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Waveform Measurements

You use many terms to describe the types of

measurements that you take with your oscilloscope.

This section describes some of the most common

measurements and terms.

Frequency and Period

If a signal repeats, it has a frequency. The frequency

is measured in Hertz (Hz) and equals the number of

times the signal repeats itself in one second (the

cycles per second). A repeating signal also has a

period – this is the amount of time it takes the signal

to complete one cycle. Period and frequency are reci-

procals of each other, so that 1/period equals the

frequency and 1/frequency equals the period. So, for

example, the sine wave in Figure 20 has a frequency

of 3 Hz and a period of 1/3 second.

Voltage

Voltage is the amount of electric potential (a kind of

signal strength) between two points in a circuit.

Usually one of these points is ground (zero volts) but

not always – you may want to measure the voltage

from the maximum peak to the minimum peak of a

waveform, referred to as the peak-to-peak voltage.

The word amplitude commonly refers to the

maximum voltage of a signal measured from ground

or zero volts. The waveform shown in Figure 21 has

an amplitude of one volt and a peak-to-peak voltage

of two volts.

Phase

Phase is best explained by looking at a sine wave.

The voltage level of sine waves is based on circular

motion, and a circle has 360 degrees (°). One cycle of

a sine wave has 360°, as shown in Figure 21. Using

degrees, you can refer to the phase angle of a sine

wave when you want to describe how much of the

period has elapsed.

Phase shift describes the difference in timing

between two otherwise similar signals. In Figure 22,

the waveform labeled “current” is said to be 90° out

of phase with the waveform labeled “voltage,” since

the waves reach similar points in their cycles exactly

1/4 of a cycle apart (360°/4 = 90°). Phase shifts are

common in electronics.

Performance Terms

The terms described in this section may come up in

your discussions about oscilloscope performance.

Understanding these terms will help you evaluate

and compare your oscilloscope with other models.

Bandwidth

The bandwidth specification tells you the frequency

range the oscilloscope accurately measures.

As signal frequency increases, the capability of the

oscilloscope to accurately respond decreases. By

convention, the bandwidth tells you the frequency at

which the displayed signal reduces to 70.7% of the

applied sine wave signal. (This 70.7% point is

referred to as the “–3 dB point” – a term based on a

logarithmic scale.)

Rise Time

Rise time is another way of describing the useful

frequency range of an oscilloscope. Rise time may be

a more appropriate performance consideration when

you expect to measure pulses and steps. An oscillo-

scope cannot accurately display pulses with rise

times faster than the specified rise time of the oscil-

loscope.

Effective Bits

Effective bits is a measure of a digitizing oscillo-

scope’s ability to accurately reconstruct a signal by

considering the quality of the oscilloscope’s ADC

and amplifiers. This measurement compares the

oscilloscope’s actual error to that of an ideal digi-

tizer. Because the actual errors include noise and

distortion, the frequency and amplitude of the signal

as well as the bandwidth of the instrument must be

specified.

Figure 21. Sine wave degrees.

Figure 22. Phase shift.

Figure 20. Frequency and period.

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