In 1988, TNO developed a so-called “Travelling Wave Tube (TWT) simulator”. The simulator made it possible to extensively test high-voltage power supplies for microwave tube amplifiers both under operating conditions and under extreme conditions such as overload. As a result, the risk that the expensive and vulnerable tube amplifier is destroyed by faulty high-voltage control is reduced to a minimum.
To generate a higher microwave output, tubular amplifiers (Traveling Wave Tubes, microwave ovens and klystrons) were used almost without exception in 1988. The main characteristics of such tube amplifiers are:
- a high cost (5,000 – 5 million guilders for a TWT),
- a high required supply voltage (several to many kV),
- a large vulnerability to incorrect settings because large capacities are processed by a physically small structure.
The power supply equipment usually consists of:
- high voltage power supply(s),
- a modulator,
- auxiliary power supplies, and
- measurement, control and protection circuits.
Both during the development and the maintenance of such power supplies the use of a tube simulator is a great advantage. After all, before connecting the expensive tube to the power supply, one will want to thoroughly test the power supply even under conditions that normally will not occur. The latter is easily possible with a tube simulator, unlike with a tube amplifier. This approach is also quite attractive from a financial point of view.
The simulator system
For those reasons, TNO developed the universal tube simulator which replaces a TWT. Because of the modular design, the simulator could easily be made suitable for other types of microwave amplifiers. Conventional TWT simulators at that time (end of the 1980s) used a high-voltage triode. The auxiliary equipment for the triode setting, however, resulted in a large and bulky system. The TNO simulator was developed entirely in semi-conductor technology which was more convenient and reliable due to the absence of auxiliary equipment.
The main problem with using semiconductors is the fact that the circuit must be able to operate with a voltage of many kilovolts. TNO used a technique in which an arbitrary number of FETs is put in series. This stacking of FETs behaves like a single FET with a very high permissible drain voltage. The whole is unconditionally stable and exhibits excellent pulsing behaviour. The stack is therefore extremely suitable as a building block for a tube simulator. Almost every type of tube amplifier can be simulated with such a stack. Complex tube amplifiers can be simulated by using more stacks. The behaviour of the stack(s) is adapted to that of the tube amplifier using a simple network. The specific components of the network are therefore placed on an interchangeable printed circuit board (PCB). The simulator is internally protected so that the power supply can not destroy the simulator under any circumstance. The simulator can operate far beyond the specifications of the TWT, for example, to test the safety systems of the power supply.
As an example of the simulator’s performance, a TWT tube amplifier was taken with the helix and collector connected. The Vg-1g and Vg-1a characteristics of the simulator are exactly the same as those of the TWT to be simulated.
Maximum values of the TWT to be simulated:
- Voltage 15 kV
- Current 3 A
- Pulse duration 6 µs
- Frequency 500 Hz.
The simulator has the following maximum values:
- Voltage 25 kV / 30 kV non-operating
- Current 20 A limited to 6 A
- Pulse duration 100 µs, up to 6A over 10 msec
- Frequency: any value for which the average power is less than 2 kW.