B&K Precision 1856D - Manual - Manual (Page 20)

20
standing waves and cable shunt capacitance.
Waves propagating along a transmission line will be reflected at its ends if the
line is not terminated in its characteristic impedance. These reflections will set up
standing waves in the line, which may cause damage to the signal source or
produce inaccurate measurements. This effect increases as the cable length
reaches one-fourth of the wavelength for the frequency being measured. Standing
waves can be minimized by keeping cable lengths short, or eliminated by providing
a proper termination. RG58 cable, for example, has a characteristic impedance of
50

. The proper termination for this cable would, therefore, be a 50

resistor
between the center conductor and the shield. A standard BNC, 50

cable
termination is most often used for this.
Input A has an input impedance of 1 M Ω shunted by < 40 pF. Radio frequency
measurements made at this input will experience standing waves and attenuation
due to the shunt capacitance of the cable which adds to the shunt capacitance at
the input. For RF measurements at input A, it is recommended that a 50

termination be used, or that the cable be no longer than three feet (91 cm).
Input C has a built in impedance of 50

, thus the need for an external
termination is eliminated.
In situations where a 50

termination could affect bias voltages or other DC
voltages in the circuit under test, a blocking capacitor should be used.
5-2. Attenuator Probes
When measuring signals at input A that will exceed its maximum input voltage
level (See FIG. 1), a 10:1 attenuating probe such as an oscilloscope probe should
be used.
NOTE