Tektronix 310A, Instruction Manual, Page: 18 / 132

Operating Instructions-Type 310A
ment under test may not produce a normal signal. The 10X
attenuator probes mentioned previously offer the least cir­
cuit loading. See the probe instruction manual fo r loading
characteristics o f the individual probes.
When the signal is coupled directly to the vertical INPUT
connector of the Type 31OA, the input impedance is about
one megohm paralleled by about 40 picofarads. When the
signal is coupled to the vertical INPUT connector through a
coaxial cable, the effective input capacitance is increased.
The actual input capacitance depends upon the type and
length of cable used and the frequency of the signal.
Ground Considerations
Reliable signal measurements cannot be made unless this
oscilloscope and the unit under test are connected together
by a common reference (ground) lead in addition to the
signal lead or probe. Although the three-wire AC power
cord provides a common connection when used w ith equip­
ment w ith similar power cords, the ground loop produced
may make measurements impossible. The ground straps
supplied w ith the probes provide an adequate ground. The
shield of a coaxial cable provides a common ground when
connected between two coaxial connectors (or with suit­
able adapters to provide a common ground). When using
unshielded signal leads, a common ground lead should be
connected from the Type 310A chassis to the chassis of the
equipment under test.
Input Coupling
The Input Coupling switch (AC DC) allows a choice o f
the input coupling method. The type o f display desired and
the applied signal w ill determine the coupling used.
The DC coupling position can be used fo r most applica­
tions. This position allows measurement of the DC compo­
nent of a signal. It must also be used to display signals
below about 20 hertz (two hertz w ith a 10X probe) as they
w ill be attenuated in the AC position.
In the AC coupling position, the DC component o f the
signal is blocked by a capacitor in the input circuit. The
low-frequency response in the AC position is about tw o
hertz (—3 dB point). Therefore, some low-frequency atten­
uation can be expected near this frequency lim it. Attenua­
tion in the form of t ilt w ill also appear in square waves
which have low-frequency components. The AC coupling
position provides the best display of signals w ith a DC com­
ponent that is much larger than the AC component.
Deflection Factor
The amount of vertical deflection produced by a signal is
determined by the signal amplitude, the attenuation factor
of the probe (if used), the setting of the VO LTS/DIV
switch and the setting of the Variable VO LTS/D IV control.
The calibrated deflection factors indicated by the VO LTS/
DIV switch apply only when the Variable VO LTS/D IV con­
trol is set to the C ALIBR ATED position.
The Variable VO LTS/D IV control provides variable (un­
calibrated) vertical deflection factors between the cali­
brated settings of the VO LTS /D IV switch. The Variable
V O LTS/D IV control extends the maximum vertical deflec­
tio n factor o f the Type 31 OA to at least 125 volts/division
in the 50 position.
Trigger Source
I NT. For most applications, the sweep can be triggered
internally. In th e IN T position of the TRIGGER switch, the
trigger signal is obtained from the vertical deflection sys­
tem. The sweep is triggered from the same waveform that is
displayed on the CRT.
EXT. An external signal connected to the EXT TRIG
INPUT binding post can be used to trigger the sweep in the
EXT position o f the TRIGGER switch. The external signal
must be time-related to the displayed signal to produce a
stable display. An external trigger signal can be used to
provide a trigger display when the internal signal is too low
in amplitude fo r correct triggering, or contains signal com­
ponents on which it is not desired to trigger. It is also useful
when signal tracing in amplifiers, phase-shift networks,
wave-shaping circuits, etc. The signal from a single point in
the circuit under test can be connected to the EXT TRIG
INPUT binding post. Then the sweep is triggered by the
same signal at all times. This allows amplitude, time rela­
tionship or wave shape changes o f signals at various points
in the circuit to be examined w ith o u t resetting the trigger
controls.
LINE. The LINE position of the TRIGGER switch con­
nects a sample o f the power- line frequency to the trigger
circuit. Line triggering is useful when the input signal is
time-related to the power-line frequency. It is also useful
fo r providing a stable display o f a line-frequency compo­
nent in a complex waveform.
Trigger Coupling
AC. In the AC position o f the Trigger Coupling-Mode
switch, the DC component of the trigger signal is blocked.
Signals w ith low-frequency components below about 30
hertz are attenuated. In general, AC coupling can be used
fo r most applications. However, if the sweep is to be trig ­
gered at a low repetition rate or a DC level, use the DC
position o f the Trigger Coupling-Mode switch.
The triggering point in the AC position depends upon
the average voltage level o f the trigger signal. If the trigger
signals occur in a random fashion, the average voltage level
w ill vary, causing the triggering point to vary also. This shift
of the triggering point may be enough to make it impossible
to maintain a stable display. In such cases, use DC trigger
coupling.
DC. DC trigger coupling can be used to provide stable
triggering w ith low-frequency signals wnich should be atten­
uated in the AC position, or w ith low-repetition rate sig­
nals. It can also be used to trigger the sweep when thp
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