DE19838029A1 - Coaxial high frequency power measurement head for calorimetric measurement of high frequency power; uses Bragg grating or Fabry-Perot resonator as absorber - Google Patents

Abstract

The head has an absorber consisting of an optical waveguide with a Bragg grating (8) or a Fabry-Perot resonator in a doped core. The waveguide is coated with a termination resistor (5) connected to inner and outer wave guides (1,2). The high frequency power causes temperature changes in the resistance, changing the transmission characteristics of the wave guide or detuning the Fabry-Perot resonator, to provide a measure of the power.

Description

The invention relates to the field of broadband calorimetric measurement of small RF powers with the help of coaxial waveguide structures in which absorbers Find use in the form of sheet metal resistances or as Individual resistances on flat or cylindrical carrier substrates in different geometrical arrangements as low reflection shaft resistance-adapted line termination are constructed. The invention thus relates a large bandwidth coaxial RF power sensor.

The temperature change of these absorbers when RF power is applied is either directly the change in resistance of the absorbers when using temperature-dependent metal layers and termistors or indirectly with sensors, e.g. B. with thermocouples, determined (Reichelt T .: NRVD u. NRVS, the new thermal power meter. News from Rohde u. Black (1992) No. 137 pp. 4-7.

An RF power measurement by changing the resistance depending on the temperature can also be carried out with the help of bolometers in thin film technology. Such bolometers are from H. Taddiken, D. Janik: Bolometer in thin-film technique as a power transfer standard for the 3.5mm coaxial line system. MIOP 97 congress documentation; known.

From Nak Sam Chung et .: Coaxials and waveguide micro-calon.meters for RF and microwave power standards. WEE Transactions on instrumentation and measurement (43/1998) Vol. 38, No. 2) Another method for HF power measurement is known.

The problem with direct resistance measurement is electrical separation between low-frequency resistance measuring circuit and high-frequency coaxial Line termination without significant mutual interference and measurement errors, e.g. B. To increase the reflection factor. Increased with indirect temperature measurement the measuring inertia and the measuring sensitivity decrease. At high cut-off frequencies it also occurs due to the small dimensions of the coaxial lines used and connectors to problems with the absorber geometries.

The object of the invention is to provide an arrangement that is small Absorber geometries with integrated temperature sensors and absolute electrical isolation between the RF circuit and the sensor measuring circuit, the small RF power with can detect low inertia.  

According to the invention, the object is achieved by an arrangement having the features of claim 1 solved. Advantageous embodiments are the subject of dependent claims 2 to 5.

An optical fiber is used as the absorber body, in the case of the Ge-doped core a Bragg grating is introduced photorefractive by known methods. At the location of the Bragg Lattice zone is a resistance layer, z. B. Ni or CrNi, with for the chosen coaxial absorber termination necessary surface resistance and one optimal geometry deposited. The temperature dependence of the Bragg grid, caused by the temperature dependence of the effective refractive index, is used as a sensor effect used. The change in optical reflection or transmission behavior with respect The center wavelength and optical bandwidth of the Bragg grating is detected and serves as a measure for the temperature change of the absorber layer.

Instead of a Bragg grating, a Fabry-Perot resonator can also be used. By the temperature change caused by the resistance layer then causes a detuning of the resonator, which is used as a sensor effect.

The coaxial line at the termination level is advantageously two or four radially symmetrical terminating resistors broadband and low reflection completed.

For the broadband HF adjustment of the coaxial power sensor, you can also adjustable metal surfaces are provided at a distance from the end plane of the coaxial line become.

A special version is achieved by the coaxial line through a conical line in their dimensions is reduced, so that the reduced diameter inner conductor indirectly can be formed with a metal-coated optical fiber.

Another temperature waveguide with a resistance layer and Bragg grating or Fabry-Perot resonator arranged in the outer conductor so that only one thermal There is contact with the immediate vicinity of the coaxial line without a thermal Influenced by the RF power.  

The advantage of the solution according to the invention is that by using the Optical waveguide with Bragg grating or Fabry-Perot resonator and absorber a complete electrical separation of the HF circuit from the sensor evaluation is achieved.

Additional electrical components in network structures for electrical decoupling of the Measuring circuit, e.g. B. separation capacities are eliminated.

The influence of parasitic electrical quantities is small for the absorber high measuring frequencies lower and the prerequisites for a large HF bandwidth of the Measuring head improve. The measuring times for the calorimetric HF power measurement become shorter and the sensitivity to small RF power increases. Measurement error caused by the The influence of ambient temperature can be influenced by an additional reference sensor the same design, which only has thermal contact with the surroundings of the coax line, be compensated. On the fiber optic connections between the measuring head and Evaluation unit have electromagnetic disturbances even with long connection lengths and large RF powers have no influence so a large spatial distance to Evaluation unit for special measurement configurations can be realized.

The invention is explained in more detail below using exemplary embodiments. In the Drawings show:

Fig. 1 shows a line end of a coaxial line with four radially symmetrically arranged terminating resistors

Fig. 2 shows a line end of a coaxial line with two radially symmetrically arranged terminating resistors

Fig. 3 shows a line end of a coaxial line, which is tapered by a conical line and an axially symmetrical terminating resistor

Fig. 1 shows a cable end of a coaxial line consisting of an outer conductor 2, an inner conductor 6 and a dielectricum 1 between the external conductor 2 and the inner conductor 6. The outer conductor 2 has an axis-normal termination. Two single-mode optical fibers 3 ; 4 , which are each coated with a resistance layer 5 made of Ni or CrNi over the inner diameter length of the outer conductor 2 of the coaxial line, are guided radially symmetrically at the location of the metal layer in a 90 ° angle across the line end of the coaxial line. The electrical contact points 7 are located on the inner conductor 6 of the coaxial line and on the outer conductor ring 2 of the coaxial line in the form of soldering points or conductive adhesive points. At the location of the contact points 7 , the resistance layers 5 are provided with gold contact rings.

Longitudinally symmetrical under the resistance layers 5 are in the cores of the optical fibers 3 , 4 photorefractive continuously generated Bragg grating zones 8 , the total length of which is in each case greater than the length of the two resistance layers lying one behind the other. Instead of the Bragg grating 8 , a Fabry-Perot resonator can also be provided in the optical waveguide.

The four partial resistors, which are connected in parallel, are dimensioned with respect to the surface resistances in such a way that a termination which is correct for the wave resistance is implemented for the coaxial line component used. The absorbers and Bragg grating or Fabry-Perot resonator contained in the two optical waveguides 3 , 4 are constructed optically and electrically in the same way. To improve the broadband adaptation values of the HF termination, the cross-wise arranged and resistance-coated optical fibers can be partially sunk into the dielectric 1 of the line end.

At the four ends 9 of the optical fibers 3 , 4 , the transmission or reflection behavior can optionally be detected depending on the RF power applied to the input of the coaxial line. If two optical fibers 3 , 4 and four partial resistors are used, the light outputs that are coupled in and out can be combined using couplers. A stabilized laser light source is used on the input side, the spectral optical bandwidth of which is greater than that of the Bragg gratings used and corresponds to the optical center frequency of the Bragg gratings used. The difference between the center wavelength and the bandwidth of the transmission or reflection signals of the Bragg grating 8% r at the coaxial measuring head input of differently applied HF powers is converted with the aid of known methods of optical spectral analysis into electrical measuring signals for determining the HF power values.

The optical waveguide 10 with the same absorber 5 and Bragg grating 8 is only in thermal contact with the immediate vicinity of the coaxial line, without thermal influence by the RF power. The measurement signal obtained from this additional fiber is used to compensate for the influence of the ambient temperature.

Fig. 2 shows an end of a coaxial line with two radially symmetrically arranged terminating resistors. In contrast to the illustration in FIG. 1, an optical waveguide 3 is sufficient here. For the HF adjustment of the power measuring head, two adjustable metal surfaces 11 are provided at a distance from the end of the coaxial line.

Fig. 3 shows a cable end of a coaxial line with a tapered conduit 12. The dimensions of the coaxial line are reduced so much with the conical line that the inner conductor 6 can be extended with the resistance-coated optical waveguide 3 . The outer conductor 2 is spaced from the optical waveguide 3 or from the resistance layer 5 . Electrical contacts 7 are provided between the inner conductor 6 and the resistance layer 5 and between the resistance layer 5 and the outer conductor 2 , so that there is an axially symmetrical terminating resistance through the resistance layer 5 .

Reference list

1

dielectric

2nd

Outer conductor

3rd

first optical fiber

4th

second optical fiber

5

Resistance layer

6

Inner conductor

7

electrical contacting

8th

Bragg grid zone

9

End of the optical fiber

10th

Reference optical fiber

11

capacitive compensation area

12th

Cone pipe

Claims (6)

1. Coaxial RF power measuring head of large bandwidth, with an RF connector input and at least one terminating resistor for low-reflection wave-resistance-adapted line termination, the terminating resistor being formed by an absorber body with a resistance layer ( 5 ), characterized in that the absorber body consists of an optical waveguide ( 1 ; 2 ) which has in its Ge-doped core a photorefractive Bragg grating ( 8 ) or a Fabry-Perot resonator, the area of the optically active cladding of the optical waveguide ( 1 , 2 ) is coated with the resistance layer ( 5 ), the resistance layer ( 5 ) is electrically connected to the inner and outer conductors, the reflection or transmission behavior of the Bragg grating ( 8 ) being changed by the temperature change of the resistance layer ( 5 ) in the case of differently applied HF powers at the coaxial connector input of the power measuring head, or on e The Fabry-Perot resonator is detuned, which is used for calorimetric measurement of the RF power. 2. Coaxial RF power measuring head according to claim 1, characterized in that on the optically active cladding of the optical waveguide ( 1 ; 2 ) at the location of the Bragg grating ( 8 ) or the Fabry-Perot resonator corresponding to the length of the terminating resistor used for the Coaxial line termination a Ni or CrNi layer ( 5 ) is applied with optimized surface resistance. 3. Coaxial RF power measuring head according to claim 1 and 2, characterized in that the coaxial line at the end plane through two or four arranged radially symmetrically Termination resistors is broadband and low reflection. 4. Coaxial RF power measuring head according to one of claims 1 to 3, characterized in that adjustable metal surfaces ( 11 ) are provided for the adjustment of the coaxial power measuring head at a distance from the end plane of the coaxial line. 5. Coaxial RF power measuring head according to claim 1 and 2, characterized in that the coaxial line is reduced in diameter by a conical line so that the inner conductor ( 6 ) at the end of the coaxial line is formed by an optical fiber ( 3 ) without changing the characteristic impedance . 6. Coaxial RF power measuring head according to claim 1, 2, 3 and 4, characterized in that a further optical waveguide ( 10 ) with a resistance layer ( 5 ) and Bragg grating ( 8 ) or Fabry-Perot resonator in the outer conductor ( 2 ) so is arranged that there is only thermal contact with the immediate vicinity of the coaxial line, without thermal influence by the RF power.