Oscilloscope Basics: Multiplexed Front Panel Controls

Fig. 1. Modern,
slim front panel.

Most Teledyne LeCroy oscilloscopes are equipped with traditional front panel controls—knobs and buttons—that are a (literally) handy way to make basic acquisition settings such as gain, timebase and trigger level. While all these could be made using the oscilloscope software, using the front panel allows you to keep dialogs closed and more of the screen “real estate” available for viewing traces as you modify these settings.

In order to optimize that real estate, front panels have become increasingly slim, and many front panel controls on newer Teledyne  LeCroy oscilloscopes are multiplexed, meaning they have multiple functions or can be used to control multiple on-screen objects. Here is a list of tips to keep in mind when using the front panel.

Front panel knobs control the active trace

Depending on the oscilloscope model, it’s possible to have up-to-20 traces open on screen at once, and these may be a mix of channels, zooms, math functions or saved memories. But, there is only one front panel knob each for Vertical Scale, Offset, Horizontal Scale and Delay—the settings that allow you to control the visible acquisition “window” and scale of any trace. Therefore, these four knobs have to do the job of many. 

The front panel knobs always control the active trace, which is the one whose descriptor box is highlighted on screen.

Fig. 2. Active trace descriptor (left)
has a blue or yellow highlight.

Turning on a new trace always makes that one active, so for example, if you create a new math function or zoom, the original source channel is no longer the active trace, the function or zoom is. To use the front panel to modify the scale of the original channel, you need to re-activate it by re-selecting its descriptor box, the little box at the bottom of the grid with its name on it. The same is true for any trace: touch the descriptor box to re-activate it, then you can modify it using the front panel knobs.

Let the LED be your guide

Most front panels have LED indicators to alert you whether the front panel is controlling a math, zoom or memory trace, rather than a channel. If your front panel has actual Math, Zoom and Mem buttons, the LED is on the button. Still, you will have to look at the descriptor boxes to tell which trace is active.

Likewise, the channel buttons light up to show which one is active, the trigger buttons light to show which trigger mode is selected, and other buttons light to show the functionality is “on.”

The Adjust knob may be controlling a dialog setting or a trace

Fig. 3. Highlighted
active field.

Some software settings don’t have a dedicated front panel knob, but you may want to be able to adjust them quickly, such as Persistence Saturation level. For these, there is the Adjust knob. Just as the other knobs control the active trace, the Adjust knob controls the active field, which will be similarly highlighted on screen. However, the dialog must be open for Adjust to continue working.

Fig. 4. HDO4000A Adjust
knob with Intensity button.

One exception to this rule is when you press the Intensity button next to the Adjust knob. In that case, Adjust is now raising or lowering the Intensity of the active trace, which is not simply overall brightness but a filtering factor that makes high occurrence samples more visible. Press Intensity again, and now the Adjust knob has toggled back to controlling the last active field.

Not all fields can be controlled using Adjust, generally only those that take a continuous range of numeric values. You'll have to try Adjust to see if it works for you.

You don’t have an Adjust knob? See if the Cursor knobs will do the trick…or maybe try the User button.

Knobs do one thing when turned, another when pushed

See how underneath many knobs there is the legend: “Push ---“? That describes what happens if you push the knob like a button. For instance, Offset and Delay will return to Zero if you push the knob. The oscilloscope will Find the trigger Level that works best with your signal to display a full amplitude waveform.

If the Push action of any knob is Variable (e.g., V/div), pushing it will toggle the way the setting is incremented as you turn the knob. By default, most numeric settings increment in steps of 1, 2, 5, 10. For example, at a 1 V gain setting, the first slight turn of the knob to the right (higher) raises Vertical Scale to 2 V, the next to 5 V, the next to 10 V, and so on. However, if you want to raise/lower a setting by a small increment every time you turn the knob (how much depends on the overall vertical setup), pushing the knob like a button will toggle into variable mode. Pushing the knob again will toggle back to stepped. The same occurs when the legend is Coarse/Fine, as shown on the Adjust knob above.

The Cursor controls are really multiplexed

Fig. 5. Coarse and Fine Cursor knobs.
These can also be the Adjust knobs.

Cursor controls have always had multiple functions, and they've varied with the introduction of new oscilloscope designs.

If you don’t already have a cursor on screen, just press the Cursor (or Cursor Type) button to place one. It’s not working? Well, you need to have a waveform somewhere for cursors to deploy. Not the cursor you’d hoped for? Just keep pressing the button, eventually the one you want will show up. As soon as you see it, use the Cursor knob to move the cursor into position. Don’t want any cursors? You guessed it, just keep pressing the Cursor button until they are off the grid and the LED on the button goes out.

Wait, your cursor has multiple lines that need to be positioned? For that, push the Cursor knob like a button to Select the line you want (it will appear brighter), then turn the knob to move it.

Want to start over? If the Cursor knob legend is Default (rather than Select), push the knob to go back to the default positions.

You have two Cursor knobs? The one labeled Coarse will select (if you push) and move (if you turn) the absolute cursor (dashed-dotted line); the one labeled Fine will select and move the relative cursor (dotted line).

For those who don’t have an Adjust knob on their front panel, the Cursor knobs will function as the Adjust knobs when the cursors are turned off. The LED behind Cursors / Adjust will indicate which role they're filling. When they're Adjust, turning the knobs will raise/lower the setting of an active field, or raise/lower the intensity of an active trace. Since there is no Intensity button in this case, or cursors to select, push the knobs to toggle between controlling fields and controlling intensity. Use the Coarse knob to make stepped (1,2,5,10) adjustments; use the Fine knob to make variable (single-increment) adjustments.

The Single trigger button arms on one, fires on two

If you’ve wondered why you didn’t get a single-shot acquisition the first time you pressed Single, it is because unlike the other trigger buttons that operate “immediately”, Single requires two presses. The first one arms, or readies, the oscilloscope to trigger, the second actually fires the trigger. Even that is a bit of a misnomer, as the trigger conditions still need to be met for the trigger to fire, although in the case of repetitive triggers, like Edges, something will trigger the acquisition within a time that appears instantaneous to the eye.

The User button could be anything

Newer model oscilloscopes, such as the WavePro HD, WaveRunner 8000HD and WaveSurfer 4000HD have a User button instead of the old Print button. Sometimes, User takes the place of the Save button, as well. The User button is configurable, so it may be set to any number of print actions (e.g., literally send the screen image to a printer or “print” it to a file), save actions (e.g., save waveform, save panel setups) or other actions. You’ll want to check how this button is set before you use it by choosing Utilities > Front Panel Setup from the menu bar. While you’re there, you can choose an action for the User button that is helpful to you.

Check your Getting Started Guide and Operator’s Manual

The exact operation of all front panel controls is documented in the Getting Started Guide shipped with each oscilloscope, and in the Operator’s Manual, both available from our website. Since even similar front panels can have different operation, it’s always a good idea to review the manuals. Find your product manuals at the Teledyne LeCroy Technical Library.