Tektronix TYPE L, Instruction Manual

Page: 10 / 46

However, it is often neither necessary nor
desirable to display the dc component of the
input waveform. A capacitor which is in series
with the INPUT connector blocks the dc com ­
ponent when the AC-DC switch is in either
the AC or the X10 GAIN AC position. This
perm its viewing of the ac part of a large
input signal, as well as detailed viewing of
the ac part of any signal having adc component.
Connecting to the S ig n al Source
Stray electric and magnetic fields picked
up by unshielded connecting cables often change
the appearance of displayed waveforms. This
type of distortion occurs even in the audio­
frequency range, except possibly when making
measurements of low-impedance circuits. In
general, unshielded leads of appreciable length
are not suitable for connecting the input signal
to the plug-in INPUT. Shields of leads used
should be grounded to the chassis of the instru­
ment in use. Coaxial cables are recommended
fo r most purposes.
In broadband applications, it is usually neces­
sary to terminate a coaxial cable with a resistor
which presents a resistance equal to the charac­
teristic impedance of the cable. This prevents
the production of high-frequency damped o s c il­
lation. As the length of the cable is increased
it becomes more important to properly te r ­
minate the cable. The termination is placed
at the oscilloscope end of the cable, although
many sources require an additional termination
at the source end as well.
F o r large amplitude signals an attenuator
may be included in the connective system to
provide an oscilloscope display having a more
desirable amplitude than would be possible with
no attenuation. Cables, attenuators and te r ­
minating resistors are listed in the Accessories
section of any Tektronix oscilloscope Instruction
Manual.
When checking the instruments, actual operat­
ing conditions should be simulated as nearly
as possible.
F or example, the oscilloscope
should work into a load impedance sim ilar to
that which it encounters in actual use.
The input circuit of the plug-in may som e­
tim es produce loading of the signal source.
At the INPU T connector of the Type L , the
impedance of the input circuit is equivalent to
a resistance of 1 Megohm shunted with a
capacitance of 20 picofarads.
With a few
feet of shielded cable, the capacitance may
w ell be increased to 100 picofarads. Where
the effects of these resistive and capacitive
loads is significant, a probe may be used
to introduce the input signal.
Use of Probes
An attenuator probe reduces both capacitive
and resistive loading of the signal source
and decreases apparent sensitivity of the o s c il­
loscope.
Attenuation by the probe permits
observation and measurement of signal voltages
in excess of those that can be accepted by
the pream plifier alone.
The 10X probes (P6017, P6000, P510A, P410)
supplied with the oscilloscopes accepting letter-
s eries plug-ins may all be used with th eL Unit.
Input characteristics of these probes are listed
in the Characteristics section of this Manual.
Maximum voltage which may be applied to
the probes is 600 volts dc-plus-peak-ac.
Exceeding this lim it may result in damage
to the components inside the probe. The P510A
Probe can be used only from DC to 50MC,
due to the tendency for the probe to ring
at 50MC and above.
If the displayed waveform contains high-
frequency portions, it is generally necessary
to clip the probe ground lead to the chassis
o f the equipment being checked.
B efore using a probe, check its adjustment.
An adjustable capacitor in the body of the probe
is used to compensate for differences between
input capacitances of various instruments. To
insure the accuracy of pulse and waveform
measurements, the probe adjustment should
be checked when changing oscilloscopes, and
occasionally when used with any scope.
Probe Compensation
To adjust the compensation of the probe
to the instrument combination with which it
is to be used, the following procedure is
suggested:
1. Set the oscilloscope and plug-in controls as
listed above in F irs t-T im e Operation. Turn the
oscilloscope CALIBRATO R voltage selector to
2 VOLTS. Allow adequate time for the instru-
2-4
Operating Instructions - Type L Plug-In
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