ELECTRONICS
September 1945

By McGraw-Hill Staff
Page 100 - 109

er (6AC7). The negative grid voltage cutoff and positive grid current saturation convert the sinewave input into a more nearly rectangular wave which is fed to a pulse generator (6L6), also overdriven. In the plate of this high-current tube is an inductance. The sudden current changes at the sides of the rectangular wave, passing through this inductance, induce high peaks of voltage which are used to control the following sweep-voltage generator after passing through a peak filter.
The sweep generator consists of a capacitor, charged through the 6L6 sweep generator tube. The approximately constant current characteristic of the 6L6 results in a closely linear increase in voltage across the sweep capacitor until the sudden positive peak arrives from the preceding peak filter. Then the

capacitor is suddenly discharged by the heavy reverse current passed by the sweep generator tube. The result is that the voltage across the sweep capacitor has the saw-tooth form required for deflecting the electrostatic cathode-ray tube.
Before application to the c-r tube, the sweep voltage is amplified (6SJ7) and mixed with the spread voltage. The spread voltage, which we recall is synchronous with the lobe-switching, is amplified (6SJ7) and causes the saw-tooth sweep wave to be displaced bodily upward and downward, as shown in the figure. Thus the sweep associated with the one lobe-channel is displaced horizontally on the c-r tube with respect to the sweep associated with the other lobe-channel. This causes the image to appear "split" as shown at the bottom right in the diagram.

    The displaced sawtooth wave is applied to two amplifiers (6L6's) which produce symmetrical waves of opposite polarity for application to the horizontal deflection plates of the c-r tube. The symmetry is essential to produce a linear sweep.
    The receiver output is amplified in a pulse amplifier (2-61,6's) and then applied to the vertical deflection plates. The resulting screen pattern is a series of pulses in pairs. The first pair is a split image of the transmitted pulse, which is transmitted directly from the transmitting array to the receiving array. The receiver is considerably over-loaded at this instant, and the pulse saturates the amplifiers and hence is flattened at the top. No special device is provided to protect the receiver during the transmitted pulse. The next pair of pulses to the right is a split image of the pulse echo from a target.
    In operation, the azimuth and ele
vation operators adjust the spread
voltage and the amplitude of the
pulses until the split image is displayed conveniently for comparison of the amplitudes of the two components of the split image. There-upon the angular orientation of the corresponding array is adjusted until both components are of equal height. The array is then pointed directly at the target.

Range Indicator

The range indicator does not make use of the spread voltage so only a single image appears for each echo. The range operator adjusts the phase shift of the range unit until the target echo appears under a cross-hair on the associated scope screen. The range to the target is then read off the phase-shift dial, which is calibrated in thousands of yards.
The SCR-268 employs 110 vacuum tubes, which are divided among the several units as follows : high voltage rectifier 4, keyer 11, range unit 3, modulator 10, transmitter 16, two receivers 15 each, three indicators 12 each. Despite this fact, and the evident complexity of the circuits, the 268 has proved to have outstanding serviceability in the field. -D.G.F.

REFERENCES

(1) Fink, P. G., The Radar Equation, Electronics, p. 92, April 1945.

September 1945 - ELECTRONICS 109

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ELECTRONICS September 1945

ELECTRONICS
September 1945