chapter 15 psk modulator - universitas diponegoroelektro.undip.ac.id/sukiswo/?download=pt3_15_psk...

Chapter 15

PSK Modulator

15-1: Curriculum Objectives

1. To understand the operation theory of PSK modulation.

2. To understand the signal waveform of PSK modulation.

3. To design the PSK modulator by using MC 1496.

4. To understand the methods of measuring and adjusting the PSK modulation circuit.

5-2: Curriculum Theory

In communication system, besides AM and FM, there is another type of modulation which is the

phase modulation. In phase modulation, the amplitude and frequency remain the same, the only

difference is the phase. The binary signal is used to switch the phase between 0° and 180°, which

is called phase shift keying (PSK) modulation.

Generally, in order to increase the transmission rate, we need to use more bandwidth. However,

as for the variation of PSK modulation, the signal is hidden in the phase, therefore, the problem

of the consumption of bandwidth will not occur. Figure 15-1(a) is the 1-bit transmission of the

PSK modulation. If the variation located at the zero phase, it represents the data signal is zero.

On the other words, If the variation located at the 𝜋phase, it represents the data signal is 1.

Figure 15-1(b) is the 2-bit transmission of the PSK modulation, which it is also known as

quadrature phase shift keying (QPSK). If the variation located at the zero phase, it represents the

data signal is (0, 0). If the variation located at the 𝜋/2 phase, it represents the data signal is (0, 1).

If the variation located at the 𝜋 phase, it represents the data signal is (1, 0). If the variation

located at the 3𝜋/2 phase, it represents the data signal is (1, 1).

(a) Constellation diagram of PSK modulation. (b) Constellation diagram of QPSK modulation.

Figure 15-1 Constellation diagram of PSK and QPSK modulations.

Figure 15-3 is the simple circuit diagram of PSK modulator. At the Data Input port, input 5 V

voltage, then D1, D3 will ON, D2, D4 will OFF. The carrier signal inputs to the Carrier Input port

will pass through T1 and couples to the second coil. After that the signal will pass through D1, D3

and reach to the first coil of T2. Then the signal will couple to the second coil of T2, at this

moment, the phase of the waveform at PSK output terminal will similar to the phase of the

Carrier Input, as shown in figure 15-2. On theother hand, if we input —5 V voltage at the Data

Input port, then D1 , D3 will OFF, D2, D4 will ON. At this moment, the phase of the waveform at

PSK output terminal will opposite to the phase of the Carrier Input. This type of modulation is

known is PSK modulation.

Figure 15-2 Signal waveforms of BPSK modulation.

Figure 15-3 Simple circuit diagram of PSK modulator.

M-ary PSK can be expressed as

XPSK (t) = A cos [𝜔ct + 2m𝜋

𝑀) ; m = 1,2, ….. ,≤M (15-1)

M: 2N

N: Numbers of bit during transmission.

If the data signal is 1-bit, that is M = 2. So, XPSK(t) will transmit binary bits signal and the phase

shift of the modulated signal is 180° out of phase. Figure 15-3 shows the signal waveform of

binary phase shift keying, (BPSK). The BPSK signal at logic 1 is represented as A cos(𝜔ct +

𝜋)and the BPSK signal at logic 0 is represented as A cos(𝜔ct+ 2𝜋).

Figure 15-4 shows the block diagram of PSK modulator. This block diagram is similar to the

block diagram of ASK modulator in chapter 11. The only difference is the PSK modulator

converts the unipolar data signal to bipolar data signal before sending the signal to the balanced

modulator. Therefore, phase modulation can be achieved by using the balanced modulator. The

bandpass filter will remove the high frequency signal, which make the PSK signal waveform

more perfect.

In this experiment, MC1496 is used to implement the balanced modulator. Figure 15-5 is the

internal circuit diagram of MC1496. From the circuit diagram, D1, RI, R2, R3, Q7 and Q8

comprise the current source, which provides Q5 and Q6 with DC bias current. Q5 and Q6 comprise

thedifferential transistor, which is used to drive the Q1, Q2, Q3 and Q4 that is the dual

differential amplifier. Pin 1 and pin 4 are for data signal input. Pin 8 and pin 10 are for carrier

signal inputs. The gain of balanced modulator is controlled by the external resistor between pin 2

and pin 3. The bias voltage of the amplifier can be determined by the external resistor connected

at pin 5.

Figure 15-4 Basic structure diagram of PSK modulator.

Figure 15-5 Internal circuit diagram of MC1496

Figure 15-6 Circuit diagram of PSK modulator by using MC1496.

Figure 15-6 is the circuit diagram of 1-bit PSK which the carrier signal and data

signal are single-ended input. Pin 10 is the carrier input and the data signal is passed

through the unipolar to bipolar converter which is comprised by 74HCU04, 74HC126,

3904, 3906, D1, D2, D3 and R1 to R8. The converted bipo lar signa l will be sent to

pin 1 o f MC1496. R 2 2 determines the gain of the circuit and R23 determines the bias

voltage of the circuit. If we adjust VR1 or change the amplitude of the data signal, then

we can prevent the PSK modulation signal from distortion. This signal will be sent to

the filter, which is comprised by 𝜇A741, C4, C6, R26, R27 and R28. Then the high

frequency signals, which are produced by the balanced modulator will be filtered and a

better PSK signal will be performed.

15-3: Experiment Items

Experiment 1: PSK modulator

1. Refer to the circuit in figure 15-6 or refer to figure DCT 15-1 on GOTT DCT-6000-08 module.

2. At the input terminal of modulation signal (Data 1/P), input 5 V amplitude and 100 Hz TTL

signal. By using oscilloscope, observe on the output signal waveforms of the unipolar to bipolar

converter output terminal TP 1, then record the measured results in table 15-1.

3. According to the input signal in table 15-1, repeat step 2 and record the measured results in

table 15-1.

4. At the input terminal of modulation signal (Data I/P), input 5 V amplitude and 100 Hz TTL

signal with 50 % duty cycle, i.e. data signal streams with "10". By using oscilloscope, observe on

the output signal waveforms of TP1, then record the measured results in table 15-2.

5. According to the input signal in table 15-2, repeat step 4 and record the measured results in

table 15-2.


signal with 50 % duty cycle, i.e. data signal streams with "10". At the input terminal of carrier

signal (carrier I/P), input 400 mV amplitude and 20 kHz sine wave frequency.

7. By using oscilloscope, observe on the output signal waveforms of the output terminal of

modulated PSK signal (PSK O/P). Adjust VR1 and observe on the modulated PSK signal

waveform until the waveform does not occur distortion. Slightly adjust the VR2 to avoid the

asymmetry of the waveform. By using oscilloscope, observe on the output signal waveforms of

TP1, bipolar signal test point (TP2), carrier signal (TP3), balanced modulator (TP4) and

modulated PSK signal output port (PSK O/P), Finally, record the output signal waveform in table

15-3.

8. According to the input signal in table 15-3, repeat step 6 to step 7 and record the measured

results in table 15-3.



signal (carrier I/P), input 400 mV amplitude and 20 kHz sine wave frequency.





TP1, TP2, TP3, TP4 and PSK O/P. Finally, record the output signal waveform in table 15-4.





signal (carrier I/P), input 400mV amplitude and 20 kHz sine wave frequency.





TP1, TP2, TP3, TP4 and PSK O/P. Finally, record the output signal waveform in table 15-5.



Table 15-1 Observe on the output signal of unipolar to bipolar converter by changing the

frequencies of data signal.

Data Signal

Frequencies

Data I/P TP1

100 Hz

1 kHz

10 kHz

Table 15-2 Observe on the data signal of unipolar to bipolar converter by changing the duty

cycle of data signal. (fData = 100 Hz)

Data Signal

Duty Cycles

Data I/P TP1

50%

33%

66%

Table 15 -3 Observe on the PSK modulation signal by changing the frequency of carrier signal.

( Vc = 400 mV , fData= 100 Hz )

Carrier Signal Frequencies Carrier I/P TP1

20 kHz

TP2 TP3

TP4 PSK O/P


(Continue) ( Vc = 400 mV , fData = 100 Hz )

Carrier Signal

Frequencies

Carrier I/P TP1

50kHz

TP2 TP3

TP4 PSK O/P


(Continue) ( Vc = 400 mV , fData = 100 Hz )

Carrier Signal

Frequencies

Carrier I/P TP1

100kHz

TP2 TP3

TP4 PSK O/P

Table 15 -4 Observe on the PSK modulation signal by changing the amplitude of carrier signal.

(fc= 20 kHz , fData = 100 Hz )

Carrier Signal

Amplitude

Carrier I/P TP1

400 m V

TP2 TP3

TP4 PSK O/P

Table 15 -4 Observe on the PSK modulation signal by changing the amplitude of carrier signal.

(Continue)(fc = 20 kHz , fData = 100 Hz )

Carrier Signal

Frequencies

Carrier I/P TP1

1 V

TP2 TP3

TP4 PSK O/P

Table 15 -5 Observe on the PSK modulation signal by changing the duty cycle of carrier signal.

( Vc = 400 mV , fc = 20 kHz , fData = 100 Hz )

Data Signal

Duty Cycles

Data I/P TP1

33 %

TP2 TP3

TP4 PSK O/P

Table 15 -5 Observe on the PSK modulation signal by changing the duty cycle of carrier signal.

(Continue) ( Vc = 400 mV , fc = 20 kHz , fData = 100 Hz )

Data Signal

Duty Cycles

Data I/P TP1

66 %

TP2 TP3

TP4 PSK O/P

15-5: Problems Discussion

1. What is the operation theory of the unipolar to bipolar converter as shown in figure 15-6?

2. What are the differences between PSK and ASK modulation circuits?

chapter 15 psk modulator - universitas diponegoroelektro.undip.ac.id/sukiswo/?download=pt3_15_psk...

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