Tektronix 1S1 Instruction Manual page 23

Operating Instructions – Type 1S1
sine-wave triggering, an amplitude of 50 mv or more (peak
to peak) is required. Since the maximum amplitude to be
applied to the SIGNAL IN connector is ±2 volts, combined
dc and peak ac, this is also the upper limit obtainable for
internal triggering. The TRIGGER SENSITIVITY control has
adequate range for triggering on any signal between these
amplitude limits.
External triggering on fast-rise waveforms requires an
amplitude of 7mv or more at the EXT TRIG connector. Ex-
ternal sine-wave triggering requires an amplitude of 8mv
or more, peak to peak. Maximum amplitude of the input
triggering signal applied to the EXT TRIG input connector
should not be over ±200mv. The input circuitry will not be
damaged by voltages up to ±2 volts, but above 200mv
the TRIGGER SENSITIVITY control may not be able to con-
trol the triggering operation. External 50-ohm attenuators
may be used to reduce the amplitude to the required level.
(A convenient amplitude for the signal is about 150mv.)
Triggering From an External Source
To be able to view the signal that triggers the Type 1S1,
triggering must take place approximately 40nsec before the
input signal reaches the sampling gate. For internal trigger-
ing, this time difference is provided by the internal delay line
in the vertical signal path between the trigger takeoff and
the sampling gate.
Since external triggering signals are applied to the trig-
ger circuit in the same manner as internal triggering signals,
the external triggering signal will have to reach the EXT
TRIG connector at approximately the some time as the verti-
cal signal reaches the SIGNAL IN connector. Otherwise,
the desired signal may occur outside the "time window" of
the crt screen.The time positioning controls of the Type
1S1 provide a relatively wide range of time positioning of
the display that is adequate for most purposes. However,
to view some portion of the signal that is not within the
range of the time positioning controls, it is necessary to use
external triggering to advance or delay the triggering signal
with respect to the vertical input signal.
Coupling
The EXT TRIG input is connected to the trigger circuit
(ac-coupled) when the TRIGGER SOURCE switch is set to
either EXT+ or EXT-. When the switch is set to either of
the INT positions, the EXT TRIG input is terminated in 50
ohms. The impedance of the cable or other device used
to apply the external triggering signal should be approxi-
mately 50 ohms. An impedance above 50 ohms will atten-
uate the signal and result in poor triggering.
Even though the input to the trigger circuit is capaci-
tively coupled, the triggering signal should not have a large
dc component. If the signal has a dc component greater
than ±2 volts at the signal source, connect it to the EXT
TRIG connector through a coupling capacitor. Be sure to
take this capacitance into consideration when determining
the low-frequency cutoff of the input coupling.
Sources
External triggering signals may be obtained from a test
device, from a pulse generator that is pulsing a test device,,
2-14
or from the same signal that is applied to the vertical input.
Several types of connecting devices are available for con-
necting the triggering signal to the Type 1S1. These include
attenuator probes, voltage or current pickoff devices, T-con-
nectors and coaxial cables. Some of these are suggested
in Table 2-2. Be sure the amplitude of the triggering sig-
nal does not exceed the limits given above under "Trigger-
ing Signal Requirements".
Probes. For many applications, a trigger probe can be
used to trigger the Type 1S1 if a signal that is time-related
to the input signal can be taken from the device under
test. The probe is connected to the EXT TRIG connector
through an adapter or through a coupling capacitor and an
adapter. The Tektronix P6034 Probe provides 10X attenua-
tion with an input resistance of 500 ohms and input capaci-
tance of about 0.8 pf. The Tektronix P6035 Probe provides
100X attenuation with 5 k ohms input resistance and about
0.7pf capacitance. For amplitudes above about 16 volts,
use a coupling capacitor with the probe.
Current Takeoff. When a portion of the input signal
must be taken off for triggering, some means of dividing
the signal without introducing severe distortion must be
found. One suitable takeoff device is the Tektronix CT-3
Current Takeoff. This is a feed-through device that is in-
serted into the 50-ohm signal path and takes off a small
portion of the signal. The output voltage to the trigger
circuit, through a BNC connector, is about 1/10th of the
amplitude of the feed-through signal.
Voltage Pickoff. The Tektronix VP-1 Voltage Pickoff may
be used in conjunction with a P6034 Probe or P6035 Probe to
take a portion of the signal from the 50-ohm system. The
VP-1 is inserted into the 50-ohm signal path and allows the
probe to take the signal from the line.
T-Connectors. An impedance-matched T-connector (e.g.
General Radio Type 874-TPD) can also be used, dividing the
input signal into two equal signals for viewing and trigger-
ing. If an unmatched coaxial T-connector is used to obtain
the triggering signal, the impedance transition may cause
reflections in the system. Most of these reflections can usual-
ly be absorbed by placing attenuators in series with the
signal.
Coax Cables. If a 50-ohm signal source that is time-
related to the vertical input signal is available, it may be
used as an external triggering signal. By selecting the length
of the triggering signal cables with respect to the input
signal cables, the triggering point may be adjusted as de-
sired to shift the "time window" of the crt display. The
delay provided by coax cables with polyethylene dielectric
is about 1 nsec for each 8 inches of cable. If cables are
used, however, keep them as short as possible.
If the triggering signal occurs slightly early with respect
to the portion of the vertical signal to be viewed, the dis-
play can often be returned to the screen by setting the TIME
POSITION RANGE switch to a higher numerical value
and using the MAGNIFIER knob to return the TIME/CM
switch to the desired sweep rate.
If the desired portion of the signal cannot be located on
the screen by use of the TIME POSITION RANGE switch,
triggering is occuring too early. In this case, the triggering
signal should be delayed with coax cables. If any portion