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CONSIDERATIONS
Display Instability
An uncertainty of ± I least significant digit is inherent in all
digital measurements, and greater uncertainties can result
from other factors. For example, in low frequency
measurements, high frequency noise on the input can cause
miscounting. Also, uncertainty may be introduced by
instability of the input frequency, usually common with LC -
type oscillators.
Use of Antenna
To measure transmitter frequency, it is not always necessary
to have a direct electrical connection to the transmitter. In
fact, the counter should be protected against from excessive
power levels. A preferred method of frequency measurement
is to connect an antenna to the input of the counter. The BK
Precision Model AT - 21 Antenna Kit is ideal for use with
the Model 1804D counter for measuring frequencies from
20MHz to 200MHz. The antenna should be placed in
parallel with the transmitting antenna and separate by a few
inches. The specific
distance is determined by the power level of the transmitter.
Some very low power transmitters may not provide enough
signal to the counter with this this method. An unmodulated
carrier should be transmitted and the frequency will appear
on the display of the counter.
Cable Considerations
Cable connections in RF measurements should be aimed at
reducing standing waves and shunt cable capacitance, both
of which can affect measurement accuracy.
Standing waves can be minimized by matching impedances
of signal source, cable, and termination. For example, for a
50 ohm source, use a 50 ohm cable and terminate with a 50
ohm
resistive load. Both standing waves and shunt cable
,
capacitance can be reduced by keeping cable lengths short,
under three feet (91cm).
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