Compact photoconductive-based sampling system with electronic sampling delay

Summary form only given. As electronic signals move to higher frequencies and wider bandwidths, there is need for new methods of measuring these high-frequency/high-speed (tens of GHz) and/or high bit rate (tens of GBs/s) signals. Critical technical issues associated with the design of a rugged, compact, "real-time" sampling system using photoconductive switches as the test signal generator and sampler were investigated. The design concept is based upon an optoelectronic equivalent time sampling principle and optical-microwave signal mixing. It involves first phase locking of the periodic input signal to be measured to the periodic optical pulses from a mode-locked laser and subsequent sampling of the locked signal by the optical pulses. A photoconductive switch is used for the optical-microwave mixer and another photoconductor for the sampler. The optical pulses we use were provided by 100-fs pulses from a Ti:sapphire laser. The optical-microwave intermixing process generates a low-frequency replica of the high-frequency input signal. The ratio of the repetition rate of the input signal to its low-frequency replica is the time expansion factor. The repetition rate of the low-frequency signal provides the offset frequency for the equivalent time sampling. Because there is no electro-mechanical moving parts required to acquire a waveform, the sampling is done at a fast rate, and acquisition times of 10 ms or less are possible. The success of this technique depends critically on the stability and reliability of the optical microwave phase-locked loop (OMPLL), which locks the phase of the signal generator\´s trigger to the optical pulses.