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cable shields the central signal conductor. Scope probes are also quite conven
ient from a mechanical standpoint.
To determine if a direct connection with shielded cable is permissible, you must
, know the source impedance of the circuit you are connecting to, the highest fre
quencies involved, and the capacitance of the cable. If any of these factors are
unknown, use a 10X low-capacitance probe.
An alternative connection method at high frequencies is terminated coaxial cable.
A feed-thru terminator having an inpedance equal to that of the signal-source im
pedance is connected to the osclloscope input connector. A coaxial cable of
matching impedance connects the signal source to the terminator. This technique
allows using cables of nearly and practical length without signal loss.
If a low-resistance ground connection between oscilloscope and circuit is not
established, enormous amounts of hum will appear in the displayed signal. Generally,
the
outer conductor of shielded cable provides the ground connection. If you are
using plain lead wire, be certain to first connect a ground wire between the Model 7040A
Ground connector (39) and the chassis or ground bus of the circuit under observation.
WARNING: The Model 7040A has an earth-grounded chassis (via
the 3-prong power cord). Be certain the device to which
you connect the scope is transformer operated. Do NOT con
nect this oscilloscope or any other test equipment to
..AC/DC". "hot chassis", or "transformerless" devices.
Similarly, do NOT connect this scope directly to the AC
power line or any circuitry connected directly to the
power line. Damage to the instrument and severe injury
to the operator may result from failure to heed this warning.
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