Three Diode Balanced Mixer

The within described invention relates to the field of radar, millimeter wave devices, and to Gunn oscillators in particular.

In devising a mixing device for signals in the millimeter wave frequency range (30 to 300 GHZ) special problems may arise. A usual manner of providing a mixing function in such enviroments is to use hybrid couplers, having Schottky Barrier diodes, to create a balanced mixer. However, some of the disadvantages of such systems include excessive noise, high as 12.5 db DSB for example, and that the units can burn out at reaching a relatively low power level, 100 milliwatts CW, RF power for example. Yet there is a demand for ever higher power levels in modern communications equipment and a necessity for even greater fidelity in communication for which noise is detrimental. Improvement of devices for conventional mixing, especially in these two respects would therefore be greatly desirable.

The application relates to an improved mixer which is considered to be patentably distinct from our earlier invention described and claimed in our copending patent application Ser. No. 507,886, filed June 27, 1983, and which relates to a dual Gunn diode self-oscillating mixer.

According to the invention, it is possible to slave two self-oscillating Gunn diode cavities and inject a signal for the purpose of mixing with the self-oscillation voltage within the cavity. A line connection is made between the cavities so that power may circulate therebetween and thereby have injection locking. The line is adjusted in length so that it is compatible with a 180.degree. phase difference which exists between the two cavities. Two signals are fed along image lines into the two cavities at 180.degree. phase difference that are derived from a single signal intended for mixing by passing it through a 90.degree. splitter, with one input dielectric line terminated in a load to avoid reflections. Another 90.degree. phase shift, making 180.degree. total, is accomplished in the lines by the coupling effect. When an energy signal of different frequency impinges upon a Gunn diode self-oscillating mixer a mixing operation results. This yields an intermediate frequency (IF) signal which is tapped from the cavity by an electrical lead. The IF signal represents the mixed signal which was sought, and it can be amplified and processed or displayed in the CRT.

Accordingly, it is an object of the invention to provide a high powered mixing device, suitable for use in high frequency communications systems.

Another object of this invention is to provide a mixing device which may handle relatively high powered energy signals, yet with low noise added and relatively higher burnout threshold.

A still further object of this invention is to provide a relatively low cost, high power, dielectric compatible mixing device suitable for applications in modern communications systems.

These and other objects of the invention will be readily apparent to those skilled in the art in connection with the following description of the invention, and the appended drawings, in which:

FIG. 1 is a top layout view of slaved Gunn diode cavity oscillators according to the invention;

FIG. 2 is a cross-sectional side view of a Gunn diode cavity oscillator usable in this invention;

FIGS. 3A and 3B show respectively a plan top, and a side view for constructing an integrated V-band device according to this invention; and

FIG. 4 shows a system view for a three cavity Gunn diode balanced mixer.

In FIG. 1, two cavities 102, 103 are shown. The cavities, which may comprise internal structure such as in FIG. 2 as but one type of embodiment, are already self-oscillating at a select frequency. Both cavities are arranged to oscillate at the same frequency, and to insure this they are "injection locked", here by means of coax line 106. The purpose of line 106 is to allow energy waves from both cavities to intermix, thereby causing the two cavities to harmonize as one in effect, and oscillate at the same frequency. Other means of injection locking may instead be used; one other method is to move the cavities together so that they are back-to-back on adjacent sides, and to then delete coax 106 and replace its function by an aligned hole through both walls for the desired energy wave contact. It should be noted that a method is inherently shown here for boosting output power of cavity mixing devices by in effect having two or more cavities in "parallel", their mutual power being combined as at junction 112, e.g. In this mixing system it should be noted also that higher than conventional input power levels are being handled. The cavity-plus-Gunn diode as shown can handle approximately 30 times (3 watts average compared with 100 MW averge) as much input power as a non-cavity Schottky Barrier diode element alone, a Tunett diode, or a two-beam lead diode for instance, which elements are conventionally used for mixing in these radar type applications. A cycle of use for this mixing system, used in a radar application, might be as follows: During a transmit cycle, a relatively high powered signal out of cavity 102 and/or, 103 may be allowed to propagate along image line halves 104, 105 respectively, joining in signal 120, delivered to for example an antenna connected to the left end of image line 104. At 126, the signals are coupled together thus doubling power usually transmitted by a single cavity alone. Signal 120 might represent for example a radar broadcast signal for which a return signal 100 is expected in a next subcycle. It is this signal 100 which will ultimately be mixed with the self-oscillating signals already subsisting within cavities 102 and 103. The frequency of signal 100 will mix with the common frequency in 102, 103 and will produce IF signals 109, 111 respectively in the cavities which IF signals are representative of the desired mixed quantities. The mixing of return signal 100 with that in the cavities is useful as an operation in radar correlation to determine a target, as is known. While details of the use of the IF signal in so determining a target will not here be explained, it is understood as an application to those skilled in the art.

Image lines 104, 105 together form a pair known as a 3 db hybrid image line coupler. Functionally, the coupler and overall image lines serve to impart a 180.degree. phase shift total as will be explained and further to split a signal such as return signal 100 into signals 110 and 101, in region 126, where 90.degree. phase shift is imparted, one half power going to each cavity. In the reverse direction, split signals leaving the cavities can be coupled in region 126 into a joint signal. As a result of the hybrid coupler action, cavities 102, 103 are operating with input injected signals which are 180.degree. apart, and this 180.degree. phase difference accounts for complete noise cancellation, the noise signals being subtracted from one another, while the input signals are not. It is important that if line 106 is used, it should be made of a length consistent with the 180.degree. phase difference of the two cavities, namely it should be an odd number of half wavelengths of the cavity frequency. Half of the full 180.degree. phase shift, 90.degree., is first introduced in the region of the coupler 126 as was mentioned. In the region of 126 there are two painted magnetic dots found, there, one on each image line facing one another, which accomplish these coupling and shifting functions. Because the power was split from a common signal 100, the two resulting signals 110, 101 have identical waveforms, and are equally of half amplitude each though shifted in phase 90.degree.. An additional 90.degree. phase shift, completing the 180.degree. total needed, is imparted automatically in the lines 104, 105, as the signals 110, 101 pass to the cavities 102, 103, by the principle of induction in these lines which are sufficiently proximate to one another for field interaction. A load termination 116 here is needed to terminate the unused left end of line 105 so as to avoid line reflections in this unused branch. The image lines 104, 105, as will be explained further with reference to an alternative integrated version of the invention in FIGS. 3A and 3B, can be embodied of any high resistivity dielectric material such as quartz, alumina or even plastic. However, it is possible to also implement the lines of conventional waveguide hardware functional elements though more bulky, to have the coupling and phase shifting operations also done by conventional waveguide type functional elements though such plumbing is cumbersome, or of any other devices which will perform functions of conducting wave signals, splitting, coupling and phase shifting, as were described. Likewise, many type embodiments are possible for the self-oscillating cavities 102, 103 other than that sample shown in FIG. 2. Other substitutions of negative resistance devices than a Gunn diode are quite possible, and here are other ways possible to feed out the IF signals 109, 111 and combine them than those shown by FIG. 2 and the combination step shown by block 112. Use of grounded output resistor 107 and amplifier 108 as means of isolation and taking out the signal in a usable form can also be modified into any other arrangement for outputting which might be known in the art. Likewise, input dielectric 250 in FIG. 2 could be changed for a waveguide type input, possibly bolted onto the cavity or the like, with waveguide type images lines used for 104, 105. Any self-oscillating cavity could be used for 102, 103 provided it met the requirements explained above. The concept of combining more than one cavity for boosting output power handling capacity might also be expanded to three or more cavities, if the phase shifts could be properly arranged for noise cancellation, and other necessary functions described above were satisfied.

In FIG. 2, a cavity type oscillator capable of accepting a dielectric waveguide input 250 is shown in cross-sectional side view. This structure comprises a non-magnetic metal cavity comprising metal top 206, metal base 207, metal end 210, metal back side 280, metal front side not shown; the left end being open to face the dielectric input shown.

Within the cavity bounded by the respective metal walls is found a suitably biased, self-oscillating Gunn diode 200; or other suitable negative resistance device generally. As explained, when energy waves impinge upon self-oscillating Gunn diode 200, they cause the Gunn diode self-oscillation to include a mixing operating of the new wave frequency with the self-oscillation frequency. An intermediate frequency (IF) oscillation signal in addition to the usual self-oscillating frequency signal and other frequency signals result from such mixing. The output intermediate frequency is of course related to the desired mixed signal. The self-oscillation of Gunn diode 200 is ordinarily accomplished with aid of biasing electrode 202, to which a biasing voltage of some +5 to +10 volts typically is applied by means of conductive rod 209 fed by a bias voltage applied through an inductive choke 208. The voltage applied by electrode 202 is ordinarily sufficient to cause the self-oscillations in Gunn diode 200 to begin; a typical frequency of operation might be 94 GHZ. Electrode size, shape and proximity among other factors of course also influences the said frequency; fine tuning may be accomplished by a metal tuner 203, which may be screw advanced deeper into the cavity or withdrawn as desired. The changed position of 203 within the confines of the cavity changes the cavity Q, or quality factor, which directly affects the self-oscillating frequency. The electrode may be thought of as a coarse tuner, and 203 may be thought of as a fine tuner element.

Solid metal post 209 performs the function not only of conducting electricity down to elements 200 and 202, but also of physical anchoring support for the various elements, including elements 200 and 202. The post 209 and electrode 202 are typically round, top cross-sectionally, though they need not be so. Atop post 209 can be a Tee connection, at which place the bias line from inductor 208 may be connected such as at point 217, and from which an output line 218 could also be connected. Post 209 is electrically insulated from metal top wall 206 by low capacitance insulator 211, which is a non-conductive insert, such as a standard O.S.M. fitting. When waves impact between the self-oscillation frequency and the operational frequency of the injected waves, which presumptively are different in value, mixing can occur. If waves enter at a frequency of perhaps 95 GHZ, such signal waves will combine in the Gunn diode with the ongoing self-oscillating frequency which might be 94 GHZ for example. The different signals will mix, yielding an IF signal, among others, which will pass along post 209 eventually passing to 218 for output. This IF signal may also pass through a device for amplifying the signal if necessary so that it might be utilized. The amplifier may also posses band-pass filter properties so that only the IF signal might be amplified and passed through to output. Monitoring of cavity power levels may be done by having a metal RF probe 204 within a cavity to pick up presence of electrical power, the probe being electrically insulated from metal plate 206 by low capacitance insulator insert 212. Copper is a suitable material for the probe 204, as for the metal sides; however one could use any suitable non-magnetic conductive metal material. The coax line 106 shown in FIG. 1 is generally connected between the corresponding probes 204 of the two cavities though other methods might be used for power exchange for injection locking.

FIGS. 3A and 3B represent respectively the top and front side views of the entire network of this invention completely fabricated as a unitary structure on a single brass slab; the entire device can be commonly fabricated and made less than 21/2".times.11/2".times.3/4" in size.

Input slot 312 exposes the right most end of image guide half 305, corresponding to 104 of FIG. 1, at which place an antenna may be connected for two way transceiving as was mentioned earlier in regard to FIG. 1. It is also possible to merely inject here a signal to be mixed with the self-oscillating signal in the cavity(s). There need not be both a transmit signal and a later reflected signal in operation here, and no antenna is absolutely necessary; one could simply use the invention as an ordinary mixer without reference to a particular radar enviroment with particular meaning attached to inputs and resulting IF mixed signal. An input at 312 could simply be mixed with a self-oscillation signal in cavity 302, or 303, or both. A ground plane 309, which could be of brass or other suitable metal, is here made 21/2".times.11/2".times.3/8" thick though these and similar dimensions mentioned in this specification are generally for convenience and not critical. The hybrid 3 db image lines which may be made of quartz are small as 11/4" overall length, 1/20" .times.1/10" cross-section, and having a middle bend as shown extending for about 27/100" in the mid region of the coupler 308. The coupler comprises to dielectric image guides with an adjacent dot of metallic paint shown for obtaining 90.degree. of phase shift, though other means may be used for the coupling and phase shifting by 308. The two cavities 302 and 303 are here formed by inserting partition 306 mid way within "U" shaped housing 307. The dimensions of a cavity are, though of sufficient size to accommodate the two dielectric image guides (with adjacent dots of metallic paint shown for obtaining 900.degree. of phase shift), are in other respects relatively tiny, 9/20".times.11/20" with the small Gunn diode centered in a cavity barely 7/100" in from the housing's back wall. For the functions of biasing the Gunn diode and outputting the IF signal bath, a wire (331,332) is merely soldered to the top of the Gunn diode in each cavity, and led out to coax plugs 333,334 (not shown) respectively where coax connectors can be located. Though some of the functional parts of FIG. 2, i.e., electrodes, posts, probes, fine and coarse tuning, insulation, choke, inlet connections, may not be fabricated here in FIGS. 3A, 3B because of space limitations, a careful choice of layout, materials, sizes has been made here to ensure that the cavities will still similarly function, to self-oscillate, mix and the like, even without some of those refinements. A hole 330 in the wall 306 accomplishes the same injection locking, i.e., the maintenance of the same frequency of operation in both cavities, which function had been accomplished by coax 106 in FIG. 1. Atop housing 307 is a metal cover 311, which basically completes the cavities, they being open only on the side facing image lines 304, 305. As mentioned, cavities 302, 303, are injected with input signals from lines 304, 305, which signals are 180.degree. apart in phase. As before, a natural 90.degree. phase shift exists in the lines owing to induction, and 90.degree. further is added by coupler 308. The "L" shaped element 310 serves to mount the lines, provide place for signals at 312, and also has a like input slot for connecting a load termination at unused end of line 304 to prevent reflections as was explained previously. The element 310 is only about 11/4" on a side, and 3/8" in width and height, though such dimensions are but for convenience. The housing 307 and metal cover 311 are both only about 0.074" in height so one can appreciate the small sizes possible in this unitary fabrication. It can be readily seen that an integrated unit has now been described which is a complete mixing device in its own right. As mentioned earlier it is a mixing device capable of handling much higher input power levels than conventionally and further, deliver even higher boosted output power levels than heretofore possible based on those same input levels. The device further is relatively less expensive of construction, at least in the sense that the more expensive and often burning out Schottky barrier diodes have been replaced with the less expensive and more power handling capability of Gunn diode devices. Further, there is almost complete noise cancellation owing to the 180.degree. phase arrangement, making this a most desirable improved mixing device, particularly in radar correlator uses.

Shown therefore is a means for mixing two signals in the microwave range, yielding higher power, the power boosted by having more than one cavity, yet while introducing virtually no noise in the final mixed signal; and, of having a device with high power capacity for handling higher level inputs for the device owing to use of Gunn diode cavities of high power capacity instead of lone conventional elements which have a relatively low power level burnout capacity.

In FIG. 4, there is shown a mixing device utilizing three Gunn diode cavity oscillators 401, 402 and 403. In this arrangement, an output 409, from 403, a first of the three cavities, is used to supply the input signal to be coupled and split at 404, and eventually for feeding cavities 401, 402 with their respective input signals 411, 410, in parallel with respective input inductors 430, 431. The purpose of these inductors is to condition the signals input to the cavities 401, 402. Similarly, the respective signals 413, 412 outputted from respective cavities 401, 402 are conditioned by passage through filter networks in parallel with each output in order to produce respective conditioned IF signals 440, 441. In each respective output branch there is a circuit unit comprising the parallel combination of a grounded capacitor 422, 419 and series circuit of an inductor 423, 420 with a power source 424, 421 of negative bias, positive to ground.

The conditioned signals 440 and 441 are combined in combining means 414, whereat signal 440 is subtracted from signal 441. The combined output is next conditioned by passage through capacitor 415, and the capacitor output 416 can be tapped as the final, mixed output signal, generation of which had been sought.

The following analysis is offered as to the operation of the system in FIG. 4. Gunn diode oscillator 403 acts as the local oscillator source generating both local oscillator signal, S.sub.L, and noise, S.sub.n, unavoidable. The signal 409 coming in is designated as S.sub.s and at the 3 dB hybrid coupler 404, both inputs are split. Gunn diode oscillators 401 and 402 are injection locked by means of coax line 444 so that their own noise levels are suppressed and their oscillator frequency and phase can be made equal to that seen from the Gunn oscillator source 403. This means that they will see S.sub.L and S.sub.n taking on frequency and noise characteristics coming from the local oscillator. The phase shift at Gunn oscillator 402 will be 90.degree. ahead of 401 due to the 3 dB hybrid coupler. The signal voltage 409 will also be split with the signal at diode cavity 402, being 90.degree. ahead of the signal at diode cavity 401.

Because the diodes in the cavities 401, 402 are biased in the reverse directions it is noted that their a.c. currents will be 180.degree. out of phase for the IF currents so that

Where i.sub.o (t)=signal 414, i.sub.1 (t)=signal 440 and i.sub.2 (t)=signal 441. With the above conditions, the noise will be cancelled and the signals IF added so that a great improvement in signal to noise ratio will be obtained. More quantitatively this can be described as follows in relation to FIG. 4:

e=complex voltage coupling coefficient

t=complex voltage transmission coefficient

S.sub.s =R.F. signal voltage

Sn=L.O. noise voltage

S.sub.L =Local Oscillator Voltage

S.sub.1, S.sub.2 =signals 411, 410 incident on Gunn diode oscillators 401 and 402, respectively.

Oscillators 401 and 402 are injection locked to oscillators 403, except 401 will be 90.degree. ahead of 402 in phase for S.sub.L and Sn and 402 will be 90.degree. ahead of 401 with respect to S.sub.s. Their natural resonant frequency is locked, and noise can be suppressed, the main source of noise being that generated in the local oscillator 403.

The mixing at the diodes follows a square law response

For instance the voltage S.sub.1 developed at Gunn diode 401 is

and a similar equation for S.sub.2.

Using v.sub.1 (t)=ReS.sub.i and v.sub.2 (t)=ReS.sub.2, the IF currents can be found. Signal related 1 IF is:

and noise current at IF frequencies is:

It can be noted that the signal in each arm adds if .phi..sub.c -.phi.t=90.degree. out of the hybrid coupler, and that the noise cancels as .vertline.t.vertline..sup.2 -.vertline.c.vertline..sup.2 due to the coupling action. For 3 dB coupling this is 1/2-1/2=0, as is the case in FIG. 4.

Shown therefore is a balanced mixer which can operate with low noise yet at high powers. Because of conversion gain the system provides exceedingly high signal to noise ratios, while ordinarily, mixers give conversion loss. There is resistance to burnout for Gunn diodes can withstand 3-4 Watts CW without burnout, 30 times the value required for mixer diodes. The system is relatively economical; the diodes are low cost and the cavities easy to assemble. The system is compatible with image line and microstrip circuits.

While this invention may have been described with respect to a particular embodiment or embodiments, the description also includes all substitutions and modifications which can be made within the spirit and scope of the invention, as will be understood by those skilled in the art.