Humber Institute of Technology and Advanced Learning
Wireless Telecommunication - WLS 611
Fundamentals of Electronics and Networking
Project #3
Phase Locked Loop
Objective:
1) To analyse some of the characteristics of a phase locked loop system.
2) To gain a deeper understanding of the various blocks of a PLL system.
3) To study the operational characteristics of the phase detector, low pass filter and
Voltage Controlled Oscillator in a PLL system.
4) Identify the factor that can cause failures in a PLL circuit and learn the techniques
to troubleshoot and correct the faults.
The circuit you will be using is shown below:
+ 12 V
10
17
R3
2kΩ
4
100k
11
50%
R6
150kΩ
9
TP5
R7
150kΩ
+ 5V
18
R1
10kΩ
REF IN
12 Rx
1
TP1
2
8
14
TP2
3
1
Q1
2 7486N
33kΩ
3
6
TP3 Filter
7Output
R
47kΩ
C
100nF
7
2N2219
D1
D2
R2
33kΩ
TP4
Q2
0
R5
2kΩ
R4
2kΩ
C1
15
500pF
14
13
Q3
0
Block A
Block B
Block C
TP6
Equipments:
 DC Multimeter
 Scope
 5 and 12V Power Supplies
 Audio Generator (Function Generator)
 2 Scope Leads, 4 Power Supply leads
Block D
Q4
0
0
5
16
TP7
500pF
0
0
C2
VCO Output
Block E







Resistors (33k, 10k, 33k, 47k, 2k, 2k, 150k, 150k, 2k, )
1 rectifier Diode and 1 zener diode.
XOR gate
3 npn transistor and 1 pnp transistor
Capacitors (100nf, 500pf, 500pf)
1 switch
1 potentiometer
Procedure:
A)
1. Connect the two power supplies of 5V and 12V.
2. Name the blocks of a PLL circuit and identify those blocks on the circuit
schematic.
3. Once you have identified the blocks, label them on the schematic and show it to
your teacher and let him initial your schematic.
B)
Free running frequency of the VCO:1) Connect the +12V supply voltage and measure the free running frequency of the
VCO.
C)
Linear Range of VCO
1) Remove the jumper between point 1 and point 2 (if it is already removed, then
proceed further) and apply a DC voltage of about 12V directly to point 2. Change
the DC voltage in steps of 1V and record the frequency at the output of the VCO.
Get enough readings to plot a suitable graph of DC voltage versus frequencies.
2) Determine the linear range of VCO output frequency versus DC voltage.
Flow ………........................ and Fhigh.........................................
3) Plot the graph of your result.
4) Determine the gain of the VCO in Hz/V. The gain of the VCO is defined as
∆Hz/∆V.
5) Over the linear range, take any two voltages and the corresponding frequencies.
∆Hz/∆V= f2-f1/V2-V1.
6) What is the slope of your graph is Hz/V = …………………
7) Why it is important to know the slope.
D)
Measuring the “Capture” and Tracking or Hold range or lock range.
1) Reconnect the jumper between point 1 and point 2.
2) Make sure that the 5V power supply is connected.
3) Connect the signal generator to the reference input, REF IN. Apply a sine wave
with the amplitude set to 3-4 Vpp at a frequency of 6kHz. Connect one channel of
the scope to each input lead of the phase detector.
4) Measure the “Capture Range” of the PLL.
fimin:...............……………………. and fimax:………................…………………..
5) Measure the “Lock Range” or “Tracking Range” or “Hold Range”.
fiL(min) : ……..........…………………… fiL(max).......................................................
6) Which range is larger:……………….. and why:…………………….
7) Measure the “gain” of the phase detector and filter combination by measuring the
change in DC output voltage of the filter to the change in phase at the phase
detector input as the reference frequency is adjusted. Your gain will be in units of
V/deg or V/Hz.
Analysis:
1) Study the operation of the phase detector by adjusting the reference input over the
tracking or lock range and watching the phase detector input and output on the
scope.
What did you observe?
2) Draw the waveform of input and output over the tracking range as well as when
VCO is not in lock.
3) What happens to the filter output when VCO gets out of lock?
4) Observe the filter output as you adjust the reference frequency from a low (or
high) value closer to the VCO free running frequency until it locks. Draw the
sketch.
5) What happens to the filter output when VCO suddenly locks to the input
frequency?
6) What determines the capture range, capture time and the lock range of the PLL?
7) Calculate the cut off frequency of the low pass filter and the time constant.
Analyze and discuss the effects of the cut off frequency fc and the time constant
Tc on the operation of the PLL.
8) Change the capacity value of the LPF and observe its effects on the operation of
the PLL. What were the changes you observed?
9) Discuss the effects of the linear range of VCO on the PLL operation. How would
it affect the system operation?
Troubleshooting:Observe, measure and sketch the voltages and waveforms at TP1, TP2, TP3, TP4, TP5
TP6, TP7 using scope and multimeter as appropriate, while the system is in lock and out
of lock. Record and sketch your results and answer the following questions.
1) What is the function of diode D1?
2) What will happen if diode D1 is accidently connected other way around i.e. if
anode is at base of transistor Q1 and cathode is grounded.
3) What if diode D1 becomes shorted?
4) What is the function of block A?
5) What kind of waveform you are expecting at TP2 for normal operation?
6) What will be the effects on the PLL system, if Q1 becomes shorted. Sketch the
output at pin 3 of the phase detector if Q1 becomes shorted. You can actually
connect a jumper between the collector and ground to see the effects.
7) What is the function of Ex-OR gate?
8) What kind of waveform would you observe at TP3 and pin 3 if Ex-OR gate is
defective (or the IC is defective)?
9) Sketch the waveform you are expecting at TP3 for normal operation.
10) What would you see at TP3, if C becomes shorted.
11) What will be the effect of an incorrect C value on system operation.
12) What is the purpose of Block D?
13) What is zener diode doing in the circuit. What is its purpose?
14) Sketch the expected waveform at TP5?
15) What will you see at TP5 if zener is accidentally reversed? Or becomes open? Or
becomes shorted?
16) What will you see at TP4, TP5 if Q2’s emitter was grounded and collector was
connected to the zener diode?
17) If Q2’s C-E junction becomes open? Or shorted?
18) What is the function of block E?
19) What determines the free running frequency of the VCO?
20) What determines the linear range of VCO?
21) What is the control voltage applied to the VCO?
22) Sketch the expected waveform at TP6 and TP7?
23) Discuss the effects of incorrect R6, R7 values?
24) What if R6 or R7 becomes shorted? or open?
25) What will be the effect of incorrect C1 or C2 value on TP6 and TP7?
26) What will be the effect of C1 or C2 becoming shorted on TP6 and TP7?
27) What will be the effect on TP6/TP7 of R6 or R7 becoming open.