Tektronix 502A, Instruction Manual

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Operating Instructions — Type 502A
Fig. 2-4. Inputs to the vertical amplifiers. Waveforms applied to
input connector A are displayed in the upright position, while wave­
forms applied to input connector B are inverted.
Differential inputs must be
connected to both the A and B
input connectors of one amplifier. In the A and
B positions
of the input selector switch, the corresponding waveforms
are displayed on the crt. In the A-B (DIFF) position, the
B input is algebraically added to the A input and the
difference is
displayed. This feature permits you to elim­
inate common mode signals within specified limits.
Input Coupling
Input signals to the vertical amplifiers can be either
ac or dc coupled by placing the Input Selector switch in
the corresponding positions. DC coupling applies both the
ac and dc components of the input signal to the vertical
amplifier circuits. This permits you to measure the dc
voltage level as well as the amplitude of the ac component.
It is sometimes neither necessary nor desirable to display
the d c component. In such cases ac coupling should be used.
With ac coupling, a capacitor is placed in series with the
input connector to block the dc component while at the same
time allowing the ac component to be displayed.
Fig. 2-5. Rejection of a common mode signal by the differential
amplifier. The waveform applied to input connector B is algebra­
ically added to the waveform applied to input connector A and
the resultant waveform is displayed on the screen of the crt.
Input Connections
Here are some precautions you should observe in connect­
ing your oscilloscope to the signal source:
1. Avoid errors in readings due to stray electric or magne­
tic coupling between circuits, particularly in the leads
connected to the input connector. In general, unshielded
leads of appreciable length are unsuited for this use. When
shielded leads are used, the shields should be grounded to
the input connector shield and to the chassis of the equipment
being tested. Coaxial cables are recommended for many
purposes. Special care must be taken in the high sensitivity
ranges of the oscilloscope due to the low signal level and
high amplifier gain.
2. As nearly as possible, simulate actual operating condi­
tions 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 signal source due
to the input circuit of the oscilloscope. The circuit at the
input connectors can be represented by a resistance of 1
megohm shunted by a capacitance of 47 pf. With a few
feet
of shielded cable, the capacitance may well be 100 pf
or more. In many cases, the effects 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, at the same time reducing sensitivity. When making
amplitude measurements with an attenuator probe, be sure
to multiply the observed amplitude by the attenuation of the
probe.
A Type P6006 Probe is furnished as an accessory to the
Type 502A Oscilloscope. Connected to the Input connector
of the Type 502A, the probe presents an input characteristic
of 10 megohms shunted by approximately 9 picofarads and
has an attenuation ratio of 10:1. 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 contains fast changing
portions, it is generally necessary to clip the ground lead
of the probe to the chassis of the equipment being tested.
Select a ground point which is near the probe connection.
Probe Adjustment
An adjustable capacitor, which is built into the probe
body, is used to compensate for slight variations in input
capacitance from one instrument to another. To insure the
accuracy of pulse and transient measurements, this adjust­
ment should be checked frequently.
To make this adjustment, set the AMPLITUDE CALIBRATOR
and SENSITIVITY Controls on the Type 502A to display a
signal of suitable amplitude. Touch the probe tip to the
CAL OUT Connector and adjust the TIME/CM Control to
display several cycles of the waveform.
Loosen the locking sleeve several turns. (See figure 2-6.)
Now while holding onto the knurled part near the cable, turn
2-3