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Other specialized meter types 57

Frequency counters

The measurement of energy used by your home is an application to which digital me-
tering is well suited. It’s easier to read the drum type, digital kilowatt-hour meter than
to read the pointer type meter. When measuring frequencies of signals, digital metering
is not only more convenient, but far more accurate.
The frequency counter measures by actually counting pulses, in a manner similar
to the way the utility meter counts the number of turns of a motor. But the frequency
counter works electronically, without any moving parts. It can keep track of thousands,
millions or even billions of pulses per second, and it shows the rate on a digital display
that is as easy to read as a digital watch. It measures frequency directly by tallying up
the number of pulses in an oscillating wave, even when the number of pulses per sec-
ond is huge.
The accuracy of the frequency counter is a function of the lock-in time. Lock-in is
usually done in 0.1 second, 1 second or 10 seconds. Increasing the lock-in time by a fac-
tor of 10 will cause the accuracy to be good by one additional digit. Modern frequency
counters are good to six, seven or eight digits; sophisticated lab devices will show fre-
quency to nine or ten digits.

Other specialized meter types

The following are some less common types of meters that you might come across in
electrical and electronic work.

VU and decibel meters
In high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loud-
ness meters are sometimes used. These are calibrated in decibels, a unit that you will
sometimes encounter in reference to electronic signal levels. A decibel is an increase or
decrease in sound or signal level that you can just barely detect, if you are expecting the
change.
Audio loudness is given in volume units (VU), and the meter that indicates it is
called a VU meter. Usually, such meters have a zero marker with a red line to the right
and a black line to the left, and they are calibrated in decibels (dB) above and below this
zero marker (Fig. 3-12). The meter might also be calibrated in watts rms, an expression
for audio power.
As music is played through the system, or as a voice comes over it, the VU meter
needle will kick up. The amplifier volume should be kept down so that the meter does-
n’t go past the zero mark and into the red range. If the meter does kick up into the red
scale, it means that distortion is probably taking place within the amplifier circuit.
Sound level in general can be measured by means of a sound-level meter, cali-
brated in decibels (dB) and connected to the output of a precision amplifier with a mi-
crophone of known, standardized sensitivity (Fig. 3-13). You have perhaps heard that a
vacuum cleaner will produce 80 dB of sound, and a large truck going by might subject
your ears to 90 dB. These figures are determined by a sound-level meter. A VU meter is
a special form of sound-level meter.