Specifications

Operation of an analog scope

We'll use the B&K 2125A as an example analog scope. It is a popular two-channel 30 MHz scope. A

special feature of the scope is that it provides a component tester, which we'll look at in more detail.

Over the years there have been a variety of other features provided with analog scopes, such as both

digital and analog storage, counters, and digital voltmeters.

While the control panel of an analog scope may look intimidating to a newcomer, this will quickly pass

after some experience with using the scope. The 2125A scope is quite easy to set up to look at an

unknown waveform.

The basic strategy is to apply the signal to an AC-coupled channel, using auto triggering and set the

timebase control to about the middle of its adjustment range. Once an auto-triggered trace is seen, you

can adjust the vertical gain and timebase controls to get an acceptable display. Below is a general

procedure (numbers and letters in parentheses refer to Figure 5 above):

1. Set all lever switches to their uppermost position. Make sure the COMP TEST button (19) is not

pressed in.

2. Set control 7 to MAIN.

3. Set all potentiometer knobs so that their indicator is vertical. The exceptions are to leave the

HOLD OFF control (6) fully counterclockwise and the three variable adjustments (12, 15, and

17) fully clockwise until they click.

4. Set the scope's timebase control (21) to 1 ms/div.

5. You should see a flat horizontal trace across the screen.

6. If you don't, press the BEAM FIND button (20). This should show you a compressed view of the

horizontal line. If it doesn't, you may not have the intensity control (1) turned up high enough.

Also adjust the focus control (2) to get a sharp line.

7. If the beam find button does show you the waveform, adjust control 10 to vertically position the

horizontal line on the screen.

8. Connect a signal to the scope via channel 1's BNC jack (A). You can also use an oscilloscope

probe. If you have a probe, you can connect the probe's center conductor to the scope's CAL

waveform at (D).

9. Adjust the vertical gain knob (the inner knob of (15)) to display the waveform on the screen.

Adjust the timebase (21) to get a few periods of the waveform displayed.

This procedure should result in a waveform being displayed on the screen. If it doesn't, it's possible that

the signal is changing too slowly to be displayed easily on an analog oscilloscope (this is an advantage

of digital oscilloscopes, as they can capture and display slow waveforms that are difficult to see on an

analog scope).

One thing you'll notice from using an analog scope is that once you're familiar with the control layout,

it's often faster to change scope settings than for an equivalent operation with a digital scope. This is

one reason why troubleshooters often prefer an analog scope: the control is right there and there's no

navigating a menu to enable or adjust it. The B&K 2125A has 21 controls on its control panel (three

controls are double potentiometers and 7 of the knobs can be pulled out to switch something). In

contrast, the B&K 254xB scopes have 29 buttons and 8 knobs. Most of the functionality in the digital

scopes is accessed through menus, which require more button presses. While this is only natural

because the digital scopes have more features, it may be slower than doing an equivalent task on an

analog scope.

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