Input parameters in the Tektronix MDO3000 Series Oscilloscope are determined by the user. This is done by pressing one or more of the channel menu buttons, causing the horizontal channel menu to appear.
Menu items are:
• Scale – Fine, Offset, Position, Probe setup and Deskew
• Coupling – ac or dc
• Termination – One Megohm or 50 ohm
• Bandwidth – Full, 250 MHz or 20 MHz
• Invert – On or Off
• Label – An on-screen keyboard for creating, inserting and displaying items.
DC coupling is the default, and it is used most of the time. It really should be called dc/ac coupling. Both electrical components are conveyed to the preamplifier and processor. AC coupling should be called AC only. If your oscilloscope does not have this mode, you can obtain the same effect by inserting a series capacitance into the external signal path, upstream from the channel input. The dc component is attenuated. This mode is useful when examining the output of a high-voltage power supply in order to detect excessive ripple. That way, the high-amplitude dc component does not overwhelm the low-amplitude ac component in the auto-ranging oscilloscope.
The soft key associated with Termination toggles between 1 MΩ and 50 Ω. One Megohm is the default and is used most of the time. It is automatically set when you switch to ac coupling. Use 50-Ω termination with care. The lower impedance invites greater current in the input circuit, which can overload the oscilloscope preamplifier, probe and circuit under investigation. At 50-Ω termination, the maximum vertical scale factor is one volt per division. But if a 10X probe is used, this factor increases to 10 V.
Press the soft key associated with Invert to reverse signal polarity. Off is default, and it is used in most applications.
Pressing the soft key associated with Bandwidth, the vertical Bandwidth menu appears. In the 1-GHz Tektronix MDO3104, available choices are Full Bandwidth, 250 MHz and 20 MHz. It should be noted that activating these limits does not affect the actual bandwidth of the instrument. It merely reduces the bandwidth of the signal at the preamplifier input.
The purpose of this operation is to improve the display of signals having excessive noise. Many types of noise are broad-spectrum phenomena. Limiting bandwidth eliminates a significant amount of high-frequency noise without degrading the signal of interest provided it occupies the constrained portion of the spectrum.
You can see how effective bandwidth limiting is in cleaning up a waveform by connecting the internal arbitrary function generator to the Channel One analog input via a BNC cable. Pressing AFG, the default Sine wave and horizontal AFG menu appear. Press the soft key associated with Output Settings. In the vertical menu, the second menu item is Add Noise, which is done by turning Multipurpose Knob a. At 50%, the waveform is quite noisy, and triggering is lost.
Going back to the channel menu, press the soft key associated with Bandwidth, bringing up the vertical Bandwidth menu. Limiting the bandwidth to 250 MHz improves the signal slightly, but it is still noisy. Further reducing it to 20 MHz restores triggering, but there is still some noise as can be seen in the thickened trace.
In many situations, Waveform Averaging is more effective than Bandwidth Limiting for mitigating noise. To see Waveform Averaging at work, press Acquire>Mode>Average. In this mode, a user-selected number of consecutive waveforms is averaged. Because the sine wave is periodic and noise is random, the averaging process reinforces the signal under investigation and cancels out noise. The number of waveforms to be averaged is selected by turning Multipurpose Knob a or entering the desired number in the keypad, ranging from two to 512. As this value rises, noise is reduced and the waveform becomes clearer. At 512, the trace is exceptionally narrow, showing no presence of noise in the signal.
Pressing More, a pop-up menu permits the user to access:
• Fine Scale
• Probe Setup
Fine Scale permits the user, by turning Multipurpose Knob a, to reset the vertical scale from 10 V/div to 1 mV/div in small increments. To see how this control affects the display, use the internal AFG to display a sine wave and observe it throughout the range of fine-scale settings. Without this or any signal at the output, notice that a distinct noise floor appears at a fine-scale setting of 1.00 mV/div. This mode is useful for displaying detail in complex signals.
Offset and position, both also regulated by Multipurpose Knob a, perform similar functions. You can use Vertical Position to place the waveform for best viewing. The waveform baseline indicators show the zero level for the waveform. Adjusting the channel’s vertical scale, the waveform expands or contracts around the waveform’s baseline indicator. Moving a waveform by means of the vertical offset control, the baseline indicator no longer represents zero. Instead, it represents the level of the offset.
Probe Setup defines probe parameters. For probes that do not have a Tektronix interface, when Probe Type is set to Current, turn Multipurpose Knob a to adjust the Amps/Volts ratio (attenuation) to match the probe. To measure current by probing voltage drop across a resistor set Measure Current to Yes. Press A/V ratio on the side menu and turn Multipurpose Knob a to set the Volts/Amps ratio for your setup.
Select Deskew to make display and measurement adjustments for probes that have differing propagation delays. This is especially important when using a current probe in conjunction with a voltage probe. For best results, use a deskew fixture such as the Tektronix 067-1686-xx. Without a deskew fixture, the deskew menu controls can be used to set the oscilloscope’s deskew parameters based on each probe’s propagation delay.
Wave Inspector allows you to take a closer look at any displayed waveform. Once again, we’ll display the sine wave, provided by the internal arbitrary function generator, as a model. The controls used to operate Wave Inspector are in a dedicated square area at the top center of the front panel. The first thing to notice, in the center of the Wave Inspector section, is the large pair of concentric knobs. The inner knob zooms in on a selected section of the displayed waveform. As soon as you move this knob, an overall reproduction of the waveform appears in split-screen format above the waveform under investigation, plotted in the time domain against the usual X- and Y-axes. The upper display, the overall view, extends across a longer time period. Prominent brackets delineate the portion of the waveform that is shown in the lower display. As you turn the zoom knob clockwise, the bracketed area contracts and the waveform in the lower display zooms in until it is nearly a straight line.
The outer knob pans the brackets, maintaining the previously set zoom status. As the brackets pan across the waveform, the detail in the lower display changes to reflect this change in the bracketed segment. Using zoom and pan together, you can examine a complex waveform in great detail.
Below the magnifying glass icon, the play/pause button causes the brackets to automatically pan across the displayed waveform. The more you turn the ring, the more the waveform accelerates. If you rotate the ring as far as it can go, the play speed does not change, but the zoom box quickly moves in that direction.
You can mark locations of interest in the waveform. The marks limit the analysis to distinct regions of the waveform. This can be done automatically or manually. Use the arrow keys to navigate among marks. Search marks can mark regions of a waveform for reference. Additionally, you can set marks automatically using search criteria. For example, you can search for and mark regions with unique edges, pulse widths, runts, logic states, rise and fall times, setup and hold and bus search types.
To manually set and clear marks, move to the area of the waveform that you want to set or clear a search mark. Press the arrow buttons to go to an existing mark. Then press Set or Clear. To automatically set and clear search marks, first press Search. Then select the desired search type from the horizontal menu. In the vertical menu, activate the search function. In the second page of this menu, turn on the search mark table. It shows a time-stamped listing of each event.
On the screen, hollow triangles show the location of automatic marks and solid triangles show the location of user-defined marks. You can investigate the entire waveform by navigating among search marks using the arrow buttons.