BK Precision 4011A Betriebsanleitung PDF herunterladen (Seite 63)

Appendix D: Operating Instructions for a Typical Oscilloscope

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If several voltage waveforms are to be studied and must maintain their relative phase positions,

the sweep generator must be synchronized to the same voltage during the entire test. In this case,

one voltage is applied to the oscilloscope as an external trigger.

An independent voltage may be applied to the horizontal input in place of the sweep oscillator

voltage. In this case, two independent input voltages are displayed against one another. If the

horizontal frequency is a submultiple of the vertical frequency, the trace will form a stationary

pattern on the screen.

Today the traditional CRT oscilloscopes are rapidly being replaced with digital oscilloscopes that

have flat-panel liquid crystal displays (LCDs), some with color displays. Instead of directly ap-

plying the incoming voltages to deflection plates, the digital oscilloscopes capture the voltage

information and store it in computer memory as digital signals, which are then analyzed and dis-

played on the LCD. While the new digital scopes handle the incoming signal differently from the

CRT-based scopes, the basic purpose and many of the operational controls remain the same.

Therefore, the discussion that follows applies, for the most part, to both types of oscilloscopes.

Potential Grounding Errors With Oscilloscope

In practice the user needs to be aware that the shield or ground wire on the oscilloscope’s signal

input is connected to the oscilloscope’s chassis ground, and therefore to the ground lead on the

instrument’s electrical power connection. This fact creates the possibility of grounding errors

when making connections to circuits. Before proceeding, the user should review the article “Os-

cilloscope Grounding Errors” found in Appendix B of these appendices.

Preliminary Adjustment To Obtain a Trace

To operate the oscilloscope, first turn on the power switch and allow the unit to warm up and ini-

tialize. Place the HORIZONTAL CONTROL in the sweep position. Adjust the INTENSITY and FOCUS

controls on a CRT scope until the desired brightness and line width are obtained. Do not leave

the spot stationary on the screen, as doing so may leave a permanent burn on the screen coating.

Once a trace is obtained, some oscilloscopes must be checked for calibration and balance. For

the HP 120B CRT oscilloscope (a typical example), the procedure is as follows: Place the TRIG-

GER LEVEL on AUTO and the TRIGGER SOURCE to INT. Turn the horizontal and vertical VERNIERS to

CAL, the VERTICAL SENSITIVITY to CAL, and the HORIZONTAL DISPLAY to 1 msec/cm. With the

controls in these positions, an internal calibrating signal is produced on the screen. Turn the CAL

adjustment until the upper and lower peaks of the square wave are 6 cm apart.

Digital oscilloscopes often have automated setup and calibration upon power-up.

Waveform Observation

After the initial adjustments are made, the oscilloscope is ready for operation. To observe the

waveform of any periodic signal, apply the signal to the vertical input terminals. Use DC cou-

pling if the input is DC or very low in frequency or if you want to capture any DC offset. Now

the signal from an oscillator, signal generator, or some component of an electrical circuit may be

observed on the screen. The best resolution of the waveform is obtained when the time scale is

adjusted so one or two cycles appear on the screen and when the vertical scale is adjusted so the

amplitude occupies most of the graticule. If the waveform will not stabilize, adjust the SYNC or

TRIGGER just enough to cause the pattern to stop. Whenever possible, connect the oscilloscope

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