Month: November 2017

Finding the optimal sample clock phase

Some time ago we talked about the advantages of synchronizing the digitizer sample clock to the laser.  Our new system incorporates an on-board PLL multi-phase clock that allows Scanbox to automatically measure the contrast in a test image as a function of sample phase delay and select the optimal value for your setup.

A typical measurement showing the change in normalized contrast of a target image as a function of phase delay (here the range from -8 to 8 covers the entire 12.5 ns period of the laser).

pll_1

Scanbox automatically shows the resulting images, on the same scale, for each phase delay value as well:

pll2

Scanbox will also plot the normalized images obtained for the settings that yield the lowest and and highest contrasts:

pll_best_worse

One can clearly see from the images that phase does not simply scale the contrasts of the images, but has an obvious effect on their SNR.  (Why this occurs exactly is still a matter of debate here, but data rule and the results are clear.)

This option will be automatically available to those who adopt the new tower system.  What can you do if you have one of the older systems?  You can simply change the sample clock phase by extending the length of the cable running from the laser SYNC OUT to the external clock of the digitizer.  Extensions of 50cm in length can be connected together to yield phase steps of 1/8th of the laser period.  So if you want to optimize the contrast and SNR of your images take a day off to find your optimal delay and improve the quality of your data.

Using the correct range and bias for PMT amplifiers

You may have one of two different PMT amplifiers in your setup.  One which has a variable gain.

DHPCA_100

The typical settings of this amplifier should be: left the switch on GND, middle switch on L or H but with the knob set to a gain of 10^4, next switch on FBW and DC for the rightmost switch.

If you have this amplifier, measure the output with an oscilloscope and making sure the channel is in high impedance mode (>1Mohm).  Adjust the output with the offset screw on the left (do not confuse with the bias screw!) so that the reading is around 380mV.  Then, edit the scanbox_config file and set pmt_amp_type to ‘variable’.

Alternatively, you may have a fixed gain amplifier that is a bit smaller and looks like this.

hca-400m-5k_w.jpg

Here the gain is fixed (your version will read 5 x 10^4 V/A and 100MHz bandwidth).  Follow the same instructions above to set the bias to 1.9V by means of the offset screw.  Then, make sure the pmt_amp_type is set to ‘fixed’ in the configuration file.

That’s all.  These settings will allow you to maximize the dynamic range of your digitizer card and the contrast of the images.

Note: the pmt_amp_type variable is only present in the latest release of the software.  So you will need to update to use this feature.