Fluke 123B/S Service Manual - Page 50

Real time sampling TRIGDT signal, Random repetitive equivalent sampling TRIGDT signal

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123/124 Service Manual TRIGQUAL 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. 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 frequency/pulse width -, and capacitance measurements. Section 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_B, 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 TRIGGER SAMPLES SWEEP 1 SAMPLES SWEEP 2 dt1 3 dt2 4 13 14 PIXEL 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Figure 3-11. Random Repetitive Sampling Mode

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Service Manual
3-22
TRIGQUAL 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.
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 frequency/pulse width -, and
capacitance measurements. Section 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_B, also provided by the D-ASIC, is used for synchronization.
Real time sampling TRIGDT signal
For time base settings of 1
µ
s/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
µ
s/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
µ
s...20
µ
s.
dt1
dt2
TRIGGER
SAMPLES SWEEP 1
SAMPLES SWEEP 2
3
4
2
1
3
4
6
5
9
10
7
8
11
14
15
12
13
16
PIXEL
13
14
Figure 3-11. Random Repetitive Sampling Mode