Operation; Signal Connections - Tektronix 3A74 Instruction Manual

Operating Instructions — Type 3A74
GENERAL OPERATION
Any of the four amplifier channels can be used independ-
ently by setting the appropriate MODE switch to the
NORM. or to the INV. position and connecting the signal
to be observed to the appropriate input. The following
discussion applies equally to each channel.
Signal Connections
The signal to be displayed is applied to the appropriate
input connector on the front panel of the Type 3A74. For
best results here are some precautions you should observe
when making the connections.
1. It is often possible to make signal connections to
the Type 3A74 with short-length, unshielded test leads*.
This is particularly true for high-level, low-frequency signals.
When such leads are used, you must also use a ground con-
nection between the Type 3A74 or oscilloscope chassis
ground and the chassis of the equipment under test. Posi-
tion the leads away from any stray electric or magnetic
field source to avoid obtaining erroneous displays.
2. In many low-level applications, however, unshielded
leads are unsatisfactory for making signal connections be-
cause of unavoidable pickup resulting from radiating
fields. To prevent unwanted signal pickup, use shielded
{coaxial} cables. Be sure that the ground conductors of the
*To connect test leads to the Type 3A74, use the single binding
post adapters (Tektronix Part No. 103-033} supplied with the
cables are connected to the chassis of both the oscilloscope
and the signal source.
3. As nearly as possible, simulate actual operating con-
ditions in the equipment under test. For example, the equip-
ment should work into a load impedance equal to that
which it will see in actual use.
4. Consider the effect of loading upon the equipment
under test due to the input circuit of the Type 3A74. The
input circuit can be represented by a resistance of 1 meg-
ohm (+1%} paralleled by a capacitance of 47 picofarads
(42%). With a few feet of shielded cable, the capacitance
is increased considerably. Where the effects of these re-
sistive and capacitive loads will affect the operation of the
device under test or distort the signal, use a probe in the
manner described next.
Use of Probes
An attenuator probe lessens both capacitive and resis-
tive circuit loading and at the same time reduces plug-in
sensitivity. The attenuation introduced by the probe extends
the vertical deflection factor of the Type 3A74 so that high-
amplitude signal voltages can be conveniently displayed on
the crt. When applying signals to the probe, consider the
input voltage rating of the probe. When making the ampli-
tude measurement, be sure to multiply the observed ampli-
tude by the probe attenuation.
unit.
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(a) Effect of compensation with a 60-cycie calibrator waveform.
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INCORRECT ADJUSTED CORRECTLY INCORRECT
{b) Effect of compensation with a 1-ke calibrator waveform.
Fig. 2-3. Probe compensation waveforms.
2-4
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