential encoding can e used with any form PSK modulation. With differential BPSK (DBPSK), for example, there are two symols, 1 and 0, and two phases in the modulated waveform. A 1 could e represented y a change in phase in the modulated waveform from one symol interval to the next, and a 0 could e represented y no change in phase. Regardless of which phase the local oscillator in the receiver initially locks on to, the data will e correctly recovered. Differential encoding can e implemented y using a differential encoder in the modulator and a differential decoder to the demodulator, as shown in Figure 3-74 and Figure 3-75. I/Q Modulator I Channel Level Converter Low-Pass Filter Binary Data Serial to Parallel Converter Differential Encoder QPSK Signal Q Channel Level Converter Low-Pass Filter QPSK Symol Generator Encoder QPSK Signal Generator Figure 3-74.Differential QPSK modulator. The differential encoder in Figure 3-74 encodes the symols generated y the Serial to Parallel Converter. It does this y generating new symols (encoded symols) that will produce the desired phase changes. The modulator modulates the I and Q carrier signals using the encoded symols. I Channel Sample & Hold Decision Circuit QPSK signal cos t sin t Q Channel Recovered Clock Sample & Hold Recovered Clock Decision Circuit Differential Decoder Figure 3-75. Differential QPSK demodulator. Parallel to Serial Converter Data Output 250 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Discussion The demodulator in Figure 3-75 demodulates the received signal in order to recover the encoded symols. These are decoded y the differential decoder in order to recover the original symols. Differential QPSK (DQPSK) With differential QPSK (DQPSK), there are four symols, or diits and four possile phases. Each of these diits is mapped to a specific change in phase. Tale 3-7 shows a commonly-used mapping of diit to phase change, although many other mappings are possile. Considering that there is only one constellation point in each quadrant of the QPSK constellation, Tale 3-7 also shows all possile quadrant changes for each phase change. The four quadrants are numered as follows: Tale 3-7.Typical QPSK differential mapping. Data Diit Phase Change Quadrant Changes 2 1 3 4 00 90 01 0 11 270 10 180 1 2 2 3 3 4 4 1 1 1 2 2 3 3 4 4 1 4 2 1 3 2 4 3 1 3 2 4 3 1 4 2 For example, consider the sequence of data its 001101. When asolute (nondifferential) coding is used, each diit is mapped to an asolute transmitted phase, as shown y the constellation points in Tale 3-8. If, at the eginning of the transmission, the demodulator locks onto the wrong phase due to phase amiguity, as shown in the tale, the demodulated constellation will always e rotated y the same angle with respect to the transmitted constellation. In this case, all decoded diits are erroneous. Festo Didactic 86311-00 251
Ex. 3-3 Differential Encoding Discussion The asolute diit-to-phase mapping used in Tale 3-8 is as follows: Tale 3-8.Asolute (non-differential) encoding example with 90 demodulation phase error. Transmitted Diit Transmitted Phase Demodulated Phase Decoded Diit 10 11 00 01 00 01 (error) 11 10 (error) 01 11 (error) Tale 3-9 shows the effect of differential encoding (using the mapping of Tale 3-7). For the purpose of this example, the initial phase is assumed to e in the first quadrant. The first diit of our sequence, 00, produces a phase change of 90, shifting the transmitted constellation point into the second quadrant. As in Tale 3-8, the demodulated constellation is always rotated y the same amount with respect to the transmitted constellation. However, since the information is contained in the phase changes, rather than in the asolute phase, this does not cause errors in the decoded diits. The decoded diits are identical to the transmitted ones. Tale 3-9.Differential encoding example with 90 demodulation phase error. Transmitted Diit Mapped Phase Change Transmitted Phase Demodulated Phase Detected Phase change Decoded Diit (initial state) (initial state) 00 90 90 00 11 270 270 11 01 0 0 01 252 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure Outline Advantages and disadvantages of differential encoding With differential encoding, the data is encoded using changes in phase, rather than using asolute phases. Phase amiguity therefore does not affect the decoded data. Immunity to phase amiguity is the advantage offered y differential encoding. Differential encoding, however, requires slightly more complex circuitry in the modulator and demodulator. There is one other disadvantage in using differential encoding. Since the phase changes convey the data, an error in detecting the phase of one modulation symol produces errors in two successive data symols. This increase in the numer of errors can often e compensated for, however, y slightly increasing the transmitted power in order to reduce the proaility of error due to channel noise. Differential encoding in the Earth Station Transmitter In the Earth Station Transmitter, the I- and Q-channels of the modulator (the Level Converters and mixers) use a fixed mapping etween the diits they receive and the resulting constellation points. In order to implement differential encoding, the Differential Encoder encodes (alters) the data diits in order to produce the required phase changes. For each unencoded diit the Differential Encoder receives, it generates a new, encoded diit that will produce the required phase change. The Differential Decoder in the demodulator reverses the process in order to recover the original data. PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Differential encoding Phase amiguity a If you are using the Telemetry and Instrumentation Add-On, the use of a conventional dc voltmeter or millimeter, in addition to the virtual instruments, will facilitate the last section of this procedure. PROCEDURE Set up and connections 1. If not already done, set up the system and align the antennas visually as shown in Appendix B. 2. Make sure that no hardware faults have een activated in the Earth Station Transmitter or the Earth Station Receiver. Faults in these modules are activated for trouleshooting exercises using DIP switches located ehind a removale panel on the ack of these modules. For normal operation, all fault DIP switches should e in the O position. 3. Turn on each module that has a front panel Power switch (push the switch into the I position). After a few seconds, the Power LED should light. Festo Didactic 86311-00 253
Ex. 3-3 Differential Encoding Procedure 4. If you are using the optional Telemetry and Instrumentation Add-On: Make sure there is a USB connection etween the Data Generation/Acquisition Interface, the Virtual Instrument, and the host computer, as descried in Appendix B. Turn on the Virtual Instrument using the rear panel power switch. If the TiePieSCOPE drivers need to e installed, this will e done automatically in Windows 7 and 8. In Windows XP, the Found New Hardware Wizard will appear (it may appear twice). In this case, do not connect to Windows Update (select No, not this time and click Next). Then select Install the software automatically and click Next. Start the Telemetry and Instrumentation application. In the Application Selector, do not select Work in stand-alone mode. If the Telemetry and Instrumentation application is already running, exit and restart it. This will ensure that no faults are active in the Satellite Repeater. Differential encoding In this section, you will determine how the Differential Encoder represents the diits to e transmitted as phase changes in the modulated signal, rather than as asolute phases. The Differential Encoder does this y mapping the diits from the Serial to Parallel Converter to new diits (encoded diits) that produce the required phase changes. The ojective of this section is to complete Tale 3-11. 5. Make the following connections: BIT CLOCK BIT CLOCK OUTPUT Earth Station Transmitter Binary Sequence Generator (BSG) DATA DATA INPUT 5 Digital Modulator I Q I Q IF 1 OUTPUT Up Converter 1 Up Converter 2 RF OUTPUT Satellite Repeater RF OUTPUT Earth Station Receiver Down Converter 2 IF 2 OUTPUT Down Converter 1 Digital Demodulator I Q I Q Figure 3-76. Connections for unmultiplexed digital transmissions. 254 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure Use the oscilloscope to oserve the following signals: Channel Connect to Signal Ch 1 Transmitter TP8 Differential Encoder I-output Ch 2 Transmitter TP9 Differential Encoder Q-output EXT TRIG BSG Sync. Sequence generator sync. signal a On some oscilloscopes, the EXT TRIG input is disaled when using the X-Y display format. In this case, the EXT TRIG connection is unnecessary. In the Telemetry and Instrumentation application, your could make the following settings and connections: Digital Output Settings Digital Output 1 Source... BSG1 Signal... Data Digital Output 3 Source... BSG1 Signal... Sync. Generator Settings Binary Sequence Generator (BSG) 1 Generation Mode... User Entry Binary Sequence... 11 Bit Rate... 20 000 000 BIT CLOCK Data Generation/ Acquisition Interface DIGITAL OUTPUT 1 (BSG1 Data) DIGITAL OUTPUT 3 (BSG1 Sync.) DATA INPUT 5 CH1 IN CH2 IN TP9 EXT TRIG Digital Modulator TP8 BIT CLOCK OUTPUT I Q Virtual Instrument I Q 6. On the Earth Station Transmitter, turn on the Scramler. (The Clock & Frame encoder can e on or off.) This will ensure that all four constellation points will e present at oth the input and the output of the Differential Encoder. Set the oscilloscope to the X-Y Display Format and adjust it to center the image on the screen. Then set the Display Mode to Dots. Figure 3-77 shows examples of what you may see. Festo Didactic 86311-00 255
Ex. 3-3 Differential Encoding Procedure Oscilloscope Settings: Channel 1 (X) Scale... 100 mv/div Channel 2 (Y) Scale... 100 mv/div Display Format... X - Y Display Mode... Dots (a) Sampling Window = 10 s () Sampling Window = 100 s Figure 3-77. Adjusting the oscilloscope to display QPSK constellations. The appearance of the display will depend on the model of the oscilloscope and on the oscilloscope settings. In this step, it is only important that the display e approximately square and centered on the screen. As mentioned in Exercise 3-2, since only four signals states are possile with QPSK modulation, an ideal QPSK constellation would have only one point in each quadrant that is visited. Imperfections in the pulse signals, however, cause multiple points to appear in each quadrant. If you are using the Telemetry and Instrumentation Add-On, set the Sampling Window to 100 s. This is a good setting for the following steps. If you are using a conventional oscilloscope, you may find that adjusting the contrast improves the appearance of the constellation. The oscilloscope displays the constellation points one after the other. On some oscilloscopes, it may e preferale to turn on persistence in order to simultaneously display all of the constellation points that are currently eing visited. Otherwise, if you freeze the display (using Single Refresh or Stop, depending on the model), or if you capture the screen, you will may see only the constellation point displayed at that instant. 7. Turn off the Scramler and the Clock & Frame Encoder. Configure the inary sequence generator to generate the first two-it inary sequence ( 11 ) shown in Tale 3-10, at a it rate of 20 Mit/s. This repeating inary sequence can only produce one sequence of unencoded diits: 11 11 11 11 at the input of the Differential Encoder. Four diits of this sequence have already een entered in the second column of the tale. 256 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure Tale 3-10.Determining the diit to phase-change mapping of the Differential Encoder. Two-Bit Binary Sequence Unencoded Four- Diit Sequence Differentially Encoded Constellation Differentially Encoded Diit Sequence Phase Change Between Encoded Diits ( ) 11 11 11 11 11 00 10 Oserve the constellation at the output of the Differential Encoder (referred to in this exercise as the differentially encoded constellation). Note that, ecause the Serial to Parallel Converter groups data its into diits starting at an aritrary it, and ecause the Differential Encoder output Festo Didactic 86311-00 257
Ex. 3-3 Differential Encoding Procedure depends not only on the diits it receives ut also on its initial state, a given two-it inary sequence at the input to the Serial to Parallel Converter can produce from one to four different differentially encoded constellations. Restart the BSG in order to change the timing relationship of the data with the other signals. Do this many times until you are sure you have oserved each possile constellation that can e generated y the repeating inary sequence 11. If you are using the Telemetry and Instrumentation Add-On, click the Restart BSGs utton on the Instrumentation ta. If you are using conventional instruments, you may have to turn the inary sequence generator off and ack on. Sketch each of the differentially encoded constellations you oserve for the inary sequence 11 in the third column of the tale. There will one to four different constellations. If there are more than one, sketch each different constellation in a separate row. If you don t need four rows, simply leave some rows lank. a When you sketch a constellation, sketch it as an ideal constellation, with only one point per quadrant visited, as shown in the examples elow: If the oscilloscope displays: Sketch the constellation as: Once again, otain each different constellation that this inary sequence can produce and, for each one, set the oscilloscope Display Format to Normal (not X-Y) and the Display Mode to Normal. This will show the corresponding sequence of encoded diits at the output of the Differential Encoder. Figure 3-78 shows an example. 258 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure Note that the output of the Differential Encoder consists of a repeating fourdiit sequence. Q axis 1 0 0 1 I-channel signal 1 I axis 0 0 1 1 1 0 0 Q-channel signal (a) Constellation (X-Y Display Format) () Diits (Normal Display Format, Time Base = 0.1 s) Figure 3-78. Constellation and diits. To display one symol per horizontal division with the Normal Display Format, set the Time Base of the oscilloscope to 0.1 s. If necessary, change the Scale setting of the oscilloscope to display the symol waveforms, ut do not change the position of the traces. Enter any four consecutive diits of this encoded diit sequence in the fourth column of Tale 3-10 (enter four diits, even if they are all the same). To determine what encoded diit sequence is displayed on the oscilloscope, first recall which channel (I or Q) represents the MSB and which represents the LSB (refer to your answer to Step Ex. 3-28 of Exercise 3-2). Below each diit you just entered in the tale, draw the constellation point corresponding to the diit, as shown in the example elow: Differentially Encoded Diit Sequence 10 10 10 10 a The mapping of diits to constellation points at the output of the Differential Encoder is identical to the mapping used at the output of the Serial to Parallel Converter, shown in Figure 3-65 of Exercise 3-2. Recall that the Differential Encoder works y modifying (encoding) the diits so that the data is represented y changes in phase rather than y the phases themselves. Festo Didactic 86311-00 259
Ex. 3-3 Differential Encoding Procedure For each other two-it inary sequence in Tale 3-10, fill in columns 3, 4 and 2 of the tale, as follows: Adjust the oscilloscope to display a constellation (using the X-Y Format). Configure the BSG to generate the two-it inary sequence at 20 Mit/s. Restart the BSG many times until you oserve all possile constellations for that sequence. Sketch each different constellation, one per row, under Differentially Encoded Constellations. Otain each constellation again and determine the corresponding Differentially Encoded Diit Sequence. If all diits are the same, simply refer to your answer for Figure 3-65 of Exercise 3-2. Otherwise, use the oscilloscope to determine the diits. Enter the Unencoded Four-Diit Sequence that produces the constellation. The repeating inary sequence 00 can only produce one sequence of unencoded diits: 00 00 00 00 There are two ways that the two-it inary sequence 10 can e divided into diits: 10 10 10 10 and 01 01 01 01. To determine which series of diits is present at the input of the Differential Encoder, temporarily connect the oscilloscope proes to TP7 and TP8 and display the signals using the Normal Display Format. The four quadrants are numered as follows: 2 1 3 4 8. When you have finished filling out columns 1 to 4 of Tale 3-10, in the last column of the tale, enter the phase change etween consecutive diits in the differentially Differential Encoded Diit Sequence (0, 90, 180, or 270). A phase change of 0 leaves the point in the same quadrant (e.g. 3 3). A phase change of 90 moves the constellation point counter-clockwise y one quadrant (e.g., 1 2, or 4 1). A phase change of 180 moves it into the opposite quadrant (e.g. 2 4). A phase change of 270 moves it clockwise y one quadrant (e.g. 2 1) Once you have completed Tale 3-10, summarize you results in Tale 3-11. Enter the phase change that is produced y each unencoded diit. Then enter the four possile quadrant changes for each phase change. 260 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure Tale 3-11.Differential encoder mapping. Unencoded Diit 00 11 10 01 Phase Change (degrees) Quadrant Changes 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 Phase amiguity In this section, you will repeatedly force the Costas loop on the Earth Station Receiver to unlock and relock and oserve the effect on the constellation representing the encoded symols. Then you will oserve the effect of the Differential Decoder on these symols. 9. Use the same connections as in the previous section (see Figure 3-76). On the Earth Station Transmitter, make the following adjustments: Channel... any Data Source... Sampler Scramler... Off Clock & Frame Encoder... Off On the Earth Station Receiver, make the following adjustments: Channel... same as transmitter Descramler... Off Center Frequency... mid position Gain... Adjust so the green LEVEL LED is lit. 10. Configure the inary sequence generator to generate the repeating sequence 110010 at 20 Mit/s. This repeating sequence can e divided into diits in two ways, which we ll call Condition A and Condition B. In Tale 3-12, enter five more consecutive diits for each condition. Festo Didactic 86311-00 261
Ex. 3-3 Differential Encoding Procedure Then sketch the constellation that will result from each of these diit sequences (refer to Figure 3-65 of Exercise 3-2). Tale 3-12. Diits and unencoded constellations. Condition: A B Diits: 11 10 Constellation: 11. The Differential Encoder will represent these diits as phase changes, rather than asolute phases. In Tale 3-13, enter the same diit sequences as you did in Tale 3-12. Tale 3-13. Diits, phase changes, and encoded constellations. Condition: A B Phase Changes: Diits: 11 10 Initial state: Constellations: In Tale 3-13, enter the phase change that each diit will produce in the encoded constellation (refer to Tale 3-11). There are four possile initial states, shown in Tale 3-13. For each initial state in the tale, sketch the constellations that will result from the phase changes. 262 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure 12. Use the oscilloscope to oserve the constellation formed y following signals: Channel Connect to Signal Ch 1 Transmitter TP6 Serial to Parallel Converter I-output Ch 2 Transmitter TP7 Serial to Parallel Converter Q-output EXT TRIG BSG Sync. Sequence generator sync. signal Adjust the oscilloscope to produce a relatively clean display. You may find that setting the Sampling Window to 100 s gives the est results. Referring to Tale 3-12, how many different constellations can this it sequence 110010 produce at the outputs of the Serial to Parallel Converter? Ignoring imperfections due to and limiting and ringing, how many points will there e in each possile constellation? Restart the inary sequence generator several times to oserve the different constellations and verify your answer. 13. Use the oscilloscope to oserve the constellation formed y following signals: Channel Connect to Signal Ch 1 Transmitter TP8 Differential Encoder I-output Ch 2 Transmitter TP9 Differential Encoder Q-output EXT TRIG BSG Sync. Sequence generator sync. signal Referring to Tale 3-13, how many different constellations can this sequence produce at the outputs of the Differential Encoder? Ignoring imperfections due to and limiting and ringing, how many points will there e in each possile constellation? Restart the inary sequence generator several times to oserve the different constellations and verify your answer. Festo Didactic 86311-00 263
Ex. 3-3 Differential Encoding Procedure 14. Restart the inary sequence generator until the signals at TP8 and TP9 produce a three-point constellation. Oserve this constellation as you turn on and off the Scramler and the Clock & Frame Encoder several times. How many points has the constellation when either the Scramler or the Clock & Frame Encoder, or oth, are on? When oth the Scramler and the Clock & Frame Encoder are turned off, note that the constellation always has three points, although it may not always have the same orientation. Turning these functions on and ack off does not change the numer of points in the constellation. a The constellation at the transmitter (from TP8 and TP9) must remain a threepoint constellation during the following steps. Do not restart the inary sequence generator or change any of its settings, unless instructed to do so. Adjusting the receiver to minimize drift in the Costas loop a The Costas loop is designed to work with a modulated signal that contains all four phases. Since this presently not the case, fine adjustments must e made so that the Costas loop will remain locked long enough to oserve phase amiguity. Using the Telemetry and Instrumentation Add-On The following steps require measuring the feedack voltage at TP1 of the Earth Station Receiver and oserving a constellation on the Oscilloscope. If you are using the Telemetry and Instrumentation Add-On, it is not possile to do oth of these at the same time using only the virtual instruments. If a conventional dc voltmeter (or multimeter) is availale, use it to monitor the voltage at TP1 as you oserve the constellation on the virtual Oscilloscope. If you do not have a dc voltmeter, you can measure the voltage at TP1 using either the Oscilloscope or the True RMS Voltmeter / Power Meter. Temporarily connect one proe to TP1 and note the AVG (average) voltage displayed elow the graticule of the Oscilloscope or the dc voltage displayed on the True RMS Voltmeter / Power Meter. Make sure the instrument you are using is in the continuous refresh mode. Then reconnect the proe to display the constellation. Since the True RMS Voltmeter / Power Meter averages the measurement over the selected Averaging Time, the voltage reading will more stale on this instrument than the AVG reading on the Oscilloscope. Using the Oscilloscope may e more convenient, however, since you will not have to switch etween the two instruments. 264 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure 15. On the Earth Station Transmitter, make the following adjustments: a Clock & Frame Encoder... On In the following steps, the state of the Scramler and Descramler make no difference. Either one can e on or off. On the Earth Station Receiver, lock the Costas loop, as shown in the procedure of Exercise 1-1. Make sure the Sync. LED on the receiver is lit. Set the Center Frequency control to its mid position. When the Costas loop is locked onto the received signal, the voltage in its feedack circuit (TP1) remains fairly constant, although it may fluctuate slightly. Measure and record this voltage (to at least three significant digits). a The voltage at TP1 of the receiver will e used for reference purposes only. If you use a proe to measure this voltage, it is not necessary to set the proe attenuation switch to x1; you can leave it set to x10 and simply record the indicated voltage. Voltage at TP1 when the Costas loop is locked: On the transmitter, turn off the Clock & Frame Encoder while oserving the voltage at TP1 of the receiver. If the Costas loop ecomes unlocked, this voltage will egin to drift. If it does, note whether the voltage drifts up or down and then: Turn on the Clock & Frame Encoder. While monitoring the voltage at TP1 of the receiver, lock the Costas loop. Then readjust the Center Frequency control as follows: If the voltage at TP1 drifted down, increase the Center Frequency (turn the control slightly clockwise). If the voltage at TP1 drifted up, decrease the Center Frequency (turn the control slightly counter-clockwise). Turn off the Clock & Frame Encoder again while oserving the voltage at TP1 of the receiver. If necessary, readjust the Center Frequency control. Repeat this several times until the Costas loop remains locked. a If you adjust the Center Frequency to its minimum or maximum position and this is not sufficient to cause the voltage to change in the desired direction, reduce the Gain on the receiver slightly and try again. Once you have found the necessary adjustments, turn the Clock & Frame Encoder off and on several times. Whenever the Clock & Frame Encoder is turned on, the Sync. LED on the receiver should light immediately, showing that the Costas loop has remained locked. Festo Didactic 86311-00 265
Ex. 3-3 Differential Encoding Procedure 16. While still monitoring the voltage at TP1 of the receiver, momentarily interrupt the received signal in order to unlock the Costas loop. There are two ways to do this: On the receiver, rapidly cycle through the Channels, returning quickly to the previously selected channel. Walk quickly in front of the antenna connected to the Earth Station Receiver in order to momentarily lock the RF signal. This momentarily interrupts the signal at the IF 1 INPUT of the Digital Demodulator, which causes the Costas loop to unlock. Hopefully, the Costas loop will relock when the signal is restored. If it does, the voltage at TP1 will remain fairly constant. If the voltage drifts after the signal is restored, the Costas loop has not relocked. If this happens, slightly readjust the Center Frequency control and, if necessary, the Gain control and try again. If may e necessary to make several readjustments. Once the controls are correctly adjusted, it should e possile, with the Clock & Frame Encoder off, to momentarily interrupt the received signal and have the Costas loop relock. 17. Use the oscilloscope to oserve the constellation formed y following signals: Channel Connect to Signal Ch 1 Receiver TP2 I Level Converter output Ch 2 Receiver TP3 Q Level Converter output EXT TRIG BSG Sync. Sequence generator sync. signal This constellation will e a three-point constellation. Figure 3-79 shows an example of one of the four possile orientations of the constellation. Figure 3-79. Three point constellation at the Level Converter outputs. 266 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Procedure Oserving phase amiguity 18. Momentarily interrupt the received signal in order to unlock the Costas loop, as you did in Setp 16. There are two was to do this: On the receiver, rapidly cycle through the Channels, returning quickly to the previously selected Channel. Walk quickly in front of the antenna connected to the Earth Station Receiver in order to momentarily lock the signal. Repeat this several times noting the orientation of the three-point constellation each time it reappears. If the Costas loop does not relock, the constellation will not reappear as a stale constellation. In this case, turn the Clock & Frame encoder on and relock the loop, then try again. If necessary, slightly readjust the Center Frequency control and the Gain control as explained aove. When the Costas loop relocks, does the three-point constellation always appear in the same orientation? Explain what is happening. 19. Use the oscilloscope to oserve the constellation formed y following signals: Channel Connect to Signal Ch 1 Receiver TP4 I DATA Ch 2 Receiver TP5 Q DATA EXT TRIG BSG1 Sync. Sequence generator sync. signal On the Earth Station Receiver, momentarily unlock the Costas loop and relock it using the same method as in the previous step. Repeat this several times noting the orientation of the constellation each time it reappears. If the Costas loop does not relock, the constellation will not reappear as a stale constellation. In this case, turn the Clock & Frame encoder on and relock the loop, then try again. If necessary, slightly readjust the Center Frequency control and the Gain control as explained aove. Festo Didactic 86311-00 267
Ex. 3-3 Differential Encoding Procedure How many phases are in this signal (how many quadrants are visited)? When the Costas loop relocks, does the constellation always appear in the same orientation? Explain. 20. Store the currently displayed constellation in the oscilloscope memory. Then use the oscilloscope to oserve the constellation formed y following signals: Channel Connect to Signal Memory Ch 1 Receiver TP4 I DATA Memory Ch 2 Receiver TP5 Q DATA Ch 1 Transmitter TP6 Serial to Parallel Encoder I-output Ch 2 Transmitter TP7 Serial to Parallel Encoder I-output EXT TRIG BSG1 Sync. Sequence generator sync. signal Compare the constellations of the unencoded symols at the transmitter and that of the decoded symols at the receiver. 21. Use the oscilloscope with the Normal Display Format to oserve the following signals: Channel Connect to Signal Ch 1 Transmitter TP4 Serial to Parallel Converter input Ch 2 Receiver TP6 Parallel to Serial Converter output On the Earth Station Receiver, momentarily unlock the Costas loop and relock it. Repeat this several times. 268 Festo Didactic 86311-00
Ex. 3-3 Differential Encoding Conclusion Is the data correctly recovered each time the Costas loop is relocked? Explain. If the receiver data appears unstale, the Costas loop may have unlocked. Turn on the Clock & Frame Encoder while measuring the voltage at TP1 of the receiver in order to relock the loop. CONCLUSION In this exercise, you oserved how differential encoding represents data diits as phase changes in the modulated QPSK signal, rather than as asolute phases. It does this using a predetermined mapping etween diits and phase changes. You oserved that phase amiguity makes it impossile to estalish a fixed relationship etween the asolute phase of the modulated signal and the diits. Differential decoding, however, correctly interprets the changes in phase in order to correctly recover the diits. REVIEW QUESTIONS 1. Explain phase amiguity and why it arises with QPSK modulation. 2. Explain is the advantage of using differential encoding? 3. What is the main disadvantage of using differential encoding? 4. How can the main disadvantage of differential encoding e compensated for? Festo Didactic 86311-00 269
Ex. 3-3 Differential Encoding Review Questions 5. Can differential encoding e used only with QPSK modulation? 270 Festo Didactic 86311-00