Note: Descriptions are shown in the official language in which they were submitted.
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SWITC9æD RING PHASE BIT CIRCUIT
Background of the Invention
The present invention relates to diode phase shifters
and more particularly, to a switched-ring phase shifting
circuit adapted to provide large phase shifts and high
peak power capability.
Phased array antennas are used to provide scanning
flexibility in an antenna system without the need to
physically move the position of the array. Phased array
systems typically include two dimensional arrays, such as
planar arrays wherein each of the elements may be
individually excited. Through individual excitation of
radiating elements of the array, or individual rows or
columns of elements, the scanning direction may be varied
15 with a high degree of discrimination, providing a narrow
beam that may be directed with precision.
The steering of the resulting beam from the antenna
array is accomplished by controlling the phase of the
signals applied to each element of the array relative to
20 the other elements. The "Radar Handbook" by Merrill
Skolnik, (McGraw Hill 1970) provides a thorough
description of contemporary phase shifting circuits.
If only on-axis radiation is required, then phase
shifters are ordinarily not required. However, if the
25 beam is to be scanned or moved in space relative to the
bore sight axis of the array, then variable or
controllable phase shifters must be used to achieve such a
result. Simple controllable phase shifters can be
implemented by the use of different lengths of
30 transmission line which are switched into or out of the
propagation path to add delay proportional to the length
of transmission line so switched. The length of the
reference and delay paths are selected in view of the
operating frequency range such that a desired phase shift
is effected. Consequently, the propagation delay, or
phase shift, corresponds to the length of transmission
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line switched. Such switched-line systems are somewhat
difficult to implement, because even the simplest
electronically-controllable type require switching diodes
series-connected with the transmission line. The biasing
of such series-connected diodes requires series-connected
capacitors to prevent the controlling bias signal applied
to one diode from effecting the diode associated with the
next transmission-line section. The series diodes and
coupling capacitors are costly and introduce losses.
In addition to the switched-line systems, other
switching phase shifters are known which switch lengths of
shunt transmission line into the circuit with the main
transmission line path. Such circuits are commonly
referred to as a "T" circuit or a "~" circuit (which is a
15 dual of the "T" circuit). The "T" circuit includes a pair
of series diodes connecting input and output terminals and
a shunt diode connected to the serial line between the
series diodes. The "~" circuit includes a single series
diode and a pair of shunt diodes, each connected on an
20 opposite side of the series diode. In the contemporary
~ circuit the series diode is either forward or reverse
biased, effecting insertion of the delay line when the
diode is reverse biased. In the forward bias state the
input RF signal is communicated direstly to the output
25 terminals through the series diode. In the reverse bias
state a portion of the input RF signal is communicated
across the diode terminals (which acts as a capacitance in
the reverse bias state), and another portion of the input
RF signal passes through the delay line. Both portions
30 are combined at the output side of the series diode
producing a composite signal. The effective phase shift
of the circuit is the output signal when the diode is
forward biased and the composite signal when the diode is
reverse biased.
One difficulty with respe~t tocontemporary series/shunt
phase shifting circuits concerns the operation of the
diode disposed in the series path between input and
output. The series diode is responsive to a D.C. bias
voltage across the diode electrodes. The field produced
by the bias voltage induces a change in electrical
characteristics of the diode, which in turn affects the
circuit impedance. The change in impedance causes a
change in phase shift through the circuit. Variations
in the impedance of the diode may unfavorably impact the
operating bandwidth of the circuit.
Additionally, the series diode may limit the power
handling capacity of the phase shifting circuit. The
power handling capacity of the diode is proportional to
the square of the total voltage across the reverse
biased diode. Because the series diode is located
closest to the power source (the shunted diodes are
spaced from the series diode by attenuating transmission
line segments), the series diode is subject to greater
power handling demands and may limit the power handling
capacity of the circuit.
Accordingly, it is desirable to provide a phase
shifter circuit which incorporates the power handling
the bandwidth advantages for a switched diode system,
wherein the series diode is eliminated from the circuit
path and phase shift may be effected without the need to
drive the input RF signal through a reverse bias diode.
As will be discussed in detail below, in accordance
with the present invention the series diode is
eliminated, thereby removing operational limitations
resulting from the characteristic operation of the
diode.
Various aspects of the invention are as follows:
A phase shifting circuit for varying the phase
shift of an output signal in relation to an input
signal, said circuit characterized by:
- 3a -
a plurality of bidirectional transmission line
segments connected to form a ring;
input and output terminals connected to opposite
ends of a first of the plurality of said transmission
line segments to form a first signal path therethrough;
and
means for selectively enabling a second signal
path, in parallel with said first signal path, through
the remainder of the plurality of said transmission line
segments;
said first and second signal paths being of
different lengths such that enablement of said second
signal path varies the phase shift of a signal at said
output terminals in relation to a signal at said input
terminal.
A phase shifting circuit characterized by:
first, second, third and fourth transmission line
segments serially connected to form a ring, said first
segment being less than 1/4 wavelength long, said second
and fourth segments each being approximately 3/8
wavelength long and said third segment being
approximately 1/4 wavelength long;
RF input and output terminals being connected to
the ring at opposite ends of said first segment;
first and second diodes connected on opposite sides
of said third segment, said diodes being connected to
substantially short a portion of the ring to ground when
the diodes are forward biased; and
means for biasing the diodes to enable a first
signal path through the ring when the diodes are forward
biased, and to enable first and second signal paths
through the ring when the diodes are reverse biased,
said first and second signal paths having different
lengths such that the phase of the RF output signal is
shifted upon forward and reverse biasing of the diodes.
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- 3b -
Summary of the Invention
By way of added explanation, a phase shifting
circuit is disclosed comprising a plurality of
bidirectional transmission line seg~ents serially
connected to form a ring. Input and output terminals
are connected to opposite ends of the first of the
transmission line segments to form a first signal path
through the ring. A second ring signal path, through
the
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remainder of the serially connected transmission line
segments, is selectively enabled to provide a
predetermined phase shift at the output of the ring. By
relocating the semiconductor devices to remove them from
both signal paths connecting input and output terminals,
the circuit permits high power signals to be phase shifted
over wide ranges.
In the preferred embodiment the second signal path is
enabled by a pair of diodes connected to the second signal
10 path on one end and to ground on the other. By forward
biasing the diodes the RF signal through the second signal
path is shorted to ground, leaving only a signal flowing
through the first signal path at the RF output. When the
diodes are reverse biased the RF ground in the second
15 signal path is removed, providing a complete second path
to the input RF signal. The RF output signal in that case
is a composite of the signals flowing through the first
and second signal paths. Because the first and second
signal paths have different lengths the phase of signals
20 at the termination of each path has a different phase.
The combination of the two signals results in a composite
signal having a phase distinct from the phase of the
signal output from the first signal path. By proper
selection of the lengths and impedances of the segments
25 forming the first and second signal paths the desired phase
difference may be effected between the signal output from
the first signal path and the composite signal.
In the preferred embodiment the first signal path is
formed of a transmission line segment having a length
30 slightly less than 1/4 wavelength of the center bandwidth
frequency. The second path is formed to include two
segments having a length of approximately 3/8 wavelength
each, and an additional segment having a length of
approximately 1/4 wavelength. Thus, the collective length
35 of the second signal path is approximately one wavelength.
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Brief Description of the Drawinqs
Figure lA is a circuit diagram illustrating a
switched ring phase bit circuit in accordance with the
present invention;
S Figure lB is a circuit diagram illustrating the
operation of the circuit of Figure lA when the shunt
diodes are in a forward bias state;
Figure lC is a circuit diagram illustrating the
operation of the circuit of Figure lA when the shunt
10 diodes are in a reverse bias state;
Figure 2A represents the equivalent circuit diagram
of the circuit of Figure lA when the shunt diodes are in a
forward bias state; and
Figure 2B represents the equivalent circuit diagram
15 of the circuit of Figure lA when the shunt diodes are in a
reverse bias state.
Detailed Description of the Presently Preferred Embodiment
The detailed description set forth below is intended
merely as a description of the presently preferred
20 embodiment of the invention, and is not intended to
represent the only form in which the present invention may
be constructed or utilized. The description below sets
forth the functions and electrical conditions that are
effected by the invention in connection with the
25 illustrated embodiment. It is to be understood, however,
that the same, or equivalent functions or electrical
conditions may be accomplished by different embodiments
that are also intended to be encompassed within the spirit
and scope of the invention.
Figure lA of the drawing illustrates a switched ring
phase bit circuit in accordance with the present
invention. The circuit 11 functions to selectively phase
shift the RF input signal by an amount corresponding to
the respective lengths of the signal paths in the circuit.
35 It is to be understood that the particular lengths and
corresponding phase shifts that are discussed below are
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intended to be exemplary only, and are not intended to
represent the only lengths or contributions that may be
implemented within the scope of the invention.
The input to circuit 11 is provided at RF input
terminal 13. The output is at RF output terminal 15. DC
bias is provided at bias terminal 17 and is used to vary
the state of shunt diodes 19 and 21. Bias terminal 17
may, for example, be connected to a power source that may
vary between a positive voltage (e.g., plus 100 volts) and
10 a small negative voltage (e.g., minus 0.75 volts). RF
choke 23 isolates the bias supply from the RF signal
passing through circuit 11. Capacitors 25 and 27 isolate
the RF input and output circuits, respectively, from the
DC bias circuit connected to bias terminal 17.
The remaining portions of circuit 11 include
transmission line segments 29, 31, 33, 35, 37 and 39. The
length of the various segments is selected in view of the
RF signal bandwidth and the characteristic impedance that
a particular length segment exhibits over the operating
20 bandwidth. Sections 51 and 53 serve as voltage
transformer sections incorporated into the transmission
line segments 35 and 39, to further reduce the voltage
level applied to the cathode of diodes 19 and 21, thereby
increasing the peak power handling ability of the circuit.
25 The transforming action of sections 51, 53 is a
consequence of their size and characteristic impedance in
relation to the overall size and characteristic impedance
of segments 35 and 39, respectively. The sections 51 and
53 may be formed to extend approximately two thirds the
30 length of the segments 35 and 39.
Selection of the actual length of the transmission
line segments will determine the characteristic impedance
of the segments. For example, it is generally known that
a short at the end of a 1/4 wavelength transmission line
35 reflects a very high impedance (theoretically infinite) at
the input to that transmission line, and that a short at
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the end of a 1/2 wavelength transmission line reflects a
very low impedance (theoretically a short) at the input.
A short at the end of a 3/8 waveltngth transmission line
reflects an impedance at the input that is a capacitive
reactance between these extremes. ~s described below the
present invention utilizes the characteristic impedances
of different lengths of transmission line segments to
provide alternate RF signal paths without the need to
utilize a series diode.
Figure lB illustrates the effective operation of the
circuit of Figure lA when the DC bias signal applied to
bias terminal 17 is negative, effectively forward biasing
diodes 19 and 21. In a forward bias condition the RF
signal input at terminal 13 passes through transmission
15 line segments 29, 31 and 33. The forward bias condition
of diodes 19 and 21 effectively grounds the connected
terminal of transmission line segments 35 and 39,
respectively. Thus, there is no alternate path for the RF
input signal other than through transmission line segment
20 31.
Figure 2A shows the forward bias equivalent circuit
representation of the conditions illustrated at Figure lB.
As shown at Figure 2A transmission line segments 35 and 39
are grounded at one end, representative of the forward
25 bias state of diodes 19 and 21. In the preferred
embodiment, segments 35 and 39 are implemented to have a
length of approximately 3/8 wavelength at center frequency
of the RF signal. As a consequence of that length,
segments 35 and 39 exhibit a characteristic impedance that
30 is a capacitive reactance represented by capacitors 41 and
43. The segment 31 is preferably implemented to have a
length of less than 1/4 wavelength, e.g. between 1/8 and
1/4 wavelength, and generates an impedance that is
effectively a series inductive reactance, represented as
35 coil 45 in the equivalent circuit shown at Figure 2A. The
characteristic capacitive and inductive reactances
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interact to form a low pass circuit, so that the input RF
signal finds a low impedance path through segment 31
trePresented as coil 45) and a high impedance path through
segments 35 and 39 (represented as capacitors 42 and 43).
Figure lC illustrates the operation of the circuit of
Figure lA when the diodes 19 and 21 are reverse biased.
In the reverse bias condition diodes 19 and 21 no longer
act as shorts, as in the forward bias condition
illustrated at Figures lB and 2A. In the reverse bias
condition diodes 19 and 21 act as capacitors, and are
therefore represented by capacitors 47 and 49 in Figure
lC. Because the ends of segments 35 and 39 are no longer
an RF short to ground, the RF input signal is now provided
with a pair of alternate signal paths through the circuit.
The first path 20, as before, is through transmission line
segment 31. The second path 40 is through segments 35, 37
and 39, the combined length of which is approximately one
wavelength in the presently preferred embodiment.
Segments 35, 39 may, for example have a length between 1/4
and 3/8 wavelength, and segment 37 may have a length
between 1/8 and 1/4 wavelength. The RF output under those
conditions is a composite signal formed of the signal
flowing through the short path 20 (through segment 31) and
the signal flowing through the long path 40 tthrough
segments 35, 37 and 39). The phase of the composite
signal is different than the phase of the signal output
from segment 31 when the diodes are forward biased.
Consequently, the enablement of the alternate signal path
40 introduces a phase shift to the RF output signal. The
resulting phase shift that is effected by the circuit may
be varied by varying the respective lengths/of theP signal
RF transmission line segments. Figure 2B illustrates the
reverse bias equivalent circuit of the circuit set forth
at Figure lC. As shown in Figure 2B the diodes 19 and 21
may again be represented as capacitors 47 and 49.
Segments 35, 37 and 39 collectively form a long, or delay
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path 40 through which the RF signal may flow without being
shorted to ground.
By proper choice of the transmission line segments,
the present invention may be utilized to implement phase
shifts in the range of up to 180 degrees. Because of the
elimination of the series diode such as that incorporated
in contemporary " ~" phase shifter circuits, the present
invention is not so constrained in its peak power
capability. Due to their location in the circuit, diodes
10 19 and 21 are not required to withstand the same
operational conditions as required of the series diode in
conventional "~" circuits.
The invention permits wide tolerance variation in
diode parameters since phase shift is implemented
15 primarily by the proper choice of transmission line
segments and impedance levels. For example, the circuit
has been used in L-band applications, producing a 90
degree phase shift wherein the diode capacitance was 2.5
picofarads, segments 35 and 39 were formed to be .31 ~,
20 segment 31 was formed to be .14~ and segment 37 was formed
to be .22~. Diodes with the capacitance of the order of
1.0 picofarads and a resistance on the order of 0.25 ohms
are satisfactory for S-band applications. Diodes with a
capacitance of the order of 2.5 picofarads and a
25 resistance of 0.25 ohms are satisfactory for L-band
application. Diodes with a capacitance of the order of
0.8 picofarads are satisfactory for C-band applications.
Circuit bandwidth is expected to be approximately 15 to
25~ of the operating frequency.
As described above, the particular selection of
diodes and transmission line segments may be selected in
view of the desired phase shift to be implemented. Thus,
the circuit of the present invention may be implemented in
redundant parallel fashion, with each separate circuit
35 designed to affect a particular phase shift, the selection
of which may be regulated by a choice of the particular
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circuit It is anticipated that other modifications and
alternate applications of the present invention may be
recognized and implemented by those of ordinary skill in
the art without departing from the broader aspects of the
invention.
