Bidirectional repeater amplifier

A double diamond configuration of phase shift circuits are coupled via circulators, amplifiers, power dividers, and power combiners. The circuit arrangement serves to amplify signals in either direction while accurately maintaining relative phase shift. There is a 180 degree phase shift between components back to the originating transmission line, to this cancel them. The circuit may be used between microwave transmission lines, such as waveguides.

BACKGROUND OF THE INVENTION 
This invention relates to repeater apparatus for amplifying two signals (at 
the same frequency) in opposite directions, and more particularly, to 
apparatus for amplifying such signals at microwave frequencies. 
In many systems it is a requirement that electrical signals be transmitted 
in both directions between two locations. The straightforward manner of 
doing this is to use two separate transmission lines. However, frequently, 
for economy or other reasons, a single transmission line is used, with the 
signals transmitted in the two directions at the same time. This usually 
presents no significant problem, unless amplification is required. In 
telephony and other arts over the years many arrangements have been used 
to provide two-way amplification. These systems may include various types 
of hybrid circuits, fast switching to select the direction of 
amplification, selective attenuation, negative impedance devices, etc. 
Problems encountered include singing (oscillation around a loop), 
imperfect impedance balance at hybrids, echoes, delay distortion effects, 
phase error, etc. 
The state of the art is indicated for example by U.S. Pat. No. 3,689,711 by 
Earle et al for a Call Diverter Repeater; U.S. Pat. No. 3,855,431 by 
Stewart for an Electronic Hybrid Amplifier; and U.S. Pat. No. 3,911,372 by 
Seidel for an Amplifier With Input and Output Impedance Match. 
In microwave systems, such as at radar frequencies of one to fifteen 
gigahertz, it may be particularly important to preserve the phase 
relation. One prior solution to the two-way amplification problem has been 
to switch an amplifier to be instantaneously operative in the direction of 
the signal. This is in effect a time division approach. However, available 
switches are high speed or high power, but not both. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide two-way amplification with 
reduction of the relative phase error between the two signals. Another 
object is to provide a circuit which passes the signals with a minimum 
delay. Still another object is to eliminate the need for high-power 
fast-acting switches. 
According to the invention, a repeater amplifier arrangement between two 
two-way transmission lines comprises a double diamond configuration of 
phase shift circuits, coupled via circulators, amplifiers, power dividers, 
and power combiners. The circuit arrangement serves as a bidirectional 
repeater amplifier which accurately maintains the relative phase of the 
two signals (of the same frequency) passing through it in opposite 
directions. 
Features include the following: 
1. Minimum relative phase shift between the two signals. 
2. Capability to simultaneously handle two signals. 
3. Minimum signal delay through the circuit (special delay lines not 
required). 
4. Switches are not required to route the signals.

DETAILED DESCRIPTION 
The invention is a circuit which is comprised of several amplifiers, phase 
shifters, power dividers, power combiners, and circulators in a 
hybrid/bridge arrangement. The circuit arrangement serves as a 
bidirectional repeater amplifier which accurately maintains the relative 
phase of the two signals (of the same frequency) passing through it in 
opposite directions. 
The external lines J and K, as well as the lines interconnecting the 
components, may be waveguides or other microwave transmission devices; in 
which case the other elements are also microwave devices adapted to 
interface with the lines. 
Circulator 10 routes the left input signal on line K to the input of 
preamplifier 14, routes the output from power combiner H to the left port 
to line K, and provides signal isolation from power combiner H to 
preamplifier 14. Similarly, circulator 12 routes signals from line J to 
preamplifier 16, from line I to line J, and provides isolation from line I 
to preamplifier 16. 
Amplifiers 14 and 16 will normally be low power preamplifiers which provide 
the desired system sensitivity and noise figure. Amplifiers 18 and 20 will 
normally be identical power amplifiers. 
Power dividers A, B, E and F serve to equally divide a single signal into 
two signals. Power combiners C, D, G and H combine signals from two 
sources into a single output signal. 
The phase shifters 22, 24, 26 and 28 in a first diamond or bridge 
configuration, and phase shifters 32, 34, 36 and 38 in a second diamond, 
are used to introduce precise phase changes in the signals passing through 
them. 
The power dividers and combiners A-G may be considered to be nodes of the 
two diamonds. Node A splits the signals from preamplifier 16 for input to 
phase shifters 26 and 28. Node B splits the signals from preamplifier 14 
for input to phase shifters 22 and 24. Node C combines the signals from 
phase shifters 22 and 26 for input to amplifier 18. Node D combines the 
signals from phase shifters 24 and 28 for input to amplifier 20. 
Node E splits the signals from amplifier 18 for input to phase shifters 32 
and 36. Node F splits the signals from amplifier 20 for input to phase 
shifters 34 and 38. Node G combines the signals from phase shifters 36 and 
38 for input to an additional phase shifter 40. Node H combines the 
signals from phase shifters 32 and 34 for input to the circulator 10. The 
phase shifter 40, connected between node G and circulator 12, provides a 
phase shift of zero degrees or pi (180.degree.). This permits control of 
the relative phase of the two signals. The device 40 could be continuously 
variable. 
In the embodiment shown in FIG. 1, the devices 22, 24, 36 and 38 each have 
a phase shift of zero degrees; the devices 26 and 34 have a phase shift of 
minus one half pi (90.degree.), and devices 28 and 32 have a phase shift 
of plus one half pi (+90.degree.). 
Circuit connections: The circuit is constructed so that the following 
signal paths are equal 
BC=BD, AC=AD, CE=DF, EH=FH and EG=FG. 
Circuit Operation 
Signals enter the circuit from lines K and J, and are routed by the 
circulators into preamplifiers 14 and 16 respectively. The outputs of 
preamplifiers 14 and 16 are divided and phase shifted (by the quantities 
given in the diagram). The phase shifted signals are recombined at circuit 
junctions C and D and are sent to power amplifiers 18 and 20 respectively. 
The outputs of amplifiers 18 and 20 are divided into two equal signals 
which are routed into the upper phase shifters. 
After being phase shifted the signals are recombined at circuit junctions G 
and H. If the shifts are accurate, the signal at node H will be the same 
signal as entered at line J except it will be amplified. Likewise, the 
signal at node G will be the same as entered at line K with amplification. 
In addition, the circuit will prevent signals from returning back in the 
direction of their origin. This is accomplished since the values of the 
phase shifters cause a 180 degree phase shift in the two signal components 
which are returning back to their source, be it either K or J. 
It is not necessary for both input signals on lines K and J to be present 
simultaneously for this amplifier circuit to amplify and route signals. 
Alternative Embodiment 
Several variations are possible for the phase shifter quantities shown in 
FIG. 1. In a first alternative, phase shifters 22, 24, 28, 32, 36 and 38 
each provide a phase shift of zero (0.degree.), while phase shifters 26 
and 34 each provide a phase shift of pi (180.degree.). In a second 
alternative, phase shifters 22, 24, 28, 34, 36 and 38 each provide a phase 
shift of zero (0.degree.), while phase shifters 26 and 32 each provide a 
phase shift of pi (180.degree.). 
These circuits perform in the same manner as the circuit in FIG. 1 in that 
the phase shifters cause the signals to flow from line K to J with 
amplification and from line J to K with amplification and at the same time 
prevent signals from returning back in the direction of their source. 
In another embodiment shown in FIG. 2, low-power, high-speed electronic 
switches are used with the preamplifiers. One switch 42 is located 
somewhere between circulator 10' and node B', and another switch 44 is 
located somewhere between circulator 12' and node A'. The switch control 
circuit (not shown) senses the signals to cause one switch to be open and 
the other closed, changing according to the instantaneous direction of 
signal flow. These switches are installed to prevent the simultaneous flow 
of signals. through the system. 
It is also possible to replace the circulators 10 and 12 by high power 
switches. At any moment, one of the switches would couple line J or K to a 
preamplifier, while the other couples the output from node H or G to the 
other line K or J. 
For optimum operation of the circuit of FIG. 1, a large emphasis should be 
put on phase matching of components. The reflection coefficients of the 
combiners and splitters need to be extremely low. The amplifiers 18 and 20 
should maintain phase linearity over the range from zero input to an input 
equal to a preamplifier output. 
Thus, while preferred constructional features of the invention are embodied 
in the structure illustrated herein, it is to be understood that changes 
and variations may be made by the skilled in the art without departing 
from the spirit and scope of my invention.