High Voltage Modulator HVM for Photoconductive Antennas

Overview

The HVM is a very compact modulated high-voltage source built for driving photoconductive antenna terahertz emitters. It takes a 3.3 to 5 V logic-level square wave between 100 Hz and 1 MHz plus a 12 V DC supply, and delivers a 100 V peak square wave with rising and falling edges under 50 ns, in a 70 x 50 x 20 mm package.

Why bias modulation instead of an optical chopper

A photoconductive antenna only radiates terahertz when it is biased, so switching the bias switches the terahertz output. That gives the same modulation a mechanical chopper would provide for lock-in detection, but with no moving parts, no acoustic noise, no vibration coupled into the table, and modulation frequencies far above what a chopper wheel can reach. Higher modulation frequency moves the measurement further from 1/f laser and amplifier noise, which directly improves dynamic range.

Where the edge speed matters

Sub-50 ns switching edges mean the antenna bias is well defined for essentially the whole of each half cycle even at the top of the frequency range. Slow edges would leave the antenna in an ill-defined partially biased state for a significant fraction of the cycle, which shows up as modulation sidebands and as a reduced effective duty cycle in the lock-in signal.

Integration notes

Drive the trigger input from the same clock that the lock-in amplifier uses as its reference, so the demodulation is exactly phase locked to the emitter. Because the unit is small enough to sit next to the antenna, keep the high-voltage lead short: a long bias cable both slows the effective edge and radiates the switching transient into nearby detection electronics.