Fundamentals Of Oscilloscope (analog And Digital)

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Fundamentals of Oscilloscope (Analog and Digital)

Shashidharan p, Former HOD, Dept. of Physics, Vartak College

The specific characteristic of a signal can be measured accurately by a variety of instruments:

for example the frequency or time period of a signal can be measured by a frequency meter,

voltage can be measured by volt meter etc. These instruments are limited to measuring only a

single characteristic.

Then, why an oscilloscope, which is not so accurate and expensive?

With an oscilloscope the user can visualize the signal of interest and also observe whether the

signal contains other properties that would not be made clear by most other instruments. For

example if an AC signal is riding on a DC signal or there is noise. Thus Oscilloscope is

indispensable tool for anyone designing, manufacturing or repairing electronic equipment or

even to scientists because with the proper sensor, an oscilloscope can measure all kinds of

phenomena.

The key to any good oscilloscope system is its ability to accurately reconstruct a waveform –

referred to as signal integrity.

Oscilloscopes can be classified – as analog and digital types. For many applications, either an

analog or digital oscilloscope will do. However, each type has unique characteristics that may

make it more or less suitable for specific applications. Digital oscilloscope can be further

classified into digital storage oscilloscopes (DSOs), digital phosphor oscilloscopes (DPOs)

and sampling oscilloscopes.

Analog oscilloscope (also called CRO, Cathode Ray Oscilloscope)

The CRO displays an input signal in VOLTS along the vertical or Y – axis as a function of

TIME along the horizontal or X – axis. The electronic circuit of a CRO is thus classified into

vertical and horizontal sections.

The vertical section has the following parts: The input selector, the input attenuator, the

vertical amplifier and the delay line. The horizontal section has the following parts: trigger

circuit, sweep generator, horizontal amplifier. The phosphor display called CRT(Cathode Ray

Tube) and graticule is common to both sections.

The Input Selector, Attenuator and Position

The input selector switch chooses direct coupling(dc), ground(gnd) or capacitor

coupling(ac).Generally the gnd is in between dc and ac (Fig.1). dc coupling allows the entire

signal (dc + ac) to be displayed on the screen (Fig.1a). ac coupling blocks the dc component

of a signal so that only the ac component centered around zero Volt is displayed (Fig.1b).

The ac coupling setting is useful when the entire signal (dc + ac) is too large. The ground

setting disconnects the input signal from the vertical system and a horizontal line on the

screen that represents zero volts can be seen in the auto trigger mode.

The input attenuator consists of a number of RC voltage dividers. The attenuator has to be a

compensated attenuator. A Compensated Attenuator is a simple two port network which

provides a constant attenuation over a wide range of frequencies (from DC to the highest

frequency the instrument can handle) (Fig.2).

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Fig.1 Fig.1a Fig.1b

A compensated attenuator has to be critically compensated (R1C1 = R2C2). An under

compensated or over compensated attenuator will distort the waveform as shown in Fig.2a.

Fig.2 Fig.2a

A CRO has multi range compensated attenuator so that various voltage levels can be

measured by the same instrument (Fig.3). Input to compensated attenuator is from the input

selector. The switch that selects the attenuation is marked on CRO as Volts/Div. The

maximum voltage you can display on the screen is the volts/div setting multiplied by the

number of vertical divisions.

Fig.3

A test signal is provided on the oscilloscope by the manufacturer to check whether the

attenuator is critically compensated.

The vertical position control allows moving the waveform up and down exactly where one

wants it on the screen. It is marked on the CRO as V POS or with an up-down arrow (↕).

Vertical Amplifier

It amplifies the input signal, which is to be displayed on the screen of CRT. Vertical

amplifier is usually fixed gain. It is easy to design a fixed gain amplifier with large

bandwidth. The vertical amplifier consists of a pre-amp and a main vertical amp. The pre-

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amp is usually a FET input followed by a phase inverter or a paraphase amp which then

drives the main amp (Fig.4). A paraphase amp provides two output signals 180o out of phase

(Fig.4a).

Fig.4 Fig.4a

Vertical amplifier determines the sensitivity and bandwidth of the oscilloscope. The vertical

sensitivity is expressed in terms of V/Cm at the mid frequency. The sensitivity of an

oscilloscope is directly proportional to the gain of the vertical amplifier.

Some oscilloscopes provide a DSP arbitrary equalization filter used to improve the oscilloscope

response. This filter improves the frequency response, phase linearity, and provides a better

match between channels. It also decreases rise time and improves the step response.

Delay Line: A part of the input signal is used to start the horizontal sweep generator for

starting the horizontal deflection of the electron beam. It takes a certain amount of time

before the sweep is initiated. This results in the loss of the leading edge of the vertical input

signal. To overcome this problem the vertical signal is passed through a delay line so that the

vertical signal arrives at the Y- deflection plates exactly when the horizontal sweep is

initiated. The signal pick-off for horizontal sweep initiation should precede the delay line.

There are two types of delay lines: 1. Lumped parameter delay line (Fig.4b) and

2. Distributed parameter delay line (Fig.4c).

Fig.4b Fig.4c

The lumped parameter delay line consists of cascaded LC network. When the network is

terminated in its characteristic impedance Zo, it appears as a low pass filter. The pass-band is

defined as the range in which attenuation is zero. The upper limit of the pass-band is called

the cut-off frequency fc and delay time is given as approx. td ≈1/πfc = √LC.

The less careful adjustment and occupation of less space makes the distributed type co-axial

cable delay line more prospective.

Sweep or Ramp Generator

The CRO displays the vertical input signal as a function of time. This requires the electron

beam of the CRT to be moved across the screen with a constant velocity and returns rapidly

to the starting point on the left. This requires a linearly rising voltage which rises from a

minimum voltage to a maximum voltage and suddenly comes back to the minimum value.

Such a waveform is generated by a sweep or ramp generator and the waveform is called a

saw-tooth wave. The linearly rising part of the saw tooth is called the run up ramp voltage.

During the ramp up the electron beam is swept from the left of the screen to the right. During

the re-trace or fly-back of the ramp the beam return from right to left of the screen. During

this return the beam is cut off using the control grid of the CRT (Voltage V2 in Fig.5).

A sweep generator is said to be free running if no external signal is used to start or stop the

ramp. A stable CRT display can be obtained only if the vertical frequency is an integral

multiple of the horizontal sweep frequency, otherwise the display becomes unstable. Thus the

sweep signal should be synchronized with the vertical input signal. This can be achieved by

the premature stoppage of the run-up ramp. For the first few sync pulses the ramp will remain

unsynchronized, eventually it get synchronized. The period of the sync signal should be

shorter than the free running ramp (Fig.5). The sweep time control on the CRO panel is

marked as TIME/DIV.

Fig.5 Fig.5a

In the absence of any sync pulse the inverting input of the op-Amp will be at Zero voltage,

thus taking its output (V2) to –VCC thus switching OFF transistor Q2. The capacitor will start