Synchronization to Laser Clock

ScanImage® (resonant scanning only) allows to synchronize the sample clock of the NI FlexRIO digitizer module to be synchronized to an external clock (e.g. the laser sync clock). The frequency of the external sample rate has to be equal or smaller than the maximum specified sample rate of the digitizer module (5732: 80MHz, 5734: 120MHz). In doubt, verify the parameters of the clock with a high speed oscilloscope before connecting it to the digitizer.

Hardware Configuration

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Laser Clock Wiring

The Laser Clock is connected to the CLK IN SMB connector on the NI FlexRIO adapter module.

ScanImage® Configuration

If using the vDAQ, see the triggers tab of vDAQ Scan System Configuration in the resource configuration window.

Otherwise, if using NI DAQs, see the advanced tab of NI Resonant Scan System Configuration.

To configure ScanImage® for the use with an external clock the following settings need to be adjusted in the Machine Data File:

%% ResScan

externalSampleClock = true;

externalSampleClockRate = 80e6;

The parameter externalSampleClockRate is the nominal rate of the laser clock and is used within ScanImage® as a first approximation to determine the scan parameters. To ensure maximum accuracy, ScanImage® will validate the stability and exact rate of the laser clock on every start of an Acquisition. If ScanImage® detects an unstable clock, it will report an error and abort the acquisition.

Troubleshooting

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Laser Clock Filtering

A noisy laser clock can cause the NI digitizer module to become unstable. An external circuit (e.g. a Schmitt Trigger) is required to clean the clock for use with ScanImage.

Laser Clock Signal Quality

The external clock connected to the NI digitizer module must comply to the parameters outlined in the NI5732/5734 user manual, page 36, section ‘CLK IN’. If the fast photodiode output of the laser (laser clock output) does not fulfill these specifications, the signal needs to be filtered by an external circuit (e.g. a Schmitt trigger) before it can be connected to the digitizer input (See figure at right).

 Alternatively, an external fast photodiode can be used that fulfills the electrical specifications required by the NI digitizer module.

Attention

For high frequency signals, electrical termination is required. The NI5732/5734 Clock input connector is single-ended with a 50Ohm termination. Use cables with 50Ohms characteristic impedance and make sure the source of the clock signal is 50Ohm terminated as well. Correct impedance matching will eliminate reflections in the transmission line.




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Sampling Phase Delay

Various delays can lead to a phase offset between the sampling sweet spot and the actual sampling of the signal.

Sampling Phase Delay

Delays between the emission of the laser pulse and the registration of a sample clock by the digitizer lead to a phase shift between the laser pulse and the acquisition of the PMT signal. This phase shift depends on delays in the photodiode that generates the laser pulse clock, cable delays and ultimately on delays in the PMT and preamplifier.

While ScanImage® 2015 supports the synchronization to an external laser clock, it cannot adjust for this phase delay in software, so an external circuit is needed to delay the sample clock.

Tip

The laser pulse separation time of a laser with 80MHz repetition rate is 12.5ns. Because of this short time interval a crude way of adjusting the phase is to vary the cable length used for connecting the laser clock output to the FlexRIO digitizer module in a range of c*12.5ns = 3.7m

Alternatively, a delay switch box can be used.

Anti-Aliasing Filter

By default, ScanImage® enables a hardware Bessel filter to eliminate aliasing effects and to reduce high frequency interference (see NI 5732/5734 manual page 17,22). When syncing to a laser clock, it might be preferable to deactivate this filter in software. To change the filter settings of the FlexRIO digitizer module, choose from one of the two methods in the info panel. The property hSI.hResScan.channelsFilter accepts three possible values: ‘none’ ‘bessel’ ‘elliptic’.

Method 1

After starting ScanImage, enter the following command in the Matlab command window to deactivate the anti-aliasing filter:

>> hSI.hResScan.channelsFilter = ‘none’;

Method 2

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  • In the ScanImage® main window choose Settings > User Settings

  • Click the v symbol in the right bottom corner to expand the window

  • Locate the entry hSI.hResScan.channelsFilter and activate the checkbox to add it to the Current USR Properties

  • Locate the setting hSI.hResScan.channelsFilter in the list ‘Current USR Properties’ and change the value to ‘none’