Upload
ngoque
View
258
Download
5
Embed Size (px)
Citation preview
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
52 FACET by Lab-Volt
Exercise Generation and Demodulation of DPSK Signal
EXERCISE OBJECTIVE
When you have completed this exercise, you will see the operation principle and
characteristics of the DPSK signal generator by measuring the waveform and spectrum
of a DPSK signal. You will also be familiar with the demodulation principle of the DPSK
signal demodulator.
DISCUSSION
Figure 4-2 illustrates the block diagram for DPSK signal generation which consists of a
DPSK encoder and a BPSK modulator.
Thus, the procedure for a DPSK signal generation takes two separate steps; the
differential encoding followed by the BPSK modulation. The DPSK encoder consists of
an EXCLUSIVE NOR and a delay circuit, and the BPSK modulator consists of a level
converter, carrier signal, and a multiplier.
A cos ct
A cos ct+_
+_1LEVEL
CONV.
DPSK ENCODER
DATA INPUT
DELAY
Figure 4-2. The block diagram for DPSK signal generation
The differential encoding refers to the procedure of encoding the data differentially,
that is, the presence of a binary one or zero is manifested by the symbol’s similarity or
difference when compared to the preceding symbol.
Figure 4-3 illustrates a differential encoding of a binary message data stream, m(k),
where k is the sample time index. The differential encoding starts (second row in the
figure) with the first bit of the code bit sequence, c(0), chosen arbitrarily (here taken to be
one).
Then the sequence of encoded bits, c(k), can be encoded in the following way:
m k c k c k1
where the symbol represents modulo-2 addition and the overbar denotes complement.
In Figure 4-3, the differentially encoded message is obtained by using the above
equation. In other words, the present code bit, c(k), is a one if the message bit, m(k), and
the prior coded bit, c(k-1), are the same, otherwise, c(k) is a zero.
The third row translates the coded bit sequence, c(k), into the phase shift sequence, θ(k), where a zero is characterized by a 180˚ phase shift, and a one is characterized by a 0˚ phase shift.
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 53
Informationmessage
1 1 0 1 1 1 0 0 1 1 0
Differentially encoded message
1(ref.) 1 1 0 0 0 0 1 0 0 0 1
Corresponding phase shift
0 0 0 ̟ ̟ ̟ ̟ 0 ̟ ̟ ̟ 0
Figure 4-3. Example of Differential Encoding
The procedure for DPSK signal demodulation also takes two separate steps; the BPSK
demodulation which uses coherent detection followed by the differential decoding.
Figure 4-4 illustrates the block diagram for DPSK decoder which consists of an
EXCLUSIVE NOR and a delay circuit. That is, the differentially encoded message is
obtained by using the following equation.
m k c k c k1
where the symbol ⊕ represents modulo-2 addition and the overbar denotes
complement. In other words, the present message bit, m(k), is a one if the code bit, c(k),
and the prior coded bit, c(k-1), are the same, otherwise, m(k) is a zero.
DELAY
Encoded
Message Decoded
Message
Figure 4-4. The block diagram for DPSK decoder
Figure 4-5 illustrates an example of a differential decoding of the sequence of encoded bits, c(k), where k is the sample time index. The sequence of encoded bits, c(k), is
delayed by a bit to give the prior coded bit, c(k-1).
Then the message data stream, m(k), can be decoded by the EXCLUSIVE NOR of the
encoded bits, c(k) and the prior coded bit, c(k-1).
Encoded message 1 1 1 0 0 0 0 1 0 0 0 1
Delayed message 1 1 1 0 0 0 0 1 0 0 0 1
Decoded message 1 1 0 1 1 1 0 0 1 1 0
Figure 4-5. Example of Differential Decoding
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
54 FACET by Lab-Volt
EQUIPMENT REQUIRED
Description
FACET base unit
QPSK/OQPSK/DPSK circuit board
Virtual Instrument*
Oscilloscope, dual trace
Spectrum Analyzer
Function Generator
Generator, sine/square wave
Power supply, 15 Vdc (2 required)**
* Throughout this manual, settings for the oscilloscope, function generator, and spec-
trum analyzer refer to Lab-Volt’s Virtual Instrument Model 1250. Equivalent instrumen-
tation may be substituted.
** Only required if the FACET base unit does not contain a power supply
PROCEDURE
1. NRZ GENERATOR module, DPSK ENCODER module, QPSK MODULATOR_I
module, CARRIER & PHASE SHIFT module, QPSK DEMODULATOR_I module,
AMPLIFIER module, and DPSK DECODER module of QPSK/OQPSK/DPSK
TRAINER are used in this exercise.
2. Locate the NRZ GENERATOR block.
3. Connect the output of the sine/square wave generator, to the CLOCK INPUT of the NRZ GENERATOR block shown in Figure 4-6.
BSM BSL
KCOLC
TUPNI
RALOPINU
TUPTUO
RALOPIB
TUPTUO
ROTARENEG ZRN
SYNC.
Figure 4-6. NRZ GENERATOR module
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 55
4. Using the oscilloscope to observe, adjust the sine/square wave generator for a 0 to 5 V
pk, 10 kHz square wave at the CLOCK INPUT. (Time Base – Sample frequency-
40µs/div, signal coupling-DC).
5. Adjust 8 DIP switches on the NRZ GENERATOR module for 8 LEDs to show the value of ‘(MSB) 10100110 (LSB)’.
6. Connect channel 1 of the oscilloscope to the UNIPOLAR OUTPUT of the NRZ
GENERATOR block.
7. Oscilloscope settings:
Measure: Channel 1
Trigger source: external trigger. Connect to the SYNC terminal of the NRZ
GENERATOR block.
Signal coupling: DC
Time Base: Sample frequency - 100 µs/div
Vertical: Auto
Sync on the negative slope and adjust the trigger level for a stable display.
8. Measure the waveform and sketch it in Figure 4-7.
Figure 4-7. The waveform of UNIPOLAR OUTPUT
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
56 FACET by Lab-Volt
9. Observe the waveform and record the values at the right-hand side of Table 4-1.
NOTE: This waveform should be the same as the theoretical value in the left column of
Table 4-1 except that the oscilloscope displays the LSB at the far left and MSB at the far
right.
Theoretical value Measured value
waveform MSB 10100110 LSB LSB 01100101 MSB
bit time interval 0.1 ms 0.1 ms
Table 4.1 UNIPOLAR OUTPUT
10. Deactivate the Scope instrument and activate the Spectrum instrument. Connect
Channel 1 of the spectrum analyzer to the UNIPOLAR OUTPUT of the NRZ
GENERATOR block. Measure and sketch the spectrum of the pulse signal in Figure
4-8. (Frequency range – 12.5 kHz, signal coupling-AC).
Figure 4-8. The spectrum of UNIPOLAR OUTPUT
11. Connect the UNIPOLAR OUTPUT of the NRZ GENERATOR block to the DATA
INPUT of the DPSK ENCODER by inserting a lead wire as shown in Figure 4-9.
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 57
ATAD
TUPNI
ATAD
TUPTUO
REDOCNE KSPD
BSM BSL
KCOLC
TUPNI
RALOPINU
TUPTUO
RALOPIB
TUPTUO
ROTARENEG ZRN
SYNC.
Figure 4-9. NRZ GENERATOR and DPSK ENCODER module
12. Deactivate the Spectrum instrument and activate the Scope instrument.
13. Connect channel 1 to the DATA INPUT and Channel 2 to the DATA OUTPUT of the
DPSK ENCODER block.
14. Oscilloscope settings:
Measure: Channel 1 and Channel 2
Trigger source: external trigger. Connect to the SYNC terminal of the NRZ
GENERATOR block.
Signal coupling: DC
Time Base: Sample frequency - 200 µs/div
Vertical: Auto
Sync on the negative slope and adjust the trigger level for a stable display.
15. Observe the oscilloscope display and take notice of the figure below that matches
your waveforms. Does your oscilloscope display correspond to one of the images
shown in Figure 4-10?
a. Yes
b. No
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
58 FACET by Lab-Volt
A B
C D
Figure 4-10. Four images
16. The DATA OUTPUT of the DPSK ENCODER has four possible waveform
sequences. This occurs because the DPSK ENCODER uses two latches resulting
in four initial startup conditions 00, 01, 10, and 11. Since the DECODER CIRCUIT
is only looking for data transitions, the data is able to be recovered correctly at the
DATA OUTPUT of the DPSK DECODER.
17. Connect the DATA OUTPUT of the DPSK ENCODER module to the DATA INPUT of
the QPSK MODULATOR_I block by connecting a lead wire as shown in Figure 4-11.
ATAD
TUPNI
SOC
TUPNI
.DOM_I
TUPTUO
I_ROTALUDOM KSPQ
ATAD
TUPNI
ATAD
TUPTUO
REDOCNE KSPD
Figure 4-11. DPSK ENCODER and QPSK MODULATOR _I
18. Connect the output of the function generator, to the CARRIER INPUT of the
CARRIER & PHASE SHIFT block shown in Figure 4-12.
19. Using the oscilloscope to observe, set the output of the function generator to a
sine wave of 500 kHz, 200mVpk-pk
(Time Base – Sample frequency- 2µs/div, signal
coupling-AC, 1:1 Probe).
20. Use a two-post connector to connect the COS OUTPUT of the CARRIER & PHASE
SHIFT block to the COS INPUT of the QPSK MODULATOR_I block. Center the
potentiometer on the QPSK MODULATOR_I block.
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 59
ATAD
TUPNI
SOC
TUPNI
.DOM_I
TUPTUO
I_ROTALUDOM KSPQTFIHS ESAHP & REIRRAC
REIRRAC
TUPNI
SOC
TUPTUO
NIS
TUPTUO
Figure 4-12. CARRIER & PHASE SHIFT and QPSK Modulator_I blocks
21. Connect channel 1 to the COS OUTPUT of the CARRIER & PHASE SHIFT block.
22. Oscilloscope settings:
Measure: Channel 1, 1:1 Probes
Trigger source: Channel 1
Signal coupling: AC
Time Base: Sample frequency - 2 µs/div
Vertical: Auto
Sync on the positive slope and adjust the trigger level for a stable display.
23. Measure the waveform and sketch it in Figure 4-13.
Figure 4-13. The carrier wave
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
60 FACET by Lab-Volt
24. The procedure for DPSK signal modulation takes two separate steps; the differential
encoding flowered by the BPSK modulation. The modulated DPSK signal appears at
the I_MOD OUTPUT of the QPSK MODULATOR_I block.
25. Connect channel 1 of the oscilloscope to the I_MOD OUTPUT of the QPSK
MODULATOR_I block and channel 2 to the DATA INPUT of the QPSK
MODULATOR_I block.
26. Oscilloscope settings:
Measure: Channel 1 and Channel 2, 10:1 Probes
Trigger source: external trigger. Connect to the SYNC terminal of the NRZ
GENERATOR block.
Signal coupling: DC
Time Base: Sample frequency - 40 µs/div
Vertical: Auto
Sync on the positive slope and adjust the trigger level for a stable display.
27. The waveforms that you observe should be similar to the ones shown in Figure 4-14
below.
Note: Remember the DATA OUTPUT of the DPSK ENCODER has four possible initial
waveform sequences. The exact waveform that you observe depends on what waveform
sequence is initialized.
1.0000
-1.0000
Ch1
V
0.5000
0.0000
-0.5000
0s
4.000
-4.000
Ch2
V
2.000
0.000
-2.000
40.00μs 80.00μs 160.00μs120.00μs 200.00μs 240.00μs 280.00μs 320.00μs 360.00μs 400.00μs
Figure 4-14. QPSK MODULATOR_I block I_MOD OUPUT and DATA INPUT
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 61
28. The top waveform shown in Figure 4-14 displays a
a. differential DPSK encoder signal.
b. unipolar signal.
c. DPSK modulated signal.
d. carrier signal.
29. Thus far a DPSK modulated signal has been generated and measured. For the
remainder of the exercise, the DPSK signal will be demodulated.
30. Using a two-post connector, connect the I_MOD OUTPUT of the QPSK
MODULATOR block to the SUM_IN of the QPSK DEMODULATOR_I block shown in
Figure 4-15.
NI_MUS
SOC
TUPNI
I_ROTALUDOMED KSPQ
.DOMED_I
TUPTUO
Figure 4-15. QPSK DEMODULATOR_I module
To keep synchronization, the COS OUTPUT of the CARRIER & PHASE SHIFT block,
which is already connected to the QPSK MODULATOR_I, is also internally connected to
the COS INPUT OF THE QPSK DEMODULATOR_I block.
31. Center the potentiometer located on the QPSK DEMODULATOR_I blocks.
32. Use a two-post connector to connect the I_DEMOD OUTPUT of the QPSK
DEMODULATOR_I block to the I_AMP INPUT Q_AMP INPUT of the AMPLIFIER
block as shown in Figure 4-16.
NI_MUS
SOC
TUPNI
I_ROTALUDOMED KSPQ
.DOMED_I
TUPTUO
.PMA_Q
TUPTUO
.PMA_ID
TUPTUO
.PMA_I
TUPTUO
.PMA_I
TUPNI
REIFILPMA
.PMA_Q
TUPNI
Figure 4-16. QPSK DEMODULATOR_I and AMPLIFIER block
33. Connect channel 1 of the oscilloscope to the I_AMP OUTPUT of the AMPLIFIER
block and channel 2 to the DATA OUTPUT of the DPSK ENCODER block.
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
62 FACET by Lab-Volt
34. Oscilloscope settings:
Measure: Channel 1 and channel 2, 10:1 probes
Trigger source: external trigger. Connect to the SYNC terminal of the NRZ
GENERATOR
Signal coupling: DC
Time Base: Sample frequency - 200 µs/div
Vertical: Auto
Sync on the positive slope and adjust the trigger level for a stable display.
35. Observe the waveforms and sketch them in Figure 4-17.
Figure 4-17. The DATA OUTPUT of the DPSK ENCODER and demodulated waveform
36. The demodulated waveform sketched at the top of Figure 4-17 should correspond,
but inverted, to the waveform at the DATA OUTPUT of the DPSK ENCODER (bottom
waveform).
If the two waveforms do not coincide, make them coincide by controlling the
potentiometers of the QPSK MODULATOR_I module and the QPSK DEMODULATOR_I
module.
Note: The DATA OUTPUT of the DPSK ENCODER has four possible initial waveform
sequences. The exact waveform pattern that you observe depends on what waveform
sequence is initialized. No matter what the initial waveform sequence, the two
waveforms displayed must match.
37. Use a lead wire to connect the I_AMP OUTPUT of the AMPLIFIER block to the DATA
INPUT of the DPSK DECODER block shown in Figure 4-18.
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 63
.PMA_Q
TUPTUO
.PMA_ID
TUPTUO
.PMA_I
TUPTUO
.PMA_I
TUPNI
REIFILPMA
.PMA_Q
TUPNI
ATAD
TUPNI
REDOCED KSPDATAD
TUPTUO
Figure 4-18. AMPLIFIER and DPSK DECODER
38. Connect channel 1 of the oscilloscope to the DATA OUTPUT of the DPSK
DECODER block and channel 2 to the DATA INPUT of the DPSK ENCODER block.
39. Oscilloscope settings:
Measure: Channel 1 and channel 2, 10:1 probes
Trigger source: external trigger. Connect to the SYNC terminal of the NRZ
GENERATOR
Signal coupling: DC
Time Base: Sample frequency - 200 µs/div
Vertical: Auto
Sync on the positive slope and adjust the trigger level for a stable display.
40. Observe the waveforms and sketch them in Figure 4-19.
Figure 4-19. DECODER DATA OUTPUT and ENCODER DATA INPUT
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
64 FACET by Lab-Volt
41. The DPSK DECODER DATA OUTPUT waveform sketched in the top of Figure 4-20
should correspond to the waveform at the DATA INPUT of the DPSK ENCODER
(bottom sketched waveform).
Note: The waveform patterns are the same (LSB 01100101 MSB) but are shifted by
several clock pulses.
Unit 4 — DPSK
QPSK/OQPSK/DPSK Manual
FACET by Lab-Volt 65
REVIEW QUESTIONS
1. The DPSK ENCODER block uses two latches resulting in undetermined initial
startup conditions. How many possible waveform sequences are at the DATA
OUTPUT of the DPSK ENCODER block?
2a.
4b.
1c.
8d.
2. The demodulated I_AMP OUTPUT waveform corresponds to the waveform at the
DATA OUTPUT of the DPSK ENCODER but it is
shifted by two clock pulses.a.
level shifted by two volts. b.
ac coupled.c.
None of the above.d.
3. What is the procedure for DPSK signal modulation?
S/P converter, 2 level converters, 2 multipliers, carrier generator, phase shifter, a.
adder
exclusive NOR, level converter, carrier generator, multiplierb.
S/P converter, time delay, 2 level converters, 2 multipliers, carrier generator, c.
phase shifter, adder
exclusive NOR, delay circuit, level converter, carrier generator, multiplierd.
4. What is the procedure for DPSK signal demodulation?
QPSK DEMODULATOR_I, QPSK DEMODULATOR_Q, AMPLIFIER, DPSK a.
DECODER
QPSK DEMODULATOR_I, AMPLIFIER, DPSK DECODERb.
QPSK DEMODULATOR_Q, DPSK DECODERc.
QPSK DEMODULATOR_I, AMPLIFIER, DPSK ENCODER d.
5. In the QPSK/OQPSK/DPSK board the input to the DPSK ENCODER comes from the
NRZ GENERATOR BIPOLAR OUTPUT.a.
CARRIER & PHASE SHIFT COSINE OUTPUT.b.
AMPLLFIER I_AMP OUTPUT.c.
NRZ GENERATOR UNIPOLAR OUTPUT.d.