Microwave double balanced mixer

A first 3 dB quadrature coupler is connected to an RF input terminal and its outputs are connected to first and second antiphase power dividers. The outputs of the first and second antiphase power dividers are individually connected to the first gate of four dual-gate field effect transistors (FET's). A second 3 dB quadrature coupler is connected to an LO input terminal and its outputs are connected to third and fourth antiphase power dividers. The outputs of the third and fourth antiphase power dividers are individually connected to the second gates of the four FET's. The drains of two of the FET's are connected together and to a low pass filter, and the drains of the other two FET's are connected together and to a second low pass filter. The outputs of the filters are connected to the two inputs of a differential amplifier, and the IF signal is taken at the output of the differential amplifier.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to application Ser. No. 642,223 entitled 
"Microwave Mixer Apparatus" of Scott J. Butler filed concurrently herewith 
and assigned to the assignee of the present application. 
BACKGROUND OF THE INVENTION 
This invention relates to microwave mixers. More particularly, it is 
concerned with double balanced mixer apparatus for frequency conversion at 
microwave frequencies. 
In order to convert a high frequency signal to a lower frequency at which 
signal processing is more readily accomplished, a mixer is employed. The 
mixer employs non-linear active devices which perform the frequency 
conversion function together with appropriate signal distribution networks 
which apply signals of desired magnitude and phase to the active devices. 
More particularly, radio frequency (RF) signals and local oscillator (LO) 
signals are applied to active frequency conversion devices by way of 
signal distribution networks, the output of the active devices being an 
intermediate frequency (IF) signal. 
A double balanced mixer is a particular type of mixer which employs four 
active devices having non-linear characteristics. Double balanced mixers 
provide high isolation between the RF, LO, and IF signals and thus have 
reduced spurious output content. Double balanced mixers of hybrid form 
have been fabricated using diodes as the active devices. Although such 
mixers can provide a low noise figure, conversion gain is not achieved. In 
order to provide both conversion gain and low noise figure at microwave 
frequencies, GaAs field effect transistors (FET's) have been used as 
active devices. At low microwave frequencies GaAs FET's have been employed 
in the signal distribution networks. At high microwave frequencies 
monolithic and hybrid microwave mixers have been fabricated employing 
passive signal distribution networks mounted on a substrate. These 
microwave mixers, however, do not provide low noise figure and conversion 
gain over a broad band of frequencies together with small physical size to 
the extent which is desirable for certain applications. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention a microwave mixer 
circuit comprises a first input terminal for receiving a first input 
signal at a first microwave frequency and a second input terminal for 
receiving a second input signal at a second microwave frequency. The 
circuit includes first and second 90.degree. phase differential means 
which in response to a signal at an input thereto produce signals of equal 
amplitude at first and second outputs which have a 90.degree. phase 
differential. The circuit also includes first, second, third, and fourth 
180.degree. phase differential means which in response to a signal at an 
input thereto produce signals of equal amplitude at first and second 
outputs which have a 180.degree. phase differential. The circuit also 
employs first, second, third, and fourth non-linear mixer devices each of 
which has first and second input electrodes and an output electrode. 
The first input terminal is connected to the input of the first 90.degree. 
phase differential means, and the first output of the first 90.degree. 
phase differential means is connected to the input of the first 
180.degree. phase differential means. The first output of the first 
180.degree. phase differential means is connected to the first input 
electrode of the first non-linear mixer device, and the second output of 
the first 180.degree. phase differential means is connected to the first 
input electrode of the second non-linear mixer device. The second output 
of the first 90.degree. phase differential means is connected to the input 
of the second 180.degree. phase differential means. The first output of 
the second 180.degree. phase differential means is connected to the first 
input electrode of the third non-linear mixer device, and the second 
output of the second 180.degree. phase differential means is connected to 
the first input electrode of the fourth non-linear mixer device. Thus the 
phase of the first signal at the first input electrode of each of the 
non-linear mixer devices differs from the phase of the first signal at the 
first input electrode of each of the other three non-linear mixer devices 
by 90.degree. or 180.degree.. 
The second input terminal is connected to the input of the second 
90.degree. phase differential means, and the first output of the second 
90.degree. phase differential means is connected to the input of the third 
180.degree. phase differential means. The first output of the third 
180.degree. phase differential means is connected to the second input 
electrode of the first non-linear mixer device, and the second output of 
the third 180.degree. phase differential means is connected to the second 
input electrode of the second non-linear mixer device. The second output 
of the second 90.degree. phase differential means is connected to the 
input of the fourth 180.degree. phase differential means. The first output 
of the fourth 180.degree. phase differential means is connected to the 
second input electrode of the third non-linear mixer device, and the 
second output of the fourth 180.degree. phase differential means is 
connected to the second electrode of the fourth non-linear mixer device. 
Thus the phase of the second signal at the second input electrode of each 
of the non-linear mixer devices differs from the phase of the second 
signal at the second input electrode of each of the other three non-linear 
mixer device by 90.degree. or 180.degree.. 
First and second filter means each have an input and an output and are 
operable to pass a third microwave frequency from the input to the output. 
The output electrodes of the first and second non-linear mixer devices are 
connected to the input of the first filter means, and the output 
electrodes of the third and fourth non-linear mixer devices are connected 
to the input of the second filter means. Thus signals at the third 
microwave frequency which are a product of the mixing action in the 
non-linear mixer devices are produced at the outputs of the first and 
second filter means. 
In accordance with another aspect of the invention microwave mixer 
apparatus comprises a substrate of insulating material having flat, 
planar, parallel, opposite surfaces. A first input terminal is on one 
surface of the substrate. A first quadrature power divider microstrip 
component having an input connected to the first input terminal and having 
first and second output connections is on the one surface of the 
substrate. A second input terminal is mounted on the one surface of the 
substrate, and a second quadrature power divider microstrip component 
having an input connected to the second input terminal and having first 
and second output connections is on the one surface. 
A first antiphase power divider microstrip component on the one surface has 
an input connected to the first output connection of the first quadrature 
power divider microstrip component and has first and second output 
connections. A second antiphase power divider microstrip component on the 
one surface has an input connected to the second output connection of the 
first quadrature power divider microstrip component and has first and 
second output connections. A third antiphase power divider microstrip 
component on the one surface has an input connected to the first output 
connection of the second quadrature power divider microstrip component and 
first and second output connections. A fourth antiphase power divider 
microstrip component on the one surface has an input connected to the 
second output connection of the second quadrature power divider microstrip 
component and has first and second output connections. 
A first field effect transistor mounted on the substrate has a first gate 
electrode connected to the first output connection of the first antiphase 
power divider microstrip component, a second gate electrode connected to 
the first output connection of the third antiphase power divider 
microstrip component, and an output electrode. A second field effect 
transistor mounted on the substrate has first gate electrode connected to 
the second output connection of the first antiphase power divider 
microstrip component, a second gate electrode connected to the second 
output connection of the third antiphase power divider microstrip 
component, and an output electrode. A first filter on the one surface has 
an input connection connected to the output electrodes of the first and 
second field effect transistors and has an output connection. 
A third field effect transistor mounted on the substrate has a first gate 
electrode connected to the first output connection of the second antiphase 
power divider microstrip component, a second gate electrode connected to 
the first output connection of the fourth antiphase power divider 
microstrip component, and an output electrode. A fourth field effect 
transistor mounted on the substrate has a first gate electrode connected 
to the second output connection of the second antiphase power divider 
microstrip component, a second gate electrode connected to the second 
output connection of the fourth antiphase power divider microstrip 
component, and an output electrode. A second filter on the one surface has 
an input connection connected to the output electrodes of the third and 
fourth field effect transistors and has an output connection. 
The apparatus also includes an output terminal on the one surface and 
output means mounted on the substrate coupling the output connections of 
the first and second filters to the output terminal. 
Microwave mixer apparatus in accordance with a third aspect of the 
invention comprises a substrate of insulating material having flat, 
planar, parallel, opposite surfaces. A first input terminal is mounted on 
one surface of the substrate. A first quadrature power divider microstrip 
component on the one surface has an input connected to the first input 
terminal and has first and second output connections. A second input 
terminal is mounted on the one surface of the substrate. A second 
quadrature power divider microstrip component on the one surface has an 
input connected to the second input terminal and has first and second 
output connections. A conductive ground plane is located on the other side 
of the substrate. 
A first antiphase power divider slotline component formed in the conductive 
ground plane has an input coupled through the substrate to the first 
output connection of the first quadrature power divider microstrip 
component and has first and second output connections. A second antiphase 
power divider slotline component formed in the conductive ground plane has 
an input coupled through the substrate to the second output connection of 
the first quadrature power divider microstrip component and has first and 
second output connections. A third antiphase power divider slotline 
component formed in the conductive ground plane has an input coupled 
through the substrate to the first output connection of the second 
quadrature power divider microstrip component and has first and second 
output connections. A fourth antiphase power divider slotline component 
formed in the conductive ground plane has an input coupled through the 
substrate to the second output connection of the second quadrature power 
divider microstrip component and has first and second output connections. 
A first field effect transistor mounted on the substrate has a first gate 
electrode connected to the first output connection of the first antiphase 
power divider slotline component, a second gate electrode connected to the 
first output connection of the third antiphase power divider slotline 
component, and an output electrode. A second field effect transistor 
mounted on the substrate has a first gate electrode connected to the 
second output connection of the first antiphase power divider slotline 
component, a second gate electrode connected to the second output 
connection of the third antiphase power divider slotline component, and an 
output electrode. A first filter on the substrate has an input connection 
connected to the output electrodes of the first and second field effect 
transistors and has an output connection. 
A third field effect transistor mounted on the substrate has a first gate 
electrode connected to the first output connection of the second antiphase 
power divider slotline component, a second gate electrode connected to the 
first output connection of the fourth antiphase power divider slotline 
component, and an output electrode. A fourth field effect transistor 
mounted on the substrate has a first gate electrode connected to the 
second output connection of the second antiphase power divider slotline 
component, a second gate electrode connected to the second output 
connection of the fourth antiphase power divider slotline component, and 
an output electrode. A second filter on the substrate has an input 
connection connected to the output electrode of the third and fourth field 
effect transistors and has an output connection. 
The apparatus also includes an output terminal on the one surface and 
output means mounted on the substrate coupling the output connections of 
the first and second filters to the output terminal.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a schematic circuit diagram of a microwave mixer in accordance 
with the present invention. The circuit includes a first 3 dB quadrature 
coupler 10; that is, a power divider which produces signals at its outputs 
that are of equal amplitude and have a 90.degree. phase differential. One 
input of the first quadrature coupler 10 is connected to an RF input 
terminal 11. The other input is connected to a resistive load 12. The 
first output of the first quadrature coupler 10 is connected to an input 
of a first antiphase power divider 14 which produces at its output 
terminals signals that are of equal amplitude and have a 180.degree. phase 
differential. The other input of the first antiphase power divider 14 is 
connected to a resistive load 15. The second output of the first 
quadrature coupler 10 is connected to an input of a second antiphase power 
divider 18. The other input of the second antiphase power divider 18 is 
connected to a resistive load 19. 
A local oscillator (LO) input terminal 21 is connected to an input of a 
second quadrature coupler 22. The other input of the second quadrature 
coupler 22 is connected to a resistive load 23. The first output of the 
second quadrature coupler 22 is connected to an input of a third antiphase 
power divider 24 which has its other input connected to a resistive load 
25. The second output of the second quadrature coupler 22 is connected to 
an input of a fourth antiphase power divider 28 having its other input 
connected to a resistive load 29. 
The circuit includes four dual-gate field effect transistors (FET's) Q1, 
Q2, Q3, and Q4. More particularly, the FET's Q1-Q4 are GaAs FET's. The 
first output of the first antiphase power divider 14 is connected to the 
first gate of FET Q1, and the second output is connected to the first gate 
of FET Q2. The first output of the second antiphase power divider 18 is 
connected to the first gate of FET Q3, and the second output is connected 
to the first gate of FET Q4. The connections of the signal distribution 
networks to the FET's are such that the RF signals appearing at the first 
gates may be expressed as: 
FET Q1: A cos (.omega..sub.RF t-.pi./4) 
FET Q2: A cos (.omega..sub.RF t+.pi./4) 
FET Q3: A cos (.omega..sub.RF t-.pi./2) 
FET Q4: A cos (.omega..sub.RF t) 
The first output of the third antiphase power divider 24 is connected to 
the second gate of FET Q1, and the second output is connected to the 
second gate of FET Q2. The first output of the fourth antiphase power 
divider 28 is connected to the second gate of FET Q3, and the second 
output is connected to the second gate of FET Q4. The connections of the 
signal distribution networks are such that the resultant local oscillator 
(LO) signals appearing at the second gates of the FET's may be expressed 
as: 
FET Q1: B cos (.omega..sub.LO t) 
FET Q2: B cos (.omega..sub.LO t-.pi./2) 
FET Q3: B cos (.omega..sub.LO t+.pi./4) 
FET Q4: B cos (.omega..sub.LO t-.pi./4) 
The mixing action performed by each of the dual-gate FET's produces an 
intermediate frequency (IF) such that .omega..sub.IF =.+-.(.omega..sub.RF 
-.omega..sub.LO). The desired IF signals appearing at the drains of the 
FET's may be expressed as: 
FET Q1: C cos (.omega..sub.IF t.+-..pi./4) 
FET Q2: C cos (.omega..sub.IF t.+-..pi./4) 
FET Q3: C cos (.omega..sub.IF t.+-.3.pi./4) 
FET Q4: C cos (.omega..sub.IF t.+-.3.pi./4) 
The drains of FET's Q1 and Q2 are connected together and to the input of a 
low pass filter 35. Similarly, the drains of FET's Q3 and Q4 are connected 
together and to the input of a second low pass filter 37. The combined 
output signals of FET's Q1 and Q2 and of FET's Q3 and Q4 are thus filtered 
to remove any extraneous high frequency components. The signals at the 
outputs of the filters 35 and 37 may be expressed as: 
Filter 35: D cos (.omega..sub.IF t.+-..pi./4) 
Filter 37: D cos (.omega..sub.IF t.+-.3.pi./4) 
As illustrated in FIG. 1 the outputs of the low pass filters 35 and 37 may 
be applied to the two input connections of a conventional differential 
amplifier 40 employing three GaAs FET's. The differential amplifier 40 
functions as an antiphase power combiner which also provides gain 
resulting in a single unbalanced IF output signal at an IF output terminal 
41. 
FIG. 2 is a plan view illustrating a microwave mixer apparatus in 
accordance with the present invention for which FIG. 1 is the equivalent 
circuit schematic. The apparatus includes a thin substrate 50 of 
insulating material having flat, planar, opposite surfaces. Most of the 
components are fabricated on, or mounted on, the upper surface as shown in 
FIG. 2. The under surface of the substrate has a conductive ground plane 
deposited thereon as is well known in the art. 
The apparatus includes a first Lange coupler 51 which is a 3 dB quadrature 
coupler of microstrip structure having an input connected to an RF input 
terminal 52. The other end of the first quadrature coupler 51 is connected 
by way of a resistive load 53 and a through connection to the ground 
plane. A second Lange coupler 55 has one input connected to a LO input 
terminal 56 and the other to a load 54 and a through connection to the 
underlying ground plane. 
The four antiphase power dividers are microstrip components which are 
modified hybrid ring, or rat race, couplers 57, 58, 59, and 60. Slotline 
portions 91, 92, 93 and 94 of the couplers 57, 58, 59, and 60, 
respectively, are formed in the conductive ground plane on the underside 
of the substrate. The inputs of the 180.degree. power dividers 57, 58, 59, 
and 60 are appropriately connected to the outputs of the Lange couplers 51 
and 55 and their other inputs are connected to the underlying ground plane 
by way of resistive loads 61, 62, 63, and 64, respectively. 
The outputs of the 180.degree. couplers 57, 58, 59, and 60 are connected in 
accordance with the showings in the circuit diagram of FIG. 1 to the gates 
of dual-gate GaAs FET's 71, 72, 73, and 74, corresponding to FET's Q1, Q2, 
Q3, and Q4, respectively, of FIG. 1. The drains of FET's 71 and 72 are 
connected together and to a first low pass filter 77. Similarly, the 
drains of FET's 73 and 74 are connected together and to a second low pass 
filter 78. The outputs from the low pass filters 77 and 78 are connected 
to the FET's of a differential amplifier 80. Biasing voltages to the 
differential amplifier 80 are applied over conductive paths 86. The output 
of the differential amplifier 80 is connected to an IF output terminal 81. 
Additional components as required such as biasing arrangements and RF 
matching networks tailored to specific applications may be mounted on the 
substrate or be external thereof. Electrical connections to the circuit 
elements may be by way of conductive strips and bonding pads on the 
substrate or by direct wiring to the elements. 
The microwave mixer circuit of FIG. 1 may also be embodied in apparatus 
having several of the components fabricated on, or mounted on, the 
underside of a substrate 100 as illustrated in FIG. 3. A conductive ground 
plane on the underside of the substrate 100 has portions removed to form 
slotline components or to provide mounting space for components. An RF 
input terminal 103 is connected to one input of a Lange 90.degree. coupler 
104. The other input of the quadrature coupler 104 is connected by way of 
a resistive load 105 and a through-contact to the ground plane. A second 
Lange coupler 106 has an input connected to an LO input terminal 107. The 
other input is connected by way of a resistive load 108 and a 
through-contact to the ground plane on the underside of the substrate 100. 
The outputs of the quadrature couplers 104 and 106 are connected by 
through-contacts to the inputs of slotline components formed in the ground 
plane on the underside of the substrate. Four slotline-to-coplanar 
waveguide transitions 110, 111, 112, and 113, function as antiphase power 
dividers. The outputs of the transitions are connected to the gates of 
dual-gate GaAs FET's 115, 116, 117, and 118 in accordance with the 
schematic circuit diagram of FIG. 1. The FET's are mounted in spaces in 
the ground plane on the underside of the substrate 100. 
Filters 121 and 122 are mounted on the upper surface of the substrate 100 
and are connected to the drains of the associated FET's. The components of 
the differential amplifier 130 are mounted on the underside of the 
substrate in an opening in the ground plane. Electrical connection 135 on 
the upper surface of the substrate are connected to the differential 
amplifier 130 by contacts passing through the substrate. The output of the 
differential amplifier 130 is connected to an IF output terminal 131 by a 
connecting strip 132 on the upper surface of the substrate. 
Microwave mixer apparatus as illustrated in FIGS. 2 and 3 may be employed 
for radio frequencies of 6 to 18 GHz with local oscillator frequencies of 
between 6 and 16 GHz to produce a resulting intermediate frequency of from 
DC to 4 GHz. The dimensions of a suitable substrate are approximately 1/4 
inch square. 
Microwave mixers as shown and described may be modified to produce an IF 
signal by up conversion rather than down conversion by employing an 
in-phase power combiner in place of the IF differential amplifier. If a 
wideband 180.degree. coupler is used, summation up conversion is obtained 
by using the sum port as the IF output and connecting a resistive load to 
the difference port. Delta up conversion can be obtained by taking the IF 
output at the difference port and connecting a resistive load to the sum 
port. In either arrangement suitable bandpass filters are employed in 
place of the low pass filters. 
Microwave mixers in accordance with the present invention are double 
balanced mixers providing both low noise figure and conversion gain over 
wide bandwidths. The signal conditioning provided by the signal 
distribution circuitry between the inputs and mixer FET's and by the IF 
power combining circuitry results in suppression of many of the spurious 
mixer delta and sigma cross products. In addition, the apparatus is 
compact and may be fabricated utilizing both sides of a substrate. 
While there has been shown and described what are considered preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the invention as defined by the appended claims.