Communications: directive radio wave systems and devices (e.g. – With particular circuit
Reexamination Certificate
2003-02-13
2004-08-17
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
With particular circuit
C342S028000, C455S332000
Reexamination Certificate
active
06778132
ABSTRACT:
The invention relates to a microwave sensor with a current or voltage supply, with a self-mixing oscillator, with an impedance which is connected between the current or voltage supply and the oscillator, with a transmitting and receiving antenna and with an evaluation circuit, the self-mixing oscillator producing both the transmitted signal and also mixing the transmitted signal with the received signal and the low-frequency mixed product being tapped on the impedance and supplied to the evaluation circuit.
In the first type of microwave sensors of the above described type, the movement of an article in an area to be monitored is ascertained by the so-called Doppler effect being evaluated. A transmitted signal radiated by the transmitter with a frequency f
1
is reflected by an object moving in the area to be monitored. Part of the reflected signal is incident as the received signal with a frequency f
2
on the receiver. In a suitable mixer the received signal is mixed with the transmitted signal and then the portion of the mixed product with the Doppler frequency f
D
is evaluated. The following equations apply to the Doppler frequency f
D
f
D
=f
1
−f
2
and
f
D
=[(2
×f
1
)/
c
0
]x v
r
c
0
being the velocity of light and v
r
being the radial velocity of the moving object.
Thus either the radial velocity v
r
of the moving object can be measured from the measured Doppler frequency f
D
or, if the microwave sensor is used only for monitoring a space or a certain area, the entry of an object into the space or area to be monitored can be ascertained. One such microwave sensor which evaluates the Doppler effect and which is often also called a Doppler sensor can thus be used as a motion detector for the most varied applications.
In a second type of microwave sensors of the above described type for detection of a moving or stationary object in an area which is to be monitored, a frequency-modulated transmitted signal with a frequency f
1
(t) is emitted by a transmitter. The modulation signal is produced in doing so by a suitable modulation generator, and the frequency f
1
(t) of the transmitted signal can change linearly, sinusoidally or according to another time function. This radar process is called FMCW radar (FMCW=frequency modulated continuous wave).
Here the transmitted signal with a frequency f
1
(t
0
) is reflected by an object which is located in the area to be monitored. Part of the reflected signal after a time interval &Dgr;t is incident as the received signal with frequency f
1
(t
0
) on the receiver. At this time the transmitted signal already has the frequency f
1
(t
0
+&Dgr;t). The received signal in its frequency thus runs behind that of the transmitted signal. In a suitable mixer the received signal is mixed with the transmitted signal and then the portion of the mixed product with frequency f
IF
is evaluated.
The microwave sensor under consideration can be used both as a Doppler sensor and also as a FMCW sensor. In doing so the frequency of the transmitter depending on the application can be between 60 MHz and 60 GHz and thus also somewhat below the actual microwave range which normally extends from 300 MHz to 300 GHz. Strictly speaking, the sensor under consideration is a radio or microwave sensor.
It was stated at the beginning that the microwave sensor has a self-mixing oscillator and a transmitting and receiving antenna. A self-mixing oscillator, which can also be called a self-oscillating mixer, is a component which is used both as an oscillator and also as a mixer. On the one hand, therefore either the self-mixing oscillator produces a transmitted signal, on the other in the self-mixing oscillator which then operates as a self-oscillating mixer, the received signal is mixed with the transmitted signal. Within the framework of this application a transmitting and receiving antenna is defined as a component which is used at the same time as a transmitting antenna and as a receiving antenna.
One such microwave sensor in which instead of the four components—oscillator, mixer, transmitting antenna and receiving antenna—only the above described two components—a self-mixing oscillator and transmitting and receiving antenna are used, is disclosed by DE 32 09 093 A1 and DE 41 27 892 A1. In the known microwave sensors the self-mixing oscillator is formed by a feedback field effect transistor with a resistor for tapping the Doppler signal in its source-drain circuit. With the known microwave sensor it has already been possible to make available a device for space monitoring by means of Doppler radar which requires only relatively few components and therefore can be produced both economically and also has only little space requirement and low weight.
Especially when one such microwave sensor is made as a 2-wire device or is to be battery-operated is there however the problem that the known microwave sensors have an overly great power consumption or at low power consumption have an overly low transmitted power. One such microwave sensor made as a 2-wire device can also be called a microwave proximity switch.
Therefore the object of this invention is to improve the initially described microwave sensor such that it has lower power consumption and can be produced as economically as possible.
This object is first achieved in the initially described microwave sensor essentially in that the self-mixing oscillator is made as a push-pull oscillator with two transistors.
The use of a push-pull oscillator has the advantage that in this way higher power and better efficiency can be achieved so that one such push-pull oscillator builds-up well even at a relatively low voltage or relatively low current. In particular a symmetrically built push-pull oscillator compared to known field effect transistors used as self-mixing oscillators is much more oscillation-friendly, additionally few harmonics also occurring.
The transmitting and receiving antenna is advantageously formed by a strip line which determines the frequency of the oscillator. By using a strip line as the transmitting and receiving antenna which is a component of the push-pull oscillator it is possible to save another component, since a separate transmitting and receiving antenna is no longer necessary. In addition, however it is also possible to use a dipole antenna as the transmitting and receiving antenna.
According to one preferred embodiment of the microwave sensor as claimed in the invention, to adjust or stabilize the working point of the microwave sensor, voltage countercoupling with at least one resistor and one lowpass is accomplished.
In the Doppler sensor known from DE 41 27 892 A1, to stabilize the working point of the transistor, current countercoupling is implemented, for which between the drain terminal and the gate terminal of the field effect transistor and ground one impedance at a time is connected. This current countercoupling for adjusting the working point leads on the one hand to an undesirable cross current; this increases the energy demand of the microwave sensor which is required overall. On the other hand, when the working point is adjusted by means of current countercoupling there is the danger that the useful signal, i.e. the Doppler signal, is “regulated out” or at least attenuated by countercoupling.
Because the working point is adjusted by voltage countercoupling, first of all an unwanted cross current is avoided. In addition, the lowpass which is provided ensures that only possible temperature drift is corrected, but the Doppler signal is not attenuated. To do this, the lowpass is adjusted such that working point control is slower than the lowest expected Doppler frequency f
D
or the lowest intermediate frequency f
IF
. In a transmitted signal with a frequency f
1
of a few gigahertz, for example 2.5 GHz, and tuning of the microwave sensor to movements executed by individuals, for example a hand approaching a doorknob, the Doppler frequency f
D
is for example between 10 and 50 Hz so that the cutoff frequency of the lowpass must according
i f m electronic GmbH
Nixon & Peabody LLP
Safran David S.
Sotomayor John B.
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