For a long time, bulk solid state lasers such as Ti:sapphire have been as the workhorse for terahertz time domain spectrometers (THz-TDS). Recently, externally amplified, mode locked fiber laser sources have been developed as an alternative to the solid state lasers as the laser driver of terahertz spectrometers. Advantages of fiber lasers include compact size, portability, low cost and generally superior robustness than their solid state counterparts. Fiber lasers have the potential to exhibit extremely low timing jitter (phase noise) if the net cavity dispersion is zero. We have constructed an Ytterbium (Yb) doped fiber oscillator; the repetition rate of which can be tuned, to drive our THz-TDS. The laser operates at a repetition rate of 51 MHz, centered at a wavelength of 1030 nm, and producing pulses that can be dechirped to 81 fs (Fig. 1). The net dispersion is 1420 fs2. Here, we report on the impact of timing jitter on the operation, including stability and signal-to-noise ratio of time domain THz spectrometer. We utilize a rapid-scan configuration [1, 2], where a pulse from the laser generates the THz signal and subsequently, the nth pulse from the second arm of the same laser is used for the detection (Fig. 2). Timing jitter of the laser is analyzed using a signal source analyzer in the RF domain.