Tektronix 516 series, Instruction Manual

Page: 14 / 132

O perating Information
O perating Instructions — Type 5 1 6
Input Coupling
.The Type 516 Oscilloscope is provided with two input con­
nectors to the Vertical Amplifier. Signals may be connected
to both of these input connectors at the same time and, by
means of the MODE switch, they may be displayed one at
a time or both together on the crt screen. When only one
signal is to be displayed, it may be connected to either input
connector.
Polarity Switches
Input signals to both channels may be either ac or dc
coupled and may be displayed either normally or inverted
on the crt screen by placing the corresponding POLARITY
switches to the appropriate positions. Dc coupling applies
both the ac and dc components of the input signal to the
amplifier circuits. This permits you to measure the dc volt­
age level as well as the amplitude of the ac component.
However, it is sometimes neither necessary nor desirable to
display the dc component, and in such cases ac coupling
should be used. Placing the POLARITY switch to either of
the AC positions inserts a capacitor in series with the input
connector. This capacitor blocks the dc component while al­
lowing the ac component to be displayed.
MODE Switch
When the MODE switch is in the A ONLY position or the
B ONLY position, only the signal which is applied to the
corresponding vertical channel will be displayed on the
crt. When the MODE switch is in the ALTERNATE position,
the oscilloscope will display the signals in each of the chan­
nels on alternate sweeps of the trace. When the MODE
switch is in the CHOPPED position, the oscilloscope will
display the signals alternately at about a 150-kHz rate; in
other words, the signal in the A Channel is displayed for
about 3'/3 microseconds and then the signal in the B Chan­
nel is displayed for about 3'/3 microseconds. At the faster
sweep rates, the CHOPPED mode of operation causes the
traces to take on a dotted appearance. At the slower sweep
rates, the ALTERNATE mode of operation causes the alter­
nate appearance of the traces to become quite noticeable
which makes it difficult to compare the two. Therefore, in
general, the ALTERNATE mode of operation is most useful
at the faster sweep rates and the CHOPPED mode of op­
eration is most useful at the slower sweep rates.
DC Balance Adjustment
Occasionally, there is need for adjustment of the dc bal­
ance of one or both input channels. This need is indicated
by a vertical shift in the position of a no-signal trace as the
VARIABLE VOLTS/DIV. control is rotated.
To make this adjustment, set the MODE switch to ALTER­
NATE. With no signal connected to the input connectors, set
the STABILITY control fully clockwise and position the two
free-running traces on the screen. Rotate each VARIABLE
VOLTS/DIV. control back and forth, and simultaneously ad­
just the corresponding DC BAL. adjustment until the trace
position is no longer affected by rotation of the VARIABLE
VOLTS/DIV. control.
Input Connections
Certain precautions must be observed in connecting the
oscilloscope to the signal source to prevent errors due to
stray electric or magnetic coupling in the leads. Shielded
cables shoula be used whenever possible, with the shield
connected to the chassis of both the oscilloscope and the
signal source. Regardless of the type of input lead used, it
should be kept as short as possible.
In broadband applications, it might be necessary to ter­
minate a coaxial input cable with a resistor or an attenu­
ating pad presenting a resistance equal to the characteris­
tic impedance of the cable. This is to prevent resonance
effects and "ringing” (high-frequency damped oscillation).
It becomes more necessary to terminate the cable properly
as the length of the cable is increased. The termination is
generally placed at the oscilloscope end of the cable, al­
though many sources require an additional termination at
the source end of the cable as well.
As nearly as possible, the actual operating conditions of
the equipment being tested must be maintained. For ex­
ample, the equipment should work into a load impedance
equal to that which it will see in actual use. The input con­
nectors of the Type 516 Oscilloscope present an input im­
pendence of 1 megohm in parallel with 20 picofarads. With
a few feet of shielded cable, the input capacitance may well
be as much as 100 picofarads. In cases where the effects
of these resistive and capacitive loads are significant in terms
of the equipment being tested, you should use an attenuator
probe as described in the next paragraph.
Use of Probes
Use of the attenuator probes furnished with the Type 516
Oscilloscope reduces the capacitive and resistive loading
effect on the equipment under test and, at the same time,
reduces sensitivity. Connected to the input connectors of the
Type 516 and properly compensated, these probes present
a characteristic input impedance of 10 megohms in parallel
with 8 picofarads and have an attenuation ratio of 10:1.
The maximum-voltage rating of the probes is 600 volts. Ex­
ceeding this rating, either in peak ac volts or dc volts, may
result in damage to the probes.
When making amplitude measurements with an attenuator
probe, be sure to multiply the observed amplitude by the
attenuation factor of the probe. If the waveform being dis­
played contains fast-changing portions, it is generally neces­
sary to clip the probe ground lead to the chassis of the
equipment being tested.
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