A&D Series 57ZZ Manual do Utilizador descarregar pdf (Página 29)

This section teaches you basic measurement tech-

niques. The two most basic measurements you can

make are voltage and time measurements. Just about

every other measurement is based on one of these

two fundamental techniques.

This section discusses methods for making measure-

ments visually with the oscilloscope screen. This is a

common technique with analog instruments, and

also may be useful for “at-a-glance” interpretation of

DSO or DPO displays.

Note that most digitizing oscilloscopes include auto-

mated measurement tools. Knowing how to make

measurements manually as described here will help

you understand and check the automatic measure-

ments of DSOs and DPOs. Automated measurements

are explained later in this section.

The Display

Take a look at the oscilloscope display. Notice the

grid markings on the screen – these markings create

the graticule. Each vertical and horizontal line

constitutes a major division. The graticule is usually

laid out in an 8-by-10 division pattern. Labeling on

the oscilloscope controls (such as volts/div and

sec/div) always refers to major divisions. The tick

marks on the center horizontal and vertical graticule

lines (see Figure 38) are called minor divisions.

Many oscilloscopes display on the screen how many

volts each vertical division represents and how

many seconds each horizontal division represents.

Voltage Measurements

Voltage is the amount of electric potential, expressed

in volts, between two points in a circuit. Usually one

of these points is ground (zero volts) but not always.

Voltages can also be measured from peak-to-peak –

from the maximum point of a signal to its minimum

point. You must be careful to specify which voltage

you mean.

The oscilloscope is primarily a voltage-measuring

device. Once you have measured the voltage, other

quantities are just a calculation away. For example,

Ohm’s law states that voltage between two points in

a circuit equals the current times the resistance.

From any two of these quantities, you can calculate

the third using the following formula:

Ohm’s Law:

Voltage = Current

Resistance

Current =

Voltage

Resistance

Resistance =

Voltage

Current

Power Law:

Power = Voltage

Current

Another handy formula is the power law: the power

of a DC signal equals the voltage times the current.

Calculations are more complicated for AC signals,

but the point here is that measuring the voltage is the

first step toward calculating other quantities.

Figure 39 shows the voltage of one peak (V

) and the

peak-to-peak voltage (V

p-p

), which is usually twice

. Use the RMS (root-mean-square) voltage (V

RMS

)

to calculate the power of an AC signal.

The most basic method of taking voltage measure-

ments is to count the number of divisions a wave-

form spans on the oscilloscope’s vertical scale.

Adjusting the signal to cover most of the screen

Measurement Techniques

Figure 38. An oscilloscope graticule.

Figure 39. Voltage peak and peak-to-peak voltage.

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