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In time correlated photon counting, the picosecond resolution offered by the reduced transit time spread of ultrafast microchannel plate (MCP) photomultipliers is fully exploited only if all other timing jitter contributions are minimized. With the ultrashort signals of MCP detectors, the commercially available circuit modules do not provide true constant fraction triggering (CFT) operation. The residual amplitude dependent time walk therefore sets the actual limit to the obtained time resolution. A new CFT scheme has been devised for obtaining true constant fraction triggering. A circuit has been accordingly developed and tested. The time walk is reduced to less than 8 ps for a 1:14 dynamic of the input signal (pulse amplitudes from 100 mV to 1.4 V) and to less than 25 ps for a 1:20 dynamic (from 70 mV to 1.4 V). It is shown that such a performance makes it possible to exploit in actual experiments the 20 ps intrinsic resolution of the fastest available MCP detectors. © 1997 American Institute of Physics.