Topic 3 Oscilloscope and Signal Generator

TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR

EE101(MEASUREMENT) Page 3-1

TOPIC 3

OSCILLOSCOPE AND SIGNAL GENERATOR

3.1 Introduction to Oscilloscope

3.1.1 Define of Oscilloscopes

Oscilloscope is a device that allows the amplitude of electrical signals, whether they be

voltage, current, power, etc., to be displayed primarily as a function of time ( the basic

instrument for the study all types of waveforms).

3.1.2 Explain the functions of Oscilloscope.

The main functions of oscilloscope are:

i. Measure the voltage (AC or DC)

ii. Measure the time and frequency

iii. Measure the phase differential between two waveforms

3.1.3 Classify types of Oscilloscope (analog and digital).

Oscilloscopes can be classified into two categories:

i. Analog →Works with continuously variable voltages.

→Works by directly applying a voltage being measured to an electron

beam moving across the oscilloscope screen. The voltage deflects the

beam up and down proportionally, tracing the waveform on the screen.

This gives an immediate picture of the waveform.



ii. Digital →Works with discrete binary numbers that represent voltage samples.

→Samples the waveform and uses an analog-to digital converter (or

ADC) to convert the voltage being measured into digital information.

It then uses this digital information to reconstruct the waveform on

the screen.

3.1.4 Draw block diagram of an analog oscilloscope.

Figure 3.1: An analog oscilloscope block diagram


EE101(MEASUREMENT) -3

3.1.5 Draw block diagram of a digital oscilloscope.

Figure 3.2: A digital oscilloscope block diagram

3.1.6 Explain the basic functional block diagram of an analog and digital

oscilloscope

Basic functional block diagram of an analog oscilloscope

i. When you connect an oscilloscope probe to a circuit, the voltage signal travels

through the probe to the vertical system of the oscilloscope. Figure 2 is a

simple block diagram that shows how an analog oscilloscope displays a measured

signal.

ii. Depending on how you set the vertical scale (volts/div control), an attenuator

reduces the signal voltage or an amplifier increases the signal voltage.

iii. Next, the signal travels directly to the vertical deflection plates of the cathode

ray tube (CRT). Voltage applied to these deflection plates causes a glowing dot to

move. (An electron beam hitting phosphor inside the CRT creates the glowing

dot.) A positive voltage causes the dot to move up while a negative voltage causes

the dot to move down.



iv. The signal also travels to the trigger system to start or trigger a "horizontal

sweep." Horizontal sweep is a term referring to the action of the horizontal

system causing the glowing dot to move across the screen. Triggering the

horizontal system causes the horizontal time base to move the glowing dot

across the screen from left to right within a specific time interval. Many

sweeps in rapid sequence cause the movement of the glowing dot to blend into a

solid line. At higher speeds, the dot may sweep across the screen up to 500,000

times each second.

v. Together, the horizontal sweeping action and the vertical deflection action

trace a graph of the signal on the screen. The trigger is necessary to

stabilize a repeating signal. It ensures that the sweep begins at the same

point of a repeating signal.

Basic functional block diagram of a digital oscilloscope

i. Some of the systems that make up digital oscilloscopes are the same as those in

analog oscilloscopes; however, digital oscilloscopes contain additional data processing

systems. (See Figure 3.) With the added systems, the digital oscilloscope

collects data for the entire waveform and then displays it.

ii. When you attach a digital oscilloscope probe to a circuit, the vertical system

adjusts the amplitude of the signal, just as in the analog oscilloscope.

iii. Next, the analog-to-digital converter (ADC) in the acquisition system samples

the signal at discrete points in time and converts the signals voltage at these

points to digital values called sample points. The horizontal systems sample clock

determines how often the ADC takes a sample. The rate at which the clock

"ticks" is called the sample rate and is measured in samples per second.

iv. The sample points from the ADC are stored in memory as waveform points.

More than one sample point may make up one waveform point.



v. Together, the waveform points make up one waveform record. The number of

waveform points used to make a waveform record is called the record length.

The trigger system determines the start and stop points of the record. The

display receives these record points after being stored in memory.

3.1.7 Compare advantages and disadvantages digital Oscilloscope with analog

Oscilloscope in term of accuracy.

a. Digital Oscilloscope

Advantages Disadvantages

High-accuracy measurements Can be more costly

Display storage Can be less intuitive to operate

because

they typically have more

features)

Bright, well-focused display at

virtually sweep speed

Pre-trigger viewing capability

Peak/glitch detection

Automatic measurements

Computer, printer/plotter

connectivity

Waveform processing capability

including waveform math functions

Display modes like averaging and

infinite persistence

Self calibration



b. Analog Oscilloscope

Advantages Disadvantages

Familiar controls Low accuracy

Instantaneous display updating for

real- adjustments Display flicker and/or dim display

Direct, dedicated controls for

often used

No pre-trigger viewing capability

Adjustments like vertical

sensitivity, time base speed, trace

position and trigger level low cost.

Limited bandwidth

Higher cost of ownership

Limited measurement capability

3.2 Understand the basic control functions of Oscilloscope

3.2.1 Explain function of Display Controls.

1) POWER SWITCH ON/OFF

Main power switches of the instrument. When this switch is turned on, the

LED above the switch is also turned ON

2) POWER LAMP

This LED lamp lights when power is turned ON

3) INTENSITY KNOB

It can control the brightness of the spot or trace.

4) FOCUS KNOB

After obtaining appropriate brightness with intensity, adjust focus for clearest

line



5) TRACE ROTATION KNOB

This knob is used to correct the horizontal trace when it becomes slanted

with respect to the horizontal scale, due to the effect of magnetic fields

6) SCALEILLUM KNOB

This is used to adjust scale brightness. If this knob is turned clockwise,

brightness is increased. This feature is useful for operation in dark places,

or when taking pictures

7) CAL 0.5V TERMINAL

Outputs a 0.5V p-p 1 KHz rectangular wave for calibrating probes

8) GND TERMINAL

This is a grounding terminal

3.2.2 Explain function of Vertical Controls.

1) CH1 INPUT CONNECTOR

This is a BNC connector used for vertical input CH1. The signal applied to this

connector when in the X-Y mode becomes the X-axis signal

2) CH2 INPUT CONNECTOR

This is a BNC connector used or vertical input CH2. The signal applied to this

connector when in the X-Y me ie becomes the Y-axis signal

3) AC-GND-DC SWITCH

Select following input coupling options for CHI and CH2

AC: blocks dc signal component allowing only AC signal to pass into attenuator



GND: input signal is switch off and attenuator is grounded

DC: dc coupling, all signal are directly connected to attenuator

4) VOLTS/DIV SELECTOR SWITCH

This is a step attenuator switch adjusting the vertical deflection sensitivity.

Set to the position which displays the input signal at the most convenient height

on the CRT

5) VARIABLE KNOB

The fine adjustment is used for varying the vertical-axis deflection sensitivity

continuously. If this knob is completely counterclockwise the vertical sensitivity

is reduced to less than 1/2.5 of VOLTS/DIV switch setting. This knob is used

for comparing two waveform and rise time measurement. However this knob is

normally in the locking position.

PULL X5 MAG

When the pull x5 Mag is pulled out, the vertical axis gain is magnified 5 times,

the maximum sensitivity becomes 1mV/div

6) 20MHz BANDWIDTH

The frequency bandwidth of vertical axis is limited to 20MHz. This knob can be

used when you cannot synchronize the signal by high frequency noise or expanded

trace

7) ALT/CHOP



When the vertical mode is in dual, this button can display ALT and CHOP mode. ALT

mode is a sequential display mode with one cycle of signal between CHI and CH2.

CHOP mode is a sequential display mode with a frequency step of approximately

220MHz between CHI and CH2

8) POSITION

Used to move the CHI or CH2 trace up or down on the CRT screen

9) INVERT SWITCH

When the invert push button is pressed, the polarity of the input signal applied

to CH2 is inverted. This function is convenient when 2 waveforms of difference

are compared, or for displaying the CHI and CH2 difference waveform using ADD

10) MODE SELECTOR SWITCH

Selects vertical axis operating mode

CHI: only the signal applied to CH1 is displayed on the screen

CH2: only the signal applied to CH2 is displayed on the screen

DUAL: when both CH1 and CH2 buttons are pushed in the signals applied to CH1

and CH2 input are displayed on the screen

ADD: display the algebraic sum of the CHI and CH2 input voltage

3.2.3 Explain function of Horizontal Controls.



1) A TIME/DIV

Selects sweep speed from 0.05µs/div to 0.2µs/div in 21 calibrated steps

2) HORIZONTAL POSITION

The trace can be moved in a horizontal direction. Tuning this knob clockwise moves

the trace towards the right, turning the knob is counterclockwise moves the trace

towards the left

PULL X10 MAG

When pulled out, the trace will be magnified by a factor of 10 times. The sweep

time becomes 1/10 of the indicated on-the time/div switch, (eg. 100µs/div

becomes 10µs/div for X10 MAG). To magnify a portion of a waveform: Move the

waveform of interest to the center gratitude on the horizontal scale

3) A, B, ALT (H DISPLAY)

This switch selects the sweep method of A, B. When both A and B buttons

are pushed in, it was display B sweep which was duplicated by A sweep and 2

trace of B sweep simultaneously.

4) X-Y

Displays the CH1 and CH2 input signal as an X, Y graph. The vertical deflection

signal is applied to the CH1 input and the horizontal deflection signal is

applied to the CH2 input. The CH2 Vertical position control is used for the

positioning the X, Y display on the vertical axis the horizontal position

control positions the X, Y display on horizontal axis of CRT.

5) TRACE SEPERATION



This trace separation controls the vertical position interval of A sweep

and B sweep at the sweep mode is A ALT B.

6) DLY POS

Adjusts to starting with B sweep during the A sweep periods.

7) VARIABLE

When this knob is turned all the way clockwise (cal) the sweep is indicated

by the A time/diy switch, if the knob is turned all the way

counterclockwise the sweep is less than ½.5 of the A time/div setting.

During normal operation, this knob is turned to the CAL position.

3.2.4 Explain function of Trigger Controls.

1) B TRIS'D

This knob selects between continuous delay and triggered delay. For

continuous delay (normal state), the B sweep starts immediately after the

sweep delay time determined by A time/div switch (19) and delay pos (25).

For triggered delay (the knob is pushed), the sweep starts with B trigger

signal after the continuous delay time.

2) TRIGGER SOURCE SELECTOR SWITCH

Selects sweep trigger signal source.

INT: The input signal applied to CH1 or CH2 becomes the trigger signal

CH2: The input signal applied to CH2 becomes the trigger signal

LINE: The power line frequency becomes the trigger signal source

EXT: The external signal applied to EXT input becomes the trigger signal.

This is used when the trigger signal is external the vertical input

signal



3) TRIG LEVEL KNOB

This control sets the amplitude point on the trigger waveform that will start

the sweep.

Pull Slope knob

Selects the polarity of the slope the trigger source waveform will start the sweep.

(+) Slope is selected when the pull slope knob is in normal position

(-)Slope is selected when the pull slope knob is pulled out.

4) TRIG-MODE SWITCH

Auto: Sweep continuously runs in the auto sweep mode. A trace will be displayed

even when there is no input signal or when the input waveform is not triggered. A

stationary waveform will be displayed when the input waveform is properly

triggered.

Norm: A trace will be displayed only when the input waveform is present and is

properly triggered. There will be no trace displayed on the CRT if there is no

input signal or if the input signal is not synchronized. Normal sweep is used when

the input signal's frequency is less than 2LHz.

TV-H: Effective when trig mode is set to TV, and is used when the horizontal

of the TV signal is to be synchronized.

TV-V: Effective only when trig mode is set to TV, and is used when the vertical

of the TV signal is to be synchronized.

*Both TV-V and TV-H are synchronized only when the trigger signal is (-).

6) HOLD OFF

By the operation of Hold Off, complicated repetitive signals can be captured.

3.3 Understand the Oscilloscope Probes

3.3.1 Define of Oscilloscope Probes.



A probe is more than a cable with a clip-on tip. It is a high-quality connector,

carefully designed not to pick up stray radio and power line noise. Probes are designed

not to influence the behavior of the circuit you are testing. However, no measurement

device can act as a perfectly invisible observer.

3.3.2 Classify types of Oscilloscope Probes.

Figure 3.3: Classify types of Oscilloscope Probes

3.4 Understand the application of Oscilloscope

3.4.1 Prepare procedure for calibrate Oscilloscope.

1. Turn on oscilloscope. Allow it to warm up for approximately 10 minutes. Letting the

oscilloscope warm up prevents damage to its cathode ray tube.

2. A green light is show on the CRT screen when the oscilloscope is ready. The green

light should run across the screen horizontally and should be in the center of the

screen. Adjust the position knob on the oscilloscope by and turn it clockwise or

counter clockwise until this line is at the center of the screen. If the line

shows up as a green dot moving across the screen, adjust the time/div knob until

the dot appear as a line and then center it.

3. Usually an oscilloscope has two channels CH1 and CH2. Connect your oscilloscope



probe to CH1.

4. Find the voltage selector switch and set it to AC volts.

5. Find CAL connector. It looks like a small enclosed hook, similar to the eye of a

needle.

6. Attach the oscilloscope probe to CAL connector. Ground the ground wire. A peak

to peak square wave one volt above the center division and one volt below the center

division. This means the oscilloscope is correctly calibrated at 2 Vpp.

3.4.2 Construct connection between Signal Generator and Oscilloscope for signal

measurement.

Figure 3.4: Construct connection between Signal

Generator and Oscilloscope



3.4.3 Use the Oscilloscope to measure voltage, frequency, time and phase angle in

sinusoidal and non-sinusoidal waveform from signal generator.

Figure 3.5: Sinusoidal and non-sinusoidal waveform

a. Voltage,V (V) :

= No. vertical division X volt/div X probe x1 or x10

b. Period, T (s) :

= No. Horizontal division X time/div

c. Frequency measurement, f(Hz):

= 1 / period



3.4.4 Construct connection between simple circuits to Oscilloscope for

in circuit waveform measurement.

Figure 3.6: Simple Circuit



3.4.5 Evaluate the specification of analog Oscilloscope

To use an analogue oscilloscope, three basic setting accommodate an incoming

signal:

a. The attenuation or amplification of the signal

Use the volt/div control to adjust the amplitude of the signal before it is

applied to the vertical deflection plates.

b. The time base

The time/div control to set amount of time per division represented

horizontally across the screen.

c. The trigger of the oscilloscope

Use the trigger level to stabilize a repeating signal, as well as triggering on

a single event.

Also adjust the focus and intensity controls to create a sharp and visible

display.

3.5 Introduction to signal generator

Define of signal generator.

A signal generator is a test device which generates an alternating voltage

signal suitable for test purposes. It is, in effect, a small radio transmitter generating

a signal of any desired frequency. The signal may be either modulated or

unmodulated and is used for the following checks or tests:

a. Alignment of tuned circuits, sensitivity measurements, and

approximate frequency measurements.

b. For frequency measurements, its use is limited because it is not a

frequency meter and cannot be used as a frequency standard.



The signal generator is used primarily in the alignment of tuned circuits. A

signal generator is classified according to its frequency and is one of two types:

audio frequency or radio frequency.

a. Audio frequency generators produce signals with a frequency range

from 20 Hz to 20 kHz.

b. Radio-frequency generators produce signals covering a range of

frequencies from 10 kHz to 10 GHz. Many radio-frequency generators

have audio outputs separately available through front panel jacks.

These outputs are normally 100 Hz and 400 Hz.

When using the generator, the output test signal is coupled into the circuit

being tested, and its progress through the equipment is traced by the use of high-

impedance indicating devices such as vacuum-tube voltmeters or scopes. In many

signal generators, calibrated networks of resistors, called attenuators, are provided.

These are used to regulate the voltage of the output signal and also provide correct

impedance values for matching the input impedance of the circuit under testing.

Accurately calibrated attenuators are used, because the signal strength must be

regulated to avoid overloading the circuit receiving the signal.

3.5.1 Classify types of signal generator.

There are many types of signal generators. They may be classified roughly by

frequency into audio signal generators, video signal generators, radio frequency

generators, frequency-modu lated RF generators, and special types which

combine all of these frequency ranges.

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3.5.1.1 Audio signal generators.



a. Audio signal generators produce stable audio-frequency signals used

for testing audio equipment. Video signal generators produce signals

which include the audio range and extend considerably further into the

RF range. These generators are used in testing video amplifiers and

other wideband circuits.

b. In both audio and video generators, the major components include a

power supply, an oscillator, one or more amplifiers, and an output

control. Voltage regulation circuits are necessary to ensure stability

of the oscillator in the generators which derive power from 115-volt AC

sources. In portable generators, battery power supplies are usually

used, and these require no voltage regulation.

c. In the audio and video generators of the beat-frequency type, the

output frequency is produced by mixing the signals of two radio

frequency oscillators, one of which is fixed in frequency and the other

variable. The difference in frequency of the two is equal to the desired

audio or video frequency.

(1) Audio signal generators often include RC oscillators in which the

audio frequency is directly produced. In these a resistance-

capacitance circuit is the frequency-determining part of the

oscillator. The frequency varies when either the resistance or

the capacitance is changed in value.

(2) In commercial generators, however, the capacitance alone is often

chosen as the variable element. The change in frequency which

can be produced by this method is limited, and it is usually

necessary to cover the entire range of the generator in steps.

This is accomplished by providing several RC circuits, each

corresponding to a portion of the entire range of frequency



values. The circuits in the oscillator are switched one at a time

to give the desired portion of the audio range.

d. The amplifier section of the block diagram (fig 3-2) usually consists of

a voltage amplifier and one or two power amplifiers. These are

coupled by means of RC networks, and the output of the final power

amplifier is often coupled to the attenuator, or output control, by means

of an output transformer.

Figure 3-2: Block diagram of audio or video signal generator

e. The output control section provides a means of matching the output

signal to the input of the equipment under test and regulating the

amplitude of the signal.

3.5.2 Classify types of signal generator.

i. Video signal generator : a device which outputs predetermined video

and/or television waveform and other signals used to stimulate faults in,

or aid in parametric measurements of television and video systems.

ii. Pitch generator: a types of signal generator optimized for use in audio and

acoustics applications. Sophisticated pitch generators will also include



sweep generators a function which varies the output frequency over a

range. Pitch generators are typically used in conjunction with sound level

meters, when measuring the acoustics of a room or a sound reproduction

system, and/or with oscilloscopes or specialized audio analyzers.

iii. Arbitrary waveform generators (AWG): Sophisticated signal generators

which allow the user to generate arbitrary waveforms, within published

limits of frequency range, accuracy and output level. Unlike function

generators, which are limited to a simple set of waveforms; an AWG allows

the user to specify a source waveform in a variety of different ways.

3.5.3 Explain Standard Signal Generators.

Produces known and controllable voltages

Used as power source for measurement of gain, signal to noise ratio,

bandwidth, standing wave ratio, and other properties.

Extensively used in testing of radio receiver and transmitter

The output signal can be Amplitude Modulated (AM) or Frequency

Modulated (FM)

3.5.4 Explain function generators

Produce different waveforms of adjustable frequency

Common output waveform are sine, square, triangular and sawtooth

The frequency may be adjusted, from a fraction of a hertz to several

hundred kHz

3.5.5 The front panel of a signal generator



Figure 3.7: The front

panel of a

signal

generator

3.5.6 Describe the function of the following items located on the panel of a signal

generator :

a. Frequency Selection Group

- Range switch: Provide seven fixed decades of frequency

- Multiplier: Variable potentiometer allowing frequency setting between fixed

range.

b. Sweep Group

- The sweep group can frequency sweep any of its function outputs. It could be

swept up or down in frequency using linear or log sweeps. Unlike function

generators, there are no annoying discontinuities or band-switching artifacts

when sweeping through certain frequencies. Two sweep marker frequencies can

be specified. When the sweep crosses either of the marker frequencies, a TTL

transition is generated at the rear-panel output to allow synchronization of

external devices.



c. Amplitude Modulation Group

- To provide 20dB of attenuation of the output waveform selected by function

switch.

d. DC offset Group

- To allow the DC level of the output waveform to be set as desire.

e. Function or Waveform group

- To provide selection of desired output waveform. (Square, triangle and sine

waveforms are provided)

f. Output Group

- Used to adjust the amplitude of the generator’s out signal. The group consists

of the amplitude control knob, the three attenuation buttons and the fused

50Ω BNC connector.

3.5.7 Construct simple circuit and see signal generator as signal source to the

circuit

Figure 3.8: Simple circuit construction of signal generator

Topic 3 Oscilloscope and Signal Generator

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