Communications: directive radio wave systems and devices (e.g. – Determining distance – With frequency modulation
Reexamination Certificate
2000-02-25
2001-11-13
Lobo, Ian J. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
With frequency modulation
C342S200000
Reexamination Certificate
active
06317075
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to an FMCW sensor.
Given sensor systems with FMCW radar, particularly microwave radar systems, it is possible to obtain simultaneous range and velocity measurements. Such sensor systems have versatile use possibilities, for example in the automotive or automation technologies. The abbreviation FM stands for frequency modulation and CW stands for continuous wave. A signal source generates a frequency-modulated signal that propagates wave-like. The signal source comprises, for example, a microwave oscillator and a modulator. The preferably linearly frequency-modulated signal is beamed out by an antenna and is reflected from a target. The received signal is mixed in a mixer with the transmission signal present at the moment. The difference frequency that derives is a measure for the distance of the target from the antenna. A monostatic or a bistatic antenna arrangement can be employed for the transmission and reception means. Given the monostatic arrangement, the transmission and the reception signals are beamed out or, received by the shared transmission and reception antenna. As a result of a circulator or directional coupler, the transmission and the reception signals are separated from one another, i.e. the transmission signal is separated by the reception mixer and the reception signal is separated from the transmission path. Given the bistatic antenna arrangement, separate transmission and reception antennas are provided.
PCT Patent Application No. 97/09637 discloses an FMCW radar sensor wherein a delay line is present as a reference means. A time-delayed signal is generated from the transmission signal with this reference means, a balancing and a correction of a measured signal being capable of being undertaken with said time-delayed signal such that a non-linear curve of the characteristic of the modulation can thus be computer-compensated. A correction of the modulation characteristic on the basis of an evaluation of the reference signal is likewise possible. PCT Patent Application No. 97/09777 discloses a device serving a similar purpose.
Further examples for linearization of the modulation given FMCW sensors may be found in the publications by B. Zimmermann et al., “24 GHz Microwave Close-Range Sensors For Industrial Measurement Applications” in Microwave Journal, May 1996, pages 228 through 238, and by P. Lowbridge et al., “A Low Cost mm-Wave Cruise Control System for Automotive Applications” in Microwave Journal, October 1993, pages 24 through 36. A closed control loop, for example with digital frequency discriminator, is employed, this being supplied with an intermediate frequency signal. An additional, frequency-stable local oscillator is required for this purpose. As an alternative thereto, U.S. Pat. No. 5,546,088 discloses a radar sensor wherein a frequency-modulated 24 GHz radar signal is generated by five-fold frequency multiplication of a 4.8 GHz VCO signal. However, a considerable circuit outlay is required for this purpose.
What is also disadvantageous about known FMCW sensors is that separate transmission and reception devices must be present given the bistatic structure and a circulator or hybrid coupler or, directional coupler for dividing transmission and reception signals onto the various signal paths is required given the monostatic structure. German Patent Application No. 196 10 850 discloses a monostatic, homodyne radar system, whereby a balanced mixer (push-pull mixer) is utilized instead of a traditional directional coupler with separate mixer. The mixer contains a hybrid coupler functioning as directional coupler with 90° phase difference between coupling arms, and comprises a reflection factor of more than 0.3, so that a coupling of the power supplied by the oscillator to the antenna ensues that is adequate for the emission of the transmission signal.
PCT Patent Application No. 95/26073 discloses a radio-frequency oscillator in planar structure, whereby a dielectric resonator is operated in higher modes and is coupled by suitable alignment to a micro strip conductor. Such an oscillator in the region around 24 GHz is suitable as a signal source for FMCW sensors.
SUMMARY OF THE INVENTION
An object of the present invention is to an FMCW sensor, particularly for range measurement, that supplies precise measured results given reduced manufacturing outlay.
This object is achieved in accordance with the invention in an FMCW sensor having a fixed frequency generator for generating a carrier frequency and having an output; a second frequency generator for generating variable modulation frequencies and having an output; a mixer for generating mixed signals from the carrier frequency and the modulation frequencies and for reducing the generated mixed signals to a predetermined frequency band, the mixer including a field effect transistor having a gate terminal connected to the output of the fixed frequency generator, a source terminal connected to ground, and a drain terminal connected to the output of the second frequency generator, and including an output, the mixed signals being supplied from the drain terminal to the output; a transmitter and receiver unit having an input connected to the output of the mixer and having an output; a reference unit having an input connected to the output of the second frequency generator and having an output; and an evaluation means having an input connected to the output of the transmitter and receiver unit, an input connected to the output of the reference unit, and an output.
The inventive sensor has components that guarantee an exact measurement but that are fashioned such that the outlay remains slight in the manufacture of the sensor. The means utilized for this purpose are not all necessary. A preferred exemplary embodiment, however, derives from a combination of all improvements recited below. The inventive sensor is especially suited for the realization as a hybrid planar circuit (for example, in micro strip line technology or coplanar technology).
The sensor employs a reference means that comprises a delay line. The non-linearity of the modulation characteristic can thus be computer-corrected or can be corrected by a corresponding pre-distortion of the modulator. The reference signal is acquired in that a modulated, comparatively low-frequency signal is time-delayed and is mixed with the undelayed signal. The modulated, low-frequency signal is inventively supplied by a frequency generator (oscillator) suitable for this purpose. Since a radio-frequency, modulated signal is required as transmission signal, the low-frequency, modulated signal is converted into a radio-frequency, modulated signal by mixing with a fixed radio-frequency signal. Only a frequency generator that supplies a signal with a high fixed frequency but no radio-frequency modulatable oscillator is therefore inventively required. In particular, an oscillator that is constructed with a dielectric resonator according to the aforementioned PCT Patent Application No. 95/26073 is suitable as a fixed frequency generator for high frequency. The technical demands in view of temperature stability, output power, center frequency reproducibility and phase noise can be met with dielectrically stabilized fixed-frequency oscillators.
A further simplification of the technical outlay given the inventive sensor can be achieved in that a monostatic transmission and reception system is employed, this achieving a separation of transmission and reception signals not with a traditional circulator or directional coupler but by employing a specific mixer operated in transmission, so that a separate separation of transmission and reception signals can be foregone.
The components having non-linear characteristic of the mixers employed in the sensor can be manufactured comparatively inexpensively by employing suitable, commercially obtainable field effect transistors instead of diodes.
These and other features of the invention(s) will become clearer with reference to the following detailed description of the pre
Heide Patric
Nalezinski Martin
Lobo Ian J.
Schiff & Hardin & Waite
Siemens Aktiengesselschaft
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