• Power ON Switch It is used to switch on/off the CRO.
• Horizontal Amplifier Input: This is used for connecting the external horizontal signal as an input for the horizontal amplifier.
• Vertical Amplifier Input: The signal under testing is connected to vertical amplifier input.
• Intensity (Brightness): This controls the brightness of the screen. When adjusted it changes the negative voltage on the control grid and brightness changes. This happens due to a change in the number of electrons in the beam. It adjusts the high brightness to lower brightness.
• Focus: This controls the sharpness of lighted spots or the sharpness of waves displayed on the screen.
• Time/Div Scale: This controls the frequency of sawtooth waves. When adjusted, it changes the number of cycles of a wave displayed on the screen Volt/Div Scale: This controls the amplification/attenuation factor (i.e., the gain) of the vertical amplifier/attenuator. When adjusted it changes the height of the wave on the screen. Thus very high voltage waveform can be made small according to the size of the screen. It adjusts the high voltage to the low voltage.
Horizontal Position Control: By adjusting this knob, we can MOVE complete waveform along the x-axis from left to right or right to left. This is sometimes required for adjusting the position of the wave on the x-y axes. It moves from left to right.
Vertical Position Control: By adjusting this knob, we can MOVE complete waveform along the y-axis upward and downward. This is sometimes required for adjusting the position wave on the x-y axes.
• X-input: This is NOT a control knob, but it is another input of CRO. When this switch is in EXT, the internal sawtooth is cut- off and some external signal can be applied to the horizontal plates of CRT through x-input.
Y-input: This is the main input of CRO. The signal observed on the screen is applied to this input.
Astigmatism: This control is generally fitted on the backside of the CRO. It is used to correct the tilting of the waveform on the screen. This happens due to the aging of CRO. This control adjusts voltages on both deflection plates and rotates the waveform to align it exactly along the x-y axes.
CONVERSION OF SIGNAL TRACE CRO INTO DUAL TRACE CRO
The popularity of dual trace oscilloscopes has increased tremendously in recent years. In this, the conventional CRT is modified to produce a dual image by means of the electronic switch.
It has two vertical input channels. They are channel A and channel B. The two channels have identical preamplifiers and delay lines. The remaining circuits are similar to single trace CRO.
The dual trace CRO operates in four modes They are:
1. Channel "A" only.
2. Channel "B" only.
3. A and B alternate.
4. A and B chopped.
• Channel A and Channel B: In channel A and channel B selection, the CRO works as a single beam CRO. When set to A or B only the input at that channel is displayed.
• Alternate Mode: In an alternate mode, the inputs are displayed alternately on the screen. In this mode, the electronic switch alternately connects the main vertical amplifier to channel A and channel B. It means that for one sweep one channel is connected to the switch, for the second sweep another channel is connected to the switch.
Chopped Mode: In the chopped mode electronic switching is completely independent of the sweep rate.
PHASE MEASUREMENT
The CRO can be used to determine the phase difference between two a.c. signals with the same frequency. For this measurement, CRO must be kept in X-Y mode.
An oscilloscope can be used to determine the phase relationship between the two voltages of the same frequency. When two signals are applied simultaneously to an oscilloscope without Internal sweep. One to the horizontal channel and the other to the vertical channel, the resulting pattern is a Lissajous Figure that shows a phase difference between the two signals.
The Lissajous patterns are useful to find out the phase difference between two sine waves. They are useful to find the unknown frequency of the sine wave. The patterns show the phase relationship between two sine waves of the same frequency.
In order to measure the phase difference between two waves having the same frequency, the two waves are connected to the X and Y plates simultaneously. The resultant pattern is an ellipse.
PRINCIPLE OF SAMPLING OSCILLOSCOPES
Most ordinary oscilloscopes have an upper-frequency limit in the range of 20 MHz to 50 MHz. Higher input frequencies cause 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 of the input signal, and each sample is taken in a different cycle. The sampling circuits must be capable of operating at very high frequencies, but the CRT and its associated circuitry may be relatively low-frequency equipment.
10 cycles of a high-frequency waveform that is to be displayed. One amplitude sample is taken at successively late times in each cycle. The resultant series of samples reproduces the original waveform at a lower frequency. Suppose that the input signal has a time period of T = 0.01 µs. Its frequency is f₁ = 1/0.01 µs = 100 MHz. The dot display has a time period of 10 x 0.01 µs or a frequency of f2 = 100 MHz / 10 = 10 MHz waveform for display. If one cycle of dot display is created by sampling 100 cycles of signal, the frequency of the displayed wave is 1/100 of the signal frequency.
One disadvantage of this system is that only purely repetitive waveforms can be investigated. If the waveform changes over several cycles, the dot display will be in error. A transient waveform cannot be investigated by this method of sampling. It displays the errors by dots.
