NF FRA51602, Instruction Manual

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4.7 High-frequency measurements
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FRA 51602
c) Cable length and routing
The output BNC connector of OSC and the input BNC connectors of CH1 and CH2 are each electrically
isolated from the instrument enclosure. The impedance is high and can be treated as approximately
infinite impedance for DC or at mains frequencies. At higher frequencies, however, the isolation
impedance decreases due to the stray capacitance to the enclosure, facilitating current flow from the
outer conductor (shielding) of the cable to ground.
If the connecting cable is long, resonance of the cable inductance and the isolation capacitance may
perturb the frequency characteristics or make measurement impossible. At the (series) resonance
frequency, the isolation impedance becomes very small, resulting in a state that is the same as if the
signal cable shielding were grounded. That condition may be improved by attaching a common mode
choke to the cable. For example, a ferrite core clamp such as used to prevent noise or electromagnetic
interference can be attached to the cable.
If the signal cables are rearranged, the coupling between cables and the impedance relative to ground
change, so changes in the measurement results may be observed. In such cases, fix the cables in place so
that they do not move to improve the reproducibility of the measurement results.
d) Impedance matching
It is possible to suppress reflection in the signal cable and achieve stable measurement at higher
frequencies by attaching a commercial feedthrough type 50 Ω terminator to the input BNC connectors of
CH1 and CH2. (It may be necessary to use a 50 Ω class coaxial cable for the signal cable.)
[Information]
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For a 50 Ω class coaxial cable (such as the included BNC cable, RG-58A/u, or 3D-2V, etc.), the
propagation delay is about 5 ns per meter. Converting to phase, that is equivalent to 1.8° at 1 MHz.
•
For a 50 Ω class coaxial cable, there is about 100 pF of electrostatic capacitance per meter (between
the core wire and shield). When driven by signal source resistance of 50 Ω, the gain change is about
−0.0043 dB and the phase change is −1.8 deg at 1 MHz.
•
Take care to keep the connector contacts clean. Under some conditions, connectors that are not clean
may result in measurement error of about 0.03 dB.