B&K Precision 1823A - Manual - Manual (Page 20)

20
5. OTHERS
5-1. BNC Cable Considerations
Accuracy of radio frequency measurements can be affected by connections
between signal source and counter. Main considerations are standing waves and
shunt cable capacitance.
Standing wave is usually present due to reflections when a transmission line is
not terminated in its characteristic impedance. These standing waves may cause
damage to the signal source or produce inaccurate measurements, and their
effects increase as cable length reaches one-fourth of the wavelength for the
frequency being measure. Standing wave can be minimized by keeping cable
lengths short, and more important, providing a proper termination.
The cable's characteristic impedance and the terminating impedance should
match the source impedance. For example, for a source impedance of 50

, use
50

coaxial cable terminated with a 50

resistive load.
Use DC blocking capacitor in situations where bias voltage or other DC voltages
could be affected by the termination resistor. Shunt cable capacitance, which can
cause undesirable signal attenuation, increases with increased cable length. It is
recommended that for radio frequency measurements, the cable be no longer
than three feet (90 cm), to keep shunt capacitance within acceptable limit.
In 50

systems the internal 50

input termination of the input B. BNC
minimizes reflections and the resulting standing waves. Thus, the need for an
external termination is eliminated. Also, shunt capacitance has a much lesser
effect at this BNC then at the input A, and the above restriction on cable length is
reduced. However, prescale measurements must always be taken from a 50

point in the circuit under test.
5-2. Use of Attenuator Probes
Input A resistance (1 M

) and input capacitance (< 40 pF) are independent of
the ATT switch. To decrease loading, a high impedance oscilloscope probe such
as the following may be used with input A. Use the probe in the x 10 position
whenever possible for less circuit loading.
NOTE
DO NOT USE A 10:1 PROBE WITH THE INPUT C, THE PROBE IS DESIGNED
FOR 10:1 ATTENUATOR WITH A COUNTER INPUT RESISTANCE OF 1 M

.
THE 50

TERMINATION OF THE INPUT C WOULD RESULT IN
UNACCEPTABLY HIGH ATTENUATION.