Crossing High-Voltage Inerfaces with Large Bandwidth Signals

In connection with a 50-kV modulator development, three methods of crossing the high-voltage interfaces at a 1-MHz bandwidth are being developed. The first system uses a pulse transformer driven from ground by solid-state electronics. The transformer requires special design to obtain a very short (≈ 500 ns) risetime combined with the ability to maintain a 1000-μs, 200-V pulse with negligible droop. The second system is an amplitude-modulated rf carrier operating at 5 MHz. The interface is crossed with an untuned toroidal rf transformer. Increased bandwidth is obtained by placing the carrier at the lower edge of the passband and operating in the single sideband full carrier mode. The signal is demodulated with a balanced diode detector having a peak output of +200 V and good linearity. The third method uses a Hewlett-Packard HPA-4309 electro-optical isolator with solid-state amplifier on the high-voltage side. The intrinsic bandwidth of the isolator unit is ~10 MHz. With suitable amplifiers to bring the pulse up to +200-V peak, the bandwidth is ~2 MHz. Because the interface is crossed at a very low level reaching an adequate signal-to-noise ratio has presented special problems. All three systems should be useful on a wide variety of high-voltage laboratory machines; each has special advantages depending on the particular application.