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LECTURE 4

Repetitive signals

Time varying signalse.g. RS-232 signals

Oscilloscope

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Repetitive SignalsA 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 CD

Pin 2 RxD Receive Data

Pin 3 TxD Transmit Data

Pin 4 DTR

Pin 5 GND Ground (0V)

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RS-232 Signals

Pin 6 DSR

Pin 7 RTS

Pin 8 CTS

Pin 9 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 - Output signal

Data is serially transmitted out

from this pin.

• RxD - Input signal

Data is serially input from this pin.

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Signal levels

• The voltage levels of its electrical specification are:

Logic Input Output

"0" +3V to +25V +5V to +15V

"1" - 3V to -25V -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

10

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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 BaseThe time base generator creates a sawtooth waveform that deflects the beam

horizontally (the sweep) across the screen.

Vertical Deflection SystemThe 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 Y-

plates 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 negative-

going 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 channelsIt 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

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.

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Fig. S4.9

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.

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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 5s/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 Fig.S4.13

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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