Section 2: Data Acquisition
Triggering
Triggering is a process to help view a single waveform instead of a display that is unstable with waves moving all over the screen. If the screen is blank, it may require triggering to see the waveform. The trigger tells the oscilloscope when to acquire data. The waveform is then displayed on the screen, originating at the designated point. Triggers are set to a certain amplitude. When the wave reaches this amplitude, data acquisition begins. However, each amplitude (with the exception of the critical points) occurs twice during a wave, once on the rising edge, and once on the falling edge. Therefore, this parameter must be specified also. These parameters are specified from the "Trigger Menu" button.
Acquisition Modes
Sample Mode: signals are sampled at evenly spaced intervals to construct waveforms. This is an accurate representation of most waves, but may not take into account rapid changes. Aliasing can occur when the sampling rate is not fast enough. This is when the frequency appears lower than it actually is due to data being taken from several different peaks and strung together as a single wave. The signal will appear unstable, even when triggered. To check this, adjust the horizontal scale. If the wave changes dramatically, increase the sampling rate or switch to Peak Detect Mode.
Peak Detect Mode: highest and lowest points are used to construct waveform. This displays narrow pulses, but also accentuates noise.
Average Mode: several waves are read and averaged to reduce noise.
Taking Measurements
There are three ways to get data from the oscilloscope. The first is by counting graticules. You can also use the cursor, or an automated measurement can be taken.
Graticule: This method is a quick estimate taken by counting the grid divisions and multiplying by the scale. The scale is displayed at the bottom of the display. For example, if there were 4 squares between the maximum and minimum values and the scale was 100 mV/division, the amplitude would be 400mV.
Cursors: This method works by moving cursors (which are always in pairs) and reading the values they display. Voltage cursors are horizontal lines and measure vertical distances. Time cursors are vertical lines and measure horizontal distances.
To access the cursor menu, push the "Cursor" button above the vertical controls. Select Voltage or Time cursors with the top menu button. The "Source" button changes the channel. Adjust the cursors with the "Cursor" dials, which are also the "Vertical Position" dials.
Automated: In this method, the original data acquired by the oscilloscope is manipulated by the oscilloscope. These are the most accurate results produced.
For measurements of this kind, press the "Measure" button to access the menu. Select "Source" to take all measurements from the correct channel. Then, select "Type" to choose the "Freq", "perios", and "Pk-Pk". The frequency, perios, and peak to peak measurements are displayedin the corresponding box.
Two channels can be compared by taking measurements of them both simultaneously. Choose "CH1" for the first measurement and "CH2" for the second.
Eliminating Noise
If the signal is noisy, and more detail is required, push "Acquire" to open the menu. The "Peak Detect" button (followed by "Display" if necessary) will bring up options for "Contrast Increase" and "Contrast Decrease". Adjusting the contrast will make the noise more visible. Next, to separate the signal from the noise, push "Acquire" and then "Average". Changing the value for "Averages" will make it easier to see the signal detail.
Single Shot Signals
It is possible to capture a single pulse using the following method. Set the scales to the ranges you are expecting. From the "Acquire" menu, select "Peak Detect". Then, from the "Trigger Menu" set "Mode" to "Single" and "slope" to "Rising". Set the trigger midway between the maximum and minimum voltages. The "Run/Start" button will start data capture. (The oscilloscope will wait until the trigger value is reached to display the pulse even after the "Run/Start" button is pushed.)
User Manuals
User Manuals should be available at each workstation for more details and troubleshooting advice.