3.4.9. Pre- and Post-Trigger Delay
To increase trigger flexibility a pre- or post-trigger delay can be applied to the trigger position.
The amount of pre-trigger delay can be adjusted between 0 and 100% of the acquisition time window (i.e. sampling interval x number of samples), whereas the post-trigger delay can be adjusted between 0 and 200 million samples.
Pre- or post-trigger delays are just different aspects of the same trigger positioning parameter:
•The condition of 100% pre-trigger indicates that all data points are acquired prior to the trigger, i.e. the trigger point is at the end of the acquired waveform.
•The condition of 0% pre-trigger (which is identical to a post-trigger of 0) indicates that all data points are acquired immediately after the trigger, i.e. the trigger point is at the beginning of the acquired waveform.
•The condition of a non-zero post-trigger delay indicates that the data points are acquired after the trigger occurs, at a time that corresponds to the post-trigger delay, i.e. the trigger point is before the acquired waveform.
The digitizer hardware accepts pre- and post-trigger adjustments in increments of 16 samples. By definition post- trigger settings are a positive number and pre-trigger settings are a negative number.
Thus it is only natural that the software drivers treat pre- and post-trigger delays as a single parameter in seconds that can vary between –nbrSamples * samplingInterval (100% pre-trigger) and +maxPostTrigSamples * samplingInterval (max post-trigger). Since the Acqiris software drivers provide very accurate trigger position information upon waveform readout, the accepted resolution of the user-requested pre-/post-trigger delay is much better than 16 samples. For more details, refer to the Programmer’s Guide.
3.4.10. Trigger Status
The front panel includes a tri-color LED indicator to show the status of the trigger. When the LED is green it indicates the trigger is armed and waiting for a valid trigger to occur. Red indicates that the trigger has occurred, the acquisition is complete and the data is waiting to be readout. The user can override the default functions and program the LED color in an application-specific manner.
3.5.External Clock and Reference
For applications where the user wants to replace the internal clock of the digitizer and drive the ADC with an external source, either an External Clock or an External Reference signal can be used. The Clock or Reference signals can be entered into the digitizer either by the dedicated MMCX (DC271-FAMILY), MCX (DP1400), or BNC (DC240) connector or via the shared External Input connector on the front panel (all other models). In addition, for the DC135/DC140/DC211/DC211A/DC241/DC241A/ DC271/DC271A/DC271AR models the PXI Bus 10 MHz system clock signal (PXI_CLK10) can be used as the reference.
With External Clock two operating modes are possible; Continuous for the case in which the clock signals are always present and Start/Stop for the situation where the user needs complete control of the sampling process. The Start/Stop mode is not available for the DP1400. When using a Continuous External Clock, the user must ensure that the input signal has a frequency between 10 MHz and 2 GHz (DP1400 and DC271-FAMILY) or between 10 MHz and 500 MHz (all other models). For the Start/Stop mode the input signal frequency must be less than 1 GHz (DC271- Family) or 500 MHz (all other models). In all cases it must have a minimum peak to peak amplitude into 50 Ω at the front of the digitizer of at least
0.5 V for DC135/DC140/DC211A/DC241A/DC271A/DC271AR/DP1400,
1 V all other DC271-Family and
2 V for other models.
The DP1400 has a fixed threshold and uses AC coupling. Otherwise, the transitions of the clock are defined with the aid of a threshold that is user selectable in the range [-2.0 V, 2.0 V] ([-3.0 V, 3.0 V] for the DC240).. The signals should not exceed ±5 V amplitude. For a detailed discussion on the programmed use of the external clock, refer to the Programmer’s Guide.
For applications that require greater timing precision and long-term stability than is obtainable from the internal clock, a 10 MHz Reference signal can be used. The External Reference is nominally at 10 MHz. However, frequencies in the range [9.0 MHz, 10.2 MHz] will be accepted for all models other than the DP1400. If you do this you may need to correct for the difference in your application since the digitizer and the driver have no way to know about such deviations. The amplitude and threshold conditions, for an External Reference, are the same as for the External Clock. If synchronization between several digitizers is required, the reference signal should be applied to all of them.
User Manual: Family of 8-bit Digitizers | Page 36 of 66 |