3-26
FREQUENCY MEASUREMENTS
Frequency measurements are an essential part of preventive and corrective maintenance for
communication and electronic equipment. Rotation frequencies of some mechanical devices must be
determined; the output frequency of electric power generators is checked when the engine is started and
during preventive maintenance routines; carrier equipment that operates in the audio-frequency range
must be adjusted to operate at the correct frequencies; and radio transmitters must be accurately tuned to
the assigned frequencies to provide reliable communications and to avoid interfering with radio circuits
operating on other frequencies. These are only a few of the applications for making frequency
measurements.
FREQUENCY-MEASUREMENT METHODS
Frequency-measuring equipment and devices, particularly those used to determine radio frequencies,
constitute a distinct class of test equipment, because of the important and critical nature of such
measurements. The requirement of precise calibration is extremely important in all frequency-measuring
work. To provide accurate measurements, every type of frequency-measuring device must be calibrated
against some frequency standard.
FREQUENCY STANDARDS
Of considerable importance in the measurements of frequency or wavelength are the standards
against which frequency-measuring devices are compared and calibrated. Frequency standards belong to
two general categories: primary and secondary standards. The PRIMARY FREQUENCY STANDARD
maintained by the U.S. National Bureau of Standards has long-term stability and an accuracy of 1 part in
1012, using an atomic clock. A SECONDARY FREQUENCY STANDARD is a highly stable and
accurate standard that has been calibrated against the primary standard. Secondary standards are
maintained by calibration laboratories that service your test equipment.
The National Bureau of Standards provides time and frequency standards from station WWV at Fort
Collins, Colorado, and from station WWVH at Kekaha, Kauai, Hawaii. The following technical radio
services are given continuously by these stations:
Standard radio frequencies
Standard audio frequencies
Standard time intervals
Standard musical pitch
Time signals
Radio propagation notices (WWV only)
Geophysical alerts
Universal Time Coordinated (UTC)
+ UT1 Corrections
The UTC scale uses the ATOMIC SECOND as a time interval. UT
1
is based on the earth's uniform
rate of rotation. Since the earth's rotation is not precisely uniform, UT1 is an adjustable interval.
