EZ Digital FC-3000 Operation Manual - 5. Others
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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
Ω, use 50Ω coaxial
cable terminated with a 50
Ω 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 (9O cm), to keep shunt capacitance within acceptable limit.
In 50
Ω 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 50
Ω point in the circuit under
test.
5-2. Use of Attenuator Probes
Input A resistance (1 M
Ω) and input capacitance (
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
Ω. THE 50 Ω
TERMINATION OF THE INPUT C WOULD RESULT IN UNACCEPTABLY HIGH
ATTENUATION.
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