Section I11 Model 686C
Paragraph 3-10 cont'd
Figure 3-10. Backward-Wave Oscillator Tube
The rf output of the backward-wave oscillator is a re- moving either to the right or to the left. Along the
sult of the interaction between the electron beam and bottom of the chain is a series of unilateral amplifiers
the electric fields accompanying a microwave signal in which signals can pass only in the left-to-right
present on the helix. The term 'backward-wave oscil- direction. Each loop then consists of a transmission
lator" is quite appropriate for this tube since the rf line, two coupling capacitors and an amplifier trans-
energy moves and builds up in a direction opposite to mittingfrom left to right. In operation, positive feed-
that of the electronbeam and is coupled out at the gun back, which leads to regenerative amplification or
end of the tube via the helix terminal. oscillation, occurs utilizing a wave going from right
to left on the transmission line when the phase delay
in a single loop is just one cycle. The total phase
The operation of the backward-wave oscillator tube delay around a group of n loops will then be n cycles.
may be explained in terms of a series of feedback
loops similar tothose common to low frequency elec-
For low values of amplification, the chain of loops
tronic circuits. Each of these regenerative loops can
will act as a regenerative amplifier operating at the
function as an amplifier or an oscillator and is de-
signed so that the phase shift around the loop is one frequency which provides positive feedback. However,
if the transmission line is terminated in its character-
cycle. One of these feedback loops is shown in figure
istic impedance at the input and the amount of ampli-
3- 11 where, using conventional terminology, the for-
fication is increased, oscillations will start. The
wardor p circuit consists of a sectionof transmission
line and the backwardor fi circuits is aunilateral am- frequency of oscillationwill be controlledby the phase
delay in the amplifier chain.
plifier connecting the output of the transmission line
to the input. In this circuit positive feedback will
With this background we can now examine the actual
occur when the amplifier gain becomes sufficiently
high to overcome the loss in the transmission line and functioning of the backward-wave oscillator tube.
Figure 3-13 shows a cross section of the helix and a
the pp loopwill oscillateat a frequencyfor which the
portion of the electron beam. The helix structure
total phase delay is one or more cycles.
consists of a cylindrically-wound flat-wire tape; the
electron beam is hollow and passes very close to the
If the amplifier is designed for limited high frequency helix turns. The strong axial magnetic field focuses
response, oscillations will occur only when the phase the electrons in the beam and allows movement only
delay is one cycle and the frequency of oscillation in the direction of the axis of the tube. The lines of
can be shifted by changing the phase delay in the force of the electric fields associated with an rf wave
amplifier. The essential feature of the voltage- traveling along the helix are also shown in figure 3- 13.
tuned backward-wave tube oscillator is that the fre- Although these fields rotate around the helix at the
quency of oscillation can be changed electrically by velocity of light, the effective axial velocity is equal
changing the phase delay in the amplifier. to the velocity of light times the ratio of the turn-to-
turn spacing of the helixdivided by its circumference.
Figure 3-12 shows a chain of identical regenerative The axial electric fields will be strong between helix
feedback loops. Along the top of the chain is a series turns and very weak unaer the turns since electric
of transmission line sections that will support a wave fields cannot exist parallel to a conductor. The strong
I 1
+ - I I TRANSMISSION LINE
1
OUTPUT TRANSMISSION INPUT
LINE OUTPUT INPUT
T T
* AMPLIFIER
CHAIN OF UNILATERAL AMPLIFIERS- - - +
Figure 3-11. A Single Regenerative Loop Figure 3-12. A Chain of Regenerative Loops
3-12 00034- 1 |
ACEC
