1. Field of the Invention
The present invention relates to a microwave detector, and in particular relates to an improved microwave detector structure capable of detecting very faint detection target microwave signals even when such microwave signals are buried within circuit noise.
2. Description of the Prior Art
Microwave detectors which generate an alarm upon detecting the specific microwave signals emitted by radar type speed measurement devices are known in the prior art. In general, such microwave detectors employ various heterodyne type reception systems to detect target microwave signals from the microwaves picked up by the antenna.
In microwave detectors equipped with heterodyne reception systems, the antenna output (reception signal) and the local oscillator output undergo frequency mixing in a mixer, and then after the intermediate frequency signal obtained by such mixing is amplified to an appropriate level, a signal process is carried out to convert detection target microwave signals into prescribed signals.
Namely, after carrying out frequency mixing and amplifying a desired band signal with an intermediate frequency amplifier, the intermediate frequency signal obtained thereby will have either a peak waveform when target microwave frequency signals are received, or a faint noise waveform when no target microwave frequency signals are received.
Accordingly, by detecting the peak waveform with a wave detector and outputting a digital pulse when the level of the peak waveform reaches or exceeds a prescribed threshold value, such arrangement makes it possible to output pulses only when detection target microwave signals are received, and this forms the basis of the detection process. Further, in the case where amplification is carried out with an amplifier having an amplification factor large enough to create noise saturation, the noise component will alternately appear as positive and negative high frequency signals, but when a detection target microwave frequency signal is received, the noise saturation will continue in either its positive or negative state without change, and this enables a pulse having a prescribed width to be outputted.
Incidentally, because the oscillation frequency of normal local oscillators repeatedly sweeps the frequency band occupied by the detection target microwave frequency, in the case where a single detection target microwave frequency signal is present, the intermediate frequency signal will output two peaks having a desired spacing. In this connection, the determination of whether or not such spacing forms a prescribed spacing is used as a means for judging whether or not a real microwave signal from a detection target is present.
Now, in order to accurately determine whether the received microwave signal is a real detection target microwave signal or a false signal, a signal in which the detection characteristics (i.e., S-curve characteristics) accurately reappear is inputted into a microcomputer, but because this signal that is inputted into the computer must have a cycle rate that is two-times or higher than the frequency of the S-shaped waveform, an excessive load will be placed on the circuit. In this connection, because the S-shaped waveform is not a sine wave, the signal described above generally uses a frequency as high as possible.
Further, because the reproducibility of the S-shaped waveform has a huge effect on the resolution of an A/D converter, the signals inputted into the microcomputer require the use of a high performance device such as a DSP or the like. Furthermore, because the amount of memory required depends on the resolution of the A/D converter, high resolution requires a large amount of memory. Moreover, the provision of elements needed to satisfy such requirements leads to high costs, and this makes it difficult to construct accurate microwave detectors.
On the other hand, there is another type of microwave detector known in the prior art, in which the oscillation frequency of the local oscillator carries out a sweeping operation until a detection target microwave frequency signal is detected, whereupon the oscillation frequency of the local oscillator is switched from a sweeping mode to a fixed frequency mode (i.e., a sweep stop is carried out).
In this way, in the case where a real microwave signal from a detection target is received, because the detection of the microwave signal will continue without alteration of the frequency state, by measuring the amount of time the sweep was temporarily suspended, the microwave detector can be arranged to use such time measurements as a base for judging whether or not a real microwave signal from a detection target has been received. In the case where random microwave signals such as noise or the like is received by this type of microwave detector, because such random signals will immediately disappear upon fixing the oscillation frequency of the local oscillator (i.e., upon stopping the sweep), the fixed state of the local oscillator will be released to allow the local oscillator to return to a sweeping mode. Accordingly, the local oscillator will be released from the fixed state (sweep suspension state) before the prescribed time interval has elapsed, and this makes it possible to prevent false alarms. An example of a microwave detector which uses this basic principle is disclosed in Japanese Laid-Open Patent Application No. HEI 7-35845.
Now, because this detection method only involves measuring the sweep suspension time interval, there is no need to carry out complicated waveform processing operations, and this makes it possible to construct a microwave detector having a relatively simple structure. However, in the case where a very faint microwave signal is received, such signal will become buried in the circuit noise. As a result, even when such faint microwave signal is a real microwave signal from a detection target, the sweep suspension state of the local oscillator will be released before the prescribed time interval has elapsed. Further, if the sweep suspension time is simply shortened in order to increase sensitivity, there will also be an increased risk of malfunctions due to degradation of the pulse characteristics and the like.
Further, when faint detection target microwave signals are received, the resulting output level may be relatively larger than the noise level, but the weak level of such faint microwave signals makes it difficult to accurately discriminate the case where a faint detection target microwave signal is present from the case where there is just noise with no microwave signals.
With a view toward overcoming the problems of the prior art described above, one object of the present invention is to provide a low-cost microwave detector which can carry out a highly sensitive detection of faint detection target microwave signals using a simple structure to enable accurate detections with few false alarms. Further, another object of the present invention is to make it possible to detect faint detection target microwave signals using a simple arithmetic processing portion and a small capacity memory.
One way to achieve the objects of the present invention is to construct a microwave detector from a heterodyne-type reception portion equipped with an antenna for picking up microwave signals, a local oscillator for repeatedly carrying out a sweep operation, and a mixer for frequency mixing the output from the antenna with the output from the local oscillator, wherein the heterodyne reception portion outputs detected wave signals based on the output from the mixer; a digital processing portion which receives the detected wave signals outputted from the reception portion, wherein the digital processing portion is designed to output a High pulse when the level of the detected wave signals reaches or exceeds a prescribed reference level, and a Low pulse when the level of the detected wave signals is below the reference level; a sweep control portion for controlling the sweep operation of the local oscillator based on the High and Low pulses outputted from the digital processing portion; and a judgement portion for judging whether or not a detection target microwave signal has been detected based on at least the total time of the High pulses.
Here the term xe2x80x9cbased on at least the total time High pulses are outputtedxe2x80x9d means that in addition to of course being able to carry out judgements based on the total time the detected wave signal from the reception portion reached or exceeded the prescribed reference level of the digital processing portion, it is also possible to carry out judgements based on a normalized figure calculated by taking the ratio or the like of the total time the detected wave signal reached or exceeded the reference level to the total time the detected wave signal was below the reference level.
Another way to achieve the objects of the present invention is to construct a microwave detector from a heterodyne-type reception portion equipped with an antenna for picking up microwave signals, a local oscillator for repeatedly carrying out a sweep operation, and a mixer for frequency mixing the output from the antenna with the output from the local oscillator, wherein the heterodyne reception portion outputs detected wave signals based on the output from the mixer; a digital processing portion which receives the detected wave signals outputted from the reception portion, wherein the digital processing portion is designed to output a High pulse when the level of the detected wave signals reaches or exceeds a prescribed reference level, and a Low pulse when the level of the detected wave signals is below the reference level; a sweep control portion for controlling the sweep operation of the local oscillator based on the High and Low pulses outputted from the digital processing portion; a judgement-use digital pulse generating portion which receives the detected wave signals outputted from the reception portion, wherein the judgement-use digital pulse generating portion is designed to output a High pulse when the level of the detected wave signals reaches or exceeds a prescribed threshold level, and a Low pulse when the level of the detected wave signals is below the threshold level; and a judgement portion for judging whether or not a detection target microwave signal has been detected based on at least the total time of the High pulses outputted by the judgement-use digital pulse generating portion.
In other words, in the first microwave detector structure described above, digital signals outputted by a digital processing portion are used by both a sweep control portion and a judgement portion, but in the second microwave detector structure described above, the sweep control portion uses the digital signals outputted by the digital processing portion, while the judgement portion uses judgement-use digital signals outputted by a separately provided judgement-use digital signal generating portion. In this connection, the operating principle for both structures is essentially the same. However, the second structure is advantageous because the threshold level used for making judgements can be appropriately set at a value different from the reference value used in controlling the sweep operations. Of course, the threshold level and the reference value may be set at the same value.
Still another way to achieve the objects of the present invention is to construct a microwave detector from a heterodyne-type reception portion equipped with an antenna for picking up microwave signals, a local oscillator for repeatedly carrying out a sweep operation, and a mixer for frequency mixing the output from the antenna with the output from the local oscillator, wherein the heterodyne reception portion outputs detected wave signals based on the output from the mixer; a digital processing portion which receives the detected wave signals outputted from the reception portion, wherein the digital processing portion is designed to output a High pulse when the level of the detected wave signals reaches or exceeds a prescribed reference level, and a Low pulse when the level of the detected wave signals is below the reference level; a sweep control portion which receives the output from the digital processing portion and carries out a sweep stop when High pulses are received; and a judgement portion for judging whether or not a detection target microwave signal has been detected based on whether or not the total time over which sweep stops are carried out at the same position reaches or exceeds a prescribed sweep stop reference time.
Here, the term xe2x80x9csame positionxe2x80x9d may mean the same frequency or the same sweep voltage corresponding thereto, and as described below in the detailed description of the preferred embodiments, the sweep voltage may of course be the same as the voltage value (level) obtained by sampling with the A/D converter, for example, using the same memory regions for carrying out memory storage over a single sweep. Further, the xe2x80x9csame positionxe2x80x9d may include the case where a prescribed width compensation is allowed. In other words, in order to cover the case where sweep stops are due to the same microwave signal, rather than being limited to exactly the same point, the xe2x80x9csame positionxe2x80x9d includes the case where the sweep stops occur in the same fixed range (such as the divided regions in the embodiments described below).
Yet another way to achieve the objects of the present invention is to construct a microwave detector from a heterodyne-type reception portion equipped with an antenna for picking up microwave signals, a local oscillator for repeatedly carrying out a sweep operation, and a mixer for frequency mixing the output from the antenna with the output from the local oscillator, wherein the heterodyne reception portion outputs detected wave signals based on the output from the mixer; a digital processing portion which receives the detected wave signals outputted from the reception portion, wherein the digital processing portion is designed to output a High pulse when the level of the detected wave signals reaches or exceeds a prescribed reference level, and a Low pulse when the level of the detected wave signals is below the reference level; a sweep control portion which receives the output from the digital processing portion and carries out a sweep stop when High pulses are received; and a judgement portion for judging whether or not a detection target microwave signal has been detected based on whether or not the total time required for a single sweep reaches or exceeds a prescribed sweep reference time.
Now, in each of the microwave detector structures described above, the reference level used for comparisons by the digital processing portion is preferably set at a level which enables detection of a portion of the noise outputted by the reception portion. In this regard, the reference level must be set below the maximum level of the noise in order to detect a portion of such noise, with the preferred reference level setting being at the central level (average level) of the noise. In particular, establishing the reference level at the central level of the noise can be simply carried out by establishing the reference level at the average of the maximum and minimum levels, or by establishing the reference level at the specific level where there is a existence probability of 1/2 for signals above and below such reference level. In this way, the reference level can be set in accordance with the fluctuations of the noise level.
Accordingly, in the microwave detector structures described above, because the reference level established for the detected wave signals outputted from the reception portion can be made smaller than the amplitude of the circuit noise waveform, it becomes possible to detect digitally processed signals even in such cases where a faint detection target microwave signal is buried in the noise waveform or has a signal level close to the noise level.
Similarly, for the same reasons explained above, the threshold level used for comparisons by the judgement-use digital pulse generating portion is preferably set at a level which enables detection of a portion of the noise outputted by the reception portion.
However, even though the reference level and the threshold level are preferably established at a level which enables detection of a portion of the noise as described above, the present invention is not limited to this arrangement, and it is of course possible to set the reference level and the threshold level at levels higher than the maximum value of the noise level. In other words, even when the reference level and the threshold level are established at values higher than the maximum value of the noise level, if the reference level and the threshold level are established close to the maximum value of the noise level, then it becomes possible to detect faint microwave signals close to the noise level so long as such microwave signals have levels at or above the reference level or threshold level.
However, if the reference level or the threshold level is established at a level far higher than the maximum level of the noise, it will not be possible for faint detection target microwave signals to reach or exceed such reference level or threshold level, and because this makes it impossible to detect such faint microwave signals, the detection accuracy will be lowered. Accordingly, reference levels and threshold levels much higher than the maximum level of the noise are not preferred. However, so long as detection of faint microwave signals is possible, the reference level and the threshold level may be appropriately established to suit particular detection requirements.
In this connection, it is of course understood that the detection levels established with a sufficient margin above the noise level used in prior art microwave detectors for the purpose of detecting normal reception strength microwave signals are too high to be used as the reference level or threshold level for detecting faint detection target microwave signals according to the present invention.
Furthermore, even though there is a relationship between the noise level and the reference level and threshold level, because the level of the noise changes every moment, when each sweep is examined individually, the maximum value of the noise level in each sweep will not always match each other. Accordingly, when the reference level or the threshold level is established at a level which enables detection of a portion of the noise outputted from the reception portion, there will occasionally be times where the noise level does not reach or exceed the reference level or threshold level during one entire sweep, but because a portion of the noise will be detected during the other sweeps, such case is included in the term xe2x80x9cestablished at a level which enables detection of a portion of the noisexe2x80x9d used in the description of the microwave detector structures given above.
Of course, because the noise level undergoes slight fluctuations, such fluctuations are preferably taken into consideration when the reference level and the threshold level are established. In this regard, as was already mentioned above, the reference level and the threshold level are preferably set at the average value of the noise level to make it possible to carry out detection operations even when there are fluctuations in the overall noise level.
In the present invention, faint detection target microwave signals buried in the noise waveform outputted from the reception portion can be discriminated from such noise waveform by comparing the output of the reception portion during a particular measurement with the output of the reception portion during the time no microwave signals are received over either the entire sweep or a portion of the sweep performed by the local oscillator at a prescribed frequency, whereby it becomes possible to judge whether or not a faint detection target microwave signal has been received. In this regard, the term xe2x80x9cburied in the noise waveformxe2x80x9d includes of course the case where the level of the microwave signal is below the maximum noise level, and also includes the case where the level of the microwave signal is close enough to the noise level to make it difficult to discriminate the microwave signal from the noise.
In other words, because the output of the reception portion during the time no microwave signals are received is the noise output of the reception portion, the level of such output will fluctuate at a high frequency. Further, the noise will alternate between levels that reach or exceed the reference level or threshold level and levels that lie below the reference level or threshold level. Accordingly, the digitalization of such noise will create alternating High and Low pulses, and due to the nature of such noise, there will be a fixed ratio between the High and Low pulses over one entire sweep. In this regard, in the case where either the High or Low pulses have a fixed low occurrence probability, the term xe2x80x9cfixed ratioxe2x80x9d includes such probability value.
On the other hand, when a detection target microwave signal is received, the reference level or the threshold level is continuously reached or exceeded. Namely, even though both the noise level and the detection target microwave signal level will reach or exceed the reference level or threshold level, the time over which the reference level or threshold level is reached or exceeded will be relatively longer for the case where a detection target microwave signal is received than for the case where only noise is present. Accordingly, if the output of the reception portion during the time that no microwave signal is received and only noise is present is established as a reference, it becomes possible to make the judgement that a microwave signal has been received in the case where the measured output from the reception portion is different from such reference by a prescribed shift or greater.
Further, the operations performed by the digital processing portion and the judgement portion on the detected wave signals outputted from the reception portion are preferably carried out over prescribed sampling intervals. When sampling is carried out in this way, operations can be simplified by storing the number of samplings of each digital signal. Namely, the actual total times of each digital signal can be calculated simply by multiplying the number of samplings of each digital signal by the sampling time.
Furthermore, the digital processing portion in each of the microwave detector structures described above may be equipped with a discrimination function which makes it easy to discriminate whether the detected wave signals outputted from the reception portion have a high probability of being detection target microwave signals or a high probability of being noise. Further, the judgement-use digital pulse generating portion may be equipped with a discrimination function which makes it easy to discriminate whether the detected wave signals outputted from the reception portion have a high probability of being detection target microwave signals or a high probability of being noise. In this way, by eliminating the need to carry out various calculations (e.g., integration intervals, sweep times, etc.) based on the reception output having a high probability of being noise, the effects of noise can be reduced to enable highly accurate detection of detection target microwave signals. Moreover, during the time that the reception portion outputs signals having a high probability of being noise with no received microwave signals, because there are no signals requiring calculations, it becomes possible to reduce the memory capacity required for storing data used in such calculations.
Alternatively, instead of providing the digital processing portion with a discrimination function, the microwave detector structures described above may be provided with a discrimination portion which carries out a discrimination process on the output from the digital processing portion to discriminate whether the detected wave signals outputted from the reception portion have a high probability of being detection target microwave signals or a high probability of being noise.
In this way, the signals used in carrying out microwave signal detection judgements may be different from the digital signals used in carrying out sweep control, and because this eliminates the need to carry out various calculations (e.g., integration intervals, sweep times, etc.) based on the reception output having a high probability of being noise, the effects of noise can be reduced to enable highly accurate detection of detection target microwave signals. Further, this arrangement also makes it possible to reduce the required memory capacity.
Furthermore, if the microwave detector structures described above carry out a prescribed plurality of sweeps and the judgement results thereof are integrated to form integrated judgement data, the judgement of whether or not a detection target microwave signal has been detected may be carried out based on such integrated judgement data.
For example, even in the case where the reception portion output signal having a high probability of being a microwave signal (i.e., the output signal which reaches or exceeds the reference level or threshold level) only occurs over a short continuous time interval for each sweep, when a plurality of sweeps is examined, the output signal can be assumed to have a high probability of being a detection target microwave signal in the case where the output signal repeatedly reappears at the same position. On the other hand, in the case where an output signal is detected during one sweep but does not reappear in subsequent sweeps, such output signal can be assumed to be due to interference waves.
Furthermore, in each of the microwave detector structures described above, the sweep range of each sweep can be divided into a prescribed plurality of divided sweep regions, and a judgement process can be carried out for each divided sweep region Accordingly, by dividing the sweep in this way, the effects caused by the dispersion of the sweep interval can be reduced, and this makes it possible to carry out highly accurate detection of microwave signals.
Further, in the microwave detector structures described above, the digital processing portion can be equipped with a discrimination function which makes it easy to discriminate whether the detected wave signals outputted from the reception portion have a high probability of being detection target microwave signals or a high probability of being noise, and a memory storing function which creates memory regions corresponding to the divided sweep regions in which the detected wave signals outputted from the reception portion have been discriminated as having a high probability of being detection target microwave signals, and stores data related to the detected wave signals in the corresponding memory regions, and the judgement portion can be arranged to carry out judgements based on the data stored in the memory regions created for the divided sweep regions. In this regard, these functions are included in the signal selection function described below in the embodiments of the present invention.
In this way, because memory regions for storing data related to the output signals is only created for a portion of the divided sweep regions for a single sweep, it is possible to reduce the amount of memory used, and this makes it possible to construct a low-cost microwave detector.
In this case, the data that is stored in the memory region may include data on the number of sweeps related thereto. Further, the number of sweeps related to the data in each memory region may be stored in the same memory region or in a different memory region.
In other words, in the case where a prescribed signal is outputted, when a memory region is created and data is stored therein, the number of sweeps carried out to obtain the data in such memory regions will be different depending on the corresponding divided sweep region. Further, even when data such as the continuation time or integration interval are the same, because the number of sweeps carried out to obtain such data is different for each divided sweep region, output signals within each divided sweep region will have different probabilities of being a microwave signal. Accordingly, by storing the number of sweeps it becomes possible to carry out judgements at an even higher accuracy level.
Furthermore, a clearing function may be provided to clear the memory regions corresponding to sweep regions that have been judged to receive no detection target microwave signals. Namely, in the case where no microwave signals are received, because the data stored at such time is based on noise, by clearing such data, it becomes possible to reduce the effects of noise when making microwave detection judgements thereafter. Further, because this eliminates the need to maintain useless data in the memory, it is possible to carry out the judgement process using a small capacity memory.
Further, in each of the microwave detector structures described above, the sweep control portion may be designed to change the sweep operation performed by the local oscillator in a stepwise manner based on the High or Low pulses outputted from the digital processing portion. In this regard, in the embodiments described below, such stepwise change of the sweep operation is carried out by controlling the sweep voltage with a CPU in order to improve the operation performance.