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Transcript
FACULTY OF ENGINEERING & TECHNOLOGY
LAB SHEET
COMMUNICATIONS ELECTRONICS
EEN3096
TRIMESTER 2 (2011/2012)
Experiment 1: CE 1
Design of RF Class-A Tuned Amplifier
Experiment 2: CE 2
Design of Phase-Locked Loop
Note : Students are advised to read through this lab sheet before doing experiment.
1
Lab CE1 & CE2: Communication electronics system
design
Objective
To sharpen students’ design skills, which after completing the two lab phases, students
will be able to design, construct and perform analysis of the basic RF Class-A amplifier
and phase locked- loop based on given specifications.
To expose students to the electronic circuit test and measurement equipments and test
station used in analogue circuit industries. During the course of the lab experiments,
students will be going through RF amplifier and phase locked-loop design steps, from
theoretical calculation to practical circuit construction and actual designed circuit
performance analysis.
Upon completion of the lab experiments, student should be able to:



Gain the principle and design of RF amplifier and phase locked-loop circuitry.
Design and construct a practical communication electronics system.
Ability to perform circuit performance analysis on designed electronic systems
using electronic circuit test station.
Programme Outcomes (% of contribution)




Ability to acquire and apply fundamental principles of science and
engineering.
Capability to communicate effectively.
Acquisition of technical competence in specialized areas of engineering
discipline.
Ability to identify, formulate and model problems and find engineering
solutions based on a system approach.
22.5%
10%
17.5%
17.5%

Ability to utilize systematic approach in design and operational 22.5%
performance evaluation.

Ability to work independently as well as with others in a team.
10%
2
LAB Sheet CE1:
Design of RF Class-A Tuned Amplifier
1.0 Objective
There are many types of RF amplifier configuration using bipolar junction transistor.
The Class-A amplifier is the most general. This experiment is designed to familiarize the
student to the practical aspects of the design of a Class-A Tuned Amplifier. Issues such as
the choice of components, the biasing requirement and prediction of the circuit
performance will be illustrated.
2.0 Equipment Required
1. Function generator.
2. 100MHz bandwidth dual channel oscilloscope.
3. 5V power supplies.
4. A piece of breadboard and some single core wires.
5. A digital multi-meter.
2.1 Components Required (Quantities shown within parenthesis)
1. Resistors – 10K (1), 4.7K (1), 1K (1), 150 (1) and 470 (1).
2. Capacitors – 0.1uF (3), 1uF (1) and 100pF(1).
3. NPN Transistor – 2N3904.
4. Inductor – 4.7uH.
3.0 Introduction
An amplifier is a circuit that provides an output signal that is an enlarged replica of the
input. They faithfully reproduce input at a higher power level. An amplifier is classified
according to how it is biased. A Class-A transistor amplifier is biased so that the
transistor conducts continuously. The transistor of the Class-A amplifier is biased in the
active region during the whole duration of operation. The operation of a transistor in
active region is almost linear, thus the Class-A amplifier is a linear amplifier. Figure 1A
and 1B shows two types of Class-A transistor amplifier circuits – the tuned and un-tuned
amplifier.
3
Figure. 1A Un-tuned Class A amplifier
Figure. 1B Tuned Class A amplifier
3.1 Difference between Tuned and Un-tuned Class-A Amplifier
An un-tuned amplifier has a wider bandwidth than a tuned amplifier. This means an untuned amplifier will enlarge an input signal for a wider range of frequencies. This is
illustrated in Figure 2 for the comparison of the amplifier voltage gain.
Figure 2 – Typical frequency response of tuned and un-tuned amplifier.
To implement a tuned amplifier, a resonance network such as the parallel RLC tank
network is used. Such a network has high impedance at the vicinity of the resonance
frequency. The voltage gain of an amplifier is dependent on its load impedance. If the
parallel RLC tank network is used as the load impedance, the gain of the amplifier will be
very high when the input signal frequency is near the RLC network resonance frequency
fo. At other frequencies, the RLC tank network has low impedance, thus forcing the
amplifier to have low voltage gain.
Figure 3 – Parallel RLC tank network.
The bandwidth of a tuned amplifier using the parallel RLC tank network invariably
depends on the bandwidth of the network. The bandwidth of the parallel RLC tank
network in turn depends on its parallel resistance R. The higher the value of R the more
narrow the bandwith (as illustrated in figure 4). This means the amplifier will become
less sensitive at other frequencies.
The bandwidth of a tuned circuit such as the parallel RLC network is defined as the
amount of frequency deviation from the resonance frequency fo where the power
delivered to the circuit is one-half that delivered at resonance. It can be shown from
equation (1) and (2) that:
4
Q is known as the quality factor of the parallel RLC network. We observe that the
higher the Q of a tuned network, the smaller will be the bandwidth. A high Q tuned
network is synonymous with low loss. In the case of the parallel RLC network, the larger
the value of R, the smaller the loss and hence a higher Q is obtained.
Figure 4 – Frequency response of the RLC tank network with different value of R.
3.2 RF Amplifier Design Considerations
Great care must be taken in designing an amplifier that could function properly in the
megahertz range. The frequencies between 1MHz to 300MHz are collectively known as
radio frequency (RF). Frequencies higher than 300MHz are categorized as microwave
frequency (MF). An RF amplifier is an amplifier that would work well into the radio
frequency region. RF amplifier is difficult to design, as most of the components will not
approach their ideal characteristics at radio frequency. A number of considerations in
designing an RF amplifier:
 Parasitic elements such as stray capacitance and inductance associated with the
components. Parasitic elements exist for all practical components such as resistors,
capacitors and inductors. It also exists between the conductors used to connect the
components. The parasitic element of a practical transistor is illustrated in Figure 5.
 Stability of the amplifier. The parasitic inductance and capacitance associated with
the components used to construct the amplifier will result in unwanted feedback. This
could result in oscillation within the circuit, rendering the circuit useless.
 The characteristic of the transistor depends on operating frequency. Particularly
the input impedance and the small signal current gain β of the transistor decreases
5
as the operating frequency is increased. This would decrease the overall gain of the
circuit at radio frequency. This effect is evident for the un-tuned amplifier frequency
response as shown in Figure 2. The drop in voltage gain as frequency is increased is
due to the decrease in βof the transistor.
Figure 5 – Parasitic elements associated with a practical transistor.
3.3 Transistor at High Frequencies
At low frequencies, it is assumed that the transistor responds instantly to changes of input
voltage or current. Actually, such is not the case, because the mechanism of the transport
of charge carriers from Emitter to Collector is essentially one of diffusion. The commonemitter circuit is the most important practical configuration. Hence, we seek a CE model
that will be valid at high frequencies. Such a circuit is called the hybrid-π, or the
Giacoletto model, it is shown in Figure 6. The hybrid-π model gives a reasonable
compromise between accuracy and simplicity up to microwave frequencies. A detailed
explanation of the model can be found in Gray & Meyer, chapter 1, Millman & Halkias,
chapter 11 or Collins, chapter 10.
Figure 6 – The hybrid-π model for a transistor in CE configuration.
6
4.0 Tuned RF Class-A Amplifier
Most RF Class-A amplifiers are of the tuned type. The reasons for using tuned amplifier
are as follows:



Higher voltage gain – The gain is dependent on the value of the load resistor and the
collector biasing resistor. If we could increase the value of this resistance, higher gain
would be obtained. However, by increasing the value of RC, we interfere with the
biasing point of the transistor. If RC is too large, we might even bias the transistor into
saturation. By using parallel RLC network, the transistor will see a large resistance of
R at the resonance frequency while at dc the collector resistance is zero.
Less susceptible to noise – If the gain of the amplifier is high, electrical noise in the
input will be amplified to a perceptible level. A highly tuned amplifier will reject
noise power that is not within its bandwidth. Since the noise power within the
bandwidth is very small, a high signal-to-noise ratio at the output can be attained.
Enable transformer coupling and impedance transformation – Sometimes the load can
be coupled to the amplifier through mutual inductance. Thus, the inductor L in the
RLC network can be magnetically coupled to another inductor, forming a
transformer. The transformer has the advantage of being able to reflect a low
impedance load into a high impedance value at the RLC network. Usually
transformer-coupling circuit is used to cascade two single stage amplifiers together to
obtain larger gain. At radio frequency, the input impedance of the transistor is small
due to Ce and CC in the hybrid-π model of the transistor. Thus, a transformer is used
to reflect the small input impedance of the second stage into large impedance at the
output of the first stage.
Analysis of tuned RF amplifier proceeds in the sequence as in the analysis for
unturned amplifier. Instead of using RL for the load, the load is complex by virtue of the
load inductor and load capacitor.
Figure 7 – Using transformer to connect two single stage amplifiers together.
7
In particular, at resonance, the complex load will become real. For instance the
impedance of the parallel RLC network becomes R at resonance f = fo. The voltage gain
for the tuned amplifier at resonance is given by:
5.0 The Experiment
Figure 8 – Class-A tuned transistor amplifier.
1. Connect the Class-A RF amplifier circuit as shown in Figure 8 using the prototype
board (or breadboard as it is usually known among the electronic hobbyist
community).
2. Be extra careful when building the circuit for high frequency operation. Keep the
connecting wires on the prototype board as short as possible to reduce unwanted
inductance and capacitance in your circuit. An example of the circuit
construction is shown in Figure 9. Note the pin assignment on the general
purpose NPN switching transistor 2N3904 in Figure 9.
DC biasing measurement [15 marks]
3. Temporarily remove the transistor from the prototype board. Use the digital multimeter
to measure the exact resistance of RB1 and RB2. Reinsert the transistor after the
resistance measurement is done. Calculate the voltage at the base of the transistor
8
using the resistive voltage divider formula
. Then calculate the
voltage at the emitter of the transistor VE using VE=VB-0.7.
4. Connect the circuit to a 5V-power supply. Do not connect the function generator to the
input yet. Measure the voltages at VC , VB and VE at C, B, E pins of the transistor using
the digital multi-meter.
Figure 9 – Photograph of the experiment setup.
9
5. Verify that VB-VE ≈ 0.7V.
6. Check if the calculated and measured VB and VE agree. If the values do not agree, can
you explain why?
7. Estimate IC using
.
8. Evaluate the transconductance gm of the transistor at T=25°C using
AC Measurement [15 marks]
9. Now connect the function generator to the input of the circuit as shown in Figure 9. Set
the function generator to output a sine wave at a frequency of about 5MHz.
10. . Set the oscilloscope to dual trace mode. Using Channel 2 of the oscilloscope, probe
the base of the transistor. Using Channel 1of the oscilloscope, probe the load resistor
RL as shown in Figure 10. Set the trigger of the scope to internal, with the triggering
signal from Channel 1. Adjust the trigger level on the oscilloscope to obtain a stable
waveform. Also, adjust the vertical scale and horizontal scale of the oscilloscope to
obtain the waveform as in Figure 11.
11. Adjust the output level of the function generator so that the peak-to-peak voltage
swing is approximately 36mV or ΔVB = ±18mV. You would have to use the built-in
attenuator of the function generator. Select attenuation of –40dB. Actually, we should
keep the peak-to-peak voltage below 30mV. This will result in almost perfect sine
wave obtained at the output (no harmonic distortion). However, for the current setup,
large amount of noise is observed for signal voltage below 30mV, hence the
compromise of 36mV.
Figure 10 – Probing locations on the circuit.
10
12. Now sweep the frequency of the function generator from 1MHz to 10MHz. Record
the peak-to-peak voltage at the input (Channel 2) and output (Channel 1) and
determine the magnitude of the voltage gain AV as a function of frequency.
Figure 11 – Sample waveforms as observed on Channel 1 and Channel 2 of the
oscilloscope.
13. Plot a curve of AV versus frequency; estimate the resonance frequency of the
amplifier from the curve. The input and output waveform should be approximately
180° out of phase during resonance. Furthermore the voltage gain AV is maximum
when resonance occur, with the theoretical resonance frequency being approximated
by equation (2), repeated here:
The above equation is accurate if the series resistance of the inductor L is negligible,
or the Q of the inductor is high. At L = 4.7μH, the series resistance of the inductor is
small and hence can be omitted.
14. Practical Hints - Usually there will be a significant deviation of the calculated
resonance frequency to the measured resonance frequency. This is due to stray
capacitance between the transistor collector and ground/VCC rails on the prototype
board and the tolerance of the inductor LC and capacitor CC. Acceptable resonance
frequency is between 5.5MHz to 7.5MHz. Sketch the input and output waveform at
resonance frequency on your lab report.
11
15. Use the digital multi-meter to measure the actual resistance of the load resistor R L.
Substitute the value of RL, gm (calculated in 8.) and AV at resonance into equation (6)
to estimate the value of rce. Ignore base spreading resistance rb’b.
(7)
16. From the value of the estimated rce, find the Early voltage VA of the transistor 2N3904
using
.
Using the Hybrid-π parameters to Estimate Voltage Gain [15 marks]
17. Now change the load resistor RL to 470Ω.
18. Set the function generator output frequency at the resonance frequency measured
earlier. Adjust the output level of the function generator so that VB peak-to-peak is
approximately 36mV.
19. Measure VL peak-to-peak and determine the voltage gain of the circuit.
20. Measure the exact value of RL using the digital multi-meter. Using the value of rce, gm
calculated earlier, determine the voltage gain of the amplifier using equation (6).
Again, ignore the base spreading resistance rb’b. Compare the voltage gain AV
obtained through measurement with the value obtained using equation (6).
21. Repeat 17. To 20. For RL=1470Ω. (1000Ω and 470Ω in series). Records the
readings of VL, VB and AV at different RL(470Ω, 1000Ω, and 1470Ω) and comments.
12
22. Practical Hints - At radio frequencies above 10MHz, the typical voltage gain of a
tuned Class A amplifier will be smaller than the value calculated using equation (6)
due to reactance from capacitance Cc, Ce and stray capacitance between the leads of
the transistor. Thus to have a reasonable gain at higher frequency, we could increase
the value of RL, the parallel resistance of the RLC tank network or increase the
transconductance gm. Usually RL is fixed, as it is the load resistor, the only recourse
available is to increase gm. The gm can be increased by increasing the dc collector
current IC. Hence, for this experiment you will notice that IC is in the range of 6-7mA.
Typical IC for small signal amplifier is approximately 0.1-1mA. Usually in calculating
the voltage gain of a transistor amplifier, rb’b and rce are ignored. The base spreading
resistance rb’b is more or less independent of the transistor biasing and is small enough
to be omitted for practical application. However, rce, which is due to Early effect, is
inversely proportional to IC. For a transistor with VA = 30V, rce=30kΩ at IC=1mA and
is big enough to be ignored. At IC=6mA, rce is only 5kΩ, its effect must be included
in the AV formula of (6). The preceding steps illustrate a simple approach to
estimating rce.
23. Finally, the circuit of Figure 8 using 2N3904 is capable of operating up to a frequency
of 120MHz, although the effective voltage gain will drop to around -5.
Maximum Output Swing of the Tuned Class-A Amplifier [15 marks]
24. Change the load resistor RL to 1kΩ again. With the function generator output
frequency at the resonance frequency, slowly increase the output level of the function
generator until the output voltage waveform is clipped at the lower level (Figure 12).
Determine the maximum output voltage swing and the lower voltage threshold.
25. Compare the lower voltage threshold with the emitter voltage VE measured in the dc
measurement. This level should be slightly above VE (about 0.1V-0.3V). The reason
this clipping occurs is when VC (the transistor collector voltage) approaches VE, VCE
is reduced. As VCE approaches 0V, the transistor shifts from forward active region to
saturation region. When the transistor saturates the impedance between collector and
emitter is low, thus the output voltage at collector is effectively shorted to the ground
through the bypass capacitor CE (Figure 8). This results in the clipping of the output
voltage.
Figure 12 – Clipping at the output.
13
26. Therefore to allow maximum swing, the dc level VE of a Class A amplifier must be
kept as low as possible, or alternatively a higher operating voltage can be used to shift
the dc collector voltage higher.
Discussion [40 marks]:
1) During DC biasing measurement, if CE changes from 0.1 µF to 1 µF, what happen to
the readings in VE (assume CE is ideal capacitor, no parasitic resistance or
inductance)?
2) The input and output (VB and VL) waveform should be approximately 180° out of
phase during resonance. VB is leading VL or in reverse way? Why?
3) Based on the information obtained in Step 21, calculate the efficiency η of the Class A
amplifier at RL= 470Ω, 1000Ω, and 1470Ω respectively.
4) What is the power dissipation PD of the Class A amplifier at RL= 470Ω, 1000Ω, and
1470Ω respectively?
Hints:  
2
Po
1 VL
; Pdc  Vcc I c ; Po 
;VL is the peak AC output voltage PD  Pdc  Po
2 RL
Pdc
IMPORTNANT NOTE:
Report submission: Submit your hand-written report within 10 days of performing
the experiment to the same laboratory. Your report should include 1-2 pages of
background information on RF Class-A amplifiers, with neat diagrams of circuits,
waveforms, and data recorded. Include the discussion on the results obtained.
References
[1] P.R. Gray, R.G. Meyer, “ Analysis and Design of Analog Integrated Circuits “, 3rd
edition, 1993. John-Wiley & Sons.
[2] J. Millman , C.C. Halkias, “Integrated Electronics”, 1972, McGraw-Hill.
[3] R.E. Collin, “Foundation for Microwave Engineering”, 1992, McGraw-Hill.
[4] P.H. Young,”Electronic Communication Techniques”, 3rd edition, 1994, PrenticeHall International.
14
LAB Sheet CE2:
Design of Phase Locked-Loop
1.0 Background
The Phase-Locked Loop (PLL) was described for the synchronous reception of radio
signals as far back as 1932 by H. de Bellescizi of France and has found use in numerous
applications. Its widespread use today in all areas of electronics is due to the convenience
and small size of the integrated circuit. A Phase-Locked Loop can generate, modulate,
select, demodulate, and multiply signals. Instrumentation, television, space telemetry, and
many other applications requiring a high degree of noise immunity and a narrow
bandwidth use PLLs. PLLs are used in FM and AM demodulation because they do not
require complex coil adjustments. PLLs are also used for frequency-shift keying (FSK)
operations in digital communications.
2.0 Introduction
A basic PLL consists of three basic components:
1. A phase detector.
2. Low pass filter
3. Voltage controlled oscillator (VCO)
Sometimes an optional dc amplifier is connected between the output of the low pass
filter and the input of the VCO to increase the dynamic range of the loop. A block
diagram of the Phase-Locked Loop is shown in Figure 1.
Figure 1 – Essential components of a Phase-Locked Loop.
• Phase detector - whose role is to allow the relative phase relationship of two inputs to
be compared with each other. A larger phase difference produces a larger magnitude
output. The phase detector is often implemented with a multiplier circuit, and the
output is the product of the input signals.
• Low pass filter (LPF) - to filter out the high frequency components of the
multiplication output of the phase detector. The output of the filter is a dc voltage
corresponding to the phase difference of the two-phase detector inputs. This dc voltage
15
acts as the control voltage to the next element, the voltage controlled oscillator. The
PLL configuration may be designed so that it acts as a narrow band tracking filter
when the LPF is a narrowband filter. In another mode of operation, the bandwidth of
the LPF is wider so that the VCO can track the instantaneous frequency change of the
input signal.
• Voltage controlled oscillator - which has a natural, free running frequency when the
applied control voltage is 0V (or some other nominal dc offset value), and whose
frequency varies up and down around this free running frequency in proportion to the
control voltage. In other words, the VCO is an oscillator that produces a periodic
waveform, the frequency of which may be varied about some free running frequency,
fFR, according to the value of the applied voltage.
These three basic elements combine to form a powerful system in the PLL operation
as follows: The input signal and the VCO signal are compared in the phase detector
(multiplier), and the result is fed into low pass filter to produce a dc signal proportional to
this phase difference. The difference between them represents the error between the two
signals. The error signal controls the VCO frequency. If there is no error (the two signals
are matched in phase), the VCO control voltage is 0 V (or at the nominal dc offset value)
and the VCO frequency stays unchanged. If there is a difference, the error voltage is
nonzero (or different than the nominal dc offset) and acts to cause the VCO output to
change its phase and frequency so that it matches the input signal. The net effect is that
this is a closed loop, negative feedback system that serves to compare the VCO
constantly against the input signal and automatically correct the VCO phase for any
difference between them.
Phase-Locked
The loop is said to be phase-locked when the VCO frequency fo and the input signal
frequency fi are identical (fo = fi), and only a phase difference between the VCO and input
signal exist. The concept of phase-locked is illustrated in Figure 2, where the first figure
shows the VCO frequency fo not synchronized with the input frequency fi. The second
shows the VCO output in synchrony with the input signal.
fo=fi
Figure 2 – Comparison between free-running and phase-locked condition.
16
Without an input signal the VCO is free-running at frequency fFR. When a signal
arrives, the system enters into acquisition or capturing mode. After the PLL is phaselocked to the input signal, the VCO output will track any frequency changes of the input
signal. This is called the tracking or locking mode. The sequence of acquisition and
tracking is depicted in Figure 3.
Figure 3 – Three sequences of PLL operation.
Important
Note that in Figure 2 and Figure 3, periodic sinusoidal signal is used for vo(t) and vi(t).
Actually there is no restriction on the shape of vo(t) and vi(t), the Phase-Locked Loop
should work with any periodic signals. In most cases the output of the VCO is a periodic
square wave instead of a sinusoidal wave.
17
2.1 Capture and Lock Range
For a practical PLL, there is a limited frequency range in which the loop can achieved
phase-locked to an input signal from the free-running condition. This frequency range is
known as the Capture Range. Upon achieving phase-locked, the VCO will track any
changes in the input signal frequency. Again there is a limit to the frequency where the
VCO can remain in synchronous with the input signal. This frequency range is known as
the Lock Range. If the input signal’s frequency exceeds the Lock Range, the PLL will
return to the initial free-running condition. Most of the time the Lock Range is greater
than the Capture Range. Moreover the free-running frequency is usually at the center of
the Capture and Lock Range.
Figure 4 – Capture and Lock Range of a typical Phase-Locked Loop.
2.2 Application of Phase-Locked Loop
• Synchronization of the horizontal and vertical scan in television receivers. Also for
synchronization of the colour burst in a colour television receiver.
• Spaceflight requirements inspired intensive application of phase-lock methods, where
frequency drift, noise and Doppler frequency shift are prevalent. The PLL offers
another key advantage: as a precise yet tunable filter. Using regular components, a
traditional filter cannot achieve narrow bandwidth at high center frequencies: A 20 Hz
bandwidth around 50 MHz is not realizable in practice. With a PLL, the center
frequency is set by the VCO, while the low pass filter sets the bandwidth. These two
are completely independent of each other, and a PLL can easily be designed for 20 Hz
around 50 MHz (See Figure 5). For this reason the PLL is often used in recovering
data from space probes, where extremely narrow bandwidths are used to minimize
received noise, although the carrier may be at a high frequency. Finally, a change in
one parameter is accomplished by changing only the VCO free running frequency, or
the low pass filter bandwidth, without affecting the other parameter.
• PLL can be used as a frequency demodulator. This is evident from Figure 3 where the
output Vo changes with the input signal frequency. PLL is used as demodulator in
Frequency Modulation (FM) and Frequency Shift Keying (FSK) systems.
• Noisy oscillators can be enclosed in a PLL and locked to a clean signal. If the loop has
a wide bandwidth, the oscillator tracks out its own noise and its output are greatly
cleaned up.
• Frequency synthesizer - Frequency multipliers and dividers can be built using PLLs as
shown in Figure 6.
18
• Synchronization of digital transmission is typically obtained by phase-lock methods.
Figure 5 – PLL realization of a bandpass filter, pass band of 20Hz
and center frequency at 50MHz.
Figure 6 – Frequency synthesizer using PLL.
The Experiment
Equipment Required
1. Dual channel oscilloscope.
2. Digital multi-meter.
3. Function generator.
4. A piece of breadboard and some single core wires
5. 74HC4046 IC (1 unit).
6. Ceramic type Capacitor(s) (50V): 1nF (3 units), 10nF (1 unit), 100nF (1 unit).
7. Electrolytic Capacitor (16V): 10uF (1 unit).
8. Resistor(s): 470R (1 unit), 3.3K(1 unit).
Procedures
Introduction
In this simple exercise you would build a phase-locked loop (PLL) using the popular
CMOS integrated circuit (IC) 74HC4046. This IC has a built-in relaxation voltage
controlled oscillator (VCO) and three type of phase detectors (PD). This IC contains an
19
XOR gate PD, RS flip-flop PD and a PFD with charge pump. In this lab we would utilize
the XOR gate PD and the PFD to built a PLL. The block diagram of the 74HC4046 is
shown in Figure 7.
Figure 7 – 74HC4046 block diagram
Part 1: XOR Gate PLL
Connect up the circuit as shown in Figure 8 on a project board. Be careful in handling the
74HC4046 IC as it is susceptible to electrostatic discharge damage. An example of the
completed PLL is shown in Figure 9.
20
Figure 8 – PLL schematic using XOR gate PD
Figure 9 – The hardware based on schematic of Figure 8
21
In this schematic, R2 and C4 determine the built-in VCO characteristics. These values
allow the VCO oscillation frequency to vary from roughly 450 kHz to 3 MHz, for an
input voltage of 0.5 V to 5V. Capacitor C3 serves to stabilize the potential on pin 12 of U1
(This pin is supposed to be left as open circuit). Pin 2 of U1 is the output of the XOR gate
PD while pin 3 is the common input to all the internal PDs of the 74HC4046.
Step 1 – Evaluate Measure the VCO gain [8 marks]
Open the PLL loop by disconnecting pin 9 of U1. Power up the circuit with a power
supply, setting the output voltage to around 5V. Connect pin 9 of U1 to a second power
supply. Vary the second power supply output voltage from 0.5 V to 4.5 V in 0.5V step.
Record the frequency observed from the VCO. Plot a graph relating the frequency of the
VCO’s output versus VCO input. Calculate ko. A sample result is shown in Table 1.
To evaluate:
(i) Tabulate fvco versus Vo
(ii) Draw a graph between Volts and fvco.
(iii) Evaluate ko
Step 2 – Connect signal generator – Phase locked [8 marks]
Setup a signal generator to produce a 50% duty cycle square wave with peak-to-peak
voltage of around 4.0 V and frequency of 1.5 MHz. Connect the output of the signal
generator to pin 14 of U1 via C1. The PLL should immediately lock to the input signal.
Observe the voltage Vo, Vin and Vvco during phase-locked.
To evaluate:
(i) Draw two waveforms: (i) Vo versus time and (ii) Vin versus time on same graph
(ii) Draw two waveforms: (i) Vvco versus time and (ii) Vin versus time on same graph
Step 3 – Tracking mode observation and hold-in range measurement [8 marks]
Now you can increase and decrease the frequency of the input signal and watch the PLL
tracks the input signal. Measure the lock or hold-in range (fH) of this PLL and compare
this with theory.
Step 4 – Static phase error measurement [8 marks]
Set the input signal frequency to 1.0 MHz. From the waveforms, compute the difference
between the excess phase of the input signal and the VCO, θe. The phase difference can
be computed from the timing difference by (see Figure 5):
(1)
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Repeat this with other frequencies, for instance 750 kHz and 1.5 MHz. Check the
values of θe obtained, average value of Vo (this can be obtained from pin 10 of U1) and
VCO’s oscillating frequency all correlate well with theory.
Step 5 – Determine the capture range [8 marks]
Disconnect the signal generator from the PLL. Set its frequency to 450 kHz and connect
it back to PLL. See if the PLL can lock to the input signal. Repeat this at 250 kHz step
until you read 2.5 MHz. From this result we can estimate the capture of this PLL. Here it
would be observed that the capture range for XOR gate PLL depends on the cut-off
frequency of the RC filter. The lower the frequency, the smaller the capture range. You
can verify this by changing C1 to 10 nF and repeat the above steps.
Part 2: PFD with Charge Pump PLL [40 marks]
Modify the circuit as in Figure 10. Here we are using the internal PFD with charge pump
of 74HC4046. The loop filter is also modified accordingly to convert current into voltage.
Figure 10 – PLL using PFD with charge pump
Repeat Step 2 and Step 4 of Part 1 for this PLL. As an interesting experiment change C 2
to 1 nF afterwards, see if the PLL is stable.
To evaluate: [20 marks]
(i) Draw waveforms to do a comparison of Vvco and Vin during phase-locked at 1.5
MHz.
(ii) Draw voltage spikes at charge pump output during phase-locked at 1.5 MHz.
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IMPORTNANT NOTE:
Report submission: Submit your report within 10 days of performing the
experiment to the same laboratory. Your report should include 1-2 pages of
background information on Phase Locked-Loop, with neat diagrams of circuits,
waveforms, and data recorded. Include the discussion on the results obtained.
References
[1] P.R. Gray, R.G. Meyer, “ Analysis and Design of Analog Integrated Circuits “, 3rd
edition, 1993. John-Wiley & Sons.
[2] J. Millman , C.C. Halkias, “Integrated Electronics”, 1972, McGraw-Hill.
[3] R.E. Collin, “Foundation for Microwave Engineering”, 1992, McGraw-Hill.
[4] P.H. Young,”Electronic Communication Techniques”, 3rd edition, 1994, PrenticeHall International.
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