The green AUTO button was introduced on the very first ScopeMeter® 90 Series oscilloscopes back in 1992. Its purpose was to make informed guesses as to the settings that would make waveforms visible on the screen for the A and B channel inputs. The intent was to give the operator a starting point, from which adjustments could be made to the desired display.
The Auto Set mode, available on all Fluke ScopeMeter products today, is known as Connect-and-View™ (see sidebar).
This signal finder works pretty well. For example, if you have a probe connected to an ac receptacle when you push the button, then you will likely see the time base set to 5ms/div to display 2+ cycles of 50 Hz, or 3 cycles of 60 Hz voltage. It will also automatically select appropriate vertical sensitivities to show an on-screen voltage waveform of more than two divisions, if possible.
The feature has its limitations, though. Take the example above, where you intend to measure low level 60 Hz voltages and you haven't yet hooked up the test probes, or you haven't activated the circuit to be measured. Without clear clues at the input, the AUTO mode is likely to end up at a few µs/div and a few millivolts sensitivity, or default to the last used settings.
Is there a quick and convenient way to get to the desired settings for a measurement that you will be making repeatedly, say on a daily or weekly basis? With a little planning ahead of time, the answer is yes.
With the A channel input connected to my function generator, I pressed the green AUTO button. See Figure 1.
A sine wave of about 500 Hz appeared, but the numeric displays were not quite what I wanted—the larger digital display was showing the dc zero offset, so I pressed the MENU button to see what was going on.
From Figure 2, I learn that both the A and B channels have DC coupled inputs, and that Channel B is turned off. I set Channel B input to be AC coupled.
I decided to parallel connect the output of the function generator to both the A and B inputs.
The Channel B input settings menu (yellow button, marked "V Hz A," see Figure 3) was where I could verify that the B channel was on. I then set the primary digital measurement at the top of the display to VAC.
At this point, I switched the function generator to square wave output, added a 9 volt dc offset to simulate a biased audio transistor circuit, and made some A and B channel positioning and vertical sensitivity adjustments to clearly separate the two signals on the screen. Figure 4 shows the results. Note that the 123 is no longer in the Auto-Set mode.
As you can see from the above process, I've gone through many steps to get the display in the desired configuration. In the A Channel display, the Vdc reading is highlighted with the Vac shown in the secondary display. The vertical sensitivity is 5V/division, and the square wave is well above the zero reference mark.
The B Channel waveform is centered on the zero reference mark (AC coupled), and the vertical sensitivity is set to 2V/division. Vac is the primary display with an added digit of resolution.
This is the display that I want to use for repeated measurements over several days, so now it's time to use the SAVE SETUP feature of the ScopeMeter 123.
As you will see in Figure 5, I have stored my careful setup of the two channels of the 123 in Memory 3. Now, any time I want to repeat this complex measurement, all I have to do is RECALL Setup 3.
In the left SETUP window, you can see the dc coupling and readout settings for A and the ac coupling and measurement settings for B, along with the vertical range and time settings. Also note that the 123 is now in the MANUAL mode – Auto Set is disabled when using a recalled setup.
So, why did I switch to square waves for my test?
In last November's column, I discussed some of the issues around the use of the ScopeMeter 123 oscilloscope and FlukeView ScopeMeter software to evaluate harmonics. What better way to demonstrate the optimal setup for such a process than to use a square wave—rich in harmonics.
You will note in Figure 4 that the B Channel has a signal that is 2½ vertical divisions in magnitude. In my earlier column I indicated that the spectrum analysis software required a minimum of two vertical divisions for satisfactory results.
It's also optimal if the display shows no more than two or three full cycles of the fundamental. Figure 4 confirms that I have 2 ½ cycles available. FlukeView Spectrum Analysis requires at least 1½ cycles on screen to work, so you can usually find one or two time settings that will give you satisfactory results.
My final image shows a near-perfect square wave spectrum image that includes the 49th harmonic.
This shows once again that the ScopeMeter 123 and companion FlukeView ScopeMeter software can be a powerful combination. However, it does pay to experiment a bit to find your way around their many capabilities.