ELECTRONICS LAB. OSCILLOSCOPE

Transcription

1 ELECTRONICS LAB. OSCILLOSCOPE Yrd. Doç. Dr. Taha İMECİ Arş. Gör. Ezgi YAMAÇ Arş. Gör. Ufuk ŞANVER İSTANBUL COMMERCE UNIVERSITY

2 Contents OSCILLOSCOPE... 2 CRT PART VERTICAL CONTROL PART HORIZONTAL CONTROL PART TRIGGER PART OSCILLOSCOPE PROBES MEASURING VOLTAGE WITH OSCILLOSCOPE MEASURING FREQUENCY WITH OSCILLOSCOPE... 8

3 OSCILLOSCOPE Oscilloscopes are the devices that show the electrical signals as graphs and allow them to be measured. The inner structure of oscilloscopes is highly sophisticated. We will discuss the usage of oscilloscopes. A good oscilloscope should has two abilities. The first ability is to show and measure two different signals simultaneously. The oscilloscopes that can show and measure two electrical signals are called dual channel oscilloscopes. The second ability is to show and allow the measurement of high frequency alternating current signals. The measure of this ability can be found on the oscilloscope device and its user s manual. A medium quality one has maximum working frequency at around 40MHz-60MHz. Oscilloscopes are composed of four main parts. CRT (Cathode Ray Tube) part Vertical control part Horizontal control part Trıgger part There are oscilloscopes that have more properties than the parts we have mentioned above, such as displaying values and frequencies of electrical signals directly on the screen, memory capturing of the wanted signals, etc. The front panel of a standart oscilloscope is shown in Figure. The adjustable components are numbered in Figure. At each part, the functionality of the adjustable components will be explained in order. Figure 2

4 CRT PART CRT stands for (Cathode Ray Tube) and these tubes are special electron lamps that have a screen that can show electrical signals. Most television screens that we use and watch daily are again CRT s The inner surface of the screen is covered with phosphorus. An electron beam created by an electron gun at the rear of the tube is projected onto this covered surface and the line that the beam is projected lighted. The outer surface of the screen has vertical and horizontal lines with distance cm for measuring purposes. Those cm spaces are further divided with small ticks for increasing accuracty and preventing measurement errors. In CRT part, there is generallay six control and configuration components. Power on-off switch: is the push-type switch that is used to power the oscilloscope. When pushed, power is applied to the oscilloscope. Trace rotation (horizontal smoothess control): is the potnatiometer that adjusts the paralelness of the screen output to the horizontal axis. This adjustment is only used when it is needed. Intensity: is the potantiometer that adjusts the light intensity of the screen output. Focus: is the potantiometer that adjusts the sharpness of the screen output. Scale (screen light adjustment): is the potantiometer that is used to lighten the screen with an extarnal light source when needed. It is used to see the screen lines when the environment is dark. Calibration terminal: is the terminal that is used to verify the measurements done with the oscilloscope. This terminal generally has a standart khz frequency Vpp = 0.5V square wave on it. This terminal is also used to investigate the measurement correctness of the measurement probes. The live point of the probe is connected to this terminal and the standart signal can be observed on the screen. 3

5 2 VERTICAL CONTROL PART The amplitude of the applied signal is investigated in this part of the oscilloscope. The oscilloscope shown in Figure is a dual channel one. At each channel, one independent same-type amplifier for vertical controls is present. The control components of each channel(potantiometers and switches) are controlled independently with different buttons. The left and right sides of the MODE switch in the vertical part is channel one (CH) and two (CH2), respectively. Control components at each channel accomplishes same goal for its channel Her kanaldaki aynı isimli kontrol elemanı kendi kanalında aynı işi yapar. Both two channels can be used independently or at the same time. Vertical mode select switch (MODE): This is the switch that selects the working mode of the vertical axis Dikey eksen çalışma şeklinin seçimini yapan anahtardır. The signal applied to CH and CH2 can be seen on the screen when this switch is on CH and CH2, respectively. Both signals can be seen on the screen when this switch is on DUAL position. The sum or difference of the two signals can be seen on the screen when this switch is on ADD position. CH IN: This is the connector where the electrical signal to be measured and observed is applied to the oscilloscope. The input connector for oscilloscopes are word-standart female BNC connector, with is convenient for oscilloscope probes. GND switch: When this button is pressed, the input of the oscilloscope is connected to the oscilloscope ground. A horizontal line is observed in the screen if this button is pressed. AC/DC switch: Generally input signals have both AC and DC components. When this button ispressed, both AC and DC components are reflected to the output screen symbol. When it is released, the input signal is passed over a capacitor and applied to the oscilloscope. In this released case, only AC components are shown in the screen. Position control: This is the potantiometer that shifts the screen output vertically. Volt/div control: This control component is composed of two equicentric switches. The outer switch is a multi leveled switch, whereas the inner switch is a a single switched potantiometer.the outer switch has a crucial function. There are scaled ticks in the perimeter of this switch, labeled in units of milivolt (mv) and volt (V). The vertical side of a square on a vertical axis has the voltage value determined by this switch s arrow. This process will be explained in measuring voltages with oscilloscope. The inner switched potantiometer is vertical attenuator in the input channel. When the switch of the potantiometer is open (the arrow in CAL position ), potantiometer has no effect to the circuit. The measurement verification of the oscilloscope is tested in this position. The vertical side of the squares of the screen can be adjusted to a ratio of the input signal amplitude by opening this potantiometer switch. This adjustment is necessary when following high amplitude signals. 4

6 3 HORIZONTAL CONTROL PART The variation of the applied signal is investigated in this part of the oscilloscope. This part provides the behaviour in time of the applied signal to be measured. Position control: This is the potantiometer that shifts the screen output horizontally. Time/div control: This control component is a multi leveled switch. There are scaled ticks in the perimeter of this switch, labeled in units of microseconds (μs), miliseconds (ms) and seconds (s). The horizontal side of a square on a horizontal axis has the time value determined by this switch s arrow. This process will be explained in measuring frequency with oscilloscope. Variable control: This component is a switched potantiometer. When the switch of the potantiometer is open (the arrow in CAL position), potantiometer has no effect to the circuit. In this case, the value provided by time/div switch is the value of the horizontal side of a square on the screen. By opening this potantiometer and adjusting it, the time that is shown by a horizontal side can be lowered. Considering an example, if horizontal side of a square on a screen represents second when this potantiometer is open, if this potantiometer is applied and turned to the rightmost point, the horizontal side of the square represents 0. seconds. 4 TRIGGER PART The stability of the electrical signal (no shifts, no blinks) is provided with this part of the oscilloscope. For this part, a scan signal is produced in the inner part of the oscilloscope. This scan signal is can be produced internally or using DC input from city network or using an extarnal signal. Source control: This switch is used to select which source will be used for producing the scan signal. The scan signal is produced from CH / CH2 when this switch is in position CH / CH2, respectively and the source signal of scan signal is produced more stable in these conditions. When this switch is in LINE position, the scan signal is produced from the city network DC input of the oscilloscope. Finally, if this switch is in EXT position, the scan signal will be produced from an external signal applied from a connector shown with the scan symbol. This connector is shown as a line with an arrowhead in the panel. Level control: This potantiometer allows us to control the amplitude of the scan signal. If a stable signal can not be achieved on the screen, the problem is eliminated by adjusting this potantiometer. Mode switch: This switch determines the appliance mode of the scan signal. Scanning is done with a triggering signal when this switch is in AUTO position. In this case, a horizontal line can be seen when there is no input signal. The scanning is triggered with the input signal when this switch is in 5

7 NORM position. In this case, there will be no images when there is no input signal. The scan signal is adjusted with the vertical and horizontal synchronization pulses and the input signal when it is in TV/V and TV/H positions,respectively. The image on the screen is seen more stable when the switch is in TV/V and TV/H positions. Inverter switch: This button changes the polarity of the signal on the screen. When it is pressed, the image shown in the screen is mirror-imaged of the original signal. Generally oscilloscopes has z axis input jack at the backplane. This connector is again BNC. By applying an extarnal signal to this connector, the light intensity of the cathode ray tube can be controlled. 5 OSCILLOSCOPE PROBES Oscilloscope probes are the connection cables that are used to apply the electrical signal to be measured to the oscilloscope inputs. The structure of the cables are composed of two conductors one within the other. The outer conductor has the mesh shape. The inner conductor is placed to the center of the outer conductor. There is no electrical connection between two conductors. There is a male BNC connector to be plugged to the oscilloscope at one end of the probe. The outer and inner conductors are connected to the oscilloscope ground and oscilloscope amplifiers respectively over the connector. The outer and inner conductors are taken out by using a crocodile clip and a conducting pin respectively, on the other side of the cable. The pin can also be used by an attachable hat with a hook as shown in Figure 2. There is a swtch on the probe that can bypass the input signal with no attenuation or with 90% attenuation(/0 of the original signal) selectable, used when the input signal has high amplitude. An example oscilloscope probe is shown in Figure 2. Figure 2 6

8 6 MEASURING VOLTAGE WITH OSCILLOSCOPE Measuring voltage with oscilloscope provides much more insight than measuring with multimeters. There is no information about the shape of the electrical signal or any unwanted signals over the electrical signal when the voltage is measured with the multimeter. For this reason, researches in the lab are done by using oscilloscopes. The easiest values to obtain are peak to peak voltage (Epp) and maximum voltage (Emax) when using the oscilloscope for voltages. The electrical signal to be measured is applied to one of the oscilloscope channels. A shaped signal on the screen is achieved by adjusting TIME/DIV and VOLT/DIV switches. The voltage measurement is done on the voltage axis. The effective measurement component in voltage measurement is only VOLT/DIV switch. Figure 3 The signal to be measured, displayed in order on the screen, is shown in Figure 3. At this Figure, VOLT/DIV switch s arrow is at 2Volt level. This value is the voltage value of cm at amplitude axis on the screen. The space between the positive Emax point and negative Emax point of the signal is 4 cm s. This measurement is shown graphically at the left of the figure. The voltage value at the amplitude axis for 4cm is; 4cm.2V/cm=8V. Since this value is between the positive and negative maximum values, it is peak to peak voltage (Epp). The maximum value of the signal can be clearly seen on the screen and is Emax=4Volts. The effective voltage value (E) can be evaluated easily as E=Emax.0,707 E=4.0,707 E=2,8Volts. 7

9 7 MEASURING FREQUENCY WITH OSCILLOSCOPE The signal whose frequency will be measured is applied to any of the channels of the oscilloscope. A shaped signal on the screen is achieved by adjusting TIME/DIV and VOLT/DIV switches. The frequency measurement is done on the time axis. The effective measurement component in voltage measurement is only TIME/DIV switch. Figure 4 The signal to be measured, displayed on the screen, is shown in Figure 4. For clearance of the example, only one period of the signal is shown in the figure, wherease in reality the image outside of the period is also on the screen. Measuring frequency requires the total time of one period of the signal, and a simple mathematical calculation. The arrow of TIME/DIV switch is on 5 milisecond (ms) level. This value is the value in time of cm on the time axis. Since a period of the signal to be measured is 6 cms, the time of one period is; T=6cm.5ms/cm=30ms. Frequency is defined as the number of periods in one second. The matematical relation between the frequency and period is the reciprocal operation, shown below; f T 8

10 In which f= Frequency (Hertz) T= Period (seconds). The time unit in electronics is seconds. However, one second is a large time unit, so mostly smaller units of second are used. The relation between second and smaller units and their mathematical relations are shown in Figure 5. Figure 5 If we evalute the frequency of the measured signal; f T f T ,3Hertz If we assume the TIME/DIV switch is at 2 microseconds level when the measurement was taken and evaluate the frequency; f T f T ,000Hz KHz 9