Agilent Technologies DC152 - Manual (Page 33)

3.4.8. 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 within the time interval corresponding
to [0, 2
35 – 1 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.
3.4.9. 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 by the dedicated MMCX. In addition, the PXI Bus 10 MHz system clock
signal (PXI_CLK10) can be used as the reference.
The External Clock must be continuously present for use by these digitizers. The input signal must have a frequency
between 200 MHz and 2 GHz and a minimum amplitude of at least 0.5 V peak to peak into 50
Ω at the front of the
digitizer. The transitions of the clock are defined with the aid of a threshold that is user selectable in the range [-3.0
V, 3.0 V]. 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 [9.97 MHz, 10.03 MHz] will be accepted. 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.
3.6. Internal Calibration
The software drivers supplied include calibration functions for the timing, gain and offset settings, which can be
executed upon user request. The digitizers are never calibrated in an “automatic” way, i.e. as a side effect of another
operation. This ensures programmers have full control of all calibrations performed through software in order to
maintain proper event synchronization within automated test applications.
The digitizers include a high precision voltage source and a 16-bit DAC, used to determine the input voltage and
offset calibration.
For accurate time and voltage measurements it is recommended to perform a calibration once the module has attained
a stable operating temperature (usually reached within 15 minutes of digitizer operation after power on). Further
calibration should not be necessary unless temperature variations occur.
User Manual: Family of 10-bit Digitizers Page 33 of 55