Tektronix 502 series, Instruction Manual

Page: 14 / 114

INPUT CONNECTIONS
Here are some precautions you should observe
in connecting your oscilloscope to the signal
source.
1. Avoid errors in readings due to stray electric
or magnetic coupling between circuits, partic­
ularly in the leads connected to the input con­
nector. In general, unshielded leads of appreci­
able length are unsuited for this use. When
shielded leads are used, the shields should be
grounded to the oscilloscope chassis and to the
chassis of the equipment being tested. Coaxial
cables are recommended for many purposes.
Special care must be taken in the high sensi­
tivity ranges of the oscilloscope due to the low
signal level and high amplifier gain.
2 .
As nearly as possible, simulate actual operat­
ing conditions in the equipment being tested.
The equipment should have a load on it which is
approximately equal to the load encountered in
normal operation.
3. Consider the effect of loading upon the sig­
nal source due to the input circuit of the oscillo­
scope. The circuit at the input connectors can be
represented by a resistance of 1 megohm
shunted by a capacitance of 47 jujaf. With a
few feet of shielded cable, the capacitance may
well be TOO
jujuf or more. In many cases, the ef­
fects of these resistive and capacitive loads are
not negligible, and to minimize them, you might
want to use a probe in the manner described in
the next section.
USE OF PROBES
An attenuator probe lessens both capacitive
and resistive loading while at the same time re­
ducing sensitivity. When making amplitude
measurements with an attenuator probe, be sure
to multiply the observed amplitude by the attenu­
ation factor of the probe (marked on the probe).
Type P510A Probes are furnished as acces­
sories to the Typo 502 oscilloscope. These
probes are easily identified by their black,
molded-plastic nose. Connected to the input
connectors of the oscilloscope, these probes pre­
sent input characteristics of 10 megohms shunted
by 14
jul
/
jl
f and have an attenuation factor of 10.
The maximum voltage which may be applied to
the probe is 600 volts. Exceeding this rating,
either in peak ac volts or dc volts, may result in
damage to the components inside the probe
body.
If the waveform being displayed has rapidly
rising or falling voltages, it is generally neces­
sary to clip the probe ground lead to the chassis
of the equipment being tested. Select a ground
point near the point of measurement.
Before using a probe, always check that if is
correctly compensated. An adjustable capacitor
in the probe body compensates for variations in
input capacitance from one instrument to an­
other. To insure the accuracy of pulse and tran­
sient measurements, this adjustment should be
checked frequently.
To properly compensate the probe, set the
SQUARE-WAVE CALIBRATOR control for a suit­
able amplitude. Touch the probe tip to the CAL.
OUT connector and adjust the oscilloscope con­
trols to display several cycles of the waveform.
Adjust the probe capacitor for the best possible
square wave.
Fig. 3-3. Compensating the probe.
When the variable capacitor in the probe body has been adjusted properly,
the Calibrator waveform will be undistorted.
3-2
OPERATING INSTRUCTIONS — TYPE 502
®