Module 7 (Maintenance Practices) Sub Module 7.4 (Avionic General Test Equipment).pdf

ISO 9001:2008 Certified For Training Purpose Only

PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES

Category – A/B1 Sub Module 7.4 – Avionic General Test Equipment

PTC/CM/B1.1 Basic/M7/02 Rev. 00

7.4 Mar 2014

MODULE 7

Sub Module 7.4

AVIONIC GENERAL TEST EQUIPMENT

ISO 9001:2008 Certified For Training Purpose Only For Training Purpose Only




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Contents

OPERATION, FUNCTION AND USE OF AVIONICS GENERAL

TEST EQUIPMENT ------------------------------------------------------- 1





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OPERATION, FUNCTION AND USE OF AVIONICS GENERAL TEST EQUIPMENT Cathode ray oscilloscope

Cathode ray oscilloscope (CRO) is test equipment that facilitates display of waveforms and measurement of its parameters. It is a very versatile tool widely used in electronic measurement and testing.





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Cathode ray tube (crt)

Cathode ray tubes used in oscilloscopes consist of an electron

gun, a deflection system, and a fluorescent screen. All these

elements are enclosed in an evacuated glass tube.

The electron gun generates electrons and focuses them into a narrow beam. The deflection system moves the beam horizontally and vertically across the screen. The screen is coated with a phosphorous material that glows when struck by the electrons. The electron gun consists of a heater, a cathode, a control grid, and two anodes. The cathode is usually indirectly heated and emits a cloud of electrons. The control grid is a hollow metal tube placed over the cathode. It is maintained at a negative potential relative to the cathode to keep the electrons bunched together. The brightness, or intensity, can be adjusted by varying the voltage on the control grid. The first anode, known as focusing anode, focuses the electrons into a narrow beam. A fixed positive voltage of several thousand volts is connected to the second anode, known as accelerating anode, for accelerating the electrons in the direction of the screen.

The voltage on the focusing anode can be varied to place the focal point of the electrons beam on the screen. The deflection system moves the electron beam horizontally and vertically. If it is not deflected at all, a small bright dot will appear at the centre of the screen. The deflection system uses electrostatic fields to change the path of the beam. As the beam moves on the screen, the phosphorescence of the screen helps in tracing a continuous line. As the periodic signal repeats, the screen will be refreshed.





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Tracing a signal on a cro

Usually the horizontal (X) deflection plates are applied with a saw-tooth shaped time base signal. The signal to be displayed is applied to the vertical (Y) deflection plates. If no signal is connected to the Y-plates, the time base signal will trace a horizontal line at the centre of the screen. If necessary this can be adjusted up or down using the ‘Y shift’ control. If, by any chance, the time base signal is missing and any signal is applied to the Y-plates, it will trace a vertical line at the centre. When a signal (for example a sine wave) is applied to the Y-plates with the time base signal applied to the X-plates, the signal waveform will appear on the screen. More cycles can be displayed by decreasing the frequency of the saw tooth signal. X-amplifier and Y-amplifier can be used to adjust the scales on the X- and Y-axes. The signal can be DC or AC coupled. If AC coupled, the DC component of a signal where the AC variation is superimposed on an average DC value will be removed and the pure AC signal will be displayed.





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Dual channel oscilloscopes

Often it is necessary to display two or more waveforms on the CRT screen for comparison purposes. Dual channel (or multi-channel) oscilloscopes offer this feature. Dual trace oscilloscopes use one electron gun and one deflecting arrangement to display two traces on the screen by sharing time. When one signal is driving the electron beam, the other will be interrupted. Due to the phosphorescence of the screen, the two traces will appear to be simultaneous. The two techniques used in time-sharing are alternate (ALT) mode, and chop mode. Alt mode

In the alternate mode, each input signal is traced in alternate horizontal sweeps. Both signals appear simultaneously on the screen owing to its phosphorescence. Because the phosphorescence of the screen has got limited persistence, the first trace will decay during the second trace if the sweep duration is too high. Chop mode

In the chop mode, the horizontal sweep is divided into a number of time slots and at the end of one time slot the vertical trace jumps from one signal to the other. This mode is preferred over the ALT mode for low frequency signals and low sweep rates.





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Cro controls

The construction and controls of a typical dual trace oscilloscope are described here. Vertical controls

A dual trace oscilloscope has two channels, which facilitates tracing two signals on the screen simultaneously. This is often useful in comparing signals. For signal input, each channel has got a BNC port. The scope inputs are usually high-impedance with one-mega ohm resistance in parallel with 20pf capacitance. The signal could be either dc-coupled or ac-coupled upon selection on the associated sliding switch. With dc coupling, both dc and ac components of a signal could be displayed, whereas ac coupling, through a capacitor, would block the dc components and trace the pure ac component with a time constant of about 0.1 second. This is useful if it is necessary to isolate a small ac signal riding on a large dc voltage. There is a ground input position (GND), which can be used to see zero volts on the screen. In GND position, the signal is disconnected from the scope, and the input is grounded. Each channel has a calibrated gain switch, for selecting the vertical scale (Volts/DIVISION). Located concentrically on it is a variable gain knob (VAR), which could be used to set a given signal to a certain number of divisions. For voltage measurements, the variable gain knob must be at the calibrated position (CAL).

The INPUT MODE switch provides a choice of views. At CH1 or CH2 position, only the selected signal is displayed. There are two ways to see both inputs simultaneously: alternate (ALT) mode and chop (CHOP) mode. In alternate mode, alternate inputs are displayed on successive sweeps of the trace. In chop mode, the trace jumps back and forth rapidly between the two signals. Alternate mode is generally better except for slow signals. The sum of the two signals can be viewed with ADD position of the selector. The difference of the two could be viewed using the same selection together with INVERT selection for channel 2. The POSITION selectors allow shifting of the image up and down.





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Horizontal controls

An internal ramp generator giving deflection proportional to time generates the horizontal sweep signal. As with the vertical amplifiers, there is a calibrated selector for time scale (time/DIV) and a variable (VAR) concentric knob. The XY position allows a special feature called generating Lissajous figures, which is useful in phase comparison of two signals. In this technique, one of the inputs is used for horizontal deflection, in place of the time base. The POSITION selector allows shifting of the image left or right, whereas the MAGNIFIER switch facilitates a scale adjustment, which is 10 times the scale selected with the scale selector knob. Triggering

The trigger circuitry allows the selection of a level and a slope on the waveform at which the sweep should begin. A LEVEL selector and SLOPE selector are provided for this purpose. NORMAL mode produces a sweep only when the applied signal crossed through the trigger point set by LEVEL, moving in the direction set by SLOPE. In practice, the level control is adjusted for a stable display. AUTO position is better if a number of different signals are to be displayed. This precludes the necessity for setting the trigger level for each signal. In AUTO, the display will ‘free run’ in the absence of a signal.

SINGLE SWEEP is used for non-repetitive signals. LINE causes the sweep to trigger on the AC power line. The external trigger (EXT) inputs are used in situations where synchronization with some external test signal or a clock signal is necessary.





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The coupling mode selection is useful in viewing composite signals. HF REJ position puts a low-pass filter in front of the trigger circuit preventing false triggering due to spikes. If spikes need to be investigated, LF REJ position can be used. BEAM FINDER is useful if the trace is lost from view. TRIGGER VIEW is useful when triggering from external sources. Probes

High impedance probes are used to minimize the effects on the circuit caused by the attachment of the CRO. The probe schematic illustrates the popular 10X probe. For DC, it acts as a 10X voltage divider. By adjusting C1 to be 1/9th of the parallel capacitances C2 and C3, the circuit becomes a 10X divider for all frequencies. This adjustment is done using the calibration (CAL) setting, which displays a square wave. When the probe capacitance is properly adjusted, a pure square wave without overshoot will be displayed. However, for low strength signals, a 1X probe should be used. Some probes feature a convenient choice of 1X or 10X attenuation, switchable at the probe tip.





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Module 7 (Maintenance Practices) Sub Module 7.4 (Avionic General Test Equipment).pdf

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