Triggering Sources - Tektronix 1S1 Instruction Manual

Fig. 2-8B shows the series method of coupling to a circuit.
Resistor R plus the 50-ohm input of the Type 1S1 replaces
S
the impedance of the circuit under test. If R
simply substitute the 50-ohm test cable with no additional
series resistance. It is best to locate Rs in the original position
of R and to ground the coax where R
L
A variation of the parallel method is the reverse-termi-
nated network shown in Fig. 2-8C. This system may be used
across any impedance up to about 200 ohms. At higher
source impedances, circuit loading would require more than
20% correction. The two 100-ohm resistors across the cable
input serve to reverse-terminate any small reflections due
to connectors, attenuators, etc. The series capacitor, which
is optional, blocks any dc component and protects the re-
sistors.
TRIGGERING THE DISPLAY
Selection of the triggering source and method of coupling
will depend primarily on the amplitude of the applied signal,
the frequency or repetition rate of the signal and the time
relationship of the signal to the desired crt display.
Each sample to be displayed on the crt screen requires a
separate trigger recognition by the trigger circuit. The sam-
pling rate is determined by the frequency of the input signal,
the settings of the triggering controls and the setting of the
TIME POSITION RANGE switch. For triggered operation on
signal frequencies below the maximum repetition rate of the
trigger circuit (e.g. 80 kc on the 500 µS and 50 µS time posi-
tion ranges), each cycle of the input triggering signal pro-
duces one trigger pulse and displays one sample. Above
the repetition rate of the trigger circuit, one trigger pulse
is produced and one sample is displayed for every two or
more cycles of the input triggering signal, due to the count-
down operation of the circuit.

Triggering Sources

The triggering signal may be obtained either internally
from the trigger takeoff in the vertical channel or externally
from the front-panel EXT TRIG connector, as selected by the
TRIGGER SOURCE switch. The methods of applying the
external triggering signal are discussed below under "Trig-
gering From an External Source".
Internal Triggering. When the TRIGGER SOURCE switch
is set to either INT+ or INT-, the portion of the vertical
signal taken off by the trigger takeoff transformer at the
vertical input is applied to the trigger circuit. This allows
the trigger to start the sampling operation while the input
signal is traveling through the delay line to the sampling
gate. Triggering internally from the vertical input signal is
the most convenient method under most circumstances, since
no external triggering connections are required. The time
delay provided by the internal delay line, the time position-
ing capability of the TIME POSITION controls and the ability
to trigger on either the positive-going or negative-going
slope of the signal permit internal triggering on most com-
mon input signals. With minimum sampling delay (TIME
POSITION controls clockwise), sampling of the input signal
starts approximately 4nsec (on the 500nS ramp) before the
triggering signal reaches the sampling gate. (Minimum delay
is greater on the slower time position ranges.)
is 50 ohms,
L
was grounded.
L
Operating Instructions – Type 1S1
External Triggering. When the TRIGGER SOURCE switch
is set to either EXT+ or EXT-, the signal applied to the
front-panel EXT TRIG connector is applied to the trigger
circuit. For the crt display to be meaningful, this triggering
signal must be time-related to the vertical input signal
applied to the SIGNAL IN connector. In addition to pro-
viding the advantage of triggering on smaller signals, ex-
ternal triggering also establishes a time relationship between
different input signals, provides time positioning outside the
time domain of the Type 1S1 TIME POSITION controls, and
avoids the readjustment of triggering controls when a series
of time-related input signals is to be observed.
Free Run. Both trigger sources (the trigger takeoff and
the EXT TRIG input) are disconnected from the trigger circuit
when the TRIG SOURCE switch is set to FREE RUN position.
In addition, the circuit operation is changed so that it will
free run regardless of the setting of the TRIGGER SENSI-
TIVITY control.
Free run operation is provided primarily to present a trace
on the crt for setting up the display. It is also used when
displaying a spot on the crt screen with the DISPLAY MODE
switch set to MAN or EXT HORIZ, and when using real-time
sampling at very low frequencies (see "Real-Time Sampling
Operation" later in this section).
Normally, the trace may also be free run by turning the
TRIGGER SENSITIVITY control fully clockwise. If an input
signal is connected, however, the trigger circuit may syn-
chronize with the signal and it may not be possible to free
run the trigger. In this case, if a free running display is
desired, the TRIGGER SOURCE switch should be set to
FREE RUN position.
Triggering Signal Requirements
Repetition Rate or Frequency. The input to the trigger
circuit is capacitively-coupled, with an RC time constant
that limits sine-wave triggering to frequencies above approxi-
mately 100kc. Pulse triggering, however, provides stable
triggering down to single pulses. With either sine waves or
pulses, high-frequency triggering is stable up to at least
1 Gc.
For triggering on low repetition-rate pulses, the rate of
pulse rise must be 7 mv/µsec or faster for internal trigger-
ing, or 1 mv/µsec for external triggering, This means, for
example, that for triggering internally on a 350-mv pulse,
the risetime must be 40 µsec or less. That is, the rate of
rise between the 10% and 90% levels (280 mv) must be
7 mv/µec or faster.
Pulse triggering below approximately 50 cps is not gen-
erally practical, since it takes at least 50 dots to make one
sweep of the crt, and the display dots may drift slightly at
these very low repetition rates.
Amplitude. The amplitude of the internally-derived trig-
gering signal is only about 1/7th the amplitude of the
vertical input signal. Therefore, a signal applied to the
SIGNAL IN connector must be proportionately greater in
amplitude than a triggering signal applied through the EXT
TRIG connector to provide the same triggering capability.
Internal triggering on pulses and other fast-rise wave-
forms requires an amplitude of 40 mv or more. For internal
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