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 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: - Check Trigger Source - Set Mode Auto 1 Coupling DC AC

The Digitizing Oscilloscope Status bar: Vertical sensitivity Sweep speed Trigger time reference Channel(s) on Run/stop f Autoscale Print Calibrate Measure: Vrms, tr, ,f Trace /Setup Memory Display: grid, vectors, averaging 1 0s 2 us/ RUN 1.00V ----Setup memory---- Undo Default Storage: Auto-store, Erase, RUN/STOP Softkey labels Triggering: Modes: Auto, Normal,TV Sources: Ch1, Ch2, Ext, Line Calibration source Power ON/OFF Screen brightness Vertical position, sensitivity Math on/off Horizontal: Main Sweep Speed, Sweep Modes: Delayed, XY, Roll t = 0 Reference

Getting Started

Getting Started: 4 Line 2 1 0 1 (Power ON) (All settings to default ) Compensate probes: (Power ON) Setup Default (All settings to default ) Auto- scale 1 2 4 3 5 Line 0 1 2 3 4 5 1 Connect probe to calibrator 1 1.00V 200 us/ 1 RUN 0.00s 1 Adjust screw to eliminate: Default Setup Overshoot Undershoot

Set probe attenuation factor: 10:1 1 0.00s 200 us/ RUN 1.00V Probe 1 100 10 Match Set probe attenuation factor: Press Channel key for selected probe. Toggle softkey for setting that matches probe

m Trigger slope What the display says: is positive (rising edge) Blinks if no trigger Vertical sensitivity of CH1 is 1Volt per major division Time=0 t < 0 t > 0 Trigger source is Channel 1 1 1.00V 200us/ 0.00s 1 RUN Scope is ready for trigger 1 Ground (V=0) [If dc part of CH1 signal is too big, ground arrow points off-screen. If this happens, adjust vertical sensitivity] 1 Off On Probe 1 100 10 Channel 1 is ON Horizontal sweep speed is 200 sec per major division Readings scaled for a 10:1 probe m

What the main controls do: Horizontal delay Trigger level Line is only visible when you turn the knob. When signal is smaller than trigger level, scope stops sampling. Grid full Display: Vectors off Time/division (sweep speed) Vertical Volts/division Vertical Position

-FFT (if equipped with module) 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. Trigger Holdoff Holdoff=11.5 us + Math Functions -Add waveforms -FFT (if equipped with module) 1 - Enable channel - Set up probe

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 Answer V1= 31.25mV V2= 5.031V V= 5.000V D Cursor adjustment knob 2 Display None 3 Cursors Clear Cursors Clears any cursors already on the screen Source 4 1 Set for the correct channel |- Active Cursor -| Toggle to highlight the V1 cursor; Rotate knob for waveform minimum 5 V1 V2 T1 T2 5 |- Active Cursor -| Change to V2 cursor; Use knob to set to waveform minimum V1 V2 T1 T2 OR: Measure Vp-p, the easy way: Voltage |- Voltage Measurements -| Vp-p Vavg Vrms Simply select Vp-p from the Voltage menu. 1

Making Measurements: RISETIME Next Menu 1 Time |----------- Time Measurements -----------| 2 Rise(1) <4.000 us +Width -Width RiseTime FallTime Risetime Answer If answer needs more resolution: Rise(1) 920 ns Rotate for best display: 3 Time/Div 90% line 10% line

RISETIME, Using DELAYED SWEEP: Making Measurements: RISETIME, Using DELAYED SWEEP: Time/Div 1 Rotate to show multiple cycles on screen Rise(1) <4.000 us 2 Main Delayed Main/Delayed |-------Horizontal Mode --------| Main Delayed XY Roll Time/Div 3 Rotate. See how upper bracketed part is exploded into lower window Rise(1) 920 ns Delayed Cntr Time/Div

Making Measurements: FFT (Frequency Domain) To do FFT, a Measurement/Storage Module must be installed on back of scope. 1 Setup: Default 2 Auto- scale 3 + |-- Function 2 --| Off On Menu 4 |-- Function 2 --| Off On Menu (Hit Menu key) Operation FFT 5 Hint: To look ONLY at FFT signal without time domain signal, turn channel off: 1 Off 1 Use Time/Div to set FFT resolution On Hint: To return to FFT menu at any time, use Math key +

Storing Waveforms: AUTOSTORE 1 2 3 4 5 Displays all waveforms (Good for looking at jitter, noise, glitches) 1 2 Example: Calibrator signal in CH 1. Setup Default. AUTOSCALE. Touch AUTOSTORE. Now move horizontal delay knob and watch all waveforms stay on screen, making an overlapping pattern. 3 4 Auto- store 5 Touching Autostore again returns display to normal mode.

Storing Waveforms: TRACE MEMORY Vertical position Horizontal delay 1 Example: Calibrator signal in CH 1. Setup Default. AUTOSCALE. Save to Trace 1 2 Trace Trace 1 Off On 3 Now move waveform with horizontal delay and vertical position knobs. Note that Trace 1 still stays on screen, allowing comparison with stored waveform. 4