United States Patent [19]
`Louttit
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,513,443
`Apr. 23, 1985
`
`[54] RADIO RECEIVER SUITABLE FOR USE IN A
`SPACED CARRIER AREA COVERAGE
`SYSTEM
`
`4,387,469 6/1983 Miyazaki et a]. ................. .. 455/161
`4,419,766 12/1983 Goeken et a1.
`455/62
`4,430,753 2/1984 Shiratani ............................. .. 455/52
`
`[75] Inventor: Duncan R. Louttit, Cambridge,
`England
`[73] Assignee: U.S. Philips Corporation, New York,
`N_Y_
`[21] AppL No‘: 612,995
`
`[22] Filed:
`
`May 21, 1984
`
`[63]
`
`_
`_
`Related {15' Apphcatlon Data
`Continuation of Ser. No. 340,161, Jan. 18, 1982, aban-
`clonedv
`
`.
`-
`-
`-
`-
`Forelgn Apphcatlon Pnonty Data
`[30]
`Feb. 4, 1981 [GB] United Kingdom ............... .. 8103413
`[51] Int. Cl.3 ....................... .. H04B 7/24; H04B l/26;
`H03] 7/18
`[52] US. (:1. ...................................... .. 455/52; 455/62;
`455 A65; 455 A66
`[58] Field of Search ................. .. 455/52, 62, 161, 165,
`455/166, 168, 169, 194, 196
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`455/205
`3,673,499 6/1972 Avins et a1. ........... ...
`455/62
`3,983,492 9/1976 Fisher etaL "
`455/196
`4,069,455 l/1978 Sherman ..... ..
`455/62
`4,l97,500 4/1980 Klein et a1.
`4,380,826 4/ 1983 Usugi ................................ .. 455/165
`
`_
`_
`Pr'm‘m’ Exammer_Marc E" Bookbmdel:
`Attorney, Agent, or F1rm~Thomas A. Brlody; W1ll1am
`J. Streeter
`ABSTRACT
`[57]
`A radio receiver suitable for use in a spaced carrier area
`coverage system in which two or more carriers are
`transmitted_
`
`In order to reduce the overall noise which is present in
`existing systems and to enable carriers to be spaced
`more regularly and/or closer, the radio receiver is
`adapted to measure the strength of each signal, in turn,
`and to determine which of the received signals has the
`greatest strength. A local oscillator, conveniently in the
`form of a frequency Synthesizer is adjusted so that the
`Output frequehey to the mixer of the RF eeetieh is such
`as to enable the signal having the greatest strength to be
`reeeived- The System remains leeked Oh to the eeleeted
`signal unless or until the incoming signal decreases in
`amplitude by more than a predetermined amount If this
`occurs, the scanning sequence is repeated to ?nd the
`signal having the greatest strength, but in the event of
`all the signals scanned not being greater than a mini
`mum squelch threshold value, then a squelch signal is
`i’goduceq wh‘lch 519111;”: the audtllcl’ frequfancy Section 9f
`e recelver. n 1s a er case e receiver remains in
`stand'by mode
`
`5 Claims, 6 Drawing Figures
`
`AGE
`
`"—{_l
`i
`AMPLIFIER
`
`148
`[
`
`52
`
`AUDIO £31m
`55011011
`
`‘_ out?t
`mm
`
`1r AMP. 11
`&
`LEVEL OH.
`
`55\
`
`L0
`RCVRLRZF
`st 011
`
`FREQUENCY
`H
`SYNTHESIZER
`
`/62
`
`LOGIC
`56
`
`Juniper Ex 1014-p. 1
`Juniper v MTel891
`
`

`
`Juniper Ex 1014-p. 2
`Juniper v MTel891
`
`

`
`U.S. Patent Apr. 23, 1985
`
`Sheet2 of4
`
`4,513,443
`
`I
`
`I
`
`ll 32/ I
`
`30
`
`l\\\
`
`l
`
`//
`
`'
`
`I
`
`I \BLL I
`
`—6'25I<HZ
`
`f
`
`+6'25kHZ
`
`Fig. 3.
`
`[+0
`Y I
`CHANNEL
`RCVR RF
`52 - .13
`SECTION
`FILTER
`
`.,
`
`-
`
`'
`
`[+8
`I
`
`“
`
`'
`
`52
`
`(AGE /
`
`AUDIO FREQ.
`23.
`SECTION
`
`IF
`2
`AMPLIFIER I
`
`IF AMP. &
`E
`
`LEVEL DET.
`
`.
`
`FREQUENCY
`5.4
`SYNTHESIZER
`
`/62
`
`LOGIC
`56
`
`I\ I\
`
`60
`
`S8
`
`55\
`
`Fig.4.
`
`Juniper Ex 1014-p. 3
`Juniper v MTel891
`
`

`
`U.S. Patent Apr. 23, 1985
`
`Sheet 3 of 4
`
`4,513,443
`
`START
`
`ENABLE
`AUDIO
`/ OUTPUT
`
`82/
`
`MEAS. CARRIER
`STRENCTH/
`/CONVERT A/D
`
`MEASURE
`CARRIER WAVE
`
`CALCULATE /
`STORE
`/THRESHOLD
`
`DISABLE
`AUDLO
`OUTPUT
`
`SELECT
`STRONGEST
`[CARRIER WAVE
`
`Juniper Ex 1014-p. 4
`Juniper v MTel891
`
`

`
`Juniper Ex 1014-p. 5
`Juniper v MTel891
`
`

`
`1
`
`4,513,443
`
`RADIO RECEIVER SUITABLE FOR USE IN A
`SPACED CARRIER AREA COVERAGE SYSTEM
`
`This is a continuation of application Ser. No. 340,161,
`?led Jan. 18, 1982, now abandoned.
`
`BACKGROUND OF THE INVENTION
`A radio receiver suitable for use in a spaced carrier
`area coverage system. The radio receiver may be used
`with an area coverage system for land mobiles and in a
`conceivable maritime area coverage system.
`Spaced carrier area coverage systems are used where
`it is desired to transmit a signal to, say, a vehicle moving
`within an area and which signal, if transmitted from a
`single aerial, could be subject to extensive fading or loss
`at the receiver. In a known amplitude modulated spaced
`carrier area coverage system, typically three aerials are
`located at vantage points within the area to be covered.
`Each aerial is associated with its own transmitter which
`transmits a signal on its respective carrier wave. The
`frequencies of these carrier waves are chosen so that all
`the signals including their sidebands fall substantially
`within a 25 kHz channel. The narrow bandwidth modu
`lation signals have to be synchronized so that they are
`all in phase. The selection of the frequencies of the
`carrier waves is such that the primary, secondary and
`subsequent beat notes fall outside the audio bandwidth,
`typically 200 Hz to 3 kHz, of the receiver.
`In order to receive the signal, the known receiver has
`an IF noise bandwidth (that is the passband between the
`:3 dB points) of typically 20 kHz so that it is able to
`receive all three carrier signals and at least one sideband
`of each carrier. Because of this wide bandwidth the
`receiver picks up unwanted noise. Trying to select any
`one of these carriers by having say three channel ?lters
`each with a response of 13.75 kHz, instead of having a
`single channel ?lter with a response of :10 kHz, and
`switching between these ?lters may cause problems of,
`for example intermodulation. Furthermore, such addi
`tional channel ?lters are undesirable because channel
`?lters are regarded as being an expensive part of the
`receiver circuitry.
`An alternative area coverage system which operates
`within a 12.5 kHz channel is termed a quasi-synchro
`nous system in which several transmitters, for example
`three transmitters, operate at different but very closely
`related frequencies, the differences being less than 10
`Hz. While the system works in principle, there are sub
`jective problems due to the very low frequency beating
`of the transmitter carrier waves and the very high fre
`quency stability requirements of these transmitters rela
`tive to one another.
`
`40
`
`45
`
`SUMMARY OF THE INVENTION
`An object of the invention is to provide a radio re
`ceiver suitable for use in a spaced carrier area coverage
`system which has an improved signal-to-noise ratio.
`According to the present invention there is provided
`a radio receiver suitable for use in a spaced carrier area
`coverage system including at least two transmitters
`transmitting different carrier waves within an overall
`channel, the radio receiver comprising an RF section
`including a mixer, a channel ?lter coupled to an output
`of the RF section, means for producing a voltage indic
`ative of the strength of the signal at an output of the
`channel ?lter, means for scanning the signal strength
`voltage associated with each of the received carrier
`
`55
`
`60
`
`65
`
`2
`waves and determining which carrier wave has the
`greater (or greatest) signal strength and means for lock
`ing the injection frequency to the mixer so that the RF
`section locks onto the carrier wave which had the
`greater (or greatest) strength when the signal strength
`voltages were scanned.
`The advantages of the present invention over the
`known systems and possible modi?cations thereof are
`that the radio receiver only utilizes one narrow band
`width channel ?lter to receive spaced carrier and quasi
`synchronous area coverage systems. The switching of
`the signal is done in those parts of the radio receiver
`where one does not have interference and/or intermod
`ulation problems.
`A further advantage of the radio receiver in accor
`dance with the present invention is that it will enable a
`regular spacing of the carrier wave frequencies within a
`channel to be carried out as compared with an irregular
`spacing which is used at present to avoid beat frequency
`problems. This will enable the overall bandwidth of the
`area coverage signals to be reduced by 5.5 kHz and
`thereby enable true 25 kHz channelling to become pos
`sible.
`
`DESCRIPTION OF THE DRAWINGS
`The present invention will now be explained and
`described, by way of example, with reference to the
`accompanying drawings, wherein:
`FIG. 1 illustrates the carrier frequency arrangement
`and response of a known system;
`FIG. 2 illustrates the application of the receiver in
`accordance with the present invention to an existing
`area coverage system;
`FIG. 3 illustrates a symmetrical carrier frequency
`spacing which is possible with the radio receiver made
`in accordance with the present invention;
`FIG. 4 is a block schematic circuit diagram of the
`relevant part of a radio receiver made in accordance
`with the present invention;
`FIG. 5 is a ?ow-chart illustrating the basic operating
`steps of the micro-computer used in a logic system in
`the radio receiver illustrated in FIG. 4; and
`FIG. 6 serves to illustrate the situation when the
`received signal strength falls by more than a predeter
`mined amount below its measured value thereby caus
`ing the receiver to scan again the incoming signals.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`Referring to FIG. 1, there is shown the distribution of
`transmitted carrier wave frequencies 10, 12 and 14 in a
`known spaced carrier area coverage system and the
`response curve 16 of a known radio receiver. The am
`plitudes of the three carrier waves are intended to indi
`cate the strength of the respective signals as received at
`an arbitrary receiver in the area covered by the system.
`Ignoring the IF response curve 16 for the moment: In
`the area coverage system exempli?ed there are three
`transmitters each operating on its own carrier wave
`frequency located within a 25 kHz channel, the modula
`tion signal on each carrier wave is synchronized with
`that on the other carrier waves so that information from
`all three transmitters can be demodulated without any
`interference problems from the respective modulation
`signals.
`The carrier waves 10, 12 and 14 are distributed irreg
`ularly with respect to the center frequency fa of the 25
`kHz channel. Using a notation of a minus sign and a plus
`
`Juniper Ex 1014-p. 6
`Juniper v MTel891
`
`

`
`20
`
`35
`
`4,513,443
`3
`sign indicating frequencies below and above the center
`frequency f0, respectively, then the carrier waves 10, 12
`and 14 are at —3 kHz, —9 kHz and +9 kHz, respec
`tively, with respect to the center frequency f0. The
`selection of the frequencies for the carrier waves 10, 12
`and 14 is such that the primary beat frequencies be
`tween them are 6 kHz, 12 kHz and 18 kHz. All of which
`frequencies are outside the bandwidth of the audio am
`pli?er of the known receiver. The carrier waves 10, 12
`and 14 are amplitude modulated with the sidebands
`being typically :3 kHz. It will be noted that the lower
`sideband of the carrier wave 10 overlaps the upper
`sideband of the carrier wave 12. Although not illus
`trated a carrier wave of +3 kHz could be used instead
`of the —3 kHz one. Also, if only two transmitters are
`used then generally their frequencies are +9 kHz and
`—9 kHz.
`The known type of mobile radio receiver is tuned to
`receive all three carrier waves and their sidebands, the
`actual strength of the signals depending on the location
`of the receiver with respect to the respective transmit
`ter aerials. The channel ?lter has a bandwidth of typi
`cally :10 kHz so that inevitably part of the lower side
`band of the carrier wave 12 and part of the upper side
`band of the carrier wave 14 are omitted from the signal
`25
`passed by the channel ?lter. Furthermore the known
`system, which works satisfactorily, is subject to un
`wanted receiver noise because of the relatively wide
`bandwidth of the channel ?lter in order to receive all
`three signals. Also, as the sidebands overlap the 25 kHz
`channel, this means that it is not possible to use the
`adjacent higher and lower channels which causes dif?
`culties in allocating channel frequencies in a particular
`geographical area.
`The receiver in accordance with the present ‘inven
`tion overcomes some of these problems because instead
`of selecting all the carrier wave signals and their side
`bands within a channel, it selects only the strongest
`signal at a particular instant, such as at switch-on and
`remains locked to that frequency until its strength falls
`by a predetermined amount and/or a particular trans
`mission ceases. By selecting only one of the three car
`rier waves rather than all three as in the known re
`ceiver, then the noise-bandwidth of the channel ?lter
`can be 13.75 kHz so a great deal of the unwanted re
`ceiver noise can be avoided. This situation is illustrated
`in FIG. 2 where the same reference numerals as used in
`FIG. 1 have been used to identify the same features.
`The channel ?lter characteristic curves 20, 22 and 24 of
`a nose-bandwidth of £3.75 kHz are arranged regularly
`within the 25 kHz channel rather than centered on a
`respective transmitted carrier wave. Nevertheless, de
`pending on which of the three signals is selected by the
`receiver at least one sideband plus carrier of a particular
`carrier wave is passed by the channel ?lter.
`The radio receiver in accordance with the present
`invention will also permit the introduction of a spaced
`carrier area coverage system in which the transmitted
`carrier waves 30, 32 and 34 can be regularly spaced for
`example at f,,, +6.25 kHz and +6.25 kHz as shown in
`FIG. 3. The theoretical beat frequencies of such regu
`larly spaced carrier waves are 6.25 kHz and 12.50 kHz
`all of which lie outside the bandwidth typically 200 Hz
`to 3.0 kHz of the audio ampli?er of the receiver. Addi
`tional protection against the 12.50 kHz beat note is
`provided by IF selectivity. By being able to space the
`carrier waves regularly as illustrated then the entire
`area coverage signals are contained within a 25 kHz
`
`4
`channel with a margin of the order of 1 kHz at each end.
`This means that the frequency spectrum can be used
`more effectively because adjacent channels. rather than
`alternate channels, can be allocated in a particular geo
`graphical area.
`Also with the regular spacing of the carrier wave
`frequencies, there is no obligation on the modulation
`signals being synchronized, and. as is evident from com
`paring FIGS. 2 and 3, the change from irregular to
`regular spacing of carrier waves will involve no change
`to the channel ?lter characteristic.
`FIG. 4 illustrates a block schematic circuit diagram ot‘
`an embodiment of the relevant part of a radio receiver
`made in accordance with the present invention.
`A signal from an aerial 40 passes to a conventional
`front end 42 which comprises an R.F. ampli?er with
`RF. selectivity components and a mixer. A local oscil
`lator conveniently in the form of a frequency synthe
`siser 44 provides an injection signal to the mixer (not
`shown) in the front end 42. The signal from the mixer 15
`applied to a channel ?lter 46 from which the IF signal
`is derived. The amplitude of the IF signal at the output
`of the channel ?lter 46 varies with the amplitude of the
`signal received at the aerial 40. for convenience of de
`scription the output of the ?lter 46 will be identi?ed as
`the node 48. The channel ?lter 46 has a noise-bandwidth
`of $3.75 kHz on an IF frequency of 10.7 MHz.
`Ari IF ampli?er 50 is connected to the node 48 in
`order to derive the modulation signal by way of a con
`ventional audio frequency section 53. The IF ampli?er
`50 may be regarded as being conventional although it is
`necessary to ensure that an automatic gain control
`(AGC) signal is applied to a point in the IF ampli?er 50
`such that it has no effect on the signal at the node 48.
`that is, it does not compress this signal. As IS conven
`tional a delayed AGC signal is applied via a line 52 to
`the front end 42 in order to maintain the signal at the
`mixer constant to avoid non-linearity.
`The signal at the node 48 is applied to an IF amplifier
`1 level detector 54 which serves to measure the ampli
`tude of the signal at the node 48. For optimum results.
`the output from the IF ampli?er I level detector 54 is of
`logarithmic form so that there is adequate dynamic
`range and also the measurements can be expressed in
`decibels, dB. Conveniently the IF amplifier 1 level de
`tector 54 comprises an integrated circuit type CA 3089
`supplied by the Radio Corporation of America. with
`the output taken from the tuning meter output.
`The output of the IF ampli?er 1 level detector 54 is a
`voltage indicative of the signal strength. This voltage
`has frequency components up to a few kHz because of
`the presence of the modulation voltage which has been
`distorted by the logarithmic operation applied to the
`signal at the node 48. The signal strength voltage is
`applied via a line 55 to a logic system 56 which mav be
`a dedicated circuit but in the present embodiment com
`prises a programmed micro-computer based on a micro
`processor Intel/Philips Type 8048. together with ancil
`liary components. A ?ow-chart for the operation of the
`logic system 56 is illustrated in and will be described
`with reference to FIG. 5 of the accompanying draw
`ings.
`The logic system 56 includes an integrator for inte
`grating the signal strength voltage over a period of time
`to avoid the effects of modulation. The integrated signal
`is applied to an analog-to-digital converter to convert
`the signal into a form suitable for processing in the
`micro-computer.
`
`45
`
`55
`
`Juniper Ex 1014-p. 7
`Juniper v MTel891
`
`

`
`0
`
`45
`
`4,513,443
`
`5
`Before discussing FIG. 5 in detail the operations to be
`carried out by the logic system 56 will be summarized.
`Once a signal is received which is above a squelch
`threshold then a signal is applied to the logic system 56
`via an input 58. The logic system scans the strength of
`the signal associated with each carrier wave by apply
`ing a frequency control signal to the frequency synthe
`siser 44 so that each carrier wave is selected in turn for
`about 5 milliseconds (mS). This is done by instructing
`the synthesizer 44 to increment or decrement by a refer
`ence of 6.25 kHz, the injection frequency and hence the
`receiver passband, which passbands are indicated by the
`reference numerals 20, 22 and 24 in FIGS. 2 and 3.
`From the measurements, the biggest signal is selected.
`A check is made after 500 m5 to see if the decision is a
`good one and if not the carrier wave then having the
`biggest signal is selected and the injection frequency to
`the mixer in the front end 42 is locked to the carrier
`wave. The receiver remains locked to that carrier wave
`frequency, irrespective of whether another or all of the
`other input signals has or have a greater strength, unless
`the signal strength falls by more than a predetermined
`amount in which case the signal strength voltages are
`rescanned and provided that one or more of them are
`above the squelch threshold a selection is made and the
`receiver is locked to that frequency. The output line 60
`is used to enable/disable the audio output of the re
`ceiver.
`Preferably the frequency control signal on a line 62
`from the logic system 56 to the frequency synthesizer 44
`is a parallel digital signal, but if necessary a serial feed
`may also be used. In either case the synthesizer 44 may
`comprise a Phillips type HEF 475OV synthesizer, and in
`a situation where a serial feed is acceptable, then a Phil
`ips type HEF 4751V divider may be used in conjunc
`tion with the synthesizer. The signal on the line 62
`instructs which multiple of the synthesizer reference
`frequency of 6.25 kHz should be applied as an injection
`frequency to the mixer in the front end 42.
`Referring now to the flow-chart shown in FIG. 5, the
`various operating steps are represented by a rectangle
`and the decision steps are represented by a diamond, the
`letter N being used to represent a negative answer to the
`question posed while the letter Y represents an affirma
`tive answer to the same question.
`The block 64 represents the entry situation. The next
`step represented by the block 66 is to disable the audio
`output so that spurious noises are not supplied to a
`loudspeaker or other transducer connected to the out
`put of the audio section 53 (FIG. 4). The block 68 repre
`sents the step of selecting the strongest of the carrier
`waves and their associated sidebands. This is done using
`a sub-routine in which each carrier wave is sampled for
`5 m5 and a signal strength voltage is obtained, these
`voltages are compared with each other and the stron
`gest one is selected. The time taken to carry out this
`operation is of the order of 16 mS.
`The block 70 represents the step of waiting for the
`squelch circuitry to settle. The block 72 represents the
`step of enabling the audio output. The block 74 asks the
`question “Is the signal above the squelch threshold?”. If
`the answer is in the affirmative then the flow-chart
`continues to the block 76. However, if it is in the nega
`tive then the steps identi?ed by the blocks 66 to 74 are
`repeated until an affirmative answer is received. The
`65
`reason why a negative answer may arise is that none of
`the received signals is of such a strength as to exceed the
`squelch threshold. The overall time for carrying out the
`
`6
`steps denoted by the blocks 66 to 72 is of the order of 20
`mS.
`Returning to the block 76, this instructs that the pro
`gram pause for 500 m8 during which time the selected
`carrier wave is received and the modulation repro
`duced. The reason for the pause is that it is possible that
`not all of the transmitters may have turned-on simulta
`neously due, for example to line delays, and therefore
`the initially selected strongest carrier wave may not in
`fact be the strongest one. In order to check this, the
`blocks 78, 80 and 82 repeat the steps previously de
`scribed with reference to the blocks 66, 68 and 72. Hav
`ing decided on the strongest of the incoming carrier‘
`waves, its strength is measured and a digital value is
`produced, this operation is indicated by the block 84.
`The block 86 denotes the step of storing in a memory a
`threshold value obtained by subtracting a constant num
`ber from this digital value.
`The next step, block 88, is to check that the correct
`data is being presented to the synthesizer. As part of this
`routine which is regarded as an optional, housekeeping
`one, a decision has to be made on whether the data
`supplied is correct, block 90. If the answer is in the
`negative (N) then one repeats the sequence of opera
`tional steps and decisions beginning at the block 66. In
`the case of an affirmative answer (Y) one then checks to
`see if the synthesizer is out of lock, block 92. If it is then
`the sequence is repeated at the block 66. If the synthe
`sizer is in lock then the next step, block 94, is to measure
`the signal strength.
`As a result of monitoring the signal strength, a check
`is made as to whether the signal strength exceeds the
`threshold value which was stored during block 86, this
`is block 96. A negative answer (N), which may arise due
`to fading of the signal from the selected transmitter,
`causes the sequence of steps beginning at block 66 to be
`repeated. In the case of an affirmative answer (Y) the
`sequence proceeds to block 98 which indicates the mak
`ing of a check as to whether the squelch is open. If it is
`then the sequence of steps commencing at block 88 is
`repeated, alternatively if it is not, for example because
`the incoming signal is too weak or non-existent, then the
`whole sequence repeats beginning at the block 66.
`These operations ensure that once a signal has been
`received, the receiver is locked onto the strongest car
`rier wave until its strength decreases by more than a
`predetermined amount or below the squelch threshold.
`The sampling rate is typically 200 times per second and
`each sample measures the average signal strength over
`substantially 5 mS.
`FIG. 6 illustrates diagrammatically the strength of
`the incoming signal, the setting of a threshold value for
`that signal; the decision points and the squelch thresh
`old. The curve shown is of Received Signal Strength
`(RSS) against Time (T). The line ST parallel to the
`abscissa indicates the squelch threshold. Below the line
`ST the signal strength is regarded as being so small so
`that the squelch comes into operation.
`In this example, following the commencement of
`transmission the receiver locks on to a carrier wave
`having a strength MVI at decision point DPl. In conse
`quence a threshold value NTl is stored (block 86, FIG.
`5). As the signal falls to NTl, then the receiver scans the
`received signals and, for example say locks on to the
`same signal because it has the greatest strength (MVZ)
`and is still above the squelch threshold ST. This second
`point is indicated as decision point DP2. A new thresh
`old point NT2 is stored. In view of the fact that the
`
`Juniper Ex 1014-p. 8
`Juniper v MTel891
`
`

`
`O
`
`5
`
`20
`
`25
`
`7
`signal is still falling and reaches NT2, then a new mea
`sured value MV3 is obtained, decision point DP3. The
`new theshold NT3 is stored, but as this is below the
`squelch threshold ST then the squelch threshold ST
`becomes effective as the signal strength continues to
`fall. Accordingly, the receiver returns to a standby
`status until an incoming signal having a strength exceed
`ing ST is received, and consequently, the sequence of
`steps beginning at block 64 of FIG. 5 commences.
`The present invention has been described with re
`spect to an amplitude modulated spaced carrier area
`coverage system which operates typically at V.H.F.
`However, the receiver could be used with a frequency
`modulated system with more widely spaced carrier
`waves in a wider channel. A consequence of this is that
`the frequency synthesizer is switched in 12.5 kHz steps
`rather than 625 kHz steps as with AM. in order to
`avoid sideband overlap. This doubling of the size of the
`steps can be achieved either by changing the software
`of the logic system or the synthesizer reference from
`6.25 kHz to 12.5 kHz.
`I claim:
`1. In a method for receiving signals in a spaced carrier
`area coverage system having at least two transmitters
`with modulated carrier waves having different carrier
`frequencies in a given frequency channel, wherein the
`method includes receiving signals in said channel, mix
`ing said received signals with oscillations of a local
`oscillator to produce intermediate frequency signals in a
`given frequency band, and demodulating said interme
`diate frequency signals to produce output signals; the
`improvement further comprising the steps of:
`a. limiting said intermediate frequency signals to a
`band width approximately equal to the band width
`of each one of said modulated carrier waves,
`b. determining the signal strengths of each of said
`limited band width intermediate frequency signals,
`c. selectively varying the frequency of said local
`oscillator to sequentially convert the signals re
`ceived from said transmitters to the frequency band
`of said intermediate frequency signals,
`d. scanning the signal strengths of each of said limited
`band width intermediate frequency signals to de
`
`4,513,443
`8
`termine the received signals having the greatest
`signal strength,
`e. adjusting the frequency of said local oscillator to
`select output signals from said coverage system.
`said selected output signals corresponding to re
`ceived signals of the greatest signal strength as
`determined in d, when the signal strength of at least
`one of said limited intermediate frequency signals
`corresponding to at least one transmitter is greater
`than a ?rst given threshold.
`W
`. determining a second threshold as a function of the
`strength of the signal determined in d and adjusted
`to in e,
`g. locking the frequency of said local oscillator onto
`a value so as to lock onto the transmitter signal of
`greatest strength determined in d until the signal
`strength of said limited intermediate frequency
`signal falls below said second threshold, and
`h. repeating said steps d-g of scanning, adjusting the
`frequency of said local oscillator, determining a
`second threshold and locking, when the signal
`strength of said limited band width intermediate
`frequency signal to which said local oscillator has
`been locked, falls below said second threshold.
`2. The method of claim 1 wherein an audio output is
`produced from said output signals, and comprising the
`further step of disabling said audio output during said
`step of scanning.
`3. The method of claim 1 wherein said step of scan
`ning comprises a first scanning step for selecting the
`transmitter with the strongest carrier wave followed by
`a second scanning step to select the transmitter with the
`strongest carrier wave, whereby said step of ad justing is
`effected if the same transmitter is selected in said first
`and second steps of scanning.
`4. The method of claim 3 further comprising a step of
`pausing between said first and second scanning steps.
`5. The method of claim 3 further comprising repeat
`ing said first step of scanning if none of the received
`signals is above said first threshold following said first
`step of scanning.
`
`35
`
`t
`
`t
`
`x
`
`a
`
`i:
`
`45
`
`55
`
`65
`
`Juniper Ex 1014-p. 9
`Juniper v MTel891