Models 69 1 A/692 A/693 A/694A Section IV
Paragraphs 4-1 to 4-15
SECTION IV
PRINCIPLES OF OPERATION
4-1. INTRODUCTION. and collector in BWO tubes having a collector. Helix
and anode operating currents are critical operating
4-2. This section contains explanations of the oper- parameters and must not exceed the power dissipating
ation of the Sweep Oscillator circuits. Figure 4-1 is
a simplified block diagram showing principal circuit capability of the element.
sections and operating controls. Figure 4-2, a more
4-11. Operating current maximums for the anode and
complete block diagram, shows the main constituents helix are specifiedon the data sheet accompanying each
of the circuit sections. Each circuit section and im- new BWO tube. Because of wide variations in opti-
portant individual circuits are explained in succeed- mum operating currents and current division among
ing paragraphs.
BWO tubes of the same type, RF power output is the
4-3. As illustrated in Figure 4-2, the Sweep Oscil- primary indicator of tube performance.
lator consists of a Voltage-Tuned Oscillator, a Fre-
quency Control Section, an Amplitude Modulation Sec- 4-12. As the BWO is turned on, oscillation evolves
tion, and a Power Supply Section. The Frequency
Control Section includes the frequency-modulating from the shot noise in the electron beam. The shot
circuits. The Amplitude Modulation Section includes noise in the beam induces noise voltages on the
a square-wave generator, two marker generators, helix, and the noise voltages on the helix produce
electron bunches in the beam. These electron bunches
and circuits for automatic RF output level control move toward the collector at a velocity controlled
(ALC). The Power Supply Section includes automatic by accelerating potentials. As the electron bunches
over-current and over-voltage protection for the
Voltage-Tuned Oscillator. pass the spaces between helix turns their electric
fields appear outside the helix. At some frequency
these electric fields are in step (resonate) with
4-4. THE VOLTAGE-TUNED OSCILLATOR. the electron bunches along the helix and a back-
4-5. The Voltage-Tuned Oscillator section (Figure ward-moving wave is generated. The backward
4-2) consists of Backward-Wave Oscillator Tube V4. wave further bunches the beam, the beam in turn
amplifies the backward wave, and so on, until a
4-6. THE BACKWARD-WAVE OSCILLATOR TUBE. maximum bunch density is reached. At this state
the backward wave has maximum amplitude for the
4-7. The Backward-Wave Oscillator (BWO) tube, a existing operating conditions.
voltage-tunable microwave oscillator, is the radio-
frequency source in the Sweep Oscillator.
4-13. Frequency of oscillation, a function of beam
4-8. The BWO tube is an electron tube in which an velocity, is set by the accelerating electrode which
electron beam interacts with aguided electromagnetic has the same potential as the helix. Frequency of
wave in a way to transfer energy from the beam to oscillation varies linearly with time as helix voltage
the wave. The elements of the tube and their arrange- varies exponentially with time: the more positive
ment are shown in Figure 4-3. the voltage, the higher the frequency of oscillation.
In the Sweep Oscillator, the frequency-changing volt-
4-9. The tube consists of an electrongun, awire helix age is applied to both the helix and collector. In
through which an electron beam is directed, and a some BWO tubes the helix and collector are con-
collector to receive the beam, all within a vacuum nected internally; in others, the collector element
tube. The electron beam is hollow and focused to is operated at a more positive potential than the
travel as close as possible to the helix without touching helix,
it. Beam focus is maintained by the field of a cylindri-
cal permanent magnet encircling the vacuum tube.
Physical construction is such that the magnet or its 4-14. Output power is a function of beam current.
housing completely covers the tube and cannot be re- Beam current is primarily controlled by grid bias or
moved or adjusted in any way. Operating potentials by the current supplied to the cathode. A secondary
control of beam current is anode voltage, but anode
are supplied to the tube elements through wire leads. voltage also affects output frequency. Power output
increases noticeably as the frequency is increased,
4-10. The wire helix is a microwave transmission
line equal in length to several wavelengths. of the low- peaks near the center of the band and decreases
est output frequency. The RF output signal is gener- slightly at the high end of the band.
ated on the helix and is coupled out of the tube at the
gun end through a dc blocking capacitor or balun. Ap- 4-15. Amplitude modulation is accomplished by a cur-
proximate potentials required to operate the BWO are rent generator in series with the BWO cathode. Cath-
shown in Figure 4-3. Typical cathode current is less ode (beam) current can thus be controlled without
than 30 milliamperes and divides between anode and changing the accelerating anode-cathode voltage, thus
helix in BWO tubes having no collector, between helix avoiding frequency pulling.
02280-1 4-1
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