The signal to be measured is fed to one of the input connectors, which is usu
ally a coaxial
connector such as a
BNC
or
UHF type
.
Binding posts
or
banana plugs
may be used for lower
frequencies. If the signal source has its own coaxial connector, then a simple
coaxial cable
is used;
otherwise, a specialised cable called a "
scope probe
", supplied with the oscilloscope, is used. In
general
, for routine use, an open wire test lead for connecting to the point being observed is not
satisfactory, and a probe is generally necessary. General
-
purpose oscilloscopes usually present an
input impedance of 1
megohm
in parallel with a small but known capacitance such as 20 picofarads.
[2]
This allows the use of standard oscilloscope probes.
[3]
Scopes for use with very high frequencies may
have 50
-
ohm inputs, which must be either connected directly to a 50
-
ohm signal source or used with
Z
0
or active probes. Less
-
frequently
-
used inputs include one (or tw
o) for triggering the sweep,
horizontal deflection for X
-
Y mode displays, and trace brightening/darkening, sometimes called "Z
-
axis" inputs