Tutorial on the Ortec 935 Quad Constant Fraction

Tutorial on the Ortec 935 Quad Constant Fraction Discriminator Kevin Carnes Fall, 2004

Threshold Monitor, -200 m. V to -10 V Threshold Adjust, -20 m. V to -1000 m. V

This is the input pulse used for simulating the operation of the Ortec 935.

This shows how the input signal is split, one half attenuated to 20% of its original height, and the other half delayed. The two are then combined to produce the signal that can be seen at the CF Monitor.

Three CF Monitor signals produced with different delays.

The equations below are from the 935 manual. They give the amount of delay produced from the external cable to achieve a specific internal delay T. Note that the value depends on whether or not the internal jumper W 1 is in or out. tdext = T + 1. 0 ns, W 1 out tdext = T – 0. 7 ns, W 1 in Constant Fraction Monitor

This is an actual scope trace of the CF Monitor output from the 935 with a reasonably long delay cable.

CF Monitor scope trace with much shorter delay cable.

Walk Adjust Test Point, reads actual voltage Walk Adjust, +/- 15 m. V

Zero crossing point of simulated CF Monitor, with long cable (lg) and short cable. Walk adjust is set to + 10 m. V and – 10 m. V.

Width Adjust, <4 ns to >400 ns, depending on internal jumpers, update or block

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

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

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