The saturable reactor is an iron-core inductance so designed that its magnetic core is driven into saturation for normal values of coil current. The advantage of the saturable reactor in radar modulator application is its relatively long life. It is a passive device and uses neither electronic tubes nor mechanical moving parts.
Figure 8.36 Two-stage saturable-reactor modulator, or pulsactor
· From Fig.8.36 the circuit may be considered as a series of saturable reactors arranged in resonant circuits in which the networks are charged stage by stage.
· Two stages are illustrated.
· Assume that the energy source feeding the modulator is a sine wave whose frequency is the same as the desired pulse repetition frequency.
· The charging inductance LCh and the capacitance C1 form a resonant circuit.
· Initially, the two inductances other than the charging inductance are biased to operate in the unsaturated condition and have high reactance.
· When the voltage across the capacitor C1 reaches a maximum, the inductance L1 saturates and its reactance is lowered to a considerably smaller value L1.
· The switching action of L1 allows the charge on C1 to transfer to C2.
· This is indicated by the current iC2 in Fig. 8.37.
Figure 8.37 Build-up of the pulse in the two-stage saturable-reactor modulator
The combination of Lls , C1; and C2 forms a resonant circuit with a higher resonant frequency than that of LCh and C1, since C1 and C2 are in a series and Lls < LCh-.
· The current iC2 rises rapidly.
· As the current builds up in the resonant circuit, the voltage across the capacitor, being 90° out of phase with the current, decreases and the inductance L1 returns to its unsaturated state.
· The voltage across C2 reaches a maximum at the same time that L2 saturates.
· The energy stored in C2 is then transferred to the load via the current iC3, which is the output pulse waveform.
· In essence, the chain of saturable reactors acts to compress the energy-transfer interval, increasing the peak power and decreasing the pulse duration from stage to stage.
· The output waveform is more like that of a resonant sine wave than a rectangular pulse.
· This shape is not always well suited to radar application and is one of the limitations of this modulator.
· A more rectangular pulse may be had by tapping the last-stage, saturable-reactor winding and adding capacitance to ground so as to simulate a lumped-constant delay line.
· The capacitors from the taps to ground have negligible effect on the unsaturated operation, but at and during the switching operation, they produce an improved output pulse waveform.
· The output pulse width may be changed to some extent by adjusting the polarizing current.
· The nonrectangular pulse shape is not a disadvantage in all cases.