CATHODE RAY OSCILLOSCOPE Popular instrument to show time, voltage both DC and AC. Shows Volts / Time. Display waveforms. Spectrum scope shows volts to Frequency Cathode (-ve ) is heated, emits electrons, accelerated toward a (+ve) fluorescent screen. Intensity grid, Focus grid, Accelerating anode. (Electron gun) Horizontal deflection plates. Vertical deflection plates 1
OSCILLOSCOPE: CONTROL PANELPanel FunctionVOLTS/DIV Select the vertical deflection factor of the CRT displayTIME/DIV Select the horizontal deflection factor of the CRT displayINTEN (Intensity) The INTEN (intensity) control (sometimes called BRIGHTNESS) adjusts theControl brightness of the beam on the CRT. The control is rotated in a clockwise direction to increase the intensity of the beam and should be adjusted to a minimum brightness level that is comfortable for viewing.FOCUS and ASTIG The FOCUS control adjusts the beam size. The ASTIG (astigmatism) control(Astigmatism) Controls adjusts the beam shape. The FOCUS and ASTIG controls are adjusted together to produce a small, clearly defined circular dotTRACE ROTATION The TRACE ROTATION control allows for minor adjustments of the horizontalControl portion of the trace so that you can align it with the horizontal lines on the graticule.BEAM FINDER Control When pushed, the BEAM FINDER pulls the beam onto the screen so that you can use the horizontal and vertical POSITION controls to center the spot.Horizontal and Vertical The horizontal and vertical POSITION controls are used to position the trace.POSITION Controls Because the graticule is often drawn to represent a graph, some oscilloscopes have the positioning controls labeled to correspond to the X and Y axes of the graph. The X axis represents horizontal movement; the Y axis represents the vertical movement.AC-GDN-DC AC- Ac Signal GDN - grounding the signal DC- Dc Signal
FREQUENCY AND PERIOD. Frequency: measured in Hertz Definition: number of times the signal repeats itself in one second
FREQUENCY AND PERIOD. Period Definition: the amount of time it takes the signal to complete one cycle.
PHASE 1 cycle of a sine wave: 360⁰ Phase shift 90⁰
CATHODE RAY OSCILLOSCOPE When electrons hit the screen the phosphor is excited and emits light. Persistence. How long the display glows. May need to reduce ambient light for older instruments. Connect a signal to Vertical deflection plate. At same time a voltage that increases linearly with time (Ramp) is applied to the Horizontal deflection plates. 23
This horizontal linear deflection is produced by the Sweep generator. Sawtooth wave. When the sweep signal returns to zero ie the end of the sweep, the beam flies back to the start position. The beam is cut off during the flyback time. 24
CROS The display is made to appear stationary. This controlled by your adjustment settings. The eye sees a waveform. X is <----> Horizontal Y is ^ Vertical Height of trace 25
The signal is amplified by the vertical amplifier, applied to the vertical plates. A portion of the vertical amp signal is applied to the Sweep Trigger. The sweep trigger generates a pulse coincident with a selected point in the cycle of the trigger signal. This pulse turns on the sweep generator initiating the sawtooth wave form. The sawtooth wave is amplified by the horizontal amp and applied to the horizontal deflection plates 26
The trigger can be based on 50 (60) Hz Provision is made for an external trigger. 27
CRO TUBE CONTROLS POWER on / off Scale Illumination Focus. Create spot on screen Intensity. Brightness (Don’t burn a spot on your screen) 28
VERTICAL AMP Position on display Sensitivity of vertical amp Calibrated. Cal fully clockwise. Variable sensitivity. Continuous range between calibrated steps. AC - DC - Gnd. Selects desired coupling for incoming signal, or grounds amp input. DC couples signal directly to amp. AC connects via a capacitor. (Blocks DC) Gnd = no signal. Gnd connects Y input to 0 volts. Checks position of 0v on screen. 29
VERTICAL AMP Position on display Sensitivity of vertical amp Calibrated. Cal fully clockwise. Variable sensitivity. Continuous range between calibrated steps. AC - DC - Gnd. Selects desired coupling for incoming signal, or grounds amp input. DC couples signal directly to amp. AC connects via a capacitor. (Blocks DC) Gnd = no signal. Gnd connects Y input to 0 volts. Checks position of 0v on screen. 30
HORIZONTAL SWEEP Sweep time / Div (or CM) Select desired sweep rate, or admits external sig to horiz amp. Sweep time / Cm Variable Continuously variable sweep rates. Cal is fully clockwise. Position Controls horizontal position of trace. Horizontal variable controls attenuation of signal applied to Horz amp through Ext Horiz connector. 31
TRIGGER SET TO AUTO OR NORMAL Trigger selects timing of the beginning of the Horizontal sweep. Slope selects trigger at +ve increasing or -ve decreasing portion of signal. Coupling Selects whether trigger is at a specific DC or AC level. Source: Int from Vertical Amp Ext from Ext Trig Input. Line AC line 50 (60) HZ 32
VOLTS /DIV SWITCH Volts / Div Variable Fine adjustment these controls can have a Pull out switch position. May be 5 times mag. 33
VERTICAL MODE The operation of vertical deflection plates Chan 1 and Chan 2 can each operate separately. Dual. Ch1 and Ch2 are swept alternatively. Why Dual? Used to measure input and Output signals of a device under test. Ch1 and Ch2 can be added 34
TIME BASE Main, Max, Min, delay. Selects the sweep for the main mix or delay mode and also X-Y switch Time/Div provides selection of sweep rates. Range of 0.1 Second, 50 to .1 mS, 50 to 0.1uS per div. Note 5,2,1, sequence. To determine a frequency use reciprocal. Frequency = 1/time period (50Hz = 1/20mS) Time period = 1/Frequency (number of div * ?ms/div. Eg 4div*5ms/div = 20 ms) 35
OTHER Comp Test. Allows individual components to be tested. Connect via banana jacks to test resistors, capacitors, diodes, transistors, etc Cal delivers calibrated voltage e.g. 2v p-p 1KHz square wave for setting scale. GND. Earth terminal of scope 36
BANDWIDTH A 10MHz CRO does not mean it will correctly measure signals at 10MHz. Vertical Amps are not so wide-band as to amplify all signals. 10MHz is the 3dB point. A 10MHz signal of 1v will measure 0.707v on the screen. Clipping introduces odd order harmonics. A CRO operating near the max freq. will not show the harmonics and you think you are reading a clean signal. Square waves begin to look like sine waves. A rule of thumb is 5 times. To measure 2MHZ use a 10MHz CRO. 3 times is suitable for most Amateur work. For 7MHz. Times 3 = 21. Use a 20 MHz CRO. 38
EVERY CRO WILL BE DIFFERENT Many instruments made for specific work. Beam Finder push button Trace rotation Chan 1 Vertical input. During X-Y operation this is X axis (abscissa) Chan 2 Vertical input Chan 2. During X-Y this becomes ordinate input. 39
MINI EXERCISES Obtain a trace Brightness Focus Move trace up, down. Move trace side ways 40
A particular slide catching your eye?
Clipping is a handy way to collect important slides you want to go back to later.