  We construct a terahertz time-domain spectrometer (THz-TDS) system based on
photoconductive antenna (PCA). A 800 nm Ti sapphire femtosecond laser with 80
MHz repetition rate provides the pump and probe laser pulse, which has a 45 fs
pulse width (as short as 15 fs is available) and as much as 400 mW power. Two
commercial PCAs with 34 um and 6 um gap size are used as the emitter and
receiver, respectively. We characterize this system by measuring its absolute
radiated THz power, spectral bandwidth, signal-to-noise ratio (SNR), dynamic
range and beam profile at the focal plane. We further analyze the noise sources
of the THz-TDS system and specifically discuss the possibility to improve the
DR and SNR. In addition, using this system, we study the response of the PCA to
various parameters, such as laser power, biased voltage, beam spot's location
and laser's polarization.
