Setting Up

This section briefly describes how to set up and start using an oscilloscope - specifically, how to ground the oscilloscope, set the controls in standard positions, and compensate the probe.

Grounding

Proper grounding is an important step when setting up to take measurements or work on a circuit. Properly grounding the oscilloscope protects you from a hazardous shock and grounding yourself protects your circuits from damage.

Ground the Oscilloscope

Grounding the oscilloscope is necessary for safety. If a high voltage contacts the case of an ungrounded oscilloscope, any part of the case, including knobs that appear insulated, it can give you a shock. However, with a properly grounded oscilloscope, the current travels through the grounding path to earth ground rather than through you to earth ground.

To ground the oscilloscope means to connect it to an electrically neutral reference point (such as earth ground). Ground your oscilloscope by plugging its three-pronged power cord into an outlet grounded to earth ground.

Grounding is also necessary for taking accurate measurements with your oscilloscope. The oscilloscope needs to share the same ground as any circuits you are testing.

Some oscilloscopes do not require the separate connection to earth ground. These oscilloscopes have insulated cases and controls, which keeps any possible shock hazard away from the user.

Ground Yourself

If you are working with integrated circuits (ICs), you also need to ground yourself. Integrated circuits have tiny conduction paths that can be damaged by static electricity that builds up on your body. You can ruin an expensive IC simply by walking across a carpet or taking off a sweater and then touching the leads of the IC. To solve this problem, wear a grounding strap (see Figure 1). This strap safely sends static charges on your body to earth ground.



Figure 1: Typical Wrist Type Grounding Strap

Setting the Controls

After plugging in the oscilloscope, take a look at the front panel. It is divided into three main sections labeled Vertical, Horizontal, and Trigger. Your oscilloscope may have other sections, depending on the model and type (analog or digital).

Notice the input connectors on your oscilloscope. This is where you attach probes. Most oscilloscopes have at least two input channels and each channel can display a waveform on the screen. Multiple channels are handy for comparing waveforms.



Figure 2: Front Panel Control Sections of an Oscilloscope

Some oscilloscopes have an AUTOSET or PRESET button that sets up the controls in one step to accommodate a signal. If your oscilloscope does not have this feature, it is helpful to set the controls to standard positions before taking measurements.

Standard positions include the following:

These are general instructions for setting up your oscilloscope. If you are not sure how to do any of these steps, refer to the manual that came with your oscilloscope. The Controls section describes the controls in more detail.

Probes

Now you are ready to connect a probe to your oscilloscope. It is important to use a probe designed to work with your oscilloscope. A probe is more than a cable with a clip-on tip. It is a high-quality connector, carefully designed not to pick up stray radio and power line noise.

Probes are designed not to influence the behavior of the circuit you are testing. However, no measurement device can act as a perfectly invisible observer. The unintentional interaction of the probe and oscilloscope with the circuit being tested is called circuit loading. To minimize circuit loading, you will probably use a 10X attenuator (passive) probe.

Your oscilloscope probably arrived with a passive probe as a standard accessory. Passive probes provide you with an excellent tool for general-purpose testing and troubleshooting. For more specific measurements or tests, many other types of probes exist. Two examples are active and current probes.

Descriptions of these probes follow, with more emphasis given to the passive probe since this is the probe type that allows you the most flexibility of use.

Using Passive Probes

Most passive probes have some degree of attenuation factor, such as 10X, 100X, and so on. By convention, attenuation factors, such as for the 10X attenuator probe, have the X after the factor. In contrast, magnification factors like X10 have the X first.

The 10X (read as "ten times") attenuator probe minimizes circuit loading and is an excellent general-purpose passive probe. Circuit loading becomes more pronounced at higher frequencies, so be sure to use this type of probe when measuring signals above 5 kHz. The 10X attenuator probe improves the accuracy of your measurements, but it also reduces the amplitude of the signal seen on the screen by a factor of 10.

Because it attenuates the signal, the 10X attenuator probe makes it difficult to look at signals less than 10 millivolts. The 1X probe is similar to the 10X attenuator probe but lacks the attenuation circuitry. Without this circuitry, more interference is introduced to the circuit being tested. Use the 10X attenuator probe as your standard probe, but keep the 1X probe handy for measuring weak signals. Some probes have a convenient feature for switching between 1X and 10X attenuation at the probe tip. If your probe has this feature, make sure you are using the correct setting before taking measurements.

Many oscilloscopes can detect whether you are using a 1X or 10X probe and adjust their screen readouts accordingly. However with some oscilloscopes, you must set the type of probe you are using or read from the proper 1X or 10X marking on the volts/div control.

The 10X attenuator probe works by balancing the probe's electrical properties against the oscilloscope's electrical properties. Before using a 10X attenuator probe you need to adjust this balance for your particular oscilloscope. This adjustment is called compensating the probe and is further described in the next section. Figure 3 shows a simple diagram of the internal workings of a probe, its adjustment, and the input of an oscilloscope.



Figure 3: Typical Probe/Oscilloscope 10-to-1 Divider Network

Figure 4 shows a typical passive probe and some accessories to use with the probe.



Figure 4: A Typical Passive Probe with Accessories

Using Active Probes

Active probes provide their own amplification or perform some other type of operation to process the signal before applying it to the oscilloscope. These types of probes can solve problems such as circuit loading or perform tests on signals, sending the results to the oscilloscope. Active probes require a power source for their operation.

Using Current Probes

Current probes enable you to directly observe and measure current waveforms. They are available for measuring both AC and DC current. Current probes use jaws that clip around the wire carrying the current. This makes them unique since they are not connected in series with the circuit; they, therefore, cause little or no interference in the circuit.

Where to Clip the Ground Clip

Measuring a signal requires two connections: the probe tip connection and a ground connection. Probes come with an alligator-clip attachment for grounding the probe to the circuit under test. In practice, you clip the grounding clip to a known ground in the circuit, such as the metal chassis of a stereo you are repairing, and touch the probe tip to a test point in the circuit.

Compensating the Probe

Before using a passive probe, you need to compensate it - to balance its electrical properties to a particular oscilloscope. You should get into the habit of compensating the probe every time you set up your oscilloscope. A poorly adjusted probe can make your measurements less accurate. Figure 5 shows what happens to measured waveforms when using a probe not properly compensated.



Figure 5: The Effects of Improper Probe Compensation

Most oscilloscopes have a square wave reference signal available at a terminal on the front panel used to compensate the probe. You compensate a probe by:

When you compensate the probe, always attach any accessory tips you will use and connect the probe to the vertical channel you plan to use. This way the oscilloscope has the same electrical properties as it does when you take measurements.

Next Chapter: The Controls

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