6-37
Since the probe resistor is in series, the oscilloscope input resistance is 10 megohms when the probe
is used. Thus, using the attenuator probe with the oscilloscope causes less circuit loading than using a 1-
to-1 probe.
Before using an attenuator probe for measurement of high-frequency signals or for fast-rising
waveforms, you must adjust the probe compensating capacitor (C1) according to instructions in the
applicable technical manual. Some probes will have an IMPEDANCE EQUALIZER in the end of the
cable that attaches to the oscilloscope. The impedance equalizer, when adjusted according to
manufacturer’s instructions, assures proper impedance matching between the probe and oscilloscope. An
improperly adjusted impedance equalizer will result in erroneous measurements, especially when you are
measuring high frequencies or fast-rising signals.
More information on oscilloscope hook-ups can be found in Electronics Information Maintenance
Books (EIMB), Test Methods and Practices.
Special current probes have been designed to use the electromagnetic fields produced by a current as
it travels through a conductor. This type of probe is clamped around a conductor without disconnecting it
from the circuit. The current probe is electrically insulated from the conductor, but the magnetic fields
about the conductor induce a potential in the current probe that is proportional to the current through the
conductor. Thus, the vertical deflection of the oscilloscope display will be directly proportional to the
current through the conductor.
SPECTRUM ANALYZER
The spectrum analyzer is used to examine the frequency spectrum of radar transmissions, local
oscillators, test sets, and any other equipment operating within its testable frequency range. With
experience, you will be able to determine definite areas of malfunctioning components within equipment.
Successful spectrum analysis depends on the proper operation of a spectrum analyzer and your ability to
correctly interpret the displayed frequencies. Although there are many types of spectrum analyzers, we
will use the Tektronix, Model 492 for our discussion.
The spectrum analyzer accepts an electrical input signal and displays the frequency and amplitude of
the signal on a CRT. On the vertical, or Y, axis, the amplitude is plotted. The frequency would then be
found on the horizontal, or X, axis. The overall pattern of this display (figure 6-42) indicates the
proportion of power present at the various frequencies within the spectrum (fundamental frequency with
sideband frequencies).
