MICROWAVE OSCILLATOR WITH IMPROVED PHASE NOISE CHARACTERISTIC

Abstract: The phase noise characteristic of a microwave oscillator comprising a cavity (2) excited by an active element (1) is improved by introducing a frequency discriminator embodied by using the same oscillator cavity. The waves transmitted and reflected by the cavity (2) are measured, combined and detected by a pair of low noise diodes (7, 8) and the signals obtained are compared to produce a control signal for a phase shifter (10) placed between the active device (1) and the cavity (2). The oscillator is preferably of the type with dielectric resonator.

Description: "Microwave oscillator with improved phase noise characteristic" * * * * * DESCRIPTION The present invention relates to a microwave oscillator with improved phase noise characteristic near the carrier. The invention has its preferred but nonlimiting application in telecommunications systems and in particular in digital radio link systems with low and medium capacity and therefore is illustrated and described in detail below with reference to this application but its employment is not to be understood as limited to that illustrated. In low and medium capacity radio link systems it is constantly required to reduce the spectrum occupation even if this involves the use of more complicated modulation schemes. It has been found that a low phase noise microwave synthesiser is fundamental for this reduction and to this end the optimal solution is multiloop microwave frequency synthesis. But the circuitry complexity of a multiloop synthesiser is considerable because of the devices employed and among which are in particular frequency dividers, sampling phase detectors, phase comparators, crystal oscillators, etc. The purpose of the present invention is to overcome the shortcomings and limitations of the known art illustrated and in particular to provide a microwave oscillator comprising an active device and a resonant cavity with reduced phase noise near the carrier, simple in construction and highly stable to temperature, mechanical shocks and vibrations. These purposes are achieved in accordance with the present invention by a microwave oscillator comprising an active device and a resonating cavity and characterised in that it comprises a discriminator or frequency demodulator, using the said resonator cavity and measuring the power transmitted and the power reflected in the cavity; means for comparing the reflected and transmitted powers, connected at the output of the discriminator; and means for modifying the phase of the active device in response to the signal produced by the comparison means. The oscillator is preferably of the type with dielectric resonator. In accordance with the present invention there is provided a feedback of the noise produced by the active device previously extracted by a frequency discriminator for the purpose of reducing the oscillator phase noise near the carrier. The characteristic of the oscillator is thus independent of the active device and depends only on the dielectric resonator with high Q and on the 1/f-type low noise of the diodes present in the discriminator. Additional advantageous characteristics are the object of the dependent claims. The present invention is now described with reference to the annexed drawing which illustrates a preferred but nonlimiting embodiment of the present invention and which shows the general diagram of an oscillator with dielectric resonator incorporating a frequency discriminator provided in accordance with the present invention. The dielectric oscillator shown schematically in the Figure comprises an active device 1 consisting of a field effect transistor or GaAs FET in common-drain configuration which excites the cavity 2 of a dielectric resonator with relative dielectric constant « r - 38 coupled by a slot-line to the microstrip circuit. The oscillator output is available on a terminal 11 and there are provided in a known manner means 12 for compensation of the cavity temperature. A phase shifter 10 is placed between the active device 1 and the cavity 2 controlled by a feedback loop described below. In accordance with the present invention there is provided a discriminator or frequency demodulator 3 using the said cavity 2 and comprising two directional couplers 4 and 5 and in particular two one-fourth wave length lOdB couplers connected by two impedances Ro to ground, a 90" 3dB power divider indicated by reference number 6 and two low-noise 1/f-type beam-lead detector diodes 7 and 8. The other terminals of the two diodes 7 and 8 are connected to ground through two equal resistances R. The voltages present at the outputs on the anods of the diodes measure the power transmitted and reflected respectively in the cavity 2. Said signals are brought to the inputs of a differential amplifier 9, in which they are compared in such a manner that the output of the amplifier 9 is proportional to the oscillator frequency offset (phase shift) as regards cavity frequency, and this output signal controls the phase shifter 10, of the varactor type, placed between the active device 1 and the cavity 2. In this manner the phase of the reflection coefficient of the active device can be varied in such a manner as to correct the phase transients. Typically said differential amplifier has a function of integrator which receives at the inverting and not inverting inputs the outputs on the anods of the two diodes 7 and 8. Thanks to the proposed configuration, if the reflection coefficient phase of the active device changes, e.g. following temperature, phase noise of power supply variations, the phase shifter 10 introduces a phase variation equal and opposite to that occurred, in such a manner as to minimise the difference between the voltages at the ends of the diodes. In this manner the phase noise due to the active device is attenuated by a factor equal to the loop gain of the feedback circuit made up of the discriminator 3 and the integrator 9, and the residual phase noise is only that introduced by the diodes 7 and 8 and the differential amplifier 9. It has been found that as the power incident on the diodes increases, the sensitivity of the frequency discriminator increases, just as the noise figure of the diodes increases, so introducing a compensation such that the noise contribution of the diodes does not change. Thanks to the device proposed in accordance with the present invention, there is obtained a considerable improvement in the phase noise near the carrier, and in addition the frequency variations of the active device due to temperature are eliminated. The oscillator frequency is dependent only on the resonant cavity. Typically, with an active device consisting of a GaAs FET and using Schottky diodes for the diodes 7 and 8 there is obtained a reduction on the order of 15dB near the carrier, and the stability of the oscillator is less than 5 ppm (parts per million) in the range from 0 to 50°C. The addition of a demodulator circuit to the dielectric oscillator with resonator allows obtaining a direct microwave source with high frequency stability and low phase noise near the carrier, without requiring components such as frequency dividers, sampling phase detectors (SPD) or phase loop links (PLL) , with greater compactness of the mechanical construction and reduced cost. As reduction of phase noise depends on the noise of the diodes, which is the 1/f type, reducing this noise reduces correspondingly the oscillator phase noise. In accordance with an alternative embodiment of the present invention, not illustrated in the drawing, the dielectric resonator can be replaced by a low coupling coefficient (Q) cavity, for example embodied by a resonator on alumina substrate. There can thus be embodied for example a voltage controlled oscillator (VCO) in GaAs MMIC technology which comprises the active device, the phase shifter, the resonant cavity and the Schottky diodes. The technique proposed by the invention allows not only achieving low phase noise but also obtaining a considerable reduction of frequency drift and better stability in temperature, depending only on the electric resonator. Thanks to the invention there are obtained the typical performances of a highly complex synthesiser by means of a relatively simple modification of an oscillator with dielectric resonator.

Claims: CLAIMS 1. Microwave oscillator comprising an active device (1) and a resonating cavity (2) , and characterised in that it comprises a discriminator or frequency demodulator (3), using the said resonator cavity (2) and measuring the power transmitted and the power reflected in the cavity respectively; means (9) for comparing the reflected and transmitted powers and connected at the output of the discriminator (3); and means (10) for modifying the phase of the active device (1) in response to the signal produced by the comparison means (9) . 2. Microwave oscillator in accordance with claim 1, characterised in that the oscillator is of the dielectric resonator type (2). 3. Microwave oscillator in accordance with claim 1 or 2, characterised in that the discriminator or frequency demodulator (3) comprises two directional couplers (4,5) which are connected to a power divider (6) and two low-noise detecting diodes (7,8). 4. Microwave oscillator in accordance with claim 3, characterised in that the directional couplers (4,5) are 90" 3dB couplers. 5. Microwave oscillator in accordance with claim 3, characterised in that the comparison means comprise a differential amplifier (9) whose two inputs are connected to the outputs of the diodes. 6. Microwave oscillator in accordance with claim 5, characterised in that the means for modifying the phase of the active device (1) comprise a varactor phase shifter (10) controlled by the output of the differential amplifier (9). 7. Microwave oscillator in accordance with claim 1, characterised in that the oscillator comprises a cavity with low coupling coefficient.

Inventors: BUOLI CARLO, MORA GIOVANNI, TURILLO TOMMASO, BUOLI, CARLO, MORA, GIOVANNI, TURILLO, TOMMASO
Applicants: BUOLI CARLO, MORA GIOVANNI, SITS SOC IT TELECOM SIEMENS, TURILLO TOMMASO, SIEMENS TELECOMUNICAZIONI S.P.A.
Application year: 1993
Main classification: H03B5
IPC codes: H03B5, H03L7