Two diode image rejection and pseudo-image enhancement mixer

An image rejection and pseudo-image enhancement mixer wherein mixer diodes are coupled to a signal source through a power divider without resistor, to a local oscillator through directional filters and to mixer outputs through a quadrature coupler. A 90.degree. phase shifter coupled between one diode and the power divider without resistor provides for phase cancellation of the pseudo-image signal, while the image signal is terminated at one output port and the desired signal is obtained at another output port of the quadrature coupler.

BACKGROUND OF THE INVENTION 
This invention relates to image rejection and enhancement mixers and in 
particular to image rejection and pseudo-image enhancement mixers 
utilizing only two diodes. 
A common problem for wide open receivers is to differentiate between a 
signal of interest and its image, an unwanted input frequency that arises 
from a source other than that to which the receiver is tuned. In 
narrow-band communication receivers, one approach to the image problem is 
to insert a filter in front of the mixer. However, when the RF bandwidth 
stretches across several octaves, only an electronically controlled 
preselector, such as a YIG filter, can be used to differentiate the image 
frequency. In addition, these filters are relatively expensive, increase 
the receiver's noise figure and offer rather sluggish tuning speeds, 
making it difficult to continuously monitor frequency agile radar signals. 
Another approach to image rejection involves the use of a channelized 
mixer. The benefits of an image channelized mixer are twofold: they help a 
receiver operator identify whether a signal is a true signal or its image, 
and they can be used to reduce the image noise generated by an RF 
amplifier. 
Another important aspect of channelizied mixers is their ability to 
properly handle intermodulation product. In general, the mixer is a 
nonlinear device, thus it generates harmonics of the input signals. 
Without proper RF filtering, harmonics of one signal can mix with a second 
input signal to produce an in-band spurious IF response. Even with a 
single input signal, harmonics of the input signal can interact with 
harmonics of the local oscillator signal and produce harmonically related 
intermodulation products with the IF bandwidth. 
It is known that the conversion loss of a mixer can be made to approach 
zero if all the harmonic and intermodulation frequencies can be reactively 
terminated and properly phased. Where f.sub.LO is the local oscillator 
frequency and f.sub.RF is the frequency of the input signal, each 
modulation product, mf.sub.LO .+-.nf.sub.RF, possesses some energy and 
represents loss unless converted back to IF frequency. It is practically 
impossible to properly control the impedances at each of the frequencies, 
especially when the mixer is to operate over a wide frequency range. 
Therefore, the primary aim is to reactively terminate and properly phase 
the (2f.sub.LO -f.sub.RF) intermodulation product, often incorrectly known 
as the image. If this pseudo-image cannot be well shorted or opened across 
the full band, then control of the other intermodulation products will do 
no good. 
Although the (2f.sub.LO -f.sub.RF) intermodulation product frequency is 
identical to the frequency of the image, a major distinction exists, which 
has not been properly observed by many. The image is a potential, or in 
fact, an actual input signal. The (2f.sub.LO -f.sub.RF) pseudo-image is 
generated by the mixer diodes and, therefor, is not and logically cannot 
be an input signal to the mixer that generated it. 
Channelized mixers are further considered below in connection with FIG. 1 
and FIG. 2. 
SUMMARY OF THE INVENTION 
Accordingly it is an object of the present invention to provide a new and 
improved image rejection and pseudo-image enhancement mixer. 
It is another object of the present invention to provide a new and improved 
image rejection and pseudo image enhancement mixer which is less expensive 
and yet more reliable than prior art devices. 
Yet another object of the present invention is to provide a new and improve 
image rejection and pseudo-image enhancement mixer in which local 
oscillator and radiofrequency isolation is independent of the need to 
match mixer diodes. 
A further object of the present invention is to provide a new and improved 
image rejection and pseudo-image enhancement mixer capable of local 
oscillator noise rejection by the selectivity of a directional filter. 
An additional object of the present invention is to provide a new and 
improved image rejection and pseudo-image enhancement mixer offering, the 
ability to accept very wide bandwidth signals without concern for the 
degradation of the signal due to the image. 
Among the advantages of the present invention are lower local oscillator 
power requirements and a lower required level of bias current where 
necessary, as in millimeter wave frequency applications. 
These and other objects and advantages of the present invention will become 
apparent to those skilled in the art upon consideration of the 
accompanying specification, claims and drawings. 
In order to attain the above mentioned and other objects the present 
invention comprises a modified power divider coupled to a phase shifter 
which is in turn coupled to a first directional filter. A second 
directional filter is separately coupled to the power divider. Each 
directional filter is coupled to a mixer diode and both mixer diodes are 
coupled to a quadrature coupler from which the output signal is taken and 
the image signal terminated. A local oscillator signal is supplied through 
a power divider coupled separately to said first and said second 
directional filters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, one prior art approach to the image rejection problem 
is to employ channelized mixers that inherently separate image and real 
frequencies. Such a prior art device comprises a pair of balanced mixers 
12 and 14, having matched pairs of mixer diodes 11 and 13 and 15 and 17, 
respectively, a 90.degree. quadrature coupler 10 and a power divider 16. A 
first input of coupler 10 is terminated as is a first output port of 
quadrature coupler 18. An input signal, As, along with an image signal 
A.sub.i, may be applied to a second input port of quadrature coupler 18, a 
local oscillator signal A.sub.LO, may be applied to an input of power 
divider 16, and a desired output signal at IF, A.sub.IF, may be obtained 
from a second output port of quadrature coupler 18. 
In the prior art device of FIG. 1, channelization is obtained using the 
pair of balanced mixers 12 and 14 the two 90.degree. quadrature hybrids 10 
and 18 and the in-phase power divider 16. The RF signal, A.sub.s, is split 
by quadrature coupler 10 and fed into balanced mixers 12 and 14, so that 
the phase difference between the RF inputs of mixers 12 and 14 is 
90.degree.. The local oscillator signal, A.sub.L0, is split by in-phase 
power divider 16. Hence, both mixers are driven in phase. The device shown 
in FIG. 1 relies on a resistive, in-phase power divider 16 to split the, 
local oscillators signal, LO, and resistively terminate the pseudo-image. 
Alternately, the pseudo-image can be reactively terminated by using a 
reactive LO power divider. This process is commonly referred to as image 
recovery. 
As shown in the image recovery system of FIG. 2, the basic difference 
between channelized and an image recovery design is that the RF and LO 
input circuits are reversed; the RF input signal is fed into an in-phase 
resistive power divider 20 and the LO is fed into a Lange-type quadrature 
coupler 24 wherein the pseudo-image is reactively terminated. 
Quadrature couplers 22 and 26 and their respective matched pairs of diodes 
21 and 23 and 25 and 27 make up two balanced mixers. The pseudo-image 
generated by the set of mixer diodes 21 and 23 is opposite in phase to the 
pseudo-image generated by the set of diodes 25 and 27. This is the same as 
presenting a short circuit to each pseudo-image frequency and forcing it 
back through the mixer a second time to mix with the LO signal thereby 
reducing the overall conversion loss. The image frequency is again 
rejected through a resistively terminated port of an IF quadrature coupler 
28. 
Nevertheless, both of the devices of FIG. 1 and FIG. 2 involve significant 
conversion loss. Furthermore, the reduction of conversion loss achieved by 
the device of FIG. 2 is obtained by using 3 expensive RF quadrature 
couplers 22, 24 and 26, as opposed to the one RF quadrature coupler, 10, 
required for the device of FIG. 1. 
FIG. 3 illustrates a preferred image rejection and pseudo-image enhancement 
mixer embodying the present invention in which a mixer input 20 is coupled 
to an input of a power divider without resistor 31, hereinafter referred 
to as a modified power divider, having a first output coupled in turn to 
an input of a phase shifter 32. A second output of modified power divider 
31 is coupled by way of a transmission line 33 to a first directional 
filter 34 and to the anode of a first mixer diode 35. An output of phase 
shifter 32 is coupled by way of a transmission line 36 to a second 
directional filter 37 and to the anode of a second mixer diode 38. 
A second mixer input 40 is coupled to a first input of a power divider 41. 
A first output of power divider 41 is coupled to directional filter 34 by 
way of a first resistively terminated transmission line 42 while a second 
output of power divider 41 is coupled to directional filter 37 by way of a 
second resistively terminated transmission line 44. 
As is understood by one skilled in the art, an equalizing network may be 
introduced between directional filter 34 and diode 35 and/or between 
directional filter 37 and diode 38 in order to better match the diode to 
the transmitting system within the scope of the invention described 
herein. The embodiment described herein is illustrated in FIG. 5. 
A quadrature coupler 50, having ports numbered (1), (2), (3) and (4) as 
shown in FIG. 3, is coupled by way of port (1) to a cathode of mixer diode 
35 and is coupled by way of port (3) to the cathode of mixer diode 38. 
Quadrature coupler 50 is further coupled to a resistive termination 54 by 
way of port (2) and to a mixer output 52 by way of port (4). 
Modified power divider 31 must comprise a power divider without a resistor, 
such as a Wilkinson power divider without resistor. On the other hand 
power divider 41 may comprise a power divider with or without a resistor, 
such as a Wilkinson power divider, as is suited to a particular 
application. Phase shifters, matched mixer diodes, directional filters, 
and IF quadrature couplers, such as quadrature coupler 50, are available 
to one skilled in the art and will not be discussed further. 
Microstrip or stripline are the most desirable transmission media for 
realizing the mixer of the preferred embodiment. The present invention is 
applicable to other transmission media; however, planar transmission media 
are the most cost effective. 
Turning now to the operation of the preferred embodiment, of FIG. 3 an 
input signal, containing desired signal component A.sub.s and image 
component A.sub.i, is applied to mixer input 30. The input signal is 
divided in modified power divider 31 and channeled to produce an in-phase 
RF signal along line 33 to mixer diode 35 and a quadrature RF signal along 
line 36 to mixer diode 38 after a 90.degree. phase shift in phase shifter 
32. 
Power divider 41 channels the local oscillator signal A.sub.LO applied to 
input 40 to directional filters 34 and 37. Directional filters 34 and 37 
provide mixer diodes 35 and 38 with an in-phase local oscillator signal, 
isolate the RF and LO signal, and reduce the LO noise (by the selectivity 
of the directional filters). 
Mixer diodes 35 and 38 each generate a pseudo-image signal at the image 
frequency in the direction of modified power divider 31. Phase shifter 32 
provides the required phase differential so that the pseudo-image signals 
are 180.degree. out of phase in modified power divider 31 thereby 
cancelling one another. Thus phase shifter 32 provides the required phase 
differential to virtually short circuit (or open circuit depending on the 
line length between the diodes, 35 and 38, and the junction of in-phase 
modified power divider 31) the pseudo-image frequencies. 
Quadrature coupler 50 separates the desired and image frequencies. However, 
in the prior art devices of FIG. 1 and FIG. 2, the RF and LO signals are 
permitted to change to generate a constant IF signal. In the approach 
discussed in this disclosure, only the RF signal can be changed while the 
LO frequency is fixed. This, of course, generates a variable IF frequency. 
The operation of the present invention can be demonstrated by performing a 
complete theoretical analysis of the mixer. This analysis begins by 
introducing a nonlinear transfer function of the mixing device which is 
expressed as: 
EQU i=A.sub.0 +A.sub.1 V+A.sub.2 V.sup.2 +A.sub.3 V.sup.3 + (1) 
where i and V are the device current and voltage, respectively, all other 
terms being constants. The applied voltage is a composite of an RF, local 
oscillator and image signals and is given by: 
EQU V=V.sub.RF cos {.omega..sub.RF t+0}+V.sub.i cos {.omega..sub.i 
t+.theta.}+V.sub.LO cos .omega..sub.LO t (2) 
where: 
V=total applied signal voltage 
V.sub.RF =desired signal voltage 
.omega..sub.RF =angular frequency of the desired signal 
t=time 
.phi.=phase angle of the desired signal 
.omega..sub.i =angular frequency of the image signal 
V.sub.Lo =local oscillator signal voltage 
.omega..sub.Lo =angular frequency of the local oscillator signal 
.theta.=the phase angle of the image signal. 
The mixer output, after all higher frequency terms are eliminated, becomes: 
EQU V.sub.IF =K cos (.omega..sub.RF t+.phi.) cos .omega..sub.LO t (3) 
EQU V.sub.IF =K' cos (.omega..sub.i t+.theta.) cos .omega..sub.LO t (4) 
where: 
V.sub.IF =IF signal voltage 
K, K'=constants 
and where all other terms are as defined above. 
The term .phi. in the equations above allows the effect of different input 
phases to be assessed. 
Standard trigonometric manipulations on these equations produce terms 
containing the sum and difference intermediate frequency outputs. In most 
applications, the inputs are down-converted, producing generalized IF 
outputs of the form: 
EQU VI.sub.Fl =K" cos {(.omega..sub.LO -.omega..sub.RF) t-.phi.}(5) 
EQU VI.sub.F2 =K'" cos {(.omega..sub.i -.omega..sub.LO) t+.theta.}(6) 
where: 
V.sub.IF2 and V.sub.IF2 =generalized output signal voltages 
K" and K"'=constants and where all other terms are as defined above. 
Note that the coefficients of t in the cosine terms are chosen to be 
positive. This is done to maintain a consistent phase convention. 
Application of these basic equations to the two diode mixer of FIG. 3, 
where .phi. is taken as 90 and .theta. is taken as 0 degrees, generates 
the following four signals: 
EQU V.sub.SIF(1) =K" cos (.omega..sub.LO -.omega..sub.RF)t (7) 
EQU V.sub.iIF(1) =K'" cos (.omega..sub.i -.omega..sub.LO) t (8) 
EQU V.sub.SIF(3) =K" sin (.omega..sub.LO -.omega..sub.RF)t (9) 
EQU V.sub.iIF(3) =K'" sin (.omega..sub.i -.omega..sub.LO)t (10) 
where: 
V.sub.SIF(1) =desired signal voltage at port (1) 
V.sub.SIF(3) =desired signal voltage at port (3) 
V.sub.iIF(1) =image signal voltage at port (1) 
V.sub.iIF(3) =image signal voltage at port (3) 
and where all other terms are as defined above. 
Therefore, the input signal at port (1) of IF quadrature coupler 50 is 
EQU V.sub.1 =V.sub.SIF(1) +V.sub.iIF(1) (11) 
and the input signal at port (3) of coupler 50 is 
EQU V.sub.3 =V.sub.SIF(3) +V.sub.iIF(3). (12) 
Application of scattering matrix analysis shows that the output at port 
(2), V.sub.(2), and the output at port (4), V.sub.(4), are: 
EQU V.sub.(2) =.sqroot.2 K" sin (.omega..sub.LO -.omega..sub.RF)t (13) 
EQU V.sub.(4) =.sqroot.2 K'" cos (.omega..sub.i -.omega..sub.LO)t (14) 
where all terms are as defined above. 
The pseudo-image frequency is generated by the cubic term of the mixer 
diode transfer function, which produces an output of the form: 
EQU V.sub.pi =K"" cos {(2.omega..sub.LO -.omega..sub.RF)T-.phi..sub.pi }(15) 
where V.sub.pi is the pseudo-image signal voltage, K"" is a constant, and 
.phi..sub.pi is the phase angle of the pseudo-image signal. 
Comparing the pseudo-image outputs of the two diodes at modified power 
divider 31, it is established that they are opposite in phase when .phi. 
is made to be 90 degrees. This provides the desired effect of odd symmetry 
which is represented by a short circuit. 
Thus the present invention accomplishes the image rejection of the devices 
of FIGS. 1 and 2 and the pseudo-image enhancement of the device of FIG. 2 
with significantly less conversion loss because power in the pseudo-image 
signal is stored rather than dissipated. Furthermore, the replacement or 
elimination of costly RF couplers and diodes by less expensive parts in 
the present invention has the additional benefit of allowing RF and LO 
frequency isolation regardless whether the mixer diodes are matched. In 
addition the use of fewer diodes lowers the cost, lowers power 
requirements, and lowers dc bias current requirements while retaining the 
desired performance characteristics discussed above. 
While the present invention has been described in terms of a preferred 
embodiment, further modifications and improvements will occur to those 
skilled in the art. For example, where useful to do so one skilled in the 
art understands that power divider 41 could be replaced by a quadrature 
coupler and a phase shifter as illustrated in FIG. 4. I desire it to be 
understood, therefore, that this invention is not limited to the 
particular form shown and I intend in the appended claims to cover all 
such equivalent variations which come within the scope of the invention as 
described.