Tektronix 1S1 Instruction Manual page 27

Operating Instructions – Type 1S1
mum holdoff of the trigger circuit (for a given setting of the
TIME POSITION RANGE switch) occurs with the RECOVERY
TIME control set fully counterclockwise (but not switched to
SYNC position). If the control is turned clockwise, the hold-
off time of the circuit is increased and the maximum repeti-
tion rate is decreased. This control can change the repeti-
tion rate of the circuit by about 30% on any time position
range, permitting the rate to be adjusted so that the trigger
recognition circuit is armed between cycles of the input trig-
gering signal. If recovery occurs while the input signal level
is above the triggering level, an unstable display may result
(see Fig. 2-10).
Fig. 2-10. Operation of the RECOVERY TIME control,
illustrated with a 125-kc sine wave and 500AS time position
range. Arrows indicate triggering points. (A) Trigger recovery
occurs during wrong portion of cycle causing unstable
triggering. (B) RECOVERY TIME control is adjusted so that
recovery occurs between cycles and triggering takes place at
the same point on the waveform each time.
For waveforms such as pulses, that have low duty factors,
the RECOVERY TIME control is usually not needed unless
the frequency of the input signal is nearly equal to the
repetition rate of the holdoff circuit. For signals with medium
or high duty factors, such as sine waves and square waves,
use of the RECOVERY TIME control will usually improve the
triggering stability. In this case, adjust the TRIGGER SENSI-
TIVITY control to the desired level, then adjust the RE-
COVERY TIME control for a stable display.
Free Run. When the TRIGGER SENSITIVITY control is
turned fully clockwise, the trigger circuit will normally free
run at the maximum repetition rate of the holdoff circuit,
unless the trigger recognition circuit synchronizes with the
input triggering signal. This free run condition is normal
with any signal up to 80 kc and many signals up to about
10Mc. Above 10Mc, however, if the signal excursion oc-
curs just prior to the time that the circuit would be ready
to free run (following the holdoff period), the circuit may
2-18
be triggered in synchronization with the signal. Thus syn-
chronized triggering is possible with the TRIGGER SENSI-
TIVITY control set in its free run region (45° or more clock-
wise from the midrange point). Slight adjustment of the
RECOVERY TIME control or the TRIGGER SENSITIVITY
control will then usually provide stable triggering.
Normal Triggered Operation. On input triggering sig
nals up to the repetition rate of the trigger circuit (set by
the
TIME POSITION RANGE
control), one trigger occurs for each cycle of the input trig-
gering signal when the TRIGGER SENSITIVITY control is set
correctly. (This applies primarily to pulse triggering, since
sine-wave triggering rolls off below about 100 kc.)
When the frequency of the input triggering signal is above
the maximum repetition rate of the trigger circuit, trigger
recognition takes place on the first cycle of the waveform
following circuit recovery. Output pulses to the remainder of
the system then occur at or slightly below the maximum
repetition rate of the trigger circuit. In this triggering range,
the RECOVERY TIME control is effective in adjusting the trig-
ger circuit to recover between cycles of the input signal,
to provide stable triggering.
"SYNC" Triggering. When the RECOVERY TIME control
is switched to SYNC position, the trigger circuit operation
is changed so that it will synchronize with signals of approxi-
mately 20Mc and above, Basic operation of the trigger cir-
cuit is the same as for normal triggering, but with the RE-
COVERY TIME control set to a fixed position, only the TRIG-
GER SENSITIVITY control requires adjustment. The frequency
of the synchronizing circuit is adjusted by the TRIGGER
SENSITIVITY control to permit the circuit to synchronize with
the input signal during the remaining period. Thus, when
the circuit is ready to be triggered, the triggering occurs in
synchronization with the input signal.
Triggering Difficulties
The incorrect use of triggering controls and incorrect con-
nections to the signal source are the most common causes
of apparent triggering problems. Sometimes, however, the
difficulty may result from the misadjustment of a calibration
control or malfunction of a circuit component.
Always check switch and control settings and triggering
signal input connections if an apparent problem occurs.
Table 2-4 provides helpful information for alleviating certain
possible triggering difficulties.
TABLE 2-4
Possible Sources of Triggering Problems
Symptom Suggestions
Check that the DISPLAY MODE switch is at
No sweep
NORMAL position.
Check that the SAMPLES/CM control is not
at SWEEP OFF position.
Check that the horizontal deflection patch
cord is in place.
Check that the trace is not positioned off
the screen horizontally with the oscillo-
scope Horizontal Position control.
Check that the trace is not positioned off
switch and the RECOVERY TIME