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The University of Texas at Arlington Electrical Engineering Department Electric Circuit Lab The Oscilloscope Agilent 54621A One of the most often used instruments in the electrical engineering lab is the oscilloscope which not only you can measure the waveform quantities, but also it allows you to display the waveform as a function of time. The oscilloscope (or simply, “scope”) consists of a display tube on which one can trace the waveform. An electron beam which is generated by electron gun accelerate toward the part of the display and is deflected by electric fields, writes figures on the fluorescent screen. Following figure shows the general principle and major subsystems of an oscilloscope General principle of an oscilloscope showing the display tube and the deflection system. There are two types of scopes, the analog and the digital ones. Digital scopes have more features than the analog scopes. Digital scopes can process the signal and measure its amplitude, frequency, period, rise and fall time. Some of them have built-in mathematical functions and can do fast Fourier transforms in addition to capturing the display and sending it out to a printer. The oscilloscopes in the EE Undergraduate Lab are Agilent 54621A type digital oscilloscopes which have most of the above functions built-in. Following figure shows a typical probe used in measuring by oscilloscope. A probe is a high quality coaxial cable that has been carefully designed not to pick up stray signals originating from radio frequency (RF) or power lines. They are used when working with low voltage signals or high frequency signals which are susceptible to noise pick up. Also a probe has a large input resistance which reduces the circuit loading. The connections to the scope's will be made through a X 1 probe (where X 1 means that the probe does not attenuate, or reduce the signal; if your probe has a X 1/ X 10 selector, set it to X 1). If the voltage source to be measured has one terminal grounded, make sure that the ground of the probe is connected to the ground of the voltage source and not vice versa; otherwise, the source will be shorted and may be damaged. Review the front Panel of the scope Review the Rear Panel of the scope Hints The Digitizing Oscilloscope • The BNC shield is at earth ground. Use only the probe TIP for measuring high voltages. "Floating" the BNC shield or connecting it to a high voltage could cause a safety hazard. • Make sure probes are compensated. • If you can't get the signal on screen: - Check probe connection - Touch: SETUP, Default Setup - Touch: AUTOSCALE - Check for offset (ground symbol). If offscale, adjust vertical sensitivity and position - Turn up signal brightness with intensity control Digital Oscilloscope Block Diagram The Digitizing Oscilloscope Hints: * The BNC shield is at earth ground. Use only the probe TIP for measuring high voltages. "Floating" the BNC shield or connecting it to a high voltage could cause a safety hazard. * Make sure probes are compensated and set to proper scale (X1,X10,X100). * If you can't get the signal on screen: - Check Probe connection - Touch: SETUP, Default Setup - Touch: AUTOSCALE - Check for offset (ground symbol). If offscale, adjust vertical sensitivity. If still offscale, Touch: Coupling - Check Trigger Source 1 DC AC - Set Mode Auto The Digitizing Oscilloscope Status bar: Vertical sensitivity Autoscale Measure: Vrms, tr, f ,f Sweep speed Print Trace /Setup Memory Trigger time reference Calibrate Display: grid, vectors, Channel(s) on averaging Run/stop 1 1.00V 0s 2 us/ 1 RUN Storage: Auto-store, 1 Erase, RUN/STOP Softkey ----Setup memory---- Undo Default labels Triggering: Modes: Auto, Normal,TV Sources: Ch1, Ch2, Ext, Line Calibration source Power ON/OFF Screen brightness Vertical position, Horizontal: sensitivity Main Sweep Speed, Math on/off Sweep Modes: Delayed, XY, Roll t = 0 Reference Getting Started Getting Started: 4 Line 2 1 (Power ON) 0 1 Default Setup (All settings 5 2 Setup 3 to default ) 1 3 Connect probe to calibrator Auto- 4 scale 1 1.00V 200 1 RU 0.00s us/ N 5 Compensate probes: 1 Adjust screw to eliminate: Default Setup Overshoot Undershoot 1 1.00V 0.00s 200 1 RU Set probe attenuation factor: us/ N Press Channel key for 1 selected probe. Probe Toggle softkey for setting 1 10 100 1 that matches probe Probe 1 10 100 10:1 Match Trigger slope What the display says: is positive Blinks if (rising edge) no trigger Vertical sensitivity of CH1 is 1Volt per major division Time=0 t<0 t>0 Trigger source 1 1.00V 0.00s 200us/ 1 RU is Channel 1 N Scope is ready for trigger 1 Ground (V=0) [If dc part of CH1 signal is too big, ground arrow points off-screen. 1 Probe If this happens, adjust Off On 1 10 100 vertical sensitivity] Channel 1 is ON Horizontal sweep speed is Readings scaled 200 msec per major division for a 10:1 probe What the main controls do: Horizontal Trigger level Line is only Grid full delay visible when you turn the knob. Display: When signal is smaller than Vectors off trigger level, scope stops sampling. Time/division (sweep speed) Vertical Vertical Volts/division Position Other main controls: Trigger holdoff Signals with multiple zero crossings per cycle cause unstable displays. Holdoff ignores the false triggers for a given length of time. Holdoff Trigger Holdoff=11.5 us + 1 Math Functions - Enable channel -Add waveforms - Set up probe -FFT (if equipped with module) Horizontal Controls Vertical Controls Trigger Controls Display Display (continue) Other Functions Other Functions (continue) Other Functions (continue) Other Functions (continued) Other Functions (continued) Other Functions (continued) Making Measurements: Vp-p Measure Vp-p, using cursors: 1 Hook Calibrator signal to CH1 Grid Cursor adjustment knob 2 Display None 3 Cursors Clear Clears any cursors already on Cursor the screen s Source 4 1 Set for the correct channel |- Active Cursor -| Toggle to highlight 5 V1 V2 T1 T2 the V1 cursor; Rotate knob for waveform minimum D V1= 31.25mV V2= 5.031V V= 5.000V |- Active Cursor -| 5 Change to V2 cursor; V1 V2 T1 T2 Use knob to set to waveform minimum Answer OR: Measure Vp-p, the easy way: |- Voltage Measurements -| Simply select Vp-p from the 1 Voltage Vp-p Vavg Vrms Voltage menu. Time Making Measurements: RISETIME Next 1 Time Menu |----------- Time Measurements -----------| Rise(1) <4.000 us 2 +Width -Width RiseTime FallTime Risetime Answer If answer needs more resolution: Time/Div 90% line 3 Rotate for best display: 10% line Rise(1) 920 ns Making Measurements: RISETIME, Using DELAYED SWEEP: Time/Div 1 Rotate to show multiple cycles on screen Rise(1) <4.000 us 2 Main |-------Horizontal Mode --------| Main/Delayed Delayed Main Delayed XY Roll Time/Div 3 Rotate. See how upper bracketed part is exploded into lower Rise(1) Delayed Cntr 920 ns window Time/Div Making Measurements: FFT To do FFT, a (Frequency Domain) Measurement/Storage Module must be installed 1 Setup: Default on back of scope. Auto- 2 scale + |-- Function 2 --| 3 Off On Menu |-- Function 2 --| 4 Off On Menu (Hit Menu key) Operation 5 FFT Hint: To look ONLY at FFT signal without time domain signal, turn channel off: 1 1 Off On Use Time/Div to set FFT resolution Hint: To return to FFT menu at any time, use Math key + Auto- Storing Waveforms: store AUTOSTORE Auto- Displays all waveforms 1 store (Good for looking at jitter, noise, glitches) 2 Example: Calibrator signal in CH 1. 3 Setup Default. AUTOSCALE. Touch AUTOSTORE. Now move horizontal delay knob and 4 watch all waveforms stay on screen, making an overlapping pattern. Auto- Touching Autostore again returns 5 store display to normal mode. Storing Waveforms: TRACE MEMORY 1 Example: Calibrator signal in CH 1. Setup Default. AUTOSCALE. Trace Save to 2 Trace 1 Trace 1 3 Off On Now move waveform with horizontal Vertical position 4 delay and vertical position knobs. Note that Trace 1 still stays on screen, allowing comparison with stored Horizontal delay waveform.
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