Tutorial on the Ortec 935 Quad Constant Fraction Discriminator

Kevin CarnesFall, 2004

Threshold Adjust, -20mV to -1000mV

Threshold Monitor,-200mV to -10V

This is the inputpulse used for simulating the operation of the Ortec 935.

This shows how theinput signal is split, one half attenuatedto 20% of its originalheight, and the otherhalf delayed. The two are then combined to producethe signal thatcan be seen at theCF Monitor.

Three CF Monitorsignals producedwith different delays.

tdext = T + 1.0 ns, W1 outtdext = T – 0.7 ns, W1 in

Constant FractionMonitor

The equations below arefrom the 935 manual. Theygive the amount of delay produced from the externalcable to achieve a specificinternal delay T. Note thatthe value depends on whether or not the internaljumper W1 is in or out.

This is an actual scopetrace of the CF Monitoroutput from the 935with a reasonably longdelay cable.

CF Monitor scope trace with much shorter delay cable.

Walk Adjust,+/- 15 mV

Walk Adjust TestPoint, reads actual voltage

Zero crossing pointof simulated CFMonitor, with longcable (lg) and shortcable. Walk adjustis set to + 10 mV and – 10 mV.

Width Adjust, <4ns to>400ns, depending oninternal jumpers, updateor block

Scope trace of 935 output into 50 ohms, other two outputs in this channelunterminated.

Scope trace of 935output into 50 ohms, second outputterminated in 50 ohms, third output unterminated.

Scope trace of 935output into 50 ohms,both of the other outputs terminated in50 ohms.