United States Patent [19]
`Valentine et a!.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`5,068,663
`Nov. 26, 1991
`
`[75]
`
`[54J MOTOR VEHICLE RADAR DETECTOR
`INCLUDING ~"\fPLITUDE DETECflON
`Inventors: Michael D. Valentine; Stephen R.
`Scholl; Marwan E. Nusair, all of
`Cincinnati, Ohio
`[73] Assignee: Valentine Research, Inc., Cincinnati,
`Ohio
`[21] Appl. No.: 645,587
`[22] Filed:
`Jan. 24, 1991
`Int. CI.S .•••.•.•.••..................................... GOIS 7/42
`[51]
`[52] U.S. CI ....................................................... 342/20
`[58] Field of Search .................. 342/20; 455/226, 228,
`455/315
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`2,470,843 511949 Boothroyd et al. .................. 250/20
`2,977,465 3/1961 Sanders, Jr. et al. ................. 250/20
`3,201,696 8/1965 Sharp .................................. 325/423
`4,315,261 2/1982 Mosher ............................. 343/18 E
`4,581,769 4/1986 Grimsley et al. ................... 455/226
`4,613,989 9/1986 Fende .................................. 455/351
`4,622,553 11/1986 Baba et al. ............................ 342191
`4,626,8571211986 Imazeki ................................. 342/20
`4,630,054 12/1986 Martinson ............................. 342/20
`4,668,952 5/1987 Imazeki et a1. ....................... 342/20
`4,686,499 8/1987 Furnish ............................... 333/230
`4,698,632 10/1987 Baba et al ............................. 342117
`4,709,407 11/1987 Baba .................................... 455/226
`4,750,215 6/1988 Biggs ................................... 455/226
`4,862,175 8/1989 Biggs ..................................... 342120
`
`4,954,828 9/1990 Orr ........................................ 342/20
`
`Primary Examiner-Mark Hellner
`Attorney, Agent, or Firm-Killworth, Gottman, Hagan
`& Schaeff
`ABSTRACT
`[57]
`A radar detector for use in a motor vehicle employs
`amplitude detection to sense the presence of radar sig(cid:173)
`nals commonly used to monitor the speed of such motor
`vehicles. Amplitude signals are generated by down(cid:173)
`converting received signals using a series of mixers, one
`of which is swept to insure signal detection, and com(cid:173)
`pared to a threshold which is controlled such that noise
`is detected by the comparison on average a selected
`period of time. Detected amplitUde signals must persist
`for a given period of time before they are considered to
`be potentially valid radar signals. After passing the first
`test of persistence, the signals are verified by means of
`frequency modulating the first of the series of mixers,
`detecting the frequency modulation and correlating the
`detected frequency modulation to determine whether
`the signal is valid and if so, to which radar frequency
`band the signal belongs. A first embodiment of the radar
`detector monitors the X band (10.475-10.575 Ghz), the
`Ku band
`(13.400-13.500 Ghz),
`the K
`band
`(24.025-24.275 Ghz), and the Ka band (34.200-35.200
`Ghz) and a second embodiment monitors all of these
`radar signal bands plus and expanded Ka band
`(34.200-35.200 Ghz).
`
`36 Claims, 2 Drawing Sheets
`
`112
`
`100
`)
`
`CONTROL
`
`K40 Exhibit 1022, pg. 1
`IPR2013-00240
`
`

`

`
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`K40 Exhibit 1022, pg. 2
`IPR2013-00240
`
`

`

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`K40 Exhibit 1022, pg. 3
`IPR2013-00240
`
`

`

`1
`
`MOTOR VEHICLE RADAR DETECTOR
`I~CLUDIJ'liG AMPLITUDE DETECTIO~
`
`5,068,663
`
`2
`pressed noise is a complex task tending to require expen(cid:173)
`sive hardware.
`Accordingly, there is a need for an improved radar
`detector which operates effectively with IF bandwidth
`5 more closely approximating the matched filter band(cid:173)
`width and which overcomes the foregoing problems
`associated with conventional radar detector techniques
`utilizing FM discriminators whose output signals are
`filtered and passed to voltage comparators or other
`circuitry to detect the presence of radar signals.
`
`10
`
`BACKGROUND OF THE INVENTION
`The present invention relates generally to motor ve(cid:173)
`hicle radar detectors and, more particularly, to radar
`detectors which provide improved sensitivity and a
`satisfactory level of "false alarms" by means of signal
`amplitude detection.
`Radar signals have been commonly used by police for
`some time to determine the speed of motor vehicles. In
`response to radar speed monitoring and to signal motor
`vehicle operators when such monitoring is taking place,
`police radar detectors have likewise been used for al- 15
`most a coincident period of time. Radar detectors sense
`radar signals and alert motor vehicle operators of their
`presence by audible and/or visual alarms. To ensure
`that radar detectors advise operators of radar monitor(cid:173)
`ing operations at the earliest possible time, the detectors 20
`must be sufficiently sensitive to detect weak radar sig(cid:173)
`nals transmitted from as great a distance as possible. It is
`also important to minimize the number of "false
`alarms", i.e. alarms generated in response to signals/(cid:173)
`noise other than speed monitoring radar signals, pro- 25
`duced by radar detectors such that an operator can rely
`on radar alerting signals generated by the detector.
`Unfortunately, detection of weak signals and minimiz(cid:173)
`ing false alarms present conflicting goals.
`Currently available police radar detectors typically 30
`employ swept superheterodyne receivers which include
`FM discriminators whose output signals are filtered and
`passed to voltage comparators to detect the presence of
`radar signals. In an attempt to achieve both the weak
`signal detection and low false alarm goals, elaborate 35
`filters have been used that are matched to the "s-shape"
`of the detected FM signal which is generated as the
`signal sweeps through the intermediate frequency (IF)
`amplifier. Another approach is to perform digital signal
`processing on the output signal from the discriminator 40
`to improve post-discriminator dynamic range. Unfortu(cid:173)
`nately, all known approaches tend to be complex and
`expensive and the detectors still are made up of an FM
`discriminator, a filter which is possibly complex and
`circuitry to finally make a radar signal present or no 45
`radar signal present decision.
`Further, all such known detectors also employ an
`inordinately wide IF bandwidth in comparison to de(cid:173)
`sired matched filter considerations, possibly due to sig(cid:173)
`nal detection considerations. While matched filter band- 50
`width is approximately the square root of the sweep rate
`of the swept superheterodyne receiver, known detec(cid:173)
`tors use a substantially wider IF bandwidth. With wider
`IF bandwidth, the "s-curve" persists for a relatively
`longer period of time and therefore its energy is concen- 55
`trated at lower frequencies making it easier to detect in
`the presence of broadband noise from the discriminator.
`Conversely, as the IF bandwidth is narrowed the "s(cid:173)
`curve" occurs faster occupying a wider bandwidth, and
`is progressively more difficult to detect in the presence 60
`of noise occupying a similar bandwidth. Further con(cid:173)
`founding the recognition task is the fact that the desired
`"s-curve" is a bipolar signal buried in bipolar noise. Still
`another difficulty is the inherent nature of the FM limi(cid:173)
`ter/discriminator: improving input signal strength is not 65
`manifested solely by increased output signal amplitude
`but also by suppression of the noise component. Recog(cid:173)
`nizing improved signal to noise ratio arising from sup-
`
`SUMMARY OF THE INVENTION
`This need is met by an improved radar detector in
`accordance with the present invention wherein ampli(cid:173)
`tude detection rather than frequency discrimination is
`employed to detect the presence of radar signals. Am(cid:173)
`plitude detection is advantageous in that radar signal
`presence occurs as a unipolar pulse generated as the
`signal is swept through the IF amplifier. A unipolar
`pulse is easier to detect with a voltage comparator than
`the bipolar "s-curve" of the FM discriminator. Improv(cid:173)
`ing signal strength is also manifested by a larger detec(cid:173)
`tor output rather than suppression of the noise compo(cid:173)
`nent, again easing the radar signal detection. These
`properties of amplitUde detection permit the IF ampli(cid:173)
`fier bandwidth to be narrowed toward the desired
`matchea filter bandwidth thus reducing the noise power
`in the IF amplifier. Of course, amplitUde detection also
`creates a number of problems which are overcome in
`accordance with the present invention. Included within
`the problems presented and overcome are: implementa(cid:173)
`tion of amplitude detectors having wide dynamic range;
`coping with changing noise power presented to the
`amplitude detector as the receiver is swept; achieving
`sensitivity without generating undue false alarms; and,
`setting a decision voltage for signal detection to effect a
`suitable compromise between sensitivity to received
`signals and immunity to the noise in which the signals
`are embedded.
`In accordance with one aspect of the present inven(cid:173)
`tion, a motor vehicle radar signal detector for alerting
`an operator of a motor vehicle receiving incoming radar
`signals comprises receiver means for receiving incom(cid:173)
`ing radar signals at frequencies within at least one fre(cid:173)
`quency band. Radar signal processing means responsive
`to the incoming radar signals is provided for generating
`a final intermediate frequency signal within a final inter(cid:173)
`mediate frequency range. The radar signal processing
`means comprises variable frequency mixer means in(cid:173)
`cluding a local oscillator for generating a local oscilla(cid:173)
`tor signal which is swept across a frequency range cor(cid:173)
`responding to the at least one frequency band. Ampli(cid:173)
`tude detection means receives the final intermediate
`frequency signal for generating a signal strength signal
`representative of the amplitude of the final intermediate
`frequency signal. Signal verification means responsive
`to the signal strength signal identifies valid incoming
`radar signals. Finally, alarm means is provided for alert(cid:173)
`ing an operator of a motor vehicle upon identification of
`valid incoming radar signals.
`The radar signal processing means further comprises
`first mixer means for converting the incoming radar
`signals to a first intermediate frequency signal having a
`frequency within a first intermediate frequency range.
`The first intermediate frequency signal is mixed with
`the swept local oscillator signal by the variable fre(cid:173)
`quency mixer means to generate a swept intermediate
`
`K40 Exhibit 1022, pg. 4
`IPR2013-00240
`
`

`

`5,068,663
`
`3
`frequency signal having a frequency within a second
`intermediate frequency range. The swept intermediate
`frequency signal is converted to the final intermediate
`frequency signal by second mixer means.
`In accordance with another aspect of the present 5
`invention, a motor vehicle radar signal detector for
`alerting an operator of a motor vehicle receiving incom(cid:173)
`ing radar signals comprises receiver means for receiving
`incoming radar signals at frequencies within at least one
`frequency band. First mixer means is provided for con- 10
`verting the incoming radar signals to a first intermediate
`frequency signal having a frequency within a first inter(cid:173)
`mediate frequency range. Variable frequency mixer
`means are provided for mixing the first intermediate
`frequency signal with an oscillator signal which is 15
`swept across the first intermediate frequency range to
`generate a swept intermediate frequency signal having a
`frequency within a second intermediate frequency
`range. Second mixer means is provided for converting
`the swept intermediate frequency signal to a final inter- 20
`mediate frequency signal which is within a final inter(cid:173)
`mediate frequency range. Amplitude detection means
`receive the final intermediate frequency signal for gen(cid:173)
`erating a signal strength signal representative of the
`amplitude of the final intermediate frequency signal. 25
`Signal verification means responsive to the signal
`strength signal is provided for identifying valid incom(cid:173)
`ing radar signals. Finally, alarm means is provided for
`alerting an operator of a motor vehicle upon identifica(cid:173)
`tion of valid incoming radar signals.
`In one embodiment of the present invention, the re(cid:173)
`ceiver means receives incoming radar signals at fre(cid:173)
`quencies within three frequency bands: the X band
`(10.475-10.575 Ghz); the K band (24.025-24.275 Ghz);
`and, the Ka band (34.200-35.200 Ghz). The first mixer 35
`means comprises a first mixer for the X band signals to
`generate the first intermediate frequency signal there(cid:173)
`from. and a second mixer for the K and Ka band signals
`to generate the first intermediate frequency signal there(cid:173)
`from. Preferably, the first mixer is operated at a fre- 40
`quency of substantially 11.9875 Ghz, the second mixer
`is operated alternately at a frequency of substantially
`12.806 Ghz (for the K band) and a swept frequency
`ranging substantially from 16.360 Ghz to 16.860 Ghz
`(for the Ka band), and the variable frequency mixer 45
`means is swept through a frequency range of substan(cid:173)
`tially 1.752-2.000 Ghz.
`In another embodiment of the present invention, the
`receiver means receives incoming radar signals at fre(cid:173)
`quencies within four frequency bands: the X band 50
`(10.475-10.575 Ghz), the Ku band (13.400-13.500 Ghz),
`the K band (24.025-24.275 Ghz) and the Ka band
`(34.200-35.200 Ghz). The first mixer means comprises a
`first mixer for the X and Ku band signals to generate the
`first intermediate frequency signal therefrom, and a 55
`second mixer for the K and Ka band signals to generate
`the first intermediate frequency signal therefrom. Pref(cid:173)
`erably, the first mixer is operated at a frequency of
`substantially 11.9875 Ghz, the second mixer is operated
`alternately at a frequency of substantially 12.806 Ghz 60
`(for the K band) and a swept frequency ranging substan(cid:173)
`tially from 16.360 Ghz to 16.860 Ghz (for the Ka band),
`and the variable frequency mixer means being swept
`through a frequency range of substantially 1.752-2.000
`Ghz. For both the immediately preceding embodi- 65
`ments, the frequency range of the Ka band which is
`monitored is sufficiently wide to detect radar signals
`within a broader frequency range recently approved for
`
`4
`police radar by the Federal Communications Commis(cid:173)
`sion (FCC). Preferably, the second mixer is upwardly
`swept through a frequency range of substantially
`16.360-16.860 Ghz in synchronism with the upward
`sweep of the variable frequency mixer means through a
`frequency range of substantially 1.752-2.000 Ghz for its
`alternating sweeping operations.
`The signal verification means preferably comprises
`comparator means for comparing the signal strength
`signal to a verification threshold signal and threshold
`signal generator means connected to the output of the
`comparator means and a reference signal for controlling
`the verification threshold signal such that the compara(cid:173)
`tor means is tripped by noise on average a defined per(cid:173)
`centage of time selected by the reference signal. The
`signal verification means further comprises timer means
`for latching the signal verification means after the signal
`strength signal has exceeded the verification threshold
`signal for a defined period of time. The signal verifica(cid:173)
`tion means further comprises FM discriminator means
`for receiving said final intermediate frequency signal to
`detect an FM signal therein and dither means connected
`to the first mixer means for frequency modulating a
`local oscillator of the first mixer means which receives
`incoming radar signals from the receiver means with a
`low frequency sine wave signal and signal processing
`means connected to the FM discriminator means for
`correlating demodulated output signals from the FM
`discriminator means with the low frequency sine wave
`30 signal.
`It is an object of the present invention to provide an
`improved radar detector wherein amplitude detection
`rather than frequency discrimination is employed to
`detect the presence of radar signals; to provide an im(cid:173)
`proved radar detector wherein amplitUde detection of
`radar signals permits the IF amplifier bandwidth to be
`narrowed toward a desired matched filter bandwidth
`thus reducing the noise power in the IF amplifier; and,
`to provide an improved radar detector wherein ampli(cid:173)
`tude detection is employed to detect the presence of
`radar signals and the detector is controlled such that it
`is tripped by noise on average a selected percentage of
`the time.
`Other objects and advantages of the invention will be
`apparent from the following description, the accompa(cid:173)
`nying drawings and the appended claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic block diagram of a radar detec(cid:173)
`tor in accordance with the present invention; and
`FIG. 2 is a schematic diagram of the control circuit of
`FIG. 1.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`As previously noted, the present invention relates to
`a motor vehicle radar detector which provides im(cid:173)
`proved sensitivity and a satisfactory level offalse alarms
`by means of radar signal amplitUde detection as opposed
`to frequency discrimination which is commonly used in
`such detectors. While amplitude detection provides
`several advantages such as the generation of a unipolar
`pulse as the signal is swept through the IF amplifier and
`the ability to narrow the IF amplifier bandwidth toward
`a desired matched filter bandwidth, a number of prob(cid:173)
`lems are also encountered and addressed as will be
`described herein. Apparently the problems have out(cid:173)
`weighed the advantages to this time since applicants are
`
`K40 Exhibit 1022, pg. 5
`IPR2013-00240
`
`

`

`5,068,663
`
`6
`5
`and Hi band2 being alternately scanned by alternate
`unaware of any motor vehicle radar detector which
`activations of the veo 126 and the local oscillator 124.
`utilizes amplitude detection.
`The La bands and HI bands are thus inspected in alter(cid:173)
`The present invention will be described with refer(cid:173)
`nating, "ping-pong" fashion with the HI bands them(cid:173)
`ence to a radar detector which detects radar signals
`selves alternating between HI bandl and HI band2 in
`within four bands; however, it is to be understood that 5
`"ping-pong" fashion.
`the amplitude detection of radar signals of the present
`invention is generally applicable to radar detectors in(cid:173)
`A filter 132 serves to reduce noise and unwanted
`cluding detectors which detect radar signals within
`signal inputs from the upper sideband image for variable
`three bands, two bands, a single band or more than four
`frequency mixer means comprising the mixer 134 which
`JO is driven by swept local oscillator means comprising a
`bands.
`voltage controlled oscillator (VeO) 136. In the illus(cid:173)
`The radar detector 100 of FIG. 1 monitors four sepa(cid:173)
`trated embodiment, the filter 132 is a notch filter block(cid:173)
`rate police radar bands to sense radar signals 102 inci(cid:173)
`ing signals from 2.000 to 2.350 Ghz. The veo 136 is
`dent upon receiver means comprising an antenna 104 of
`driven by control circuit 138 and the microprocessor
`the detector 100. The four police radar bands monitored
`include the X band (10.475-10.575 Ghz), the Ku band 15
`128 to sweep the mixer 134 through the appropriate
`(13.400-13.500 Ghz), the K band (24.025-24.275 Ghz)
`range for the band of signals to be processed to generate
`and the Ka band (34.200-35.200 Ghz). Radar signal
`a swept intermediate frequency signal having a fre(cid:173)
`processing means comprising the circuitry connected
`quency within a second intermediate frequency range.
`between the antenna 104 and an amplitude detector 106,
`In the illustrated embodiment, the mixer 134 is swept
`through a frequency range of 1.752-2.000 Ghz.
`amplitUde detection means, and a FM discriminator 108, 20
`FM discriminator means, will now be described.
`Second mixer means extend between the mixer 134
`Signals received by the antenna 104 are passed to a
`and the amplitude detector 106 and FM discriminator
`108 for converting the swept intermediate frequency
`diplexer 110 which separates the signals into signals
`above 19.6 Ghz. referred to as the HI bands, and signals
`signal to the final intermediate frequency signal. Fol(cid:173)
`below 19.6 Ghz, referred to as the La bands. Thus, the 25
`lowing the mixer 134, an 800 Khz bandpass filter 138
`signals within the La bands include the X and Ku band
`centered on 400.9 Mhz provides initial selectivity and
`signals while the signals within the HI bands include the
`image rejection of noise or signals at 423.3 Mhz. A
`K and Ka band signals. The X and Ku band signals are
`mixer 140 driven by an oscillator 142 operated at a
`received simultaneously by a dual RF response down(cid:173)
`frequency of 411.6 Mhz down-converts to a 10.7 Mhz
`converter system comprising an amplifier 112 and a first 30
`IF signal which passes through a 270 Khz bandpass
`mixer 114 driven by a local oscillator 116 operated at a
`filter 144 to the amplitude detector 106 and the FM
`frequency of 11.9875 Ghz with the down-converted
`discriminator 108.
`signals ultimately appearing in a relatively wide IF band
`The sweep control circuitry for sweeping the veo
`between 1.4125 and 1.5125 Ghz.
`136 and hence the mixer 134 through the appropriate
`The K and Ka band signals are received by a second 35 frequency range for radar signal detection is illustrated
`in FIG. 2. Operational amplifier 202 together with resis-
`dual RF response down-converter system comprising a
`second mixer 118 and an amplifier 122. The down-con-
`tor 204 and capacitors 206, 208 form a ramp generator
`verted HI band signals from the second dual RF re-
`which tunes the veo 136 via the conductor 162
`sponse dovin-converter system ultimately appear in the
`through the desired frequency range. Analog switch
`same IF band as the down-converted La band signals, 40 210 permits the rise in ramp voltage to be halted as
`necessary to allow detailed inspection of any detected
`the K band signals from 1.362 Ghz to 1.562 Ghz and the
`Ka band signals from 1.350 Ghz to 1.599 Ghz. The
`radar signals. The rise in sweep voltage is halted when
`second mixer 118 is alternately driven by a local oscilla-
`the microprocessor 128 senses via an analog to digital
`tor 124 operated at a frequency of 12.806 Ghz and a
`(A/D) converter 212 and connecting conductor 212A
`voltage controlled oscillator (VeO) 126 which is swept 45 that the required voltage has been reached for a given
`sweep. A switch 214 is operated by the microprocessor
`through a frequency range of 16.360 Ghz to 16.860
`Ghz. Both local oscillators 116. 124 and the veo 126
`128 via a conductor 214A to reset the ramp to the initial
`for the first and second mixers 114, 118 receive fre-
`voltage which begins the alternately repeated sweep
`cycles of 1.752-2.000 Ghz. If desired. a second sweep
`quency modulation signals or dither D which is pro-
`vided by a detector controlling microprocessor 128 for 50 cycle can bl.! provided by a switch 216 which is oper-
`verification of receipt of actual radar signals as will be
`ated via a conductor 216A. The veo 126 can be oper-
`described hereinafter. The microprocessor 128 also
`ated directly by the microprocessor 128 or preferably is
`controls the selection of signals from either the HI
`operated from the signal which drives the veo 136 as
`bands, HI ENABLE. HI ENABLEI and HI ENA-
`will be described hereinafter.
`BLE2, or the La bands, La ENABLE, for a given 55
`The illustrated embodiments of the present invention
`detection operation as will become apparent. The first
`employ a sweep rate of approximately 8.8 Ghz/sec
`and second mixers 114. 118 comprise first mixer means
`which is comparable to or larger than the sweep rates
`employed by prior art radar detectors. The approxi-
`for converting the incoming signals to a first intermedi-
`mately 270 Khz final IF bandwidth is somewhat narrow
`ate frequency signal having a frequency within a first
`60 since prior art radar detectors employ final IF band-
`intermediate frequency range.
`The down-converted radar signals from both the HI
`widths of a few hundred Khz to 1.3 Mhz. This narrow
`bands and the La bands lying in the relatively wide first
`final IF bandwidth is an important feature of the present
`IF band are received by an amplifier 130 to be further
`invention and produces improved sensitivity, as previ-
`down-converted by the remainder of the radar signal
`ously described. given the wide received bandwidth
`processing means to a final IF frequency of 10.7 Mhz 65 and the high sweep rate.
`The cumulative cascade of veo 136, freauencv con-
`where detection ultimately occurs. It should be under-
`version via the mixer 140 and frequency dis~rimi~ation
`stood that the La bands are scanned followed by a
`bandswitching operation to the HI bands with HI bandl
`via the filters 138, 150 has been arranged to permit the
`
`K40 Exhibit 1022, pg. 6
`IPR2013-00240
`
`

`

`7
`application of FM negative feedback. Such FM nega(cid:173)
`tive feedback is essentially a form of automatic fre(cid:173)
`quency control that aids tuning of the receiver to the
`correct frequency when a radar signal has been de(cid:173)
`tected. This is desirable since the radar detector of the 5
`present invention analyzes the validity of a received
`signal before issuing an alarm to the operator of a motor
`vehicle including the detector 100.
`The amplitude detector 106 and FM discriminator
`108 are provided by a single integrated circuit 160 in the 10
`illustrative embodiment of the present invention, see
`FIG. 1. While a number of devices are commercially
`available, the illustrated embodiment of the present
`invention utilizes a Signetics broad band FM receiver
`integrated circuit chip designated as a NE604 as the 15
`demodulator. The use of the integrated circuit 160 pro(cid:173)
`vides an amplitude detector having wide dynamic range
`which is required for the radar detector of the present
`invention. The received signal strength indicator RSSI
`output signal generated on the conductor 164 is moni- 20
`tored by the control circuit 138 for initial detection of
`radar signals. In the control circuit 138, the RSSI output
`signal is initially passed through a low pass filter 218.
`The output of the filter 218 can be monitored by the
`microprocessor 128 via an analog to digital (A/D) con- 25
`verter 219 and connecting conductor 219A.
`Another problem associated with the use of ampli(cid:173)
`tude detection in a radar detector is that the noise
`power appearing in the final IF signal varies as the
`receiver is swept due to variations in gain and noise 30
`figure distributed throughout the receiver. The noise
`variations cause variations in the RSSI output signal
`that must be rejected by the detector. Referring to FIG.
`2. rejection of this noise is performed by a high pass
`filter formed by a resistor 220 and a capacitor 222. Note 35
`that the analog switch 224 is closed during the sweep
`process. The noise variation is a relatively slow phe(cid:173)
`nomenon since the sweep period is on the order of 1/10
`second. Occurrence of a signal pulse is a fast event since
`the IF bandwidth is 270 Khz. Selection of the time 40
`constant for the high pass filter is a compromise be(cid:173)
`tween rejecting noise floor variation and degrading
`sensitivity to the signal strength pulses. A 1 msec time
`constant proved suitable in a working embodiment of
`the present invention.
`Comparator 226 performs an initial tentative decision
`between a suspected radar signal and noise. When a
`sufficiently positive voltage is sensed, the output signal
`of the comparator 226 switches from a high voltage
`level to a low voltage level. The low voltage level 50
`signal passes through NAND gates 228, 230 and opens
`the analog switches 210 and 224. Opening the switch
`210 halts the advance of the sweep voltage so that the
`receiver "parks" to monitor the suspected radar signal.
`The switch 224 is opened so that if a radar signal is 55
`indeed present, the voltage across the capacitor 222 will
`not change during the period of time that the receiver is
`parked. If the voltage across the capacitor 222 were
`allowed to change, the threshold for recognizing a
`radar signal would be shifted so that the receiver would 60
`be much less sensitive when the sweep process is re(cid:173)
`sumed.
`The low pass filter 218 which passed the RSSI output
`signal from the integrated circuit 160 has bandwidth
`that is roughly matched to the expected pulse width of 65
`a signal sweeping through the IF. When the sweep
`stops with a signal present, the voltage at the Output of
`the filter 218 continues to grow slightly as the filter
`
`45
`
`5,068,663
`
`8
`settles. As a result, hysteresis in the comparator 226
`provided by the resistors 232 and 234 is beneficial in
`distinguishing between radar signals and noise.
`Another problem encountered in using amplitUde
`detection and narrowed bandwidths is the delay associ(cid:173)
`ated with matched filter bandwidths. The delay in a
`matched filter is such that the signal has just passed
`through the filter when the output voltage peaks. Thus,
`when the comparator 226 recognizes a radar signal, the
`receiver has already tuned past the signal and needs to
`have the sweep "backed-up" to be correctly tuned for
`subsequent analysis. In the illustrated embodiment of
`the present invention, sweep back-up is performed in
`the following manner: during the sweep operation the
`series combination of resistors 236 and 238 have a small
`dc voltage developed by the current established by the
`resistor 204. When the switch 210 opens, this current
`ceases, the voltage across the resistors 236 and 238
`drops to zero and the sweep voltage decreases by a
`corresponding and appropriate amount to back-up the
`sweep. Performing the correction to the sweep voltage
`increases the likelihood that an FM feedback loop ex(cid:173)
`tending from the FM discriminator 108 and passing
`through an analog switch 240, a resistor 242, a capacitor
`244, the resistor 238 and the capacitors 206 and 208, will
`find and lock onto the suspected radar signal. The resis(cid:173)
`tor 238 also adjusts frequency compensation of this FM
`feedback loop.
`Another feature of the radar detector of the present
`invention is the ability to maintain high sensitivity to
`radar signals without spending undue time inspecting
`noise. Since the detector stops the sweep or parks the
`receiver to verify suspected radar signals before gener(cid:173)
`ating an alarm, excessive sweep stopping could slow the
`effective sweep rate and thereby increase detection
`latency.
`To overcome this potential problem in the present
`invention, the radar signal detection threshold applied
`to the comparator 226 is set so that the comparator 226
`is sensitive to radar signals but also susceptible to false
`alarms. Then, at least most false alarms are screened out
`by requiring that the suspected radar signal persist for a
`minimum period of time before the microprocessor 128
`performs signal verification. A switch 246, a resistor
`248, a capacitor 250 and a comparator 252 perform the
`timer function. The timer period is chosen to be several
`times the time constant associated with the IF band(cid:173)
`width and the associated RSSI lowpass filter 218. Ap-.
`proximately 300 microseconds has proved appropriate
`in a working ~mbodiment of the p