WO1999048209A2

WO1999048209A2 - Interacting oscillator and frequency discriminator with a common resonator or time delay unit

Info

Publication number: WO1999048209A2
Application number: PCT/SE1999/000245
Authority: WO
Inventors: Stefan Andersson
Original assignee: Celsiustech Electronics Ab
Priority date: 1998-03-03
Filing date: 1999-02-23
Publication date: 1999-09-23
Also published as: AU2753399A; SE9800648L; SE9800648D0; WO1999048209A3; SE513808C2

Abstract

An oscillator (1) is stabilized by a frequency discriminator in order to reduce its phase noise. A microwave amplifier (4) supplies sufficient power to maintain the oscillation. The oscillator and the frequency discriminator use a common resonator or time delay unit (6). A pair of directional couplers (9, 11) deflect a little of the microwave incident upon the resonator (6) and a little of the microwave transmitted by the resonator. Resistors (12, 13) terminate the unconnected ports of the directional couplers. A fixed phase rotator (14) compensates for the longer route of one deflected microwave. A frequency discriminator signal (18) is generated by phase comparison (15), base-band amplification (16) and filtering (17) of the deflected and phase-compensated microwaves. The frequency discriminator signal controls the phase in the oscillator by means of, for example, a controllable phase rotator (2). In this connection, a control system which measures and corrects the phase error in the oscillator is obtained.

Description

TITLE

Interacting oscillator and frequency discriminator with a common resonator or time delay unit.

TECHNICAL FIELD

The present invention relates to an interacting oscillator and frequency discriminator arrangement which comprises a common resonator or time delay unit . The oscillator comprises or works with a phase rotation arrangement for tuning the oscillation frequency. A frequency discriminator signal generated by the frequency discriminator is also coupled to the phase rotation arrangement .

STATE OF THE ART

It is known that phase errors occur in oscillators, which errors result in the oscillation frequency not being identical with the resonant frequency of a resonator or time delay unit included in the oscillator. It is also known that the phase error occurs on the one hand statically on account of manufacturing variations and on the other hand dynamically owing to the fact that low- frequency noise modulates voltage-dependent reactances in the oscillator and thus the total phase length. It is also known that the dynamic phase error in the oscillator results in a corresponding dynamic variation of the oscillation frequency, which constitutes the phase noise of the oscillator. Frequency discrimination is a well- known technique for reducing these phase errors and in so doing reducing the phase noise from the oscillator. When a frequency discriminator is used, it is known that the resonator or time delay unit of the oscillator and the resonator or time delay unit of' -the frequency discriminator have to be synchronized in order to achieve optimum phase noise reduction. From American Patent Specification 5,032,800, it is known that it is possible to stabilize a loop oscillator with a frequency- discriminator where one resonator is common to the loop oscillator and the frequency discriminator, the problems of synchronization being eliminated. In the known arrangement according to the American patent specification, use is made of a circulator.

From American Patent Specification 5,463,360, it is known that it is possible to stabilize a reflection oscillator with a frequency discriminator where one resonator is common to the reflection oscillator and the frequency discriminator, the problems of synchronization being eliminated.

DESCRIPTION OF THE INVENTION

TECHNICAL PROBLEM

It is known that the phase noise is the limiting factor in virtually all modern microwave systems. The phase noise causes reduced resolution in, for example, radar systems, inter-symbol interference in transmission links, and so on. If it were possible to reduce the phase noise, new high-performance transmission links would be made possible, which would be capable of providing greater capacity in, for example, telephone networks, mobile telephone networks and the internet. It would also make possible new refined radar systems with greater capacity for distinguishing between small radar responses in, for example, cars, and open the way for new systems with safety functions such as, for example, an anti-collision function. For these reasons, industry, institutions and authorities are investing a great deal of technical resources in research and development in the area.

There is a requirement for oscillators which work with low phase noise for the purpose of improving the resolution in radar systems, increasing the transmission capacity in microwave links, and so on. The invention aims to solve this problem inter alia.

When frequency discriminators are used, there is a requirement to be able to synchronize the resonator or time delay unit of the oscillator with the resonator or time delay unit of the frequency discriminator in order to achieve optimum phase noise reduction. The invention aims to solve this problem also.

There is an urgent requirement to be able to avoid designs with circulators which today are space-consuming and difficult to produce and thus have expensive components. The invention aims to solve this problem also.

THE SOLUTION

In the main, the interacting oscillator and frequency discriminator according to the invention can be considered to be characterized inter alia in that the interacting oscillator and frequency discriminator arrangement generates the frequency discriminator signal from the phase difference between a small part, for example -2 dB to -100 dB, of the microwave incident upon the resonator or the time delay unit and a small part, for example -2 dB to -100 dB, of the microwave transmitted by the resonator or the time delay unit.

Further developments of the invention emerge from the subsequent subclaims .

ADVANTAGES

By means of what is proposed above, an interacting oscillator and frequency discriminator is produced, which has a technically simple and economically advantageous construction with an insignificant requirement for fine adjustments in connection with manufacture. Performance is improved in systems in which the new loop oscillator is used. Using normal control technology analysis, it is possible to show that the spectral density of the phase fluctuations is reduced to S_φ0 ( f_)

S_φs ( f_a) =

(1+G)² as long as no new noise source begins to dominate in the phase noise generation. Here, G designates the gain of the control system, which is a product of the coupling factors of the directional couplers, the Q value of the resonator or the delay of the time delay unit, the sensitivity of the phase detector, the base-band amplification, the filter characteristic and the sensitivity of the phase-controlling arrangement in the oscillator.

Moreover, only components known per se are included in the new interacting oscillator and frequency discriminator .

DESCRIPTION OF THE FIGURES

A presently proposed embodiment of an oscillator having the significant characteristics of the invention is described below with simultaneous reference to the appended drawings, in which

Figure 1 shows in principle a loop oscillator which is stabilized by a frequency discriminator and in which the resonator in the loop oscillator and the frequency discriminator is common, and

Figure 2 shows a diagram of a frequency discriminator signal and the phase rotation of the resonator in the same frequency discriminator.

DETAILED EMBODIMENT

In very general terms, the oscillator is designed in such a manner that the oscillator includes a resonator or time delay unit common to a frequency discriminator, with associated directional couplers, a power-generating part, for example an amplifier or negative resistance, and also an arrangement for controlling the phase in the oscillator. The power-generating part supplies power which is fed back via the resonator or the time delay unit and the arrangement for controlling the phase. The resonator or the time delay unit contributes to stabilization of the oscillator. The arrangement for controlling the phase makes it possible to carry out corrections of phase errors in the oscillator and thus affords a possibility for synchronizing the oscillation frequency of the oscillator with the resonant frequency of the resonator. The arrangement may consist of, for example, a controllable phase rotator constructed from a 3 dB directional coupler and a pair of varactors, or be produced by the action of direct current bias voltage on any active component included in the oscillator. The oscillator also includes an arrangement for coupling power to the output of the oscillator, for example a directional coupler, a Wilkinson divider or the like.

If it is decided to provide the oscillator with a resonator, this may then consist of, for example, a microstrip resonator, a dielectric resonator, a cavity resonator, a Fabry-Perot resonator, a YIG resonator or the like. If it is decided instead to provide the oscillator with a time delay unit, this may then consist of, for example, a coaxial cable, a fibre-optic link, an acoustic surface wave unit or the like.

The frequency discriminator is designed in such a manner that a little of the microwave incident upon the resonator or the time delay unit, and a little of the microwave transmitted by the resonator or the time delay unit, is deflected by means of directional couplers. Unused connections on the directional couplers are suitably terminated in a reflection-free manner by resistors. The microwave deflected before the resonator or the time delay unit, which corresponds to the microwave incident upon the resonator, is phase-rotated by a fixed value, so that, at the resonant frequency of the resonator or the time delay unit, it becomes phase- synchronized with the microwave deflected after the resonator or the time delay unit, which corresponds to the microwave transmitted by the resonator or the time delay unit. This fixed phase rotation is necessary as the microwave deflected after the resonator or the time delay unit would otherwise have travelled a greater distance and thus have had a different phase relationship to the microwave deflected before the resonator or the time delay unit. The fixed phase rotation is carried out with, for example, a piece of cable length. The relative phase relationship between the two microwaves that have been deflected and phase-rotated in a fixed manner will then, thanks to the rapid phase-to-frequency derivative of the resonator or the time delay unit close to its resonant frequency, be proportional there to the deviation of the frequency from the resonant frequency, and consequently be a measure of the phase error in the oscillator. The output signal of the frequency discriminator is formed by the relative phase relationship between the two deflected and phase-compensated microwaves being measured by means of a phase detector and is in this connection proportional to the phase error in the oscillator.

The output signal of the frequency discriminator is baseband amplified and filtered, after which it is coupled to the arrangement of the oscillator for controlling the phase. This provides a feedback-type control system for correcting the phase error, and thus a reduction in the phase noise.

The resonant frequency of the resonator or the time delay of the time delay unit can also be made adjustable by means of, for example, a varactor, a YIG sphere or the like. Thanks to the feedback-type control system, the oscillation frequency will slavishly follow adjustments of the resonant frequency. In this connection, an oscillator with an adjustable oscillation frequency is obtained.

In a specifically indicated embodiment according to Figure 1, a controllable phase rotator in a loop oscillator 1 is indicated by 2. An oscillation loop is shown by 3 and the loop includes a microwave amplifier 4. The loop also includes a power coupler 5 and a resonator or time delay unit 6. The power coupler 5 distributes microwave power to an output 7 and to an effected feedback to the microwave amplifier 4 via the loop 3. At the input 8 of the resonator or the time delay unit, a directional coupler, which deflects a little, for example -2 dB to -100 dB, of the microwave incident upon the resonator or the time delay unit, is indicated by 9. At the output 10 of the resonator or the time delay unit, a directional coupler, which deflects a little, for example -2 dB to -100 dB, of the microwave transmitted by the resonator or the time delay unit, is indicated by 11.

Resistors for reflection-free termination of the unused ports of said directional couplers are indicated by 12 and 13 respectively. The first directional coupler is connected to a fixed phase rotator indicated by 14. The deflected and phase-compensated phases of the microwaves are compared in a phase detector indicated by 15. The phase comparison signal is amplified in a base-band amplifier indicated by 16, and filtered in a control filter indicated by 17. The frequency discriminator signal 18 obtained is coupled to the controllable phase rotator 2 via the line 19 which then compensates for the phase error of the loop, which results in a reduction in phase noise. The components shown in Figure 1 are known per se .

The invention thus makes use of a single resonator or time delay unit 6 and effects a reduction in phase noise by the phase error of the oscillator being detected via a frequency discriminator signal 18 which is fed back to an arrangement 2 for controlling the phase, and thus compensates for the phase error. A frequency discriminator with a resonator, indicated by 20 in Figure 1, has been simulated using the "Microwave design system" software from Hewlett-Packard, which is available on the open market. The result of the simulation is shown in Figure 2 which shows clearly the rapid phase-to-frequency derivative of the resonator close to the resonant frequency and that the outgoing frequency discriminator signal has zero crossing at the resonant frequency.

In Figure 2, said phase shift is represented by a curve 21 with an associated scale graduated in degrees on the left in the figure shown by arrow 22. The frequency discriminator signal is shown by the curve 23 with an associated scale graduated in volts on the right in the figure shown by arrow 24. The horizontal axis indicates frequency for both curves. The point 25 where the phase shift of the resonator or the time delay unit is steepest and the frequency discriminator signal has zero crossing is the resonant frequency of the resonator.

The invention is not limited to the exemplary embodiment above but can be modified within the scope of the patent claims below and the inventive idea.

Claims

PATENT CLAIMS

1. Interacting oscillator and frequency discriminator (1) with a common resonator or time delay unit (6) and in which the oscillator comprises a phase rotation arrangement for tuning the oscillation frequency and also in which a frequency discriminator signal (18) generated by the frequency discriminator is coupled to said phase rotation arrangement, characterized in that the frequency discriminator signal (18) is generated from the phase difference between a little, for example -2 dB to -100 dB, of the microwave incident upon the resonator or the time delay unit (6) and a little, for example -2 dB to -100 dB, of the microwave transmitted by the resonator or the time delay unit (6) .

2. Interacting oscillator and frequency discriminator according to Patent Claim 1, characterized in that the oscillator is of the loop oscillator type.

3. Interacting oscillator and frequency discriminator according to Patent Claim 1, characterized in that the oscillator is of the reflection oscillator type.

4. Interacting oscillator and frequency discriminator according to Patent Claim 1, characterized in that no circulator is included in the design.

5. Interacting oscillator and frequency discriminator according to any one of the preceding patent claims, characterized in that the phase rotation arrangement consists of a controllable phase rotator or arrangement for direct control of the phase response in an active component, such as for example an amplifier or a negative resistance, included in the oscillator.

6. Interacting oscillator and frequency discriminator according to any one of the preceding patent claims, characterized in that directional couplers are used in order to deflect a little of the microwave incident upon the resonator or the time delay unit (6) and a little of the microwave transmitted by the resonator or the time delay unit (6) .

7. Interacting oscillator and frequency discriminator according to any one of the preceding patent claims, characterized in that a fixed phase rotator is used in order to compensate the longer route of one deflected microwave.

8. Interacting oscillator and frequency discriminator according to any one of the preceding patent claims, characterized in that the frequency discriminator comprises a phase detector which compares the phase of a little of the incident microwave and a little of the transmitted microwave.

9. Interacting loop oscillator (1) and frequency discriminator with a common resonator (6) or time delay unit and in which the oscillator comprises a controllable phase rotator (2) which controls the phase response in an amplifier (4) or negative resistance included in the oscillator, in which a frequency discriminator signal (18) generated by the frequency discriminator is coupled to the phase rotator and in which the frequency discriminator signal can be generated by means of directional couplers (9, 11) which deflect a little of the microwave incident upon the resonator or the time delay unit (6) and a little of the microwave transmitted by the resonator or the time delay unit, characterized in that an output of the phase rotator (2) is directly coupled to the amplifier or the negative resistance which is in turn connected, via a power coupler, to the input (8) of the resonator, and the output (10) of the resonator is connected to an input of the phase rotator (2) , in that the directional couplers are arranged at the inputs and outputs of the resonator and deflect -2 dB to -100 dB of said incident and transmitted microwaves respectively, and in that the first directional coupler (9) is coupled to the phase detector (15) via a phase rotator (14) and the second directional coupler (11) is coupled directly to the phase detector.