Fluke ScopeMeter 123 Service Manual page 48

123
Service Manual
3. Normal triggering.
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.
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/div 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. So 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/div, the time between two successive pixels on the
screen is smaller than the time between two successive samples. For example at 20
ns/div, 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.
3-22
from ALLTRIG, e.g. on each 10th ALLTRIG pulse a TRIGQUAL pulse is given.
The TRIGQUAL is supplied this to the synchronize/delta-T circuit via the select
logic.
The ALLTRIG signal is supplied to the synchronization/delta-T circuit.
The ALLTRIG signal is also used for frequency/pulse width -, and
capacitance measurements. Section 3.3.2.
TRIGGER
SAMPLES SWEEP 1
SAMPLES SWEEP 2
PIXEL
Figure 3-11. Random Repetitive Sampling Mode
Note
3
dt1
4
dt2
1 2 3 4 5 6 7 8
13
14
9 10 11 12 13 14 15 16