Radar Modulator - Radartutorial

Radar Modulator

The radar modulator generates a high voltage for the transmitter tube for the duration of the transmitting pulse. This radar modulator practically only switches on the anode voltage for the transmitter tube for the time of the transmitting pulse. Caused by this switching function, it is sometimes called a “keyed on/off” radar modulator.

Such a modulator is often used to drive self-oscillating high power generators like magnetrons. However, high-power amplifiers equipped with cross-field amplifiers (amplitrons) also require such a radar modulator since they may only receive high voltage for the duration of the transmit pulse.

This modulator uses a pulse forming network to store energy. This pulse forming network is charged to twice the voltage of the high-voltage power supply on the charging path with the aid of the magnetic field of the charging choke. This charging choke simultaneously limits the charging current. A charging diode is inserted to prevent the pulse forming network from discharging via the internal resistance of the power supply after charging.

The hydrogen thyratron operates as an electronic switch and is controlled by a short trigger. The R-C combination separates the thyratron input from the preamplifier’s bias voltage. The pulse transformer is used to adjust the impedances during the discharging.

As a circuit for storing energy, the thyratron modulator uses essentially a short section of an artificial transmission line which is known as the pulse forming network (PFN). Via the charging path, this PFN is charged on the double voltage of the high voltage power supply with help of the magnetic field of the charging impedance. Simultaneously this charging impedance limits the charging current. The charging diode prevents that the PFN discharge himself about the intrinsic resistance of the power supply again.

The function of the thyratron is to act as an electronic switch that requires a positive trigger of only 150 volts. The thyratron requires a sharp leading edge for a trigger pulse and depends on a sudden drop in anode voltage (controlled by the pulse forming network) to terminate the pulse and cut off the tube. The R-C Combination acts as a DC- shield and protects the grid of the thyratron. This trigger pulse initiates the ionization of the complete thyratron by the charging voltage. This ionization allows conduction from the charged pulse forming network through the pulse transformer. The output pulse is then applied to a self-oscillating stage, such as a magnetron.

The Charge Path

As an initial condition is assumed that the circuit is not energized. In the figure 3, the thyratron is shown as an open switch.

Once the power supply is switched on (look at the brown voltage jump in the diagram), the current flows through the charging diode and the charging coil and charges the capacitors of the pulse forming network (PFN). The coils of the PFN are not yet functional (having too small inductivity). However, the induction of the charging impedance offers high inductive resistance to the current and builds up a strong magnetic field. The charging of the capacitors follows an exponential function (line drawing green). The self-induction of the charging coil overlaps for this.

If the capacitors are charged with the power supplies voltage, decreases the current, and the magnetic field breaks down. The breaking down magnetic field causes an additional induction of a voltage. The inducted voltage continues the charging of the capacitors up to double the voltage of the power supply. Now the capacitors would be discharged (blue curve) about the power supplies resistance, but the charging diode cut off this current direction, and the energy remains stored in the capacitors.

The Discharging Path

When a positive trigger pulse arrives at the grid of the thyratron, the tube ionizes causes the pulse-forming network to discharge through the thyratron and the primary of the pulse transformer. (The thyratron is “fired”).

The fired thyratron grounds the pulse line at the charging coil and the charging diode effectively. Therefore, a current flows for the duration τ through the pulse transformer’s primary coil to the ground and from the ground through the thyratron, which is now conducting to the other side of the pulse forming network. The high voltage pulse for the transmitting tube can be taken on the secondary coil of the pulse transformer. For this time, a self-oscillating device swings on the transmit frequency. Because of the inductive properties of the PFN, the positive discharge voltage tends to swing negatively.

If the oscillator and pulse transformer circuit impedance is properly matched to the line impedance, then the voltage pulse that appears across the transformer’s primary coil is equal the one-half of the voltage to which the line was initially charged.