Bnc Cable Considerations; Use Of Attenuator Probes; Line Frequency Measurements - UNISource Corporation FC-8300 Operation Manual

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 50Ω, use 50Ω
coaxial cable terminated with a 50Ω 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 50Ω systems the internal 50Ω 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 50Ω point in the
circuit under test.

5-2. Use of Attenuator Probes

Input A resistance (1 M Ω) 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.
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 Ω.
THE 50 Ω TERMINATION OF THE INPUT C WOULD RESULT IN
UNACCEPTABLY HIGH ATTENUATION.

5-3. Line Frequency Measurements

Use of the attenuator, low pass filter, and/or x10 probe is advisable when
measuring line frequency because the high amplitude signal and noise can cause
1.800.561.8187
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