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Download Appendix A: Basic Operation of Tektronix TDS1002 Digital
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Transcript
Appendix A: Basic Operation of Tektronix TDS1002 Digital Oscilloscope Figure 1 below shows the front panel of the TDS1002. The power button is on the top of the case. Figure 1. TDS1002 front panel. Note the four major functional areas: Vertical, Horizontal, Trigger , and Display Settings (on the top), plus the column of five menu buttons. Vertical controls: Position 1 and Position 2: Position the waveform vertically. (Position cursors when cursor LED lit.) Ch1 Menu , Ch2 Menu: Display the vertical function menus and toggles the display of the channel waveform on/off. Math Menu: Displays the waveform math operations menu. We will not use this at present. Volts/Div: Selects calibrated scale factors. Horizontal controls: Position: Adjusts the horizontal position of the waveform. Horiz. menu: Displays the Horizontal menu. Set to zero: Sets the horizontal position to put the trigger point (see below) at the middle of the screen Sec/Div: Selects the horizontal time/div scale factor). Trigger controls: Level: Used to set up the trigger level. Trig. menu: Displays the Trigger menu. Set to 50%: Sets the trigger level to the vertical midpoint between the peaks of the trigger signal Force Trig.: Complete a waveform acquisition even if a proper trigger signal is not present. Trig. View: View the waveform seen by the trigger circuit Menu and Control buttons: Save/Recall, Measure, Acquire, Cursor, Utility buttons display the corresponding menus. Auto Set: Automatically sets the oscilloscope controls to produce a usable display of the input signal. This is your “panic” button. Punch it in anytime something goes wrong and you do not know why… Default set up: Recalls the factory set up. Single Seq: Acquires a single waveform and then stops. Run/Stop: Continuously acquires waveforms. Print: Starts print operations. You may use this in some of the individual experiments. 1 Basic Setup for 1-Channel Operation To begin, turn on the oscilloscope by pressing the POWER button on the top of the oscilloscope. Because the Tektronix TDS1002 is a dual input oscilloscope, you can choose to display a voltage waveform from either of two inputs (or both simultaneously). The input “channels” are marked CH1 and CH2, respectively, on the front panel. Repeatedly pressing the MENU button for CH 1 will turn the scope trace for Channel 1 ON and OFF. Likewise, pressing the MENU button for CH 2 will turn the scope trace for Channel 2 ON and OFF. Do not have any connections to the scope input for now. Fig. 2. Screen shot with scale factor displays. For now, we are only interested in looking at the voltage on Channel 1, so press the appropriate MENU buttons to turn CH 1 ON and CH 2 OFF. Press the Trig Menu button to bring up the TRIGGER MENU. For Mode, select Auto; for Type, select Edge; for Slope, select Rising;, for Source, CH1, and for Coupling. Next turn the SEC/DIV knob to 10.0 µs (as read out in area C of Fig. 2), and the VOLTS/DIV knob for CH 1 to get 1.00V (as read out on area A of Fig. 2). You should see a horizontal line on the screen. If not, use the VERTICAL POSITION knob to center the line so that it is at 0.00 divs (0.00 V). Because we are only looking at CH1, only the left-hand knob for the VERTICAL POSITION will be useful. Once the trace has been centered, you can use the DISPLAY option to adjust the brightness of the trace (if needed) using the Contrast Increase and Contrast Decrease buttons. Next, press the CH 1 MENU button and set the Coupling to Ground. Adjust the VERTICAL POSITION knob so that the trace is exactly centered at 0.00 div (0.00 V), then return Coupling to the DC setting. Other CH1 menu settings should be as follows: BW limit: off; Volts/Div: Coarse; Probe: 1X (you will change this later when using a 10X probe); Invert: Off. You are now ready to connect the BNC cable between the scope Chan 1 input and the Teachscope output and proceed with the measurement of the signal on Teachscope Channel 1. Scale Factor Displays As we have seen, the horizontal scale factor (time/division) is shown in area C of Fig. 2. Areas A and B show respectively the CHAN 1 and CHAN 2 scale factors (V/div). Area D shows a high precision readout of the frequency of the trigger source signal, which will normally be the CHAN or 2 input. Vertical Coupling Control From the CH 1 MENU, you can choose what kind of Coupling you wish to have for the oscilloscope input: Ground, DC, or AC. When the Coupling is set to Ground, the input signal is disconnected and the input to the oscilloscope is connected to “ground” – the local zero potential reference. While the Coupling is set to Ground, wherever the beam is positioned vertically on the screen (which can be done arbitrarily using the VERTICAL POSITION knob) will determine the vertical location that corresponds to 0 V, or ground. You should always know where ground is referenced to on the display for both CH1 and CH2 while looking at signals on the scope. Unlike Ground, when the Coupling button is in the AC position, only the DC (constant) component of the signal will be filtered from the input, which means that the AC (changing) component of the signal will still be displayed. And finally, 2 it is only when the button is in the DC position that the oscilloscope will display all components of waveform input into the device— both the DC and AC components of the signal. The advantage of using AC coupling over DC coupling occurs when you are trying to measure small variations of a signal that contains a large DC component. As an example, consider the relatively small ”ripple” (5 mV) on the output of a typical DC power supply (say 5 V). With DC coupling, increasing the input sensitivity in an attempt to see the small variation will send the trace off the top of the screen. However, with AC coupling, the 5 V level will be suppressed and the sensitivity can be greatly increased while retaining the trace near the center of the display. Trigger Controls The triggering circuitry is the means by which the oscilloscope is able to provide a stable “overlay” of repeated ”snapshots” of a repetitive signal. By controlling the threshold voltage and slope polarity at which the waveform plot begins, the user is provided with great flexibility in the appearance of the display and the use of the scope for capturing elusive signals. The occurrence of the trigger event determines the start of the waveform plot. The TRIGGER LEVEL control sets the threshold voltage for the trigger. The current trigger level is read out in display area D (Fig. 2) and is also indicated by a left-pointing arrow at the righthand edge of the main display grid. Display area D also shows the trigger source, type (edge, video, or pulse), and slope, as well as the frequency of the signal from which the trigger is derived. Fig. 3. Display of a triangle wave with different trigger conditions. The SLOPE selector chooses whether the trigger event will occur when moving through the threshold from a high voltage to a low voltage (falling edge/negative slope) or from a low to high voltage (rising edge/positive slope). Refer to Figure 3 for a pictoral representation of the result of various trigger level and slope combinations. Setting the trigger level too low or too high (outside the bounds of the signal amplitude) results in loss of triggering and an unstable “running” waveform. The trigger source selection EXT allows an external, better-conditioned signal to be used to trigger the scope while observing a relatively weak signal—one that is too weak for good internal triggering. External triggering is accomplished by connecting the signal to be used for triggering to the external trigger input EXT TRIG of the scope and selecting Ext on the trigger Source button. Cursors and Cursor Controls Push the CURSOR button to display the measurement cursors and the Cursor Menu. (The oscilloscope must show a waveform for the cursors and cursor readouts to appear.) The Cursor LEDs near the Vertical position knobs will light, indicating that those knobs now can be used to position the cursors. The cursor menu choices are shown on the following page. 3 Automatic Measurement Functions Push the MEASURE button to access automatic measurements of the waveform. There are eleven possible measurements available, and you can display up to five at a time. Push the top option button to display the Measure 1 menu, then choose the channel on which to take a measurement in the Source option. Choose the type of measurement in the Type option, then push the Back button to return to the MEASURE menu and display the measurement results. Measurement Type Freq Period Mean Pk-Pk CycRMS Min Max Rise Time Fall Time Pos Width Neg Width Definition Calculates frequency of waveform by measuring first cycle Calculates time of first cycle Calculates arithmetic mean voltage over the entire record Calculates the absolute difference between the min & max peaks of entire record Calculates a true RMS measurement of the first complete cycle of the waveform Minimum value in entire record (2500 points) Maximum value in entire record (2500 points) Measures time between 10% and 90% of the first rising edge of waveform Measures time between 90% and 10% of the first falling edge of waveform Measures time between first rising edge and next falling edge at 50% level Measures time between first falling edge and next rising edge at 50% level 4