FR2545278A1 - Circuit enabling the resonant frequency of a dielectric microwave cavity to be varied - Google Patents

Abstract

Circuit enabling the resonant frequency of a dielectric microwave cavity to be varied. This circuit comprises a coupling loop 12 arranged in the vicinity of the cavity 10 and tapping off a portion of the energy established in the latter and a means inserted into this loop and able to vary the reactance of the loop. According to the invention, this means consists of at least one Schottky diode 13, 14 loaded by an element equivalent to a resistor, the value of which is adjustable (for example a transistor 18). Application in telecommunications.

Description

The present invention relates to a circuit for varying

  the resonant frequency of a dielectric microwave cavity It finds

  an application in telecommunications ;.

  A dielectric cavity is generally made up of a cylinder of dielectric material of

  low loss and high permittivity A material used

  readable for this purpose is Zr Sn Ti O 4 The characteristics of a dielectric cavity are comparable to those

  of a metal-walled cavity, but its small di-

  mensions make it particularly well suited to

  microstrip microwave circuits A dedicated cavity

  lec Erique, 5 mm in diameter and 3 mm in height, has a resonant frequency of around 11 G Hz, which corresponds to the average fr 6 quence

  work of certain bidirectional links.

  To achieve a tuning, or a frequency change with such a cavity, we have used, in the past, several techniques = by placing near the cavity a metallic or dielectric disc and by mechanically varying the distance from this disc to the cavity, by placing a ferrite near the cavity and

  quietly varying the magnetic flux to which it is subjected

  put using a current generator, by placing a conductive loop near the cavity and inserting, in this loop, a varactor diode whose capacity is varied using

of a voltage generator.

  a The present invention incorporates this last principle of the coupling loop arranged near the cavity, loop whose reactance is varied by means of an appropriate element, but it renounces

  the use of a varactor diode Indeed, this compound

  sant requires an external voltage generator, this

  which weighs down the system The invention proposes, at the

  milk, to use an element which does not require such an external source of energy This element is constituted by a Schot tky diode (or several) Such a diode is self-polarized by the ion: microwave developed in the loop and which results

  microwave energy taken from the cavity.

  In addition, -to vary the reactance of -the bou-

  key, the diode is charged by an element equivalent to one.

  t Q resistance and we act on the value of this resistance

  this. A first advantage of the invention is that it is much simpler to modify the value of a resistor than to act on the capacity of a varactor diode. In fact, the resistive element can be a transistor (bipolar or field effect) which we do

  vary the equivalent resistance by an applied voltage

  quée on the base or the drain, which is very simple A second advantage of the invention is that it allows

  developments which were hardly compatible with the utility

  reading of a varactor diode: for example, the resistive element can be constituted by a thermistor, which makes it possible to stabilize the frequency of the cavity compared to the temperature One can also use a

  phototransistor, which allows optoelect modulation

tronic.

  The characteristics of the invention appear-

  will be better after the following description, examples

  of realization given for explanatory purposes and in no way

  limiting This description refers to drawings

annexed on which:

  FIG. 1 represents a diagram of a particular mode

  link of embodiment of the invention, FIG. 2 illustrates, in section, the mounting of the circuit of the invention,

  FIG. 3 is a diagram representing the various

  its phases of a bidirectional link, Figure 4 shows the control voltages it

  must be applied to two oscillators using the invention

tion.

  Figure 1 shows a dielectric cavity

  that 10 coupled to a loop 12 into which are inserted

  two Schottky diodes 13 and 14 A connection 16 connects the loop to a transistor 18 whose base is connected to a resistor 20 receiving a signal 21 in

form of rectangular pulses.

  Figure 2 shows the practical layout of

  these elements The cavity 10 rests on a hy-

  frequency 22, itself supported by a ground plane 24 The loop 12 is constituted by a ring 15 etched in a disc 17, which is supported by a column 26 held in a cover 28 The dielectric cavity

  is also connected to microwave circuits

  these not shown, like an oscillator for example,

  by a microwave transmission line 30.

  The principle of operation of this circuit is as follows From the point of view of direct current, the assembly is equivalent to a diode in series with a resistance In alternating, each diode rectifies a

  alternation of the microwave signal Depending on whether the

  sistance connected to the diodes is high or low, the operating point of the diode moves and the reactance offered by the loop changes The frequency of the loop-cavity assembly and the control signal of the

  transistor therefore vary in concommittance.

  By way of explanation, it may be indicated that with a ring produced on epoxy glass having an average radius of 4.5 mm and a width of 1 mm and placed at

  a distance of 3 mm above a resonator-dielect

  plate stabilizing a field effect transistor oscillator at 11 G Hz, a frequency jump of 2% is obtained with a power variation of less than 1 d B.

  The construction characteristics determine

  partially undermine the performance of the circuit For example

  ple, the frequency difference increases when: the mean radius of the ring increases, the width of the line constituting the ring increases for a given radius,

  the distance from the ring to the dielectric cavity di-

minue.

  In the diagram of FIG. 1, the transistor 18 receives, on its base, a rectangular signal which

  alternately places in a blocked or passing state.

  The resonant frequency of the set therefore takes two

  distinct values But naturally, we can

  ser a resistive element having a whole range of values to obtain a frequency modulation on

  a range and no longer jumps from it.

  The invention finds a special application.

  linked in the realization of bidirectional links

  operating in the alternation Such links include

  two end stations A and B, identical and

  working alternately in transmission and in reception

  -tion, as shown in the diagram in Figure 3.

  In this diagram: E indicates the transmission phase, R the reception phase, tt represents the duration of the transmission of a data block, tp the propagation time from one end to the other, t 9 the time on call, t waiting time, a

tc cycle time.

  In such a link, each station travels

  works on two different frequencies, one used for transmission (fe) and the other for reception (fr) With the

  circuit of the invention two oscillations can be made

  identical to two frequency states, these oscillations

  readers being arranged at each of the ends A and B of the link The diagram in FIG. 1 then cov-itent, -

  the rectangular signal 21 defining the two frequencies

  transmission and reception frequencies fe and fr The si-

  general 21 A and 21 B to be applied to the transistors of the two circuits of stations A and B are linked as indicated

the diagram of figure 4.

RE.VENDICATI O NS

  1 circuit for varying the frequency

  resonance quence of a dielect hyperfrequency cavity

  stick (10), this circuit 'comprising a loop

  range (12) disposed in the vicinity of the cavity and taking part of the energy established therein and a means inserted in this loop capable of varying the reactance of the bouce 1 S caracti: se queî c -c: medium consists of at least one Schottky diode

  (13, 14) charged by an element equivalent to a resistor

  tance, whose value e St adjustable.

  2 Circuit according to claim 1, charac-

  terized in that the element equivalent to a resistor is a transistor (18), the emitter and collector of which are connected through the Schottky diodes and the base

  receives a control signal determining the value

equivalent sistive of the transistor.

  3 Circuit according to claim 2, charac-

  terized in that the control signal (21) is rectan-

  gular and determines a passing or blocked state of the tran

sistor (18).

  4 Circuit according to claim 1, charac-

  terized in that the element equivalent to a resistance

is a phototransistor.

  5 Circuit according to claim 1, charac-

  terized in that the element equivalent to a resistance

is a thermistor.