Tektronix CSA8000B - Manual (Page 119)

Triggering
CSA8000B & TDS8000B User Manual 3- 45
Gated Trigger Connector (Option GT equipped). You can attach a BNC cable to the
External Gate input at rear panel (TTL connection). Two conditions must be
satisfied to get a stable display of waveform data:
H The channel and trigger must be otherwise triggerable without the trigger
gate.
H The gating signal must be at a TTL high; the triggering system enabled and
the instrument will acquire.
Note that the function of the trigger gate is to selectively exclude data from
acquisition by means of gating the trigger on and off, and it need not be
synchronized with either channel or trigger. See procedure on page 3--50 for
more information on setting up gated trigger.
Enhanced Triggering. These features (see note) can help stabilize triggering and
perform special measurements:
H High Frequency Triggering. When you enable the High Frequency
triggering control, the instrument increases trigger sensitivity of the trigger
circuit by decreasing hysteresis (a transition or noise band), allowing
triggering on higher frequency signals.
H Metastability Reject. When you enable Metastability Reject, the instrument
replaces the acquired sample with a null sample if it detects a potential
metastable condition. A metastable condition occurs when both the trigger
input signal and the holdoff-generated enable signal arrive at the internal
trigger recognizer at virtually the same time.
H Gated Triggering. When you enable the Gated trigger control, the trigger
and the External Gate input are applied to the instrument through what is in
effect an AND function. Gated triggering is applied to acquire and to mask
test or otherwise characterize signals like those found in extremely long
transmission carriers, such as undersea communication fibres. Since long
fibres are difficult to test, user-supplied test fixtures are used to repeat the
test signal through a short loop of the cable to simulate traveling longer
distances along its entire length. The signal is picked off and connected to
the instrument.
NOTE. Gated trigger is available only when ordered as Option GT at the time the
instrument ships.
Adjusting Holdoff. Trigger holdoff can help stabilize triggering. When you adjust
holdoff, the instrument changes the time it waits before rearming its trigger
circuit after acquiring a sample. Before rearming, trigger circuitry cannot