  A characterisation of the Timepix4 pixel front-end with a strong focus on
timing performance is presented. Externally generated test pulses were used to
probe the per-pixel time-to-digital converter (TDC) and measure the time-bin
sizes by precisely controlling the test-pulse arrival time in steps of 10 ps.
The results indicate that the TDC can achieve a time resolution of 60 ps,
provided that a calibration is performed to compensate for frequency variation
in the voltage controlled oscillators of the pixel TDCs. The internal clock
distribution system of Timepix4 was used to control the arrival time of
internally generated analog test pulses in steps of about 20 ps. The analog
test pulse mechanism injects a controlled amount of charge directly into the
analog front-end (AFE) of the pixel, and was used to measure the time
resolution as a function of signal charge, independently of the TDC. It was
shown that for the default configuration, the AFE time resolution in the
hole-collecting mode is limited to 105 ps. However, this can be improved up to
about 60 ps by increasing the preamplifier bias-current at the cost of
increased power dissipation. For the electron-collecting mode, an AFE time
resolution of 47 ps was measured for a bare Timepix4 device at a signal charge
of 21 ke. It was observed that additional input capacitance from a bonded
sensor reduces this figure to 62 ps.
