Working with a Tektronix TDS 3012B Oscilloscope EE 310: ELECTRONIC CIRCUIT DESIGN I

Transcription

1 Working with a Tektronix TDS 3012B Oscilloscope EE 310: ELECTRONIC CIRCUIT DESIGN I Prepared by: Kyle Botteon Questions? kyle.botteon@psu.edu 2

2 Background Information Recall that oscilloscopes (scopes) are specialized measurement tools. While a digital multi-meter can collect measurements that are constant with time, an oscilloscope can display and measure a time-varying signal. Moreover, it can capture and present more than one signal simultaneously for comparison or to apply mathematical operations. Keep a few things in mind while working: 1. Scopes have protection circuitry, but everything has its limits. Read warnings before performing the steps that follow to avoid breaking equipment. 2. Measurement tools, no matter how well-designed, will change the circuit once one is introduced. So, measurements will also be influenced to some degree. Always consider the effects of adding a measurement tool to a circuit. For the exercises in this guide, the oscilloscope is set to behave like a 1MΩ resistor between the node under observation and circuit ground. Materials Tektronix TDS 3012B Oscilloscope A suitable circuit to examine: o 50mV < Peak Voltage < 50V o Frequency < 25 MHz o Wired on a breadboard BNC-to-alligator or BNC-to-clip/grabber probe(s); the scope will support up to 2 measurement probes BNC-to-alligator Probe BNC Connector Grabber Connector Alligator Connector 3

3 Procedure Device and Circuit Setup 1. Power on the scope by depressing the power button. It takes about 15 seconds to boot, at which time an informational menu will appear. 2. Once the informational menu appears, press the MENU OFF button, then the scope is ready for use. It is best practice to reset the scope to factory defaults prior to each use. We ll do that now. 3. Press the SAVE/RECALL Button. 4. Select RECALL FACTORY SETUP in the list of options at the bottom of the screen. 5. Choose OK CONFIRM FACTORY INIT. There are a wide variety of probes that can be used with this device. For this exercise, we will use a rudimentary passive probe: a cable with a BNC connector on one end and test leads (alligator or grabber clips) on the other. 6. Decide how many signals will be measured (up to 2). Attach one probe for each signal by screwing the BNC connector into the CH 1 or CH 2 outlets on the front of the scope. 7. If neither yellow line(s) nor blue line(s) appear on the screen, press the CH 1 button to enable the channel 1 display and press the CH 2 button to enable the channel 2 display. If a channel is not connected to a signal under observation, do not enable it. 8. Press the MENU button in the VERTICAL settings group and verify, in the bottom right of the screen, a 1X probe is attached. If it does not say 1X, unplug the probe then re-insert it. Warning: The black leads of each probe are internally connected (inside the scope) as well as tied to earth ground. Be careful not to create a short circuit by connecting the black leads to different potentials in the circuit under observation. 9. Connect the alligator or grabber clips of each probe to the circuit. Be careful not to short any nodes by touching bare wires or other probe clips together. Device and circuit setup is complete. Now, several adjustments must be made. Informational (power-on) menu Save/Recall Menu Probes connected to channels 1 and 2 4

4 Capturing a Signal With some luck, a signal might just show up once a probe is connected to the circuit. But, odds are, the timescale, vertical scale, and other settings are not configured properly. For this exercise, a 4.5V PP, 250 KHz, +1.5V DC triangle wave is applied to the CH 1 input, set to 1MΩ mode. Scaling of Offsetting Notice the timescale in the bottom center of the display (the number with units in time). If it is not % of the period of the signal under observation, something like the first image to the right might appear. 1. Adjust the timescale to be a reasonable 1 number using the SCALE knob in the HORIZONTAL settings group. Ensure the signal to be adjusted (CH 1 or CH 2 probe) is selected by pressing the CH 1 or CH 2 button before changing the scale. After properly adjusting the horizontal scale, the entire signal may not be onscreen we must adjust the vertical scale too. 2. Adjust the vertical scale to be a reasonable 2 number using the SCALE knob in the VERTICAL settings group. Ensure the signal to be adjusted (CH 1 or CH 2 probe) is selected by pressing the CH 1 or CH 2 button before changing the scale. 3. Use the POSITION knobs in the HORIZONTAL and VERTICAL setting groups to adjust the position of the wave on the display. For example, the waveform could be centered on the screen. 4.5VPP, 250 KHz, +1.5V DC triangle wave with default settings 4.5VPP, 250 KHz, +1.5V DC triangle wave with appropriate timescale 4.5VPP, 250 KHz, +1.5V DC triangle wave with appropriate vertical scale 1 Circuit-dependent. Base a starting point on the expected signal period. For instance, for a 100 Hz signal (with period 10 ms), a timescale of 2 10ms is appropriate. 2 Circuit-dependent. Use an estimated figure as a starting point then further adjust the scale to suit. 5

5 Adjusting the Trigger The trigger setting controls when the scope will capture the waveform applied to a probe. If the trigger is larger than the signal peaks, the scope will never capture a waveform. If it is too low, the scope will trigger on noise, capturing many waveforms and presenting a distorted image onscreen. For this exercise, a 2.5V PP, 250 KHz, +0V DC square wave is applied to the CH 1 input, set to 1MΩ mode. The default trigger setting, trigger on rising edge, is assumed. 1. Observe the right hand side of the gridline-filled portion of the display. A yellow arrow, pointing left, indicates the trigger level, according to the vertical scale of the display. 2. Twist the LEVEL knob in the TRIGGER setting group until the yellow arrow points to the amplitude at which the signal should be captured. For periodic signals, it is wise to set the trigger to about 75% of the maximum amplitude of the waveform. By doing so, undesirable effects of noise are mitigated. For other signals, a guess-and-check method is suitable. Poorly adjusted trigger, set to about 1V The AUTOSET Button There exists a button that can configure all of the previous setting automatically at a price. The AUTOSET button runs a software routine that locates a signal, scales and offsets it, and adjusts the trigger. But, high frequency noise and other factors can cause the incorrect settings to be applied. For this reason, blindly accepting the result is a mistake. Still, it is a useful tool when used properly. 1. Verify at least one probe is connected to a circuit node under observation. 2. Press the CH 1 button if automatic settings should be applied to channel 1, otherwise press the CH 2 button. 3. Press the AUTOSET button. At this point, it is highly likely that a properlyadjusted signal is displayed onscreen. Still, verify the settings that were automatically applied: 4. Check the timescale and compare it to the expected timescale for the signal under observation. For example, if a 100 Hz signal (with period 10 ms) is applied to CH 1, and CH 1 is selected when AUTOSET is pressed, a timescale in the range 1 10 ms should have been configured. 6 Well-adjusted trigger, set to about 750mV Poor AUTOSET result (applied to a 10Hz triangle wave with 50% output noise) The above example demonstrates why AUTOSET is not always reliable. The 10 Hz (period 100ms) signal under observation is buried in noise, so the automatic settings adjusted the timescale to a short (10µS) period.

6 Measuring a Signal The scope has software-based measurement tools available to observe and quantify signal parameters. These are especially useful for collecting exact figures, as opposed to estimating them with the gridlines onscreen. Automatic Measurements To apply an automatic measurement: 1. Select CH 1 or CH 2 based which channel a measurement should be applied to. 2. Press the MEASURE button. A measurements menu will appear. 3. Use the -MORE- button to cycle through the six menus of measurements. 4. Choose a measurement, then apply it by pressing the button next to it. A measurement name and numerical value will appear in the color corresponding to the channel it is applied to (e.g. yellow for CH 1) 5. Press the MENU OFF button to close the measurements menu and move measurements to the side of the display. Manual Measurements Automatic measurements can be erroneous, especially when applied to noisy signals. For this reason, manual measurement tools are available. H BARS and V BARS place horizontal and vertical lines onscreen and measure the difference between the values they represent. To enable these tools: 1. Press the CURSOR button on the top right of the scope. A cursor function menu will appear. 2. Press the button associated with H BARS or V BARS based on what will be measured. 3. Adjust the location of the bars using the SELECT, COURSE, and unlabeled knob on the top center of the scope. SELECT is used to switch between the two bars, while COURSE changes the speed at which the bars may be moved with the unlabeled knob. Automatic measurement menu Automatic measurements applied The position of each bar and the difference between them is located in the top right of the display. H Bars and V Bars tools 7

7 Troubleshooting Sometimes things don t always go as planned. Consider the following tips when the scope doesn t behave as expected. General Tips Verify factory defaults were recalled prior to configuring the scope or taking any measurements. Almost always, the previous user s settings are still programmed. Check the circuit configuration. Even a single missing wire or incorrect connection could change results entirely. If no signal appears onscreen, even after scaling and offsetting Check for loose connections between the probe alligator clips or grabber clips. Verify the black leads of the probes are connected to circuit ground. If earth ground is introduced, verify the black leads of the probes and earth ground are at the same potential. If an unexpected signal appears onscreen Check the timescale a small piece of a larger waveform might be displayed. Verify the probe clips are connected to the correct circuit node. If all else fails Recall factory defaults once more and try again. Call a TA over and explain the problem. 8