B&K Precision 1803D - Manual - Manual (Page 9)

9
CONSIDERATIONS
Display Instability
An uncertainty of ± 1 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
adirect 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 Mode1 1803D counter for measuring
frequencies from 20MHz to 200MHz. The antenna should be placed
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, less than three feet (91cm).