A suppressed contribution from the 230 KHz switching regulators of the 1.2 V and 1.8 V power supply can be seen in the sampling clock spectrum of the SIS3302. Both external and internal clock operation are affected.
While no effects are expected in typical detector readout applications, phase sensitive applications -as they are typically part of accelerator controls- may be affected by the imperfection unless the below described modifications on the SIS3302 are implemented.
Following three modifications are needed for improved 1.2 V and 1.8 V power supply filtering on the SIS3302 card.
- Replace choke L600B Würth part number 744.560.25/LF (2,2μH, Struck part number: 00149) with Würth part number 744.77.020/LF (100μH, Struck part number: 04517, RS Components part number 617-1596). The replacement choke can be soldered onto the existing pads, the housing touches adjacent parts though.
- Replace choke L500B Würth part 744.560.25/LF (2,2μH, Struck part number: 00149) with Würth part 744.56.20/LF (100μH, Struck part number: 04512, RS Components part number 396-3099 )
- Change capacitor C500A (1206 footprint) from 150pF to 33pF (RS Components part number 669-8527 e.g.)
SIS3302 boards with serial number 266 and higher ship with the changes implemented already.
Below you can see the clock spectra of a SIS3302 sampling at 50 MHz prior and after the modification.
SIS3302 clock spectrum prior to modification
SIS3302 clock spectrum after modification
We would like to thank Lars Soby from CERN and his team members for bringing the power supply induced clock imperfection to our attention.