MODULATOR REVIEW

Radio frequency energy in radar is transmitted in short pulses with time durations that may vary from 1 to 50 microseconds or more. A special modulator is needed to produce this impulse of high voltage. The hydrogen thyratron modulator is the most common radar modulator.

Picture 1: Thyratron Modulator

Picture 2: Thyratron Modulator of the russian

As circuit for storing energy the thyratron modulator uses essentially a short section of 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 thyratron is to act as an electronic switch which 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 Kombination is acting as a DC- shield an protect 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 pulse transformer. The output pulse is then applied to an oscillating device, such as a magnetron.

The Charge Path

The charge path includes the primary of the pulse transformer, the dc power supply, and the charging impedance. The thyratron (as the modulator switching device) is an open circuit in the time between the trigger pulses. Therefore it is shown as an open switch in the picture.

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

UC = U0 ·( 1 – cos ωr· t)     ωr2 = 1     LDr · ΣC</nobr />

If the condensers 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. This one continues the charging of the condensers up to the double voltage of the power supply. Now the condensers would discharged (ice blue curve) about the power supplies resistance, but the charging diode cut off this current direction and the energy remains stored therefore in the condensers.

The Discharging Path

When a trigger pulse is applied to the grid of the thyratron, the tube ionizes causing the pulse-forming network to discharge through the thyratron and the primary of the pulse transformer.

Therefore, a current flows for the duration PW through the pulse transformer therefore. The high voltage pulse for the transmitting tube can be taken on the secondary coil of the pulse transformer. Exactly for this time an oscillating device swings on the transmit frequency. Because of the inductive properties of the pfn, the positive discharge voltage has a tendency to swing negative.

If the oscillator and pulse transformer circuit impedance is properly matched to the line impedance, the voltage pulse that appears across the transformer primary equals one-half the voltage to which the line was initially charged.