User manual

Programmer’s Guide Page 60 of 66
The apertureTime defines the minimum time for a frequency measurement (it may be longer if the
frequency is very low!). In the Totalize by Time mode, the value of apertureTime determines the time
window during which the input pulses are counted.
The frequency counter mode is set with the function AcqrsD1_configMode, with mode = 6.
After configuring the instrument parameters, the measurement sequence is started with the function
AcqrsD1_acquire. This function returns before the measurement is terminated. The user must wait until it is
terminated with the functions AcqrsD1_acqDone or AcqrsD1_waitForEndOfAcquisition. For the case of Totalize
by Gate the program must stop the acquisition by making a call to the function AcqrsD1_stopAcquisition.
FC results can be readout with the function AcqrsD1_readFCounter. The result is always a single double precision
number whose units are those appropriate for the type of measurement chosen.
3.19.2. ‘Start on Trigger’
The „Start on Trigger‟ mode begins data recording only upon receipt of a trigger signal, and stops after nbrSamples
data points are acquired. Not all digitizers are capable of this mode;
those that never have it are the DC110, DC240, DC265, DC270, and the 12-bit digitizers ;
others (DP105, DP106, DP110, DP111, DP210, DP211, and DP212) can have it as an option only.
It is useful in the special case where the sampling rate is less than the maximum possible and where an optimum time
correlation between the trigger and the sampling clock is required (typically when averaging waveforms). This mode
also requires that the trigger is available before the waveform of interest.
In the „Normal‟ mode, data recording begins at the time of arming, with the function AcqrsD1_acquire. The trigger
occurs asynchronously to the sampling clock, and thus will fall randomly anywhere within a sampling interval. When
averaging waveforms, this will result in an effective bandwidth reduction since the waveforms are randomly shifted
with respect to each other by up to ± ½ sampling interval.
In „Start on Trigger‟ mode, the trigger occurs before recording starts. It still occurs asynchronously with respect to
the internal reference clock (which is always running). However, if the requested sampling rate is less than the
internal reference clock frequency (e.g. 100 MS/s, while the clock runs at 500 MHz), then the time correlation
between the trigger and the effective sampling clock is within ± ½ internal reference clock time interval, not ± ½
sampling interval. Therefore when averaging, the bandwidth reduction will be less than in the „normal‟ mode. In
general, the internal reference clock runs at the upper frequency shown for the model-dependent “Input Frequency
range” shown in the table of section 3.17.2 External Clock (Continuous).
The value delayTime in the function AcqrsD1_configHorizontal is ignored. As usual, the digitizer requires some
memory overhead for additional samples. The function AcqrsD1_bestNominalSamples returns the maximum
number of available samples.
Use this code to use the „Start on Trigger‟ mode:
AcqrsD1_configXXX(.. ); // configure other parameters
AcqrsD1_configMode(instrID, 0, 0, 1);
AcqrsD1_acquire(instrID);
AcqrsD1_waitForEndOfAcquisition(instrID, timeout);
// Read out data etc. before calling again “AcqrsD1_acquire”
Note that the function AcqrsD1_acquire is still needed. However, it behaves somewhat differently in that is does not
start data recording but waits until a trigger signal is received.
Due to some circuit delays, the waveform recording starts approximately 20ns after the receipt of the trigger signal.
Furthermore the first data points may be invalid. For the DC271 family, this means that the first 8 ns worth of data
should be ignored for sampling rates 4 GS/s > SR > 500 MS/s and the first 4 ns, 16 points, for SR = 4 GS/s.
3.19.3. ‘Sequence Wrap’
The normal operation of the digitizer requires that it stop recording waveforms when the pre-defined number of
segments has been acquired. Thus, nbrSegments triggers are needed to acquire the requested number of segments
into the same number of different memory sections. After the acquisition has terminated, all of the waveform
segments are finally available for readout.
The „Sequence Wrap‟ mode also pre-defines the desired number of different memory sections, but it permits a larger
number of triggers. After the first nbrSegments waveform segments are acquired, the digitizer „wraps‟ around to the
first memory segment and keeps on recording waveforms. This sequence can go on indefinitely, since no hardware
condition will stop it. The only way to terminate this infinite loop is to stop it with the function