ELECTRONICS
September 1945

By McGraw-Hill Staff
Page 100 - 109

shown in Fig. 11. Two antenna inputs are provided, for each of the two terminations of the array. These are connected to separate r-f amplifiers (type 954 acorn pentodes) which are switched alternately on and off to provide reception alternately on the respective lobes of the array.
The switching is accomplished by applying a rectangular wave of voltage to the grids of the r-f stages, the wave being obtained from a multivibrator (switch oscillator, 6N7 double triode) and a subsequent amplifier (6N7). The multi-vibrator produces a rectangular wave at a frequency of approximately 1400 cps. Approximately three pulses and echoes are passed through one r-f stage while the other is blocked. Thereafter, the first r-f amplifier is blocked and the second passes three pulses. In this manner reception is switched rapidly from one lobe to the other.
The two r-f stages are joined together at their plates. Hence at this point the successive lobe signals are interspersed in time sequence. Thereafter, the joint sig-

nal is amplified by a second r-f stage (954).
    The tuned circuits associated with the r-f stages consist of a single inductance, permeability tuned and loaded with resistance to present a bandwidth of about one megacycle. This bandwidth is somewhat greater than the spectrum occupied by the incoming pulses, the extra space being provided to cover possible shifts in transmitter frequency.
    At the output of the second r-f stage, the signal is mixed with a local oscillator signal. This signal is derived from a type-955 acorn triode oscillator in a Hartley circuit, followed by a 954 buffer stage. The mixer tube is a 954, with the pulse signal and local oscillator output applied jointly to its control grid. At the output of the mixer the difference frequency of 19.5 Me appears.
    The 19.5 Me intermediate frequency is amplified in four i-f stages (6SK7), coupled with single tuned circuits which are permeability tuned and loaded to achieve 1-Me bandwidth. The voltage gain of each stage is about 12, or 12` = 21,000 overall for the four stages.
    The second detector is a triode
(6SJ7) operated at cut-off bias. The detector feeds a video amplifier
(6SJ7) having a gain of about 2.5. The output (taken from the plate of the video amplifier) is about 10 volts peak-to-peak maximum into a 500-ohm output circuit. Any signal higher than this saturates the video amplifier.
    The sensitivity of the receiver is such that, at maximum gain, the noise developed at the receiver input will saturate the video amplifier. Sensitivity is controlled remotely, from a control associated with the cathode-ray indicator. The sensitivity control voltage is applied to the cathode of the first i-f stage.
    In addition to the video output, the receiver provides a "spread voltage" for separating the indications of the two lobes on the cathode-ray indicator screen. The spread voltage is derived from the output of the switching amplifier. To assure that the gain in the two lobe channels is equal, a balance
control is provided in one of the
first r-f stages. This balance control is set during the calibration

procedure, using a standard signal generator.

The Range Unit

The range unit is a device which controls the timing of the indicator traces relative to the production of the transmitted pulses. The range unit shown in the block diagram of Fig. 12 is essentially a phase shifter, capable of shifting the phase of the 4098-cps sine wave by any amount, continuously and smoothly.
The input 4098-cps wave is derived from the keyer (shown in Fig. 4). After passage through a limiting resistor, and through a filter to remove harmonics, the sine-wave is passed through a Helm-holtz coil. This device consists of two sets of stator coils, arranged at right angles. The current fed one set of coils is advanced 90 electrical degrees by passage through a capacitor-resistor network. The result is a rotating magnetic field at the center of the coil system. This field induces a sine-wave voltage in a small pickup coil at the center of the system. The phase of the induced voltage is fixed so long as the pick-up coil is stationary. When the coil is rotated, the phase shifts in direct proportion to the angular motion of the coil. The induced voltage is amplified in two stages and the output is fed to the indicators as subsequently described.

The Indicator Units

Three identical units are used as indicators by the range, elevation and azimuth receiving antenna array operators. These indicators are very similar in appearance and function to a conventional cathode-ray oscilloscope. Besides the basic power supply, the indicator units derive three voltages from the other units of the radar : (1) the pulse signal from the video amplifier of the associated receiver, (2) the spread voltage from the switch amplifier of the receiver, and (3) the phase-shifted 4098-cps sinewave from the range unit.
A block diagram of one indicator, showing its waveforms, is given in Fig. 13. The basic synchronizing input comes from the range unit. This sinewave voltage is first amplified in an over-driven sync ampli-

108 September 1945 - ELECTRONICS

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ELECTRONICS September 1945

ELECTRONICS
September 1945