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LECTURE 4
Repetitive signals
Time varying signals
e.g. RS-232 signals
Oscilloscope
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Repetitive Signals
A signal has its values change in a
periodic manner. The waveform of the
signal repeats itself in regular cycles
forever.
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Non-repetitive Signals
A signal that has no cyclic repeating
pattern.
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RS-232
• RS-232 is a serial specification being
widely used in communications.
• Its applications can be found in computer
terminals, serial printers, remote control
panels and short-distance communication
links.
• It became popular when it was utilized on
the COM ports of the Personal Computer.
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RS-232 (cont.)
• The standard specifies a 25-pin D-type
connector for its signal pin connections.
• To save space for the PC circuit board, most
present-day PCs use 9-pin D-type
connectors for its COM ports construction
instead.
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RS-232 Signals
• The signals names and their corresponding
pin numbers of the 9-pin connector are:
Pin 1
Pin 2
Pin 3
Pin 4
Pin 5
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CD
RxD
TxD
DTR
GND
Receive Data
Transmit Data
Ground (0V)
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RS-232 Signals
Pin 6
Pin 7
Pin 8
Pin 9
DSR
RTS
CTS
RI
Other than the RxD and the TxD which are data
signals, the rest are control signals. We will
observe their waveforms in a lab session
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Data signals
• TxD -
• RxD -
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Output signal
Data is serially transmitted out
from this pin.
Input signal
Data is serially input from this pin.
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Signal levels
• The voltage levels of its electrical
specification are:
Logic
"0"
"1"
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Input
+3V to +25V
- 3V to -25V
Output
+5V to +15V
-5V to -15V
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Data transfer rate
• Data transfer rate
It is measured by the number of bits
transmitting per second. Common data
transfer rates are 1200, 2400, 9600, 14400,
28800, 57600, 115200 bits per second (bps).
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TxD, RxD Signals
• Logic levels of TxD & RxD
1
0
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Data Frame
• Start bit – To identify the beginning of a data
frame. It uses a single bit (logic 0)
• Data bits – To store the data. It uses 4, 5, 6,
7 or 8 bits. 8 bits are mostly used.
• Parity bit – A check bit for data. A single bit for
even or odd parity, none for no parity.
• Stop bit – To identify the end of a data frame. It
uses 1, 1.5 or 2 bits (logic 1)
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Parity bit
• Even parity – number of ones in the data
bits together with the parity bit is an even
number.
• Odd parity – number of ones in the data bits
together with the parity bit is an odd
number.
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Cathode Ray Oscilloscope (CRO)
Introduction
• An important measuring instrument in electronics.
• It is used to display the waveforms of signals.
Cathode Ray Tube (CRT)
• The heart of a CRO
Fig.1 shows the basic construction of a CRT
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Cathode Ray Tube
• Fluorescent screen
– coated on the inside with phosphorous powder which gives a
visible glow when struck by accelerated electron beam.
• Horizontal deflecting plate (X-plate)
– used to produce an electrostatic deflection of the electron beam in
a horizontal direction.
• Vertical deflecting plates (Y-plate)
– used to produce an electrostatic deflection of the electron beam in
a vertical direction.
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Horizontal Time Base
The time base generator creates a sawtooth waveform that deflects the beam
horizontally (the sweep) across the screen.
Vertical Deflection System
The user can set an attenuation/amplification to the input signal by adjusting
the volts per division (VOLT/DIV) of the input channel. A selector on the
input line allows the user to select ac coupled, dc coupled or ground.
• When set to DC, the input signal is applied directly to the vertical
deflection system, permitting the entire signal (both ac and dc
components) to be displayed on the screen.
• When set to AC, the dc part of the input signal is blocked, leaving only
the ac part of the signal being displayed.
• When set to GND, a zero volt is applied to the vertical deflection system.
This allows the user to establish a 0-V baseline for measurement.
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In Fig.S4.2, a sine wave is applied to the Yplates and a sawtooth wave is applied to the
X-plates. If the waveforms are perfectly
synchronized, the resulting waveform will
be displayed as in Fig.S4.2c.
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Triggering
• The purpose of triggering is to synchronize the horizontal sweep with
the input signal in such a way that each horizontal sweep begins at the
same point on the input signal each time.
• If the time base signal sweep across the screen in a time that is equal to
an integer number of input signal periods, the input signal will then
appear locked on the CRT screen.
• Two front panel controls: the trigger level and the trigger slope
• Trigger level: determines what minimum amplitude vertical signal is
required to trigger the horizontal sweep and where on
the waveform sweep begins.[Fig.S4.5]
• Trigger slope: determines whether the trigger occurs on a negativegoing or a positive-going edge of the input waveform.
[Fig.S4.5]
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Fig.S4.5
Sweep mode control: AUTO, NORM, SINGLE
– Auto: the sweep will periodically retrigger even if no signal is present in
the input channels.
– Norm: this mode requires a vertical signal to begin sweeping the CRT, and
the screen will remain blank otherwise.
– Single: the CRT beam will sweep only once in this mode.
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Source control
This control selects the source of the signal applied to the triggering
circuits. The selections are INT, LINE, and EXT.
• INT means that the time base is triggered by one of the input
waveforms through CH1 or CH2.
• LINE means the time base is triggered from the line or ac power
frequency.
• EXT means that the signal applied to the external trigger circuit
input will trigger the sweep circuits.
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Horizontal sweep time
– used to determine the amount of time required per division to
sweep the beam across the CRT face from left to right.
– calibrated in units of Time/division
• Example:
Each complete pulse of a displayed waveform has a cycle time period
of 2.3 divisions. What is the frequency of the waveform if the sweep
time across the CRT screen is set at 0.2 s/division?
• Solution:
The sweep time of the waveform = 2.3 div x 0.2 s/div = 0.46 s
Therefore, the frequency of the waveform
= 1/T
= 1/0.46 x 10-6
= 2.17 MHz
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Dual channels
It allows the user to view and compare two waveforms simultaneously
against the same time base. The switching modes for dual channels are:
ALT, CHOP, X-Y
• ALT (alternate) mode
One input signal does not start tracing on the screen until the other
signal finishes tracing. i.e. the CRO display alternates between the two
signals of the two channels.
• CHOP mode
The electron beam is switched back and forth rapidly between channel A
and channel B.
• X-Y mode
In this mode the internal oscilloscope time base is disconnected, the
instrument becomes a vectorscope. Channel 1 becomes horizontal (X)
input, while channel 2 is the vertical (Y) input.
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Fig. S4.8 shows two low frequency
waveforms displayed in ALT mode.
The beam is seen slowly tracing
out the first wave and then the
other. Only one of the waveforms
is displayed on the screen at any
one time. However, if the input
waveforms are of high frequencies,
both waveforms will appear
displaying simultaneously on the
screen.
Fig.S4.8
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Fig. S4.9 shows two high frequency
waveforms displayed in CHOP
mode. The waveforms are displayed
as dashed-line traces with gaps.
However, if the input waveforms are
of low frequencies, the breaks in the
traced waveforms will appear as
relatively short durations i.e. the
gaps appear invisible.
Fig. S4.9
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Basic measurement of CRO
a) Peak to peak voltage measurement
V p-to-p = (vertical p-to-p divisions) x (Volts/Div)
b) Frequency measurement
Time period T = (Horizontal divisions/cycle) x (Time/Div)
Frequency = 1/T
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Fig.S4.10 shows a triangular
wave displayed on the screen
• Peak to peak voltage = 3 Div x 0.5 mV/Div = 1.5 mV
• Period: T = 2 Div x 0.1 ms/Div = 0.2 ms
• Frequency = 1/T = 1/ 0.2 ms = 5000 Hz
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c) Phase measurement
Fig.S4.11
In Fig. S4.11, d is the horizontal measurement between the two signals to
be measured. T is the period of one complete cycle of a waveform.
Phase angle = d/T x 360o = 1 Div / 6 Div x 360o = 60o
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d) Pulse measurements
Two pulse waveforms are displayed in Fig.S4.12
Fig.S4.12
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Rise time = (Time/Div) x (Horizontal Division of the leading edge
of a pulse increasing from 10% to 90% of the pulse
amplitude)
Fall time = (Time/Div) x (Horizontal Division of the trailing edge
of the pulse decreasing from 90% to 10% of the pulse
amplitude)
Delay time = (Time/Div) x (Horizontal Division measured from the
start of the input pulse until the output pulse reaches
10% of the pulse amplitude)
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Example:
Determine the pulse amplitude, frequency, rise time and fall
time of the waveform in Fig.S4.13. The CRO is set at
5s/div and 2V/div.
Solution:
Pulse Amplitude = 4 Div x 2V/Div
= 8V
Period: T= 5.6 Div x 5 s/div = 28 s
Frequency = 1/T
= 1/ 28 s = 35.7 kHz
Rise time = 0.5 div x 5 s/div
= 2.5 s
Fall time = 0.6 div x 5 s/div
= 3 s
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Fig.S4.13
Lecture 4 - 30
Differential measurements
If an oscilloscope is equipped with an A+B function, the sum of two signals
amplitudes from channel A and channel B can be displayed. If Channel B is
put on the INVERT selection together with A+B function enabled,
differential measurements can be taken. That is the A+B function can be
used to display the difference between the two signals.
A + (-B) = A - B
Fig.S4.19 shows the connection method of differential measurement
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Oscilloscope Probe
• A measuring probe of the oscilloscope.
• 1:1 Probe
• 10:1 probe attenuates the input signal by a factor
of 10
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