Oscilloscopes

An analog oscilloscope 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.

In contrast, a digital oscilloscope 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.

Fig.1 shows a general purpose oscilloscope consists of the following subsystem:(i) Cathode Ray Tube or CRT(ii) Vertical Amplifier(iii) Delay line(iv) Time base generator(v) Horizontal Amplifier(vi) Trigger Circuit(vii) High Voltage (HV) Power supply(viii) Low Voltage (LV) Power supply

Fig.1 Block diagram of a single-beam oscilloscope

Analog Oscilloscopes

When an oscilloscope probe is connected to a circuit, the voltage signal travels through the probe to the vertical amplifier of the vertical system of the oscilloscope. Block diagram of Fig.1 shows the situation.

Depending on how the vertical scale (volts/div control) is set, an attenuator reduces the signal voltage or an amplifier increases the signal voltage.

Next, the signal travels through a delay circuit 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.

The signal also travels to the trigger circuit to start or trigger a time base circuit to produce "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.

Together, the horizontal sweeping action and the vertical deflection action traces 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, resulting in a clear picture as shown in the following figure.

In conclusion, to use an analog oscilloscope, one needs to adjust three basic settings to accommodate an incoming signal:

(i) The attenuation or amplification of the signal. Use the volts/div control to adjust the amplitude of the signal before it is applied to the vertical deflection plates.

(ii) The time base. Use the sec/div control to set the amount of time per division represented horizontally across the screen.

(iii) The triggering of the oscilloscope. Use the trigger level to stabilize a repeating signal, as well as triggering on a single event.

Also, by adjusting the focus and intensity controls a sharp, visible display can be created.

The power supply block provides the voltages required by the cathode ray tube to generate and accelerate the electron beam, as well as to supply the required operating voltages for the other circuits of the oscilloscope. relativeley high voltages are required by the cathode ray tube, on the order of few thousand volts, for acceleration, as well as a low voltage for the heater of the electron gun, which emits the electrons. Supply voltages for the other circuits are various values, usually not more than a few hundred volts.

The earliest version of the CRT was invented by the German physicist Ferdinand Braun in 1897 and is also known as the Braun tube.

A cathode ray tube is a vacuum tube which consists of one or more electron guns, internal electrostatic deflection plates, and a phosphor target. Cathode ray tubes use focused electron beams to display images. Though tubes of this type are used for many purposes, cathode ray tubes are most famous for their use in such things as televisions, oscilloscopes, computer and radar displays, and automated teller machines. Cathode ray tubes are also used in video game equipment.

Fig.2 (a) Basic Cathode Ray Tube

Fig.2(b) Basic Cathode Ray Tube Construction

Fig.3 Interior of a cathode-ray tube for use in an oscilloscope

In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of diagnostic instrument.

Fig.4 Internal Diagram of an Oscilloscope

Astigmatism:It sets the voltage on the accelerating anode with respect to vertical deflection plates. This corrects any defocusing that might be present.Intensity:The intensity of the electron beam is adjusted by varying the cathode-to-grid voltage of the triode section of the CRT.

Focus:Focus to a particular point can be adjusted by increasing the negative voltage on the focusing plate with respect to the outside anode.

Screen of CRT: When the electron beam strikes the screen of the CRT, a spot of light is produced. The screen material on the inner surface of the CRT that produces this effect is the phosphor. The phosphor absorbs the kinetic energy of the bombarding electrons and reemits energy at a lower frequency in the visual spectrum. The property of some crystalline materials, such as phosphor or zinc-oxide, zinc-sulphide, to emit light when stimulated by radiation is called fluorescence. Fluorescent materials have a second characteristics, phosphorescence, which refers to the property of the material to continue light emission even after the source of excitation is cut off.

The bombarding electrons striking the phosphor release secondary emission electrons, thus keeping the screen in a state of electrical equilibrium. These secondary emission low-velocity electrons are collected by a conductive coating, known as aquadag, on the inside surface of the glass tube, which id electrically connected to the second anode.

Fig.5 Vertical Amplifier of an Oscilloscope

Fig.6 Vertical and horizontal deflection systems of an Oscilloscope

Fig.7 Sync Selector Circuit

Fig.8 Automatic time base generation and the operation of the Oscilloscope

Fig.9 Deflection system of dual-beam Oscilloscope

Fig.10 Deflection system of dual-trace Oscilloscope

Digital Oscilloscope:The block diagram of a basic sampling and storage system for a digital storage oscilloscope (DSO) is illustrated in Fig.11. The time base generates a pulse waveform at the desired sampling frequency. Each pulse switches the sampling rate and the ADC on for a brief time period. The sampling gate generates a series of analog samples, as illustrated, and the ADC converts each sample into a coded group of pulses. The pulse groups are passed to a semiconductor memory, where they are stored for a later recovery.

Fig. 11 Digital oscilloscope

The same figure also represents the recovery of the stored information to reproduce the original waveform. The digital-to-analog converter (DAC), triggered by pulse from the time base, converts each digital sample back to analog (step) form and passes it to the oscilloscope vertical deflection amplifier. The time base also generates a staircase waveform that is fed to the horizontal deflection amplifier. The vertical and horizontal inputs can be used to produce either a dot waveform or a step waveform on the oscilloscope screen, as illustrated.

Sampling Oscilloscope:Most ordinary oscilloscopes have an upper frequency limit in the range of 20MHz to 50MHz. Higher input frequencies causes the electron beam to move so fast across the screen that only a very faint trace is produced. The sampling oscilloscope overcomes this difficulty by producing a low frequency dot representation of the signal. Each dot represents an amplitude sample input signal, and each sample is taken in a different cycle.

Fig.12 Sampling oscilloscope

Fig. 13 Down-sampled Sine Wave

Fig.12 shows sampling oscilloscope. The sampling circuit must be capable of operating at very high frequencies but the CRT and its associated circuitry may be relatively low-frequency equipment.

The principle of down-sampling of an oscilloscope is illustrated in figure 13. The solid trace is the high frequency input signal. The dotted trace is the down-sampled version.

Circles mark each sample point. The samples of the input waveform are obtained in such a way that each sample is located slightly later in the incoming waveform. The effect is to display the shape of the incoming waveform, but at a much lower frequency.

In this way, the oscilloscope can display a waveform that is much higher than the sampling frequency. In figure 13 the samples are taken at each cycle of the input waveform. In practice, several cycles of the input may elapse between each sample, in which case the division ratio between input and display is larger. As well, figure 13 shows the input as a sine wave, but any wave-shape can be treated this way.

Fig.13 shows 20 cycles of a high-frequency waveform that is to be displayed. One amplitude sample is taken at successive later times in each cycle. The resultant series of samples reproduces the original waveform at a lower frequency.