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User Manual: Family of 8-bit Digitizers Page 35 of 66
3.4.2. Trigger Coupling
Trigger coupling is used to select the coupling mode applied to the input of the trigger circuitry. Modes available
include AC LF Reject and DC. The AC LF Reject mode couples signals capacitively and removes the input signal's
DC component and signals below 50 KHz (50 Hz for DC271-FAMILY digitizers). DC mode allows all signal
components to be passed through to the trigger circuit. The DC271-FAMILY digitizers have an HF Reject mode that
removes signal components above 50 KHz.
3.4.3. Trigger Level
The trigger level specifies the voltage at which the selected trigger source will produce a valid trigger. The trigger
level is defined as a set voltage. Using the internal trigger, the level is set with respect to the midpoint voltage (V
m
= –
Offset voltage)
of the digitizer’s vertical scale. Internal trigger level settings (expressed in %) must be within V
m
±
0.6 FS (0.5 FS for DC271-FAMILY digitizers), where FS is the channel Full Scale. All trigger circuits have
sensitivity levels that must be exceeded in order for reliable triggering to occur.
For most digitizers, the AC coupled mode is implemented with an auto-level trigger. Only the DC271-FAMILY
digitizers allow trigger levels to be selected in connection with the AC coupling choices.
The DC271-FAMILY digitizers allow the user to choose the external trigger Full Scale from the set of values 0.5,
1.0, 2.0 or 5.0 V. The external trigger level can then be set to values in the range ± 0.5 FS. The DP1400 has a single
external Trigger Full Scale of 10 V. All other digitizers have an external trigger range of ± 3 V.
The DC271-FAMILY digitizers will trigger on signals with a peak-peak amplitude > 15% FS from DC to their
bandwidth limit.
3.4.4. Edge Trigger Slope
The trigger slope defines which one of the two possible transitions will be used to initiate the trigger when it passes
through the specified trigger level. Positive slope indicates that the signal is transitioning from a lower voltage to a
higher voltage. Negative slope indicates the signal is transitioning from a higher voltage to a lower voltage.
3.4.5. Window Trigger
The DC271-FAMILY digitizers and the 2 channels of the DP1400 implement a Window trigger. Two trigger level
thresholds are used to define the desired range. The trigger can then be chosen to occur either when the signal exits
or enters the window range. This mode can be thought of as the appropriate OR of two edge triggers of opposite
slope.
3.4.6. HF Trigger
The DC271-FAMILY digitizers and the 2 channels of the DP1400 implement an HF trigger that allows triggers to be
reliably accepted at rates above ∼ 1 GHz. In this mode, triggers occur on every fourth positive edge. The window
trigger mode is not available.
3.4.7. Spike Stretcher
The trigger circuit of the 2 channels of the DP1400 also has a Spike Stretcher mode which ensures that even very
short pulses are capable of generating triggers. This mode is useful if the time interval during which the trigger signal
satisfies the threshold condition is less than 0.5 ns and the trigger frequency is less than 10 MHz. The trigger slope is
positive in this mode.
3.4.8. DP1400 Multi-source Trigger
This digitizer permits triggers that require a pattern condition including one of the trigger channels and the external
trigger. The trigger condition defined above, on each of the inputs, defines the TRUE/FALSE state of each input.
These states can be logically combined with AND, OR, NAND, or NOR to define the overall trigger condition.
Potential triggers can then occur on the FALSE to TRUE transitions of the combined signal.
There is a small (~ns) delay between the times at which two simultaneous inputs arrive at the logical element that
defines the overall trigger condition. If necessary, this must be corrected for by cable delay on the external input; the
delay will depend on the overall configuration and therefore must be determined by the user.