SUMMARY

The following is a brief summary of the important points of this chapter.

All IMPEDANCE BRIDGES have several things in common. Each type of bridge has a comparing circuit and a measuring circuit. They measure an unknown impedance by comparing the characteristics of the device under test with the characteristics of components within the test set.

POWER METERS that are designed to measure af power can be separated into two distinct groups. Power meters that are designed for measuring sine waves are basically electronic voltmeters calibrated in dB or dBm. VU METERS are designed to measure the average value of complex waveforms, such as a voice.

The most common type of test equipment used to measure rf power is the ABSORPTION POWER METER. Absorption power meters are designed to absorb all or part of the signal being measured. Examples of absorption power meters are output power meters, in-line wattmeters, and meters employing bolometers.

CALORIMETERS are the most accurate type of test equipment used for measuring high power. As power is applied to a calorimeter, its medium (either liquid or solid) is heated. The heat that is produced is directly proportional to the amount of applied power. The amount of applied power is determined by measuring the change in temperature of the medium.

Today's ELECTRONIC FREQUENCY COUNTERS are capable of measuring frequencies from dc to 40 GHz. Most have added features that enable period averaging, time-interval measurements, and scaling. Frequency counter accuracy can be extended by using an external frequency standard in lieu of its internal frequency standard.