Special-Purpose Meters

Frequency Meters

Most components in a power system are designed to run on a specific ac frequency or limited range of frequencies. The frequency meter is important to test these frequencies. The frequency meter works very much like the digital voltmeter. A digital meter can generate a pulse train to be counted. Since the test frequency already has waveforms that can be counted, the clock and decade divider circuits only have to supply the gate pulse. The test signal is amplified, then converted to square wave pulses, generally by a Schmill trigger stage. The pulses are gated, then further shaped, and then counted to determine their frequency for display.


Field-Strength Meters

Field-strength meters are fundamentally used with radio transmission systems, either commercial or local-based systems, to determine how well the base transmitter covers the various parts of the area within its domain. The field-strength meter is a portable radio receiver tuned to the transmitter frequency. It picks up the radio-frequency (rf) signal, amplifies it, converts it to a dc test voltage, then measures the voltage with the dc amplifier meter circuits. The meter can be either analog or digital.

Since receivers of radio signals normally deal with very small signal levels, the field-strength meter is calibrated to give readings of rf signal levels in the millivolt or even microvolt ranges. When a local mobile communications system is used, its power output is usually kept low enough to be effective within a limited range. Because of the characteristics of the local terrain (hills, valleys, woods, etc.) and building structures, the signals could be attenuated in certain areas. The field-strength meter, when brought to various sites, can identify the fringe areas of operation in terms of distance and localities within the normal sphere of operation.

Typically, some field-strength meters are sensitive down to RF signals of SO-microvolts or below, while others are sensitive down to only I -millivolt RF signals. Typical frequency ranges are from I megahertz to 1300 megahertz; or 27 megahertz to 450 megahertz.



Recorders are needed to keep an extended record of test readings to determine whether the test voltage, current, or power is functioning properly on a continuous basis, or whether there are times that aberrations occur in the readings, particularly during specific load periods. Recorders are monitors that keep a continuous record of the circuit or circuits that they are monitoring. Some recorders have more than one channel so that they can monitor more than one circuit at a time.

Recorders are especially important for use in those power line circuits that must be “clean” for special applications, such as computer rooms and medical facilities. Some characteristics that are recorded are voltage, current, frequency, phase, surge, sag, transient pulses, etc.

Paper and stylus recorders are good for analog and wave shape data, and for continuous monitoring. Some recorders can have more than one stylus. Digital recorders have memory banks like those of computers; they store the digital data, which is sampled periodically. Some of these recorders come with their own dot-matrix printers, and others allow the data to be retrieved with a visual readout, or to be attached to a printer or computer.