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Some models are also capable of a ‘Start on Trigger’ mode of acquisition (see the Programmer’s Guide for further details). The various trigger settings are outlined below. 3.4.1. Trigger Source The trigger source can be a signal applied to either an Input Channel (internal triggering) or the External Trigger Input. For the DC135/DC140/DC211/DC211A/DC241/DC241A/DC271/DC271A/DC271AR modules, a standardized trigger in signal can also be routed via the PXI Bus Star Trigger line. Most digitizers provide a separate front panel input BNC connector that can be used as an External Trigger Input. The External Input provides a fully functional trigger circuit with selectable level and slope as for the Internal Triggering source; however it does not include coupling choices nor HF, Window, and Spike Stretcher triggers. The external trigger termination (1 M. or 50 .) is also selectable on many modules. In modules with this feature, the circuit also provides an overload protection that will automatically switch the coupling from 50 . to 1 M. if the signal is greater than ±5 V DC. The DC271-FAMILY digitizers have a fixed 50 . termination impedance. They also allow the same BW limiter selections as can be found for the channels. The DC271-Family digitizers' external trigger circuit has diode protection against overload. The DP1400 also gives the choice of 50 . or 1 M. termination impedance for the MCX external input and has diode protection against overload. However, there are no BW limiters and only DC trigger coupling is available. In all 50 . cases a ±5 V limit on trigger signals should be respected, although somewhat higher voltages for short time periods will not damage the unit. For 1 M. input signals up to ±100 V (DC + peak AC < 10 KHz) are allowed. 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 (Vm= – Offset voltage) of the digitizer’s vertical scale. Internal trigger level settings (expressed in %) must be within Vm ± 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 ...

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