the operator turns the range hand-wheel,
the radar echo is moved horizontally on the indicator screen. This
permits the echo to be placed over a specific reference point, from which
the range may be read accurately. The range accuracy of the SCR-268
is plus or minus 200 yards.
The high voltage power rectifier supply for the
transmitter is mounted in a separate truck as previously mentioned.
This latter rectifier has a capacity of 500 milliamperes at 15,000 volts.
Since the output drain on this power supply is confined to periods less
than nine microseconds long, with 240 microsecond intervals between them,
the filter required is not large.
The Keying Unit
The heart of the radar is the keying unit,
which establishes the basic timing of the system. The keyer must
first establish the rate at which the pulses are sent out, 4098 per second.
Secondly, it must produce individual pulses of the requisite short duration,
and it must prevent any undue variation in the length of successive pulses.
The output of the keyer is used to drive the
modulator tubes, which are cut off between pulses by a heavy negative bias.
The keyer output is, therefore, a positive pulse capable of overcoming
this negative bias and driving the modulator grids well into the positive
region. These requirements add up to a formidable total. The
keyer must provide an accurately timed pulse of from three to nine microseconds
length, and of some 3500-volts peak amplitude. This requirement is
met by the use of eleven tubes in the keyer unit, two of which are power
supply rectifiers.
Figure 4 is a simplified schematic of the keyer
unit. The basic timing element is the 4098-cps oscillator (6SJ7 at the
top of the diagram). This is an electron-coupled oscillator of the
Hartley type. Two outputs are taken from the oscillator, one intended
for the range unit and the other for the keyer proper. The former
path consists of two amplifiers and a 4098-cps filter, the output of which
is a 4098-cps sinewave of 50 volts peak ampli- |
tude. The latter, or keyer, path starts at the oscillator plate,
which provides a flattened (non-sinusoidal) wave. This voltage is
passed through a coupling circuit (0.01 µf series capacitances, with
100,000-ohm and 50,000-ohm shunt resistance) which sharpens the oscillator
output. The sharpened wave is then applied to an over-driven (grid-current
limiting) amplifier which flattens the wave again, but by this time the
pulse possesses a much sharper leading edge. Following is a peaking
circuit of very short time constant (0.00005 µf and 50,000 ohms)
which responds only to the leading and trailing edges of the applied wave.
The output consists of a small short positive pulse, and a somewhat larger
negative pulse which is not thereafter used. These pulses are applied
to the pulse generator which is biased to cut-off. It responds to the small
pulse, but not to the negative pulse. The output of this stage is
a sharp pulse of about -170 volts amplitude.
The remaining portion of the keyer is occupied with
increasing the peak voltage of this pulse and reversing its polarity.
The first pulse amplifier (a 6F6 beam-power tube) is intended to produce
a higher current pulse, without a high voltage output. This current
pulse drives two 6L6 beam power tubes in parallel, the output of which
is conducted to a pulse trans- |
former, capable of passing very short pulses.
The output of the transformer is a double-peaked wave of about 750-volts
peak-to-peak amplitude, centered about a point two-thirds up from the negative
peak. This wave is applied to the grid of the final amplifier, a
304TL triode. This tube is a high-current thoriated-tungsten triode,
consisting of four triode units in a single envelope.
When the pulse arrives at the 304TL grid the
tube passes a current of approximately 1 ampere through a cathode coupling
resistance, thus producing a pulse of 3500 volts peak amplitude across
the out-put terminals. Since the tube is biased to cut-off, there
is no response to the negative peak. Be-tween pulses, when the 304TL is
not conducting, its output terminals are connected across the B-supply
voltage. One output terminal is some 2500 volts negative (adjust-able
by resistor R) with respect to ground. This is the bias voltage for
the succeeding modulator tubes. The use of the cathode-follower circuit
in the final keyer amplifier provides a low impedance output, which permits
conducting the short-length pulses over coaxial cables to the modulator
unit.
The Modulator Unit
The function of the modulator unit is to amplify
the keyer output to a high power level, suitable for |
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