Circuit Descriptions
3.3 Detailed Circuit Descriptions
3
3-21
1. Single shot triggering.
The DUALTRIG signal is supplied to the synchronization/delta-T circuit. The
trigger levels TRIGLEV1 and TRIGLEV2 are set just above and below the DC level
of the input signal. A trigger is generated when the signal crosses the trigger levels.
A trigger will occur on both a positive or a negative glitch. This mode ensures
triggering, when the polarity of an expected glitch is not known.
2. Qualified triggering .
The ALLTRIG signal is supplied to T-ASIC output pin 35, which is connected to the
D-ASIC input pin 21. The D-ASIC derives a qualified trigger signal TRIGQUAL
from ALLTRIG, e.g. on each 10th ALLTRIG pulse a TRIGQUAL pulse is given.
The TRIGQUAL is supplied to the synchronize/delta-T circuit via the select logic.
3. Normal triggering.
The ALLTRIG signal is supplied to the synchronization/delta-T circuit.
The ALLTRIG signal includes all triggers. It is used by the D-ASIC for signal analysis
during AUTOSET.
Traditionally a small trigger gap is applied for each the trigger level. In noisy signals,
this small-gap-triggering would lead to unstable displaying of the wave form, if the noise
is larger than the gap. The result is that the system will trigger randomly. This problem
is solved by increasing the trigger gap (TRIGLEV1 - TRIGLEV2) automatically to 80%
(10 to 90%) of the input signal peak-to-peak value. This 80% gap is used in AUTOSET.
Note
The ALLTRIG signal is also used for capacitance measurements (S. 3.3.2).
The Synchronize/Delta-t part provides an output pulse TRIGDT. The front edge of this
pulse is the real trigger moment. The pulse width is a measure for the time between the
trigger moment, and the moment of the first sample after the trigger. This pulse width
information is required in random repetitive sampling mode (see below). The
HOLDOFF signal, supplied by the D-ASIC, releases the trigger system. The sample
clock SMPCLK, also provided by the D-ASIC, is used for synchronization.
Real time sampling TRIGDT signal
For time base settings of 1 µs/d and slower, the pixel distance on the LCD is ≥40 ns (1
division is 25 pixels). As the maximum sample rate is 25 MHz, a sample is taken each
40 ns. So the first sample after a trigger can be assigned to the first pixel, and successive
samples to each next pixel. A trace can be built-up from a single period of the input
signal.
Random repetitive (equivalent) sampling TRIGDT signal
For time base settings below 1 µs/d, the time between two successive pixels on the
screen is smaller than the time between two successive samples. For example at 20 ns/d,
the time between two pixels is 20:25=0.8 ns, and the sample distance is 40 ns (sample
rate 25 MHz). A number of sweeps must be taken to reconstruct the original signal, see
Figure 3-11. As the samples are taken randomly with respect to the trigger moment, the
time dt must be known to position the samples on the correct LCD pixel. The TRIGDT
signal is a measure for the time between the trigger and the sample moment dt. The
pulse duration of the TRIGDT signal is approximately 4 µs...20 µs.
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