BACKGROUND OF THE INVENTION

The present invention relates to a microwave input circuit with parametric upper sideband down converter which is directly connected to a microwave antenna. More particularly, the present invention relates to such a parametric down converter which is terminated by a real impedance at the input signal frequency which is not much different from the intermediate frequency produced at the output of the down converter.

Parametric amplifiers per se are known, for example from the book, VARACTOR APPLICATIONS, by P. Penfield and R. P. Rafuse, published by M.I.T. Press, 1962. It is also known from U.S. Pat. No. 3,991,373, issued Nov. 9, 1976 to Maurer et al. (the subject matter of which is incorporated herein by reference) to directly connect a down converter to the receiving antenna for microwaves and to employ for this purpose a parametric upper sideband down converter which is terminated with a real impedance at the image frequency. However, the down converter disclosed in this patent will have little noise only if the input signal frequency f_s1 is much greater than the intermediate frequency f_z produced at the output of the down converter (see column 4, equation (5) of the patent).

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an input circuit for microwaves which includes a parametric down converter and which amplifies and has low noise even if the ratio f_s1 /f_z is not much greater than unity (in which case the input signal frequency f_s1 and the image f_sp =f_p1 -f_z =f_s1 -2f_z are far apart) and if the noise temperature T_sp of the image frequency termination is less than the noise temperature T_D of the reactance diode. In this case, the converter noise temperature T_m is of the same order of magnitude as the noise temperature T_D, but this is sufficient, for example, for terrestrial radiometer application.

The above object is accomplished by constructing the input circuit of the microwave receiving system in accordance with the present invention. The input circuit includes a parametric upper sideband down converter which has its signal input directly connected to the microwave antenna and terminated by the real portion of the antenna impedance.

The parametric down converter of the input circuit may be either of the series type of the parallel type and comprises the cascade connection of a parametric down converter stage and a parametric up converter stage with both stages being either of the series or of the parallel type. Each of the stages includes a reactance diode wherein the input signal to the respective stage is mixed with a respective pump frequency produced by a pump oscillator to produce a desired intermediate frequency at the output of the respective stage. The output of the up converter stage, and hence the output of the entire down converter stage, is connected to further amplifying stages of the receiving system.

If a series type parametric down converter is utilized, then the signal input of the down converter is terminated by the real antenna resistance (R_g) at the input signal frequency (f_s1) and the down converter stage is terminated at its image frequency (f_sp) by a real resistance (R_sp) and satisfies the following relationship: ##EQU3## where R_s1 is the series resistance of the reactance diode in the down converter stage.

If a parallel type parametric down converter is utilized, however, then the signal input of the down converter is terminated by the real antenna conductance (G_g) at the input signal frequency (f_s1), and the down converter stage is terminated by a real conductance (G_g) at its image frequency (f_sp) and satisfies the following relationship: ##EQU4## where G_g is the conductance loss of the reactance diode in the down converter stage, due to R_s1.

The output of the parametric down converter according to the invention is preferably coupled to a noise-matched IF amplifier. If the down converter is of the series type, then the subsequently connected noise-matched IF amplifier should satisfy the following relationship for R_s1 =R_s2 : ##EQU5## where R_E =the input resistance of the IF amplifier;

R_s,min =the resistance of the signal source of the IF amplifier for minimum noise;

f_Z1 =the intermediate frequency produced by the first or down converter stage of the down converter;

q₁ =the dynamic quality factor of the reactance diode in the down converter stage;

R_s2 =the series resistance of the reactance diode in the up converter stage.

Alternatively, if the down converter according to the invention is of the parallel type, then the subsequently connected noise-matched IF amplifier should satify the following relationship: ##EQU6## where G_E =the input conductance of the IF amplifier;

G_s,min =the conductance of the signal source of the IF amplifier for minimum noise;

f_z1 =the intermediate frequency produced by the first or down converter stage of the down converter;

q₁ =the dynamic quality factor of the reactance diode in the down converter stage.

The IF amplifiers can be realized by a number of different circuit configurations. Preferably, however, they are parametric converter cascade amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the basic scheme for the entire down converter M according to the invention which includes the cascade connection of a down converter stage M₁ and an up converter stage M₂, as well as the associated frequency scheme.

FIG. 2 is a block diagram of another possible solution of the problem, which possible solution is an unfavorable one, however.

FIG. 3 is the basic equivalent circuit diagram of the entire down converter M of the series type according to the invention.

FIG. 4 is the basic equivalent circuit diagram of the entire down converter M of the parallel type according to the invention.

FIG. 5 is an equivalent circuit diagram of the entire down converter M of the series type according to the invention and a subsequently connected IF amplifier of the series type, realized by a parametric converter cascade.

FIG. 6 is the equivalent circuit diagram of the entire down converter M of the parallel type according to the invention and a subsequently connected IF amplifier of the parallel type, realized by a parametric converter cascade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown the basic block diagram of a down converter according to the invention for converting an input signal f_s1 supplied by a directly connected antenna to an intermediate frequency output signal f_z. As shown, the entire down converter M includes the cascade connection of a down converter stage M₁ with a real image frequency termination and a known conventional up converter stage M₂, as shown, for example, in the above identified VARACTOR APPLICATIONS book in Section 5.2, pages 99 et seq. In the down converter stage M₁, the input signal frequency f_s1 is mixed in a reactance diode D₁ with the pump frequency f_p1 to form an output signal at an intermediate frequency f_z1 which, as shown, is less than the desired final intermediate frequency f_z, while in the up converter stage M₂ the intermediate frequency signal f_z1 =f_s2 is mixed in a further reactance diode D₂ with a pump frequency f_p2 to form the desired intermediate frequency f.sub. z2 =f_z at the output of the up converter stage.

The down converter stage M₁ is terminated at its image frequency f_sp =f_p1 -f_z1 with a real impedance R_sp (noise temperature T_sp) and, due to the relationship f_s1 >>f_z1, has a low noise figure as described in the above-identified U.S. Patent to Maurer et al. Moreover, the down converter stage M₁ provides an available conversion gain (see AEU (1972), Issue 11, pages 475-480, equation (20)).

The second or up converter stage M₂ likewise furnishes, by itself and with optimum dimensioning, an available conversion gain and a low noise temperature (see VARACTOR APPLICATIONS book, Section 5.2.2, pages 104 et seq.). Whether this also applies for the cascade connection as in the present invention depends on the magnitude of the "source resistance" R_A1 (=output resistance of the first or down converter stage M₁). However, a theoretical investigation has shown that this is true. The available conversion gain in the up converter stage M₂, however, is less by about the factor 2 than the optimum value f_z /f_z1 (according to the Manley-Rowe equations). Moreover, the noise temperature T₂ is only slightly higher than the diode temperature T_D. However, according to the Friis formula, this amount plays only an insignificant part since the first or down converter stage M₁ has gain.

For comparison purposes, FIG. 2 shows a block diagram for a possible arrangement for a microwave system input circuit which includes a parametric upper sideband down converter without image frequency and which directly converts the input signal frequency f_s1 to the desired intermediate frequency f_z, followed by a parametric amplifier.

Compared to this likewise possible concept of FIG. 2, the arrangement of FIG. 1 has the following advantages:

1. the converter of the arrangement of FIG. 2 will have low noise but will have a conversion loss (see VARACTOR APPLICATIONS book, Section 5.4, pages 144 et seq.). This conversion loss thus must be compensated in the subsequently connected amplifier which may possibly require a multistage amplifier. This is not the case with the arrangement of FIG. 1 according to the invention.

2. The amplifier in the arrangement of FIG. 2 must have extremely low noise since its noise contribution is significant due to the conversion loss in the converter. Thus a very high pump frequency f_p,g >>f_z (generally also f_p,g >>f_s1) must be used in the amplifier of the possible arrangement of FIG. 2. In contradiction, in the arrangement according to FIG. 1, the highest occurring frequency is the input signal frequency f_s1, and the pump frequency f_p2 for the up converter stage M₂ is significantly smaller than f_s1. Under certain circumstances it is even possible with the arrangement according to FIG. 1 to use only one pump frequency oscillator, namely the oscillator for generating the pump frequency f_p2 for the stage M₂, and to generate the pump frequency f_p1 for the stage M₁ by frequency multiplication (f_p2 =nf_p1) of the pump frequency f_p2 with a low order of magnitude n (for example n=3).

The down converter M of FIG. 1 may be realized by using either a series type down converter stage in cascade connection with a series type up converter stage, or a parallel type down converter stage in cascade connection with a parallel type up converter stage. Depending on whether series or parallel type mixer stages are used, the down converter circuit arrangement of FIG. 1 according to the invention must satisfy one of the following relationships: ##EQU7##

This is the result of theoretical consideration which are not represented here. For the total circuit or chain of FIG. 1, it then applies (in approximation) that the available conversion gain L_v of the chain is ##EQU8## where q₁ =the dynamic quality factor of the reactance diode D₁ in converter stage M₁ ; that the minimum noise temperature of the converter cascade is ##EQU9## and that the associated optimum generator resistance, i.e., the real resistance of the antenna, the following applies:

Rg,opt =q1 ·Rs1                    (5)

where R_s1 =series resistance of the reactance diode in the converter stage M₁.

In order to illustrate the operation and advantages of the present invention by means of a numerical example, consider the following numerical values:

f_s1 =28 GHz; f_z =12 GHz; f_z1 =2 GHz;

f_sp =24 GHz; q₁ =7

From equations (3), (4) and (5) the following then applies: ##EQU10##

If, instead of using a cascade connection of a down converter stage M₁ and an up converter stage M₂ as shown in FIG. 1, the down conversion from f_s1 =28 GHz to f_z =12 GHz is handled by converter M₁ alone, the following data result for the noise minimum:

______________________________________   Tsp = TD = 290° K.                ##STR1##______________________________________Rg, opt    20 · Rs1                   12.3 · Rs1 ##STR2##     7.84 = 2270° K.                    1.7 = 493° K.Lv1 2.27           3.47______________________________________

This clearly shows that this latter solution is of no use.

Referring now to FIGS. 3 and 4, there is shown the circuit principle for the entire down converter M according to the invention with FIG. 3 showing such a converter of the series type and FIG. 4 showing such a converter of the parallel type. Each figure includes one known down converter stage M₁ with resistive image termination (as disclosed, for example, in the above-identified VARACTOR APPLICATIONS book and the AEU article) and a known conventional up converter stage M₂ without image frequency (e.g. as also disclosed in the VARACTOR APPLICATIONS book). In these figures, the following legends apply:

p₁, p₂ =the pump circuit frequencies for the respective stages M₁ and M₂ ;=2πf_p1 and 2πf_pz, respectively

z₁ =the intermediate frequency produced by the stage M₁ =2πf_z1

p₂ +z₁ =z₂ =the intermediate frequency produced by the stage M₂ =2πf_z2 =2πf_z

p₁ +z₁ =s₁ =input signal circuit frequency=2πf_s1

p₁ -z₁ =sp=image frequency=2πf_sp

In the equivalent circuit of the input circuit according to the present invention shown in FIG. 3, the antenna S is effectively represented by the signal source V_p+z and the resistance R_p.sbsb.1.sup.±z.sbsb.1. The reactance of the input circuit is represented by X_p.sbsb.1.sup.±z.sbsb.1 and the current through the antenna by I_p.sbsb.1.sup.±z.sbsb.1.

The signal coming from the antenna, which is directly connected to the signal input of the parametric down converter stage M₁ of the series type, is mixed in the parametric down converter stage M₁ with a pump frequency f_p1 in order to effectively demodulate the incoming signal. The pump frequency f_p1 is generated by a pump circuit which is represented by a source V_p1, a resistance R_p1 and a reactance X_p1 and which has a current I_p1. This pump frequency f_p1 is converted with the incoming signal in the reactance diode D₁ which has a series resistance R_s1. The signal, after being processed by the parametric down converter stage M₁, is now at an intermediate frequency of f_z1. The parameters of this parametric down converter stage M₁ are selected so as to satisfy equation (1) and, as shown, the down converter stage M₁ is terminated by the real antenna resistance at both the signal frequency and at the image frequency.

The output circuit of the parametric down converter stage M₁, which is shown as a reactance X_z1, is connected to the input of the up converter stage M₂ which, as shown, includes a further reactance diode D₂, having a series resistance R_s2, wherein the intermediate frequency f_z1 produced by the down converter stage M₁ is converted with the pump frequency f_p2 to form the output frequency f_z2. The pump frequency f_p2 is produced in a pump circuit represented by a voltage source V_p2, a resistance R_p2 and a reactance X_p2. The output circuit of the parametric up converter stage M₂, which is shown as a reactance X_p2+z1, is connected to the subsequent stages of the receiving system, which are represented by a resistance R_E '. The output resistance of the parametric down converter M is R_A.

With a parallel type parametric down converter M as shown in FIG. 4, the antenna is represented by the current source I_p.sbsb.1^+z.sbsb.1, a conductance G_p.sbsb.1.sup.±z.sbsb.1 and a susceptance of B_p1±z1. The parallel type parametric down converter stage M₁ provides a current I_p1 at the pump frequency f_p1 and the pump circuit has a conductance G_p1 and a susceptance B_p1. The parameters of this parametric down converter stage M₁ and the impedance of the antenna are selected so as to satisfy equation (2). The signals from the antenna and from the pump circuit are converted in the reactance diode D₁ which has a conductance loss G_D1. The output circuit of the parametric down converter stage M₁ is represented by an impedance Y_z1 and provides a signal V_z1 to the cascade connected up converter stage M₂.

As shown, the up converter stage M₂ includes a further reactance diode D₂ having a conductance loss G_D2 and a pump circuit represented by a current source I_p2, a conductance G_p2 and a susceptance B_p2. In the output circuit of the up converter stage M₂, which is represented by the impedance Y_p2+z1, there is provided the desired current I_z at the intermediate frequency f_z which is supplied to the subsequently connected load represented by the conductance G_E '. The output conductance of the parametric down converter M is G_A.

According to a further feature of the present invention, a suitable IF amplifier A is connected in series with the output circuit of the parametric down converter M, as shown, for example, in FIGS. 5 and 6. With such an arrangement it is possible to obtain a low noise input circuit by noise matching of the IF amplifier so that its noise temperature is brought to its minimum value (T_IF)_min. For this purpose, the following condition must be met:

RA =RS,min ·u2 or GA =GS,min /u2 (6)

where R_S,min or G_S,min is the input resistance or conductance, respectively, of the IF amplifier for minimum noise and u is the transforming ratio of the transformer between the output of the parametric down converter M and the input of the IF amplifier A, as shown in each of FIGS. 5 and 6.

According to the Friis formula, this input circuit, including the converter cascade M and the IF amplifier A, then has the noise temperature ##EQU11##

However, since, according to equation (3), ##EQU12##

With the present invention, it is possible to obtain noise matching of the IF amplifier and power matching of the parametric down converter M simultaneously. To achieve these results, according to the invention, a series type IF amplifier is utilized with a series type parametric down converter M, or a parallel type IF amplifier is utilized with a parallel type parametric down converter M, and the IF amplifier must satisfy a preset condition.

In particular, if the series type parametric down converter M, e.g. as shown in FIG. 3, is selected, then the subsequently connected series type IF amplifier must meet the condition: ##EQU13## where R_E =R_E '/u² is the input resistance of the IF amplifier.

A particularly advantageous embodiment of a series type IF amplifier A satisfying this condition when utilizing a down converter M of the series type is obtained by using a parametric converter cascade amplifier of the series type, as shown in FIG. 5. Such an amplifier comprises a cascade connection of a parametric up converter and a parametric down converter whose reactance diodes D₃ and D₄ are pumped by a common pump oscillator at the same frequency but in phase quadrative. Such parametric converter cascade amplifiers are described, for example, in U.S. Pat. No. 3,711,780, issued Jan. 16, 1973, to R. Maurer. Preferably, as shown, a common pump oscillator is used for the stages of the amplifier A and for the parametric down converter stage M₁.

Alternatively, instead of the converter cascade amplifier shown in FIG. 5, other series type amplifier configurations could be used for the IF amplifier connected to the series type parametric down converter, as long as the parameters of the amplifier circuit are selected so that the condition of equation (9) is satisfied. The desired IF amplifier can be realized if the active element is, for example, any of the following: a tube connected in a grounded grid configuration; a transistor connected in a common base configuration; or a field effect transistor connected in a common gate configuration.

If, however, a parallel type parametric down converter M as shown, for example, in FIG. 4 is utilized, then the subsequently connected IF amplifier must meet the condition ##EQU14## wherein G_E =G_E 'u² is the input conductance of the IF amplifier A.

Again, a particularly advantageous embodiment of an IF amplifier which satisfies this condition for a parametric down converter M of the parallel type is a parametric converter cascade amplifier of the parallel type, as shown in FIG. 6. Such an amplifier is especially advantageous since it is nonreciprocal and provides extremely good decoupling between its input and output in that its feedback admittance is suitably neutralized, as described, for example, in U.S. Pat. No. 3,237,017. By utilizing a parallel type converter cascade amplifier A with the parallel type parametric down converter M, the desired power matching for the entire mixer is obtained. Again, as shown, a common pump oscillator or source is used for both the down converter stage M₁ and for both stages of the amplifier A.

Again, other parallel type amplifier configurations can be used for the IF amplifier connected to the parallel type parametric down converter M, as long as the parameters of the circuit are selected so that the condition of equation (10) is satisfied. Such IF amplifiers can be realized if the active element of the IF amplifier circuit is, for example, any of the following: a tube connected in a grounded cathode configuration; a transistor connected in a common emitter configuration; or a field effect transistor connected in a common source configuration.

The present invention will now be explained in greater detail with respect to the embodiment shown in FIG. 6.

In this embodiment, the down converter M is of the parallel type and the reactance mixing diodes of the stages M₁ and M₂ are identified as D₁ and D₂, respectively. Y_p1±z1 identifies the admittance of the input circuit of the down converter stage M₁, where p₁ +z₁ is the input signal frequency and p₁ -z₁ is the image frequency.

The admittance of the intermediate frequency circuit for the down converter stage M₁ is indicated by Y_z1 while the intermediate frequency circuit for the up converter stage M₂ is indicated by Y_z. Under the condition that the pump frequency f_p1 of the down converter stage M₁ is much greater than the intermediate or output frequency f_z1 of the down converter stage M₁, the two frequencies p₁ +z₁ =f_s1 and p₁ -z₁ =f_sp are situated relatively closely together. Consequently, the antenna conductance G_p1+z1 (or antenna resistance R_p1+z1 in the case of a series type down converter) which is at the antenna temperature T_A can then be used simultaneously to terminate the image frequency, i.e. T_sp =T_A. Moreover, when f_s1 ≈f_sp, then R_g ≈R_sp or G_g ≈G_sp for the respective circuit arrangements and thus the condition of equations (1) or (2), respectively, is also satisfied automatically.

With T_sp =T_A it follows from equation (4) that ##EQU15## Consequently T_case is somewhat greater than T_D for T_A =T_D, but generally T_case <T_D for T_A <<T_D.

The microwave antenna to which the entire down converter M=M₁ +M₂ is connected is shown in FIG. 6 by the signal source identified with the letter S. In this embodiment, the down converter M is of the parallel type and consequently, according to the invention, has connected to it a converter cascade amplifier A of the parallel type. The two reactance diodes of the converter cascade amplifier A are marked C₃ and C₄. At both its input and its output, this IF amplifier has a parallel resonant circuit with an admittance Y_z which is tuned to the intermediate frequency f_z. The two reactance diodes C₃ and C₄ are coupled together via a common idle circuit identified by Y_p1±z. The IF amplifier A is coupled to the down converter M by a transformer with the transforming ratio u². The output conductance of the parametric down converter M is identified as G_A, while the input conductance of the IF amplifier A is marked G_E.

Under the above given conditions and if there is power matching at the input and at the output with simultaneous high gain, an input circuit according to the invention can achieve noise temperatures which, according to equations (4) and (7) and with sufficient diode quality q₁, lie below room temperature without the input circuit being cooled.

Preferably, as shown in FIGS. 5 and 6, a common pump oscillator is utilized for the input circuit to pump the down converter stage M₁ as well as the stages of the parametric IF amplifier A.

The circuit according to the invention has the particular advantage that the pump frequencies f_p1, f_p2 lie below the input signal frequencies f_s1 =f_p1 +f_z1. Moreover, the circuit according to the invention can easily be provided in integrated techniques.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.