Input Coupling; Internal Triggering - Tektronix 3A6 Instruction Manual

When the signal is coupled directly to the input of the
Type 3A6, the input impedance is 1 megohm {21%}
paralleled by about 47 pf. When the signal is coupled to
the input of the Type 3A6 through a coaxial cable, the
input capacitance is greatly increased. Just a few feet of
coaxial cable can increase the input capacitance to well
over 100 pf.
Input Coupling
The Channel 1 and 2 AC DC GND switches allow a
choice of input coupling. To display both the ac and de
components of an applied signal set the switch to DC. In
the AC position, only the ac component of the input signal
is allowed to pass. The GND position connects the input
grid to ground but does not ground the input signal.
The DC position can be used for most applications. How-
ever, if the de component of the signal is large enough to
drive the display off the ert viewing area or if the de com-
ponent is such larger than the ac component, use the AC
position.
In the AC position of the switch, the de component of
the signal is blocked by a capacitor in the input circuit.
The low-frequency response in the AC position is about 2
cps, 3db down. Therefore, some low-frequency distortion
can be expected near this frequency limit. Distortion will
also appear in square waves which have low-frequency
components,
The GND position provides a ground reference at the
input of the Type 3Aé6. The signal applied to the input
connector is internally disconnected but not grounded.
The grid of the input tube is at ground potential, elimi-
nating the need to externally ground the input to establish
a zero dc reference.
Internal Triggering
Single-Trace Displays. In the CH 2 and ADDED positions
of the MODE switch, push the TRIGGER switch in. When
the time-base trigger coupling switch is set to de, the setting
of the POSITION controls will affect the setting of the trigger
level control. Proper triggering can be obtained in the
CH 1 position of the MODE switch in either TRIGGER switch
position, However, in the CH 1 ONLY position (TRIGGER
switch pulled out) the POSITION controls have no affect
on dc triggering.
Dual-Trace Displays. To obtain proper triggering in the
CHOP mode, puil out the TRIGGER switch. Then, the inter-
nal trigger signal coupled to the time base does not contain
the dual-trace chopping signal. It is difficult to get a stable
display if the TRIGGER switch is pushed in because of the
150 ke dual-trace chopping signal.
In the ALTER mode the TRIGGER switch can be in either
position, depending upon the application. When the TRIG-
GER switch is pushed in, the time base will be triggered from
the display of each channel. This provides stable display of
two unrelated signals but does not indicate the time relation-
ship between the signals. To display signals showing true time
relationship, pull the TRIGGER switch out to trigger from
Channel 1 only. If the signals on the two channels are not
harmonically related, the Channel 2 display will not be stable.
In the ALTER mode with the TRIGGER switch pushed in, do
not set the time-base trigger coupling switch to de.
®
Operating Instructions—Type 3A6
Dual-Trace Operation
Chopped Mode. The chopped display is produced by
electronic switching between channels. To blank out the
switching transients, set the CRT Cathode Selector switch on
the rear of the indicator unit to the chopped blanking posi-
tion.
In general, use the CHOP position (chopped-mode opera-
tion) with sweep rates slower than 0.1 millisecond/division.
At higher sweep rates the chopped switching becomes
apparent.
Two signals which are harmonically related can be dis-
played in the chopped mode showing true time relationship.
However, if the signals are not harmonically related, the
Channel 2 display will appear unstable.
Single-shot, transient or random signals which occur within
the time interval determined by the time-base time/division
switch (10 times sweep rate) can be compared using the
chopped mode. To trigger the sweep, the Channel 1 signal
must precede the Channel 2 signal. Since the signals show
true time relationship, time difference measurements can be
made.
Alternate Mode. The ALTER position (alternate-mode opera-
tion} can be used at any sweep rate. However, for time
comparison measurements requiring sweep rates below about
20 milliseconds/division, use the chopped mode for best re-
sults.
Algebraic Addition
In many applications the desired signal is superimposed
on an undesired signal such as line frequency hum, line noise,
etc. Algebraic addition in the ADDED position of the MODE
switch can be used in many cases to improve the ratio of
desired to undesired signal. To do this, set up the instrument
as follows:
1. Connect the source which contains both the desired and
undesired signals to Channel 1 input.
2. Connect a source which contains only the undesired
signal to Channel 2 input.
3. Set the MODE switch to CHOP or ALTER, whichever
mode provides the best display at the sweep rate used.
4, The undesired portions of the signal on Channel 1 and
2 must be of opposite polarity. If they are of the same
polarity, set the INV (CH 1} NORM switch to INV.
5. Adjust the Channel 1 VOLTS/DIV switch so the desired
portion of the display is large enough for proper viewing.
6. Adjust the Channel 2 VOLTS/DIV switch for about the
same amplitude as the undesired signal on Channel 1. Use
the Channel 2 VARIABLE to match the deflection more closely.
7. Set the MODE switch to ADDED.
8. Readjust the Channel 2 VARIABLE or VOLTS/DIV switch
to cancel out the maximum amount of undesired signal.
9. If the Channel 1 VARIABLE control is left in the CALIB
position (UNCAL light off}, the amplitude of the desired sig-
nal can be determined directly from the crt display and the
Channel 1 VOLTS/DIV setting.
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