RF Circutry

Timing Generator

For testing purposes, this unit in the control console can generate the four pulses needed to control the transmitter: the receiver protection command, the beam on command, the rf on command and the rf phase command. But normally these commands are generated by a programmable pulse generator that is part of the computer data-taking interface. The program-generated pulses enter the timing generator as "Request pulses." Unless they violate restrictions put on duty cycle, pulse length, etc., the requests are passed on, unaltered, as commands to the transmitter circuitry. Otherwise the timing generator modifies them to make them acceptable. When this occurs, lights on the panel of the timing generator indicate what restrictions are being violated.

Exciter

The excite circuitry is in the control console. Is output stage is a 3-watt amplifier. The output from this amplifier, 3 watts peak, is pulsed on and off synchronously with the high-power modulator. The exciter can apply bi-phase modulation to the drive signal. No amplitude modulator is included in the exciter., although the subsequent Intermediate Power Amplifier (IPA) and Power Amplifier (PA) stages are essentially linear amplifiers when they are not driven to saturation.

IPA - Intermediate Power Aplifier

The IPA uses a single Eimac 3KM3000LA external cavity klystron and is capable of producing a peak power of 20 kW or an average power of 1 kW. It drives the two klystrons in the Power Amplifier (PA). The required drive power is only about 790 watts, so the IPA is rated very conservatively and has been one of the most reliable parts of the transmitter. This amplifier operates as a fixed gain block; its supply voltages remain constant while the transmitter is in use. It has no modulator., Its output is pulsed only because the exciter gates the input signal on and off. A second-harmonic trap (shorted stub) is located at the output of the IPA. The IPA signal reaches the PA by way of a 1.5" coaxial transmission line.

PA - Power Amplifier

Two klystrons operate in parallel as a balanced amplifier with a coax power divider at the input and a waveguide 90° hybrid combiner at the output. The input power divider is actually a ring hybrid and the divided outputs have the same phase. However, one of these outputs is delayed additional 90° by a remotely adjustable motor-driven transmission line "trombone." The isolated port on the output combiner is terminated with a "wasted load." Nominally there should be no power dissipated in the waster if the trombone section is adjusted correctly but, if the two klystrons do not have identical output power, some wasted power is inevitable. If the phase is wrong by 190°, all power will be diverted to the waster instead to the antenna. Directional couplers allow the operator to monitor the total output power, the waster power, and the individual drive and output powers.

Ring Hybrid

The klystron beam current is modulated, i.e. turned on only for the duration of each RF drive pulse. During the pulse, 35 % of the beam power (beam voltage × beam current) is converted to useful RF output power and 65 % is converted to wasted heat. The output power during the pulse is 2.5 MW and the input power is 2.5/0.35 = 7.14 MW. The average powers are 150 kW and 429 kW, respectively. When there is no RF drive, 100 % of the beam power is converted to heat. If the beam were not turned off between pulses, the input power to the transmitter would be 7.14 MW/0.06 = 119 MW. Pulsing the beam at a 6 % RF duty factor reduces the input power to 429 kW, a considerable saving in power! The klystrons, of course, cannot dissipate enough heat to run with the beam on continuously; their maximum duty factor is 6 %.

Klystron A on the Klystron Vault

Beam pulsing is done by means of a "mod anode" control element built into each klystron. When the mod anode is biased about halfway between the cathode and anode voltages, the beam current is turned on. When the mod anode is biased slightly (5 kV) more negative than the cathode voltage, the beam is completely turned off. The complete turn off is needed, even in the absence of RF drive, to prevent the klystron from generating noise, some of which would leak through the vacuum tube switching circuit that connects the mod anodes (which are connected in parallel) to a -55 kV "half voltage" tap on the beam power supply or to the chassis of the "buffer deck" which is at a potential 5 kV more negative than the cathodes. For high voltage insulation, a fiber optic link to the floating deck is used to turn on the beam. A second fiber optic link, to the buffer deck, pulses the buffer for 2 microseconds following the beam pulse to bring the mod anodes back down to cathode potential, turning off the beam.

Harmonic Filter, Antenna Tuner and Waveguide

A high-power "waffle-iron" or muffler type" waveguide filter provides dissipative attenuation for any power at the second and higher harmonics. Like the klystrons, this filter was developed for the BMEWS radars. The harmonic filter is followed by an antenna tuner in the form of a waveguide magic-T hybrid with motor-driven stubs on two of its four ports. This so-called EH tuner can be adjust to present the transmitter with reflectionless load for any reflection coefficient appearing at the downstairs end of the waveguide. The tuner is followed by 1,500 ft. of WR2100 waveguide that run from the control building to the platform. When considerable EH tuner correction is needed, it is because there are large reflections from the platform-mounted components. Although the tuner eliminates standing waves on the transmitter side, reflections from the platform will, of course, result in standing waves in the 1,500 ft. run of waveguide. But, as is the case in all transmission line situations, if the VSWR on the transmission line is less than about 2:1, the power dissipated by ohmic loss in the line is an acceptably small fraction of the total power.

Platform RF Components

The waveguide passes through a rotary joint at the top center of the azimuth arm. It then proceed down to the bottom of the arm to the continuously adjustable two-way power divider. One output of the divider supplies power to the carriage house while the other output supplies power to the Gregorian dome. This dual-beam operation is equivalent to two radars pointing in different directions. Connections from the power divider to the carriage house and the dome require the equivalent of telescoping waveguides to accommodate motion along the elevation track. This is accomplished by using a slotted waveguide fixed to the bottom of the azimuth arm. The slot (which has negligible radiation loss) points downward. A pickup probe extends from the carriage house up into the slot. The probe is actually a special waveguide elbow with wheels. This "collector" travels along inside the slotted waveguide. It has a half-height output port that passes through the approximately 8" wide slot. A 5-probe tuner at the junction of the WR2100 and the slotted waveguide eliminates reflections that would be produced at this junction. Power from the collector enters the carriage house though a length of corrugated waveguide and is connected to a turnstile junction. The lengths of the side arm shorts on the turnstile are adjusted so that a) no power is transferred from the transmitter port to the opposite port (the receiver port) and b) that the power leaving the antenna port has circular polarization. When the transmitted signal is reflected by a radar target (the ionosphere, the moon), etc.) the echo returns with the opposite circular polarization. The turnstile routes this echo power to the receiver port. This turnstile junction/circular polarization setup is therefore "self diplexing" - no additional hardware is needed to switch the antenna back and forth between the transmitter and the receiver.