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We evaluated timing jitter based on a time-to-digital converter (TDC) having sub-pico seconds time resolution. The measured jitter at liquid helium temperature was 65 fs for an optimally damped simple junction composing a conventional Josephson transmission line fabricated with the NEC standard process. The temperature dependence and shunt resistance dependence of jitter indicate that the jitter originates from thermal noise generated at the shunt resistor of the junction. We also found that the timing jitter of a balanced comparator is much larger than that of a JTL. The jitter of a balanced comparator is independent of temperature. This means that the jitter of a balanced comparator does not arise from thermal noise.