Comparison of technologies
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Since the writing of this article some years back, there have been significant developement in the speed of low and modest votlage FETs,
to the extent that we can now consider shorter rise times and higher voltages with these devices.
1 Avalanche Transistors
Pulse systems using avalanche technology offer the following typical specifications:-
Rise Time
Down to 100ps. Slower risetimes to several ns can be achieved.
Voltage
Raw unformed pulses can be made up to 6kV per module. Modules may be stacked in series and parallel to give higher voltages. 24kV into 50 ohms has been built. 50kV is within our ability.
Pulse Length
Into 50 ohms, 15ns is a typical maximum but 20ns can be achieved subject to other pulser parameters, notably voltage. Capacitive loads may be pulsed for times up to several µs.
Repetition Rates
Most pulse systems can achieve 1kHz with a sufficiently large power supply. Special units have been designed to go to 10kHz at lower voltages (~1kV). Some longer pulse length systems may be limited to several hundred Hz.
Fidelity
Avalanche pulse generators exhibit pulse waveforms that have significant perturbations to ideal waveforms. Typical problems are overshoot on the rising edge, droop or other fluctuations on longer pulses, late time ringing, falling edges that are slower than rising ones and sometimes over or undershoot. Passive networks can compensate for these effects at the expense of amplitude.
Pulse Shaping
Avalanche pulser shapes are either "raw" or "shaped". Raw pulsers have little or no pulse shaping, they achieve the maximum voltages and lowest risetimes. The shapes are typically a fast rise and slower fall with little or no flat region near the peak. Shaped pulsers typically have half the output voltage of the raw waveform unit on which they are based. Risetimes can still be close to 100ps. Pulses can be flat to around ±10%. Fall times are generally several hundred ps. and will be slower on longer pulse lengths. Pulse shaping into capacitive loads may be possible.
Load Impedances
Avalanche pulser systems can be made to drive a variety of loads from about 1 ohm to 100ohms resistive and also capacitive loads or capacitive loads at the ends of cables. The pulsers are protected against open and short circuits and can generally have other pulsers fired at them without harm, although this may well trigger them.
Variable Voltage Pulsers
Avalanche pulsers can have a variable output voltage which typically can bring the output down to 60% of its maximum. At lower voltages there may be some increase in trigger delay (~50ps) and worsening of perturbations.
Variable pulse length pulsers
Various techniques are available to make variable pulse lengths. The main limitations are that the risetime will not be better than around 150ps, and that for some techniques only a limited range of pulse lengths is available. It is often possible to configure a pulse source and load to achieve variable length systems without a variable length pulser.
2 Field Effect Transistors (FETs)
Pulse systems based upon field effect transistors offer the following typical specifications.
Rise Time
Down to a few ns.
Voltage
Pulses can be made up to 10kV per module. Modules may be stacked in series and parallel to give higher voltages.
Pulse Length
f.w.h.m. pulse lengths can go down to around 5ns although we have built pulsers with an output amplitude of 100V or so with sub 1ns pulse width. Zero to zero pulse widths which are useful for pulse picking in laser systems can go down to around 13ns. Maximum pulse lengths into 50 ohms are set by power considerations. Long pulses into capacitive loads can also be achieved.
Repetition Rates
Most pulse systems can achieve several kHz with a sufficiently large power supply. Special units have been designed to go to 100kHz. Low voltage units (less than 1kV) have been built to go at several MHz.
Fidelity
Pulses have reasonable fidelity for times long compared with the rise time.
Pulse Shaping
Square pulses are easily achievable.
Load Impedances
Pulser systems can be made to drive a variety of loads from about 1 ohm to 100 ohms resistive and also capacitive loads or capacitive loads at the ends of cables. The pulsers are protected against open and short circuits as well as arc conditions.
Variable Voltage Pulsers
FET pulsers can have a variable output voltage which typically can bring the output down to 20% of its maximum.
Variable pulse length pulsers
FETs can be turned off as well as on. Consequently variable pulse lengths can be made. For shorter pulses the pulser is generally shaped rather than turned off to get the falling edge. Such pulsers can have variable pulse lengths using techniques similar to avalanche pulsers. Other pulser/load configurations can be used to provide variable pulse lengths.