  Time over dynamic threshold (TODT) method, proposed in our previous work, has
been successfully used for nuclear pulse charge measurement in PET detectors.
It has advantages of strict linearity, large dynamic range, and better energy
resolution, but requires a relatively complex circuit to generate specific
dynamic threshold. In this paper, we propose to replace the dynamic threshold
by a simpler linearly increasing threshold (called as TOLT method) with the aim
to simplify the threshold generation circuit meanwhile maintaining its high
energy resolution. Mathematical analysis on this replacement and the related
realization circuit are presented. By energy spectrum measurement of PET
detectors, the method is evaluated. The energy resolutions of PET detectors,
composed of a PMT coupled with LYSO and LaBr3 crystal, are measured as 12.54%
and 5.18% respectively, which is equivalent to the result obtained by TODT
method. The test results show that the TOLT method is more practicable for
charge measurement of nuclear detectors.
