Test Probe Frequency Compensation

Each test probe has a special compensation capacitor built into the BNC connector body. This capacitor matches the capacity of the probe and connecting cable to the input of the oscilloscope for flat frequency response. An uncompensated probe may result in waveform distortion or incorrect amplitude readings. The compensation affects both the CRT display and the digital peakto-peak function. Check the probe's compensation by connecting it to the PROBE COMP jack on the front panel. The resulting CRT display (fig. 4-28) should have a flat top and bottom with square corners (view A). Overshoot (view B) or rounding (view C) indicates the capacitor in the probe needs adjustment. Perform this adjustment as specified in the operator's manual.

Figure 4-28.\Probe compensation display.

Test Probe Ground Connections

Each test probe comes with two ground leads and a ground clip. It is important that you use the shortest possible ground connector when working in high-frequency circuits or circuits that produce square-wave signals with fast rise time. The inductance of extra ground lead length causes distortion in these signals. The long (12-inch) ground lead is to be used only when measuring low-frequency signals.

It is always necessary to ground each probe to prevent interference or other waveform distortion on high-frequency signals. The ground connection should be made as close to the test point as possible because the extra inductance of a printed circuit board or chassis wiring will have the same effect on the displayed signal as the longer ground leads-waveform distortion.

You should use the small ground clip when making tests in digital stages. The small size is designed for easy connection when the springloaded tip is removed. The clip is made of spring steel to allow connections to the closely spaced pins of an IC. And, above all, the length of the clip is the smallest possible to prevent signal distortion on fast rise-time signals.

USING THE OSCILLOSCOPE

The oscilloscope may be used to test solid-state devices; however, this may require the construction of special test circuits. This should pose no problem if you have access to a test laboratory. There is usually a multimeter, transistor tester, or similar piece of test equipment available to test components out of the circuit. The oscilloscope is particularly useful in the process of analyzing electronic circuits during operating conditions.

To analyze waveform displays effectively, you have to know the correct wave shape. The maintenance manual for each piece of equipment shows what waveforms you should observe at the various test points throughout the equipment. Waveforms that will be observed at any one selected test point will differ; each waveform will depend on whether the operation of the equipment is normal or abnormal.

Figure 4-29 represents a typical schematic diagram with oscilloscope waveforms. By attaching the oscilloscope test probe to point A in the circuit, you can compare the CRT display to the waveform shown in view A. You can troubleshoot the entire circuit by comparing the waveforms at test points A, B, C, and D with that of your scope.

TESTING CAPACITORS AND INDUCTORS

The use of capacitors in electronics has dramatically increased in the past few years and the forecast is for an even greater usage. The transistor has given way to the IC, but because of the nature and construction of the capacitor and the inductor, these are not replaced with ICs. The more ICs we use, the more capacitors and inductors we will use. The need to measure capacity value, leakage of the capacitor, inductor value, and quality of the inductor has become more important than ever before. Without a good measure of these important parameters, proper circuit operation becomes more difficult. With a capacitor-inductor analyzer, capacitors can be checked for value and for leakage at the rated working voltage on a digital readout. Inductors can be checked for inductance and for quality when a meter,, such as the one shown in figure 4-30, is used.