An improved optoelectronic harmonic mixer (OEHM) has been employed for measuring the timing noise of $320\mathrm{GHz}$ optical pulses that are generated from a $160\mathrm{GHz}$ mode-locked laser diode by the temporal Talbot effect. The OEHM makes use of a low-drive voltage $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ modulator and a W-band unitraveling carrier photodiode for converting the $320\mathrm{GHz}$ pulse intensity into a low-frequency electrical signal. The time domain demodulation technique has been used for the precise evaluation of phase noise power spectral density. The rms timing jitter has been estimated to be $311\mathrm{fs}$ for the $10\mathrm{Hz}–18.6\mathrm{MHz}$ bandwidth.