SUMMARY

The important points of this chapter are summarized in the following paragraphs:

The BEL is a unit that expresses the logarithmic ratio between the input and output of any given component, circuit, or system and can be expressed in terms of voltage, current, or power.

Any figure expressed in bels can be converted to DECIBELS by multiplying the figure by 10. The decibel cannot be used to represent actual power, only a ratio of one power to another.

The abbreviation dBm is used to represent power levels above or below a 1 milliwatt reference level.

A BOLOMETER is a device that undergoes changes in resistance as changes in dissipated power occur. The two types of bolometers most often used are the barretter and the thermistor.

FREQUENCY MEASUREMENTS can be divided into two broad categories: mechanical-rotation frequency and electrical-output frequency measurements.

MECHANICAL ROTATION frequency is measured using a device called a TACHOMETER. Three basic tachometers are used for measuring mechanical rotation frequency - the CENTRIFUGAL tachometer, the CHRONOMETRIC tachometer, and the STROBOSCOPIC tachometer.

ELECTRICAL-OUTPUT frequencies of ac generators can be measured by VIBRATING-REED devices or TUNED CIRCUITS.

AUDIO FREQUENCIES can be measured by a process known as ZERO BEATING. This is done by matching an unknown signal with a locally generated signal of the same frequency obtained from a calibrated high-precision oscillator. As the two frequencies are brought closer to the same value, they reach a point of zero beat. This is when the frequency generated in the oscillator is equal to the frequency of the unknown signal being measured. Another term for zero beating is HETERODYNING.

WAVEMETERS are calibrated resonant circuits used to measure frequency. Any type of resonant circuit can be used in wavemeter applications. The type used depends on the frequency range for which the meter is intended.

For measuring frequencies in the microwave range, the CAVITY WAVEMETER is the type most commonly used.

The CATHODE-RAY OSCILLOSCOPE and the SPECTRUM ANALYZER are used to perform WAVEFORM ANALYSIS.

A typical BACK-TO-FORWARD-RESISTANCE ratio for a diode is 10 to 1.