Note: Descriptions are shown in the official language in which they were submitted.
~ackgroun,d of the_Invention
This invention relates to locking the output phase of
a magnetron directional amplifier to the phase of the drive
signal in such a manner that high gain is achieved over a wide
range of fre~uencies of the drive signal and -temporal change
in parameters that determine free running frequency of the mag-
netron.
The background of the invention and the invention
itself are illustrated in the accompanying drawings, in which:
FIGURE 1 is a block diagram of a prior-art phase
locking circuit;
FIGUR~ 2 is a block diagram of a preferred embodiment
of the invention;
FIGURES 3 and 4 are graphs of operating characteristics
of a magnetron;
FIGURE S is a plot of the phase locking characteristics
of the circuits of the prior artan~the .invention; and
FIGURE 6 is a plot of the phase shift experienced by a
phase-locked magnetron as a function of voltage across the mag-
netron.
In a prior art magnetron directional amplifier, shown
in FIGURE 1, phase locking of a magnetron is obtained by opera
ting the magnetron 10 in combination with a passi.~e directional
device 11, such as a three-port circulator, having one port
connected to a load 12 and the remaining port connected to a
signal source 13. The injected drive signal for the signal
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source appears to the magnetron as a reflected component of
the magnetron power output which acts to pull the operating
frequency of the magnetron to that of the injected drive signal.
A deficiency of this arrangement is that the ratio of the load
power to the signal source power (the "gain"~ is low and is
normally limited by practical ~onsiderations even if the ampli-
fier is used at only a single frequency (no modulation). One
of these considerations is that the higher the gain, the more
closely the magnetron anode current level must be controlled in
order to main~ain phase lock. A further limitation to the use
of this prior
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art arrangement is that the phase shift through the device
is very sensitive to any difference between the operating
frequency of the magnetron as a free running oscillator and
the frequency of the signal source. For example, if the
temperature of the anode block changes and therefore the
natural frequency of the tube changes, a significant change
in output phase shift from that of the input drive will
occur. The sensitivity of the phase change to a number of
external influences severely restricts the usefulness of the
magnetron in phased arrays without additional phase shift
compensation, usually provided by a phase comparator in the
output and a phase shifting device connected between the
signal source and the three-port circulator. The lack of
high gain in the magnetron requires that the phase shiting
device operate at a higher power level than that at which
electronic phase shifters normally operate.
A further problem inherent in magnetron operation is
the extreme sensitivity oE its current flow (and therefore
its output power) to a change in the voltage applied to the
magnetron thereby making it necessarv to use an expensive
regulated power supply. Because of this same sensitivity,
the phase shift through the tube is highly sensitive to any
voltage ripple on the power supply.
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1~2ZO~
Summary of the Invention
The aforementioned problems are overcome and other
objects and advantages of phase locking a magnetron are
provided by a system, in accordance with the invention,
which comprises an improvement to the prior art phase locking
system of FIG. 1 which comprises additional circuitry for
comparing the phase of the output of the magnetron at the
load with the phase of the signal source to provide an error
signal which is amplified and provided to an auxiliary coil
mounted on the magnet which provides the magnetic field for
the magnetron tube.
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Description of the Preferred Embodiment
FIG. 2 is a block diagram of a preferred embodiment of
the invention which provides phase locking of the output of
a magnetron. Magnetron tube 10 is provided with a magnet 14
which may be either a permanent magnet as shown in FIG. 1
or an electromagnet. In accordance with this invention, the
magnetic field provided by the magnet 14 to the magnetron 10
is either increased or decreased by providing the magnetic
circuit of which magnet 14 is a part with magnetic windings
15', 15" which are energized in series from direct-current
from amplifier 18. FIG. 2 shows two separate windings 15',
15" on the poles 16', 16", respectively, in a series electrical
connection with a current I providing flux in the same
direction. I~owever, a single coil located on one pole has
been found to supply sufficient change in the magnetic field
in which the magnetron tube 10 is immersed to provide the
desired phase locking in accordance with the invention.
The RF output of the magnetron tube 10 is provided
through waveguide 101 as one input to a three-port circulator
11. The other input to the circulator 11 is provided by
signal source 13 whose output is provided through the circu-
lator 11 to the magnetron 10 thereby "pulling" the output
frequency of the magnetron 10 to the frequency of the source
13 as in the prior art.
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The magnetron RF power output passes through the three-
port circulator and appears at the output line 111 which
provides the input to an RF coupler 16. ~ost of the power
entering the coupler 16 on the line 111 is provided to the
load 12. However, a very small amount (comparable to that
provided by the signal source 13~ sufficient to operate the
phase comparator 17 is provided on line 161 to the comparator
17. The other input to the phase comparator 17 is provided
by the signal source 13 on RF microwave line 131. The phase
comparator 17 compares the phase of the RF signals applied
on its input lines 131, 161 which are the phases of the
signal source 13 and the magnetron 10 output at load 12.
Typically, the phase comparator will provide a zero DC levei
when its input signals are in phase with one another, and a
maximum positive and negative signal when the phase relation-
ship of its input signals is +90 and -90, respectively.
The DC signal out of the phase comparator at terminal 181 is
ampllfied by a high-gain direct current amplifier 18 and is
then provided to the windings 15', 15" on the magnet 14 to
change the flux produced in the magnetron tube 10 in accor-
dance with the amplitude of the current I provided by the
amplifier 18. The amplifier 18 may be a differential ampli-
fier that amplifies positive or negative signals on terminal
181 to provide a positive or negative current I, respectivelyO
~5 The output current I is a function of the difference between
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the signal levels applied at the inputs 181, 182~ The direc-
tion of the output current flow I from the amplifier 18 is
determined by whether the natural operating frequency of the
tube is above or below the drive frequency. The operating
point of the magnetron tube (magnetic field, voltage, and cur-
rent) is established to provide a natural operating frequency
which i5 phase locked with the signal source 13 in the middle
of the desired phase-lock frequency range when the phase
comparator 17 has near zero output.
The invention has been described thus far without utili-
zation of the lead-lag network 19. The absence of lead-lag
network 19 provides a control circuit that is of the simplest
kind and is inherently stable. ~owever, faster response
time and reduced steady-state phase shift between the signal
source 13 and output at load 12 can be achieved by lead and
lag circuitry contained in the lead-lag network 19. The
additional cost of the network 19 is substantially negligible
because of its location in the control loop where the power
level being handled is negligible, and where the additional
gain necessary when lead-lag compensation is used can be
easily obtained. In accordance with conventional servo
system design, it may be desirable that network 19 be only a
lead network or a lag network.
FIGS. 3 and 4 show typical operating characteristics of
a magnetron tube 10 such as that which might be used in the
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phase lock system of FIG. 2. In FIG. 3, curve 30 shows
the frequency output of a magnetron as a function of the
current through the tube for a fixed value of magnetic flux
density. It is observed that the output frequency is substan-
tially affected by the current through the tube. FIG. 4
shows the volt/ampere characteristic curves of the magnetron
as a function of different magnetic fields Bl, B2, B3 applied
to the magnetron tube, A load line 40 resulting from a load
connected to the output of a magnetron shows that there is
substantial change in the voltage across the magnetron and
the current through the magnetron with a change in the magnetic
field applied to the tube. These properties of the magnetron
exhibited in FIGS. 3 and 4 are utilized in the phase locking
system of FIG. 2.
In order to lock the output frequency of the magnetron
to the signal source wlth a minimum of phase shift, the
free-running frequency should be caused to be near to the
frequency of the signal source. Since the natural frequency
of the magnetron 10 is a sensitive function of the current
through the tube as seen in FIG. 3, the control of the current
through the tube can be used to control frequency. It further
appears from the characteristic curves of FIG. 4 that changing
the flux through the magnetron tube while maintaining the
load line and the voltage V applied to the magnetron at a
constant level will result in substantial change in the
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current through the tube and hence a substantial change in
the frequency of the magnetron tube 10. It is these properties
which are utilized in the circuitry of FI~. 2 where the mag-
netic flux through the tube is charged to cause the natural
frequency of the magnetron 10 to be close to that of the
signal source prior to being pulled into a phase locked
state by coupling through the three-port circulator 11.
FIG. 5 provides a comparison between the phase locking
capability of the system of this invention, shown in FIG. 2,
with the phase locking capability of the prior art circuit,
shown in FIG. 1. The frequency difference of FIG. ~ is the
difference between the frequency of the signal source 13 and
the free-running frequency of the magnetron tube when the
value of the current I through thé auxiliary coils 15', 15"
is zero. FIG. 5 also shows the phase shift of the output
signal at the load 12 relative to the phase of the signal
source 13.
Curve 51 shows the phase shift and the frequency dif-
ference locking range 52 obtained with a magnetron when used
in the prior art circuit of FIG. 1 at high gain levels of
32 db with a drive power of 0.2 watts. Typically, for a
gain level of 32 db, the prior art locking range 52 extends
over only one or two megacycles from the free-running or
natural frequency of the magnetron (2.450 GHz). The phase
2~ shift between the frequency of the signal source 13 and the
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load 12 is seen to vary from -80 to ~80 over the locking
range 52. The deficiencies of the prior art phase locking
technique of FIG. 1 is clearly demonstrated by curve 51 of
FIG. 5. The limited frequency range over which locking
occurs and the large phase shift over this locking range
substantially reduces the utility of the circuit of FIG. 1
for many magnetron applications such as in a phased array
antenna where it is desired to keep the input power level
low so that an electronic phase shifter can be used.
Referring again to FIG. 5, curve 53 depicts data obtained
using the circuit of the invention, shown in FIG. 2 with the
same drive level and gain as for FIG. 1. It is obvserved
that the locking range 54 has been increased to approximateiy
15 MHz for the same natural frequency as stated in the pre-
ceding paragraph, and the phase shift over this locking
frequency range is only 15. The improved performance of
the circuit of this invention over the prior art in pro
viding a ten fold improvement in the locking frequency range
and in minimizing phase shift is apparent. The resulting
phase locking and high gain capability are such that appli-
cation of phase locked magnetrons in phased array antenna
systems is feasible.
Referring now to FIG~ 6, there is shown a plot of the
phase shift between the output signal at the load 12 and the
signal from the signal source 13 for a fixed frequency as a
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function of the voltage V applied to the magne~xon anode in
kilovolts by the magnetron DC power supply 20. It is seen that
a change of + 8.6% i~ the voltage V applied to the magnetron
10 causes only a 16 phase shift. Therefore, the effectiveness
of the system of FIG. 2 in reducing the effects of power
supply ripple or for voltage regulation is amply demonstrated
by curve 61 of FIG. 6. ~herefore, substantial economies in
the design of the power supply for the magnetron are available
because of the use of the invention shown in FIG. 2.
Having described a preferred embodimen~ of the invention,
it will be apparent to one of skill in the art that other
embodiments incorporating its concept may be used. It is felt,
therefore, that this invention should not be restricted to the
disclosed embodiment but rather should be limited only by the
spirit and scope of the appended claims.
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