MODULATORS

                                                                                                                                                                    The function of the modulator is to turn the transmitting tube on and off to generate the desired wave from. When the transmitted wave from is a pulse, the modulator is some times called a pulser. Each RF power tube has its own peculiar characteristics which determine the particular type of modulator to be used.The magnetron modulator, for instance, must be designed to handle the full pulse power. On the other  hand, the full power of the klystron and the traveling-wave tube can be switched by a modulator handling only a small fraction of the crossed-field amplifier (C F A) is often cathode-pulsed, requiring a full-power modulator. Some CA F s are d-c operated, which means they can be turned on by the start of the RF pulse and turned off by a short, low-energy pulse applied to a cutoff electrode. Some C A F s can be turned  on and off by the start and stop of the RF pulse, thus requiring no modulator at all. Triode and trtrode grid-controlled tube may be modulator by modulated by applying a low-power pulse to the grid. plate modulation is also used when the radar application cannot tolerate the inter pulse noise that results from those few electrons that escape the cutoff of the grid                                                                                                                                                               

        Energy from an external source is accumulated  in the energy- storage element at slow ate during the inter-pulse period. The charging impedance limits the rate at which energy can be delivered to the storage element. At the proper time, the  switch is closed and stored energy is quickly discharged through the load, or RF tube, to  from the pulse.During the discharge part of the cycle, the charging impedance prevents energy from the storage element from being dissipated in the source.                        

        Line-type modulator  . A delay line, or pulse-forming network (P F N),is sometimes used as the storage element since it can produce  a rectangular pulse and can be operated by a gas- tube switch.This combination of  delay -line storage element and gas-tube switch is called a line-type modulator It has seen wide application in radar because of its simplicity, compact size, and its ability to tolerate abnormal load conditions such of magnetron sparking.                                                             

      The charging impedance is shown as an inductance. The pulse-forming network is usually a lumped-constant delay line.It  might consist of an air-core inductance with taps along its length to which are attached capacitance to ground.A perfect match is not always possible because of nonlinear impedance characteristic of microwave tubes.A gas tube such as a thyratron or ignitrn is capable of handling high power and presents alow impedance when conducting.However a gas tube cannot be turned off it has been turned on unless the plate current is reduced to a small value.The switch initiates the start of  the  modulator pulse by discharging the pulse-forming network,and the shape and duration  of the pulse are determined by the passive circuit elements  of the pulse-forming network. Since the trailing edge of the pulse depends on how the pulse- forming network discharges into the nonlinear load, the trailing edge usually not sharp and it may be difficult to achieve the desired pulse shape.                                                                                                                                         

 

   

 The charging inductance L ch and the capacitance C of the pulse-forming network forming  a resonant circuit, whose frequency of oscillation approaches .(The inductance of the pulse-forming network and the load are assumed small.) With a d-c energy source the pulse repetition frequency Fp will be twice the resonant frequency if the thyratron is switched at the peak of maximum voltage. This method of operation, ignoring the effect of the charging diode, is called d-c resonant charging. A disadvantage of d-c resonant charging is that the pulse repetition frequency is fixed once the value of the charging. inductance and the pulse-forming-network delay-line capacitance permits the modulator to operated at any pulse repetition frequency less than that determined by the resonant frequency F o.    

Hard-tube modulator :The hard-tube modulator is essentially a high-power video pulse amplifier.It derives its na,e from the fact that the switching is accomplished with "hard vacuum" tubes rather than gas tubes.Semiconductor devices such as the S C R (silicon controlled rectifiers) can also be used in this application.Therefore, the name active-switch modulator is sometimes used to reflect the that the function of a hard-tube modulator can be obtained without  vacuum tubes. Active-switch pulse modulators can be cathode pulsars that control the full power of the RF tube but with little current, or grid pulsars   that operate at a far smaller voltage than of the RF beam                                                        

Tube protection ; power tubes can develop internal flash  arcs with little warning even though they are of good design. When a flash arc occurs in an u n protection  tube, the capacitor bank discharges large currents through the arc and tube can damaged.One method for protecting the tube is to direct the arc-discharge currents with a device called an electronic crowbar. It places a virtual short circuit across the momentary short-circuit conditions.The name is derived from the analogous action of placing a heavy conductor, like a crowbar directly across the  capacitor bank . Hydrogen  thyrateons, ignitions and spark-gaps have been used as switch.The sudden surge of current due to a fault in a protected power tube is sensed and the crowbar switches.the line-type modulator does not usually require a crowbar  since it stores less energy than the hard- tube modulator and  it is designed to discharge safely all stored each time it is triggered.