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`US005710710A
`|
`tin) Patent Number:
`5,710,710
`United States Patent
`Owenetal.
`(45) Date of Patent:
`Jan. 20, 1998
`
`
`19)
`
`[54] FREQUENCY COUNTER WITH REDUCED
`FALSE CORRELATIONS
`
`[75]
`
`Inventors: William P. Owen, Fort Lauderdale;
`Judd Sheets, St. Petersburg, both of
`Fla.
`
`[73] Assignee: Optoelectronics, Inc., Fort Lauderdale,
`Fla.
`
`[21] Appl. No.: 561,692
`[22] Filed:
`Nov. 22, 1995
`Related U.S. Application Data
`oo
`[63] Continnation-in-part ofSer. No. 310,228, Sep. 21,1994, Pat.
`No.5,471,402.
`Tint. CS oeeececceeseeeteeee GO1R 23/00; GO6F 17/00
`[50]
`[52] U.S. Ch. oe 364/484; 324/76.48; 324/76.62;
`324/76.74
`[58] Field of Search .usccsssscsncsnecsssnsesere 464/484, 485,
`464/486. 487; 324/76.42, 76.47, 76.48,
`76.58, 76.62, 76.74
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,350,950
`9/1982 Waldmann et al... 324/76.61
`4,424,482
`
`1/1984 Drogin ee we 32A/76.AT
`
`4,651,089
`3/1987 Haigh .......
`we 324/76.42
`
`4,882,740 11/1989 Abe ef al...
`csccssetenencese 24/76.48
`
`we 32476.42
`5,323,104
`6/1994 Lindell
`.....
`
`.
`324/76.42
`5,373,236 12/1994 Tsui et al.
`
`1/1995 Searing etal.
`«. 364/484
`5,379,390
`D471 AQZ LIST9GS OWE ceresccersevssreererecarsecerserreoes 364/484
`
`Primary Examiner—Vincent N. Trans
`Attorney, Agent, or Firm—Malin, Haley, DiMaggio &
`Crosby, PA
`ABSTRACT
`57)
`An improved frequency counter for more reliably reading
`the frequency of low level signals by employing a method of
`separating the desired signal from undesirable noise related
`signals. wherein the frequency counter comprises signal
`input amplifier circuitry, a frequency modulator circuit for
`phase shifting the self-oscillation frequency of the undesired
`Signal to isolateit from the valid signal, a prescaler circuit,
`a frequency or pulse counter driven by the output of the
` Prescaler, and a correlator circuit for differentiating the
`self-oscillation frequencies from the main signal frequency
`so as to reduce false correlations between the self-
`oscillation, or noise, and valid signals.
`
`4,027,146
`
`5/1977 Gilmore ceccwccosscssnsennsnseenee 364/484
`
`19 Claims, 3 Drawing Sheets
`
`10
`
`CIRCUITS
`
`MODULATOR
`
`PRESCALER
`
`Ween, 60
`~
`MICRO—
`
`COUNTER
`
`
`Pura20
`
` | ‘ INPUT 30
`
`
`
`
`
`
`
`MODULATOR CONTROL
`
`Google Exhibit 1037
`Google Exhibit 1037
`Google v. Valtrus
`Googlev. Valtrus
`
`

`

`USS. Patent
`
`Jan. 20, 1998
`
`Sheet 1 of 3
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`5,710,710
`
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`

`U.S. Patent
`
`Jan. 20, 1998
`
`Sheet 2 of 3
`
`5,710,710
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`US. Patent
`
`Jan. 20, 1998
`
`Sheet 3 of 3
`
`5,710,710
`
`~~
`
`100
`
`102
`
`CHANGE DITHER
`
`CONTROL SIGNAL
`
`PREFORM FREQUENCY
`MEASUREMENT
`
`104
`
`
`
`
`
`
` TOLERANCE OF
`PREVIOUS RESULTS
`STORED IN
`
`
`STACK
`?
`
`
`
`FREQUENCY IS
`VALID; STORE
`RESULT IN MEMORY
`
`
`PUSH RESULT ONTO
`STACK; DISCARD
`
`
`OLDEST RESULT
`
`
`
`FIG. 4
`
`

`

`5,710,710
`
`1
`
`FREQUENCY COUNTER WITH REDUCED
`FALSE CORRELATIONS
`
`This application is a continuation-in-part of U.S. appli-
`cation Ser. No. 08/310,228,filed Sep. 21, 1994 now U.S.
`Pat. No. 5,471,402.
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention generally relates to an improved frequency
`counter, and more particularly, to an improved frequency
`counter having a modulator circuit for shifting the self-
`oscillation frequency of an undesired signal and a correlator
`circuit for differentiating the self-oscillation frequencies
`from the desired signal frequency to improve the overall
`accuracy of the frequency counter by reducing false corre-
`lations between the self-oscillation and desired signal fre-
`quencies.
`2. Description of the Background Art
`A frequency counter is a test instrument which measures
`a frequency of an electrical signal. Frequency counters
`typically measure frequency by counting the number of
`cycle pulses crossing a predetermined voltage threshold,
`usually zero volts, over a precise time interval. The input
`signal is normally conditioned by known input circuitry,
`such as a Schmitt trigger circuit, which receives an input
`signal and outputs one pulse per cycle of input. A problem
`noted by conventional frequency countersis their inability to
`accurately measure the frequency of low level signals. These
`signals are difficult to accurately measure for several rea-
`sons. First, low level input signals must be amplified to
`produce pulses which will cross the cycle counttriggering
`threshold. A large amount of gain is required to best count
`these low level signals but the realization of this gain
`introduces broadband noise which is added to the desired
`signal. Second, incidental coupling around the input cir-
`cuitry can also form near oscillatory paths. Finally,
`the
`prescalers used in some types of frequency counters include
`input circuits that tend toward oscillation in the absence of
`an overriding input signal. All of these factors combine to
`form a complex input signal that is hard to distinguish from
`a desired signal. This self generated signal is here referred to
`as self-oscillation. As a result, the frequency counter may
`experience problems differentiating the main signal readings
`from self-oscillation readings since amplified self-
`oscillations can cause the counter to trigger and produce
`random measurements. This problem of making false read-
`ings is further aggravated with increasingly sophisticated
`frequency counters having higher sensitivity for measuring
`lower level signals.
`Self-oscillation is a byproduct of amplifying signals,
`especially low level signals, as noted above. The amplifier
`circuit produces noise which for large values of gain can be
`incidentally fed back from the output into the input resulting
`in an increase in noise output levels and noise peaks. Since
`it is amplified into a relatively strong signal,
`this self-
`oscillation is readable by the counter. The self-oscillation
`signal also varies in apparent center frequency with a variety
`of factors, including antenna impedance, antenna coupling,
`amplifier gain, etc. Consequently, this self-oscillation signal
`produces serious difficulties in distinguishing the counts of
`self-oscillation from those of true input signals of arbitrary
`frequency.
`Other reasons also exist for the inaccurate readings of low
`level signals. For instance. prescalers used in conventional
`counters typically include regenerative input circuits to
`
`10
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`25
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`65
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`2
`make the prescaler more sensitive. Regeneration in a circuit
`refers to partial feedback of an output signal to the input.
`Unfortunately, this regeneration cannot be suppressed with-
`out forfeiting needed sensitivity in the counter, since the
`prescaler feeds the signal pulses to the counter. By contrast,
`high level signals usually have a signal amplitude that
`overrides any self-oscillation, thereby increasing the likeli-
`hood of correct counts or readings. The difficulties in accu-
`rately measuring frequency of low level signals, and the
`increased sensitivity and amplification required for measur-
`ing low level signals contribute to false correlations and
`inaccurate measurements.
`In certain manually operated counters, the operator may
`be familiar with the input signal and expect a certain
`frequency count so that adjustments to the counter, such as
`a threshold signal level or sensitivity, can be made in
`anticipationof the input signal’s amplitude and frequency to
`increase the probability of correct readings. The problems
`discussed herein are more apparent with automated fre-
`quency counters and increasingly sophisticated frequency
`counters with higher sensitivity and improved amplifiers that
`allow the frequency counter to receive lower level signals,
`such as signals received from an antenna. Although recent
`advances in counter technology have allowed better fre-
`quency measurements of smaller input signals, achieving
`More accurate readings consistently is still problematic
`because ofthe difficulty in distinguishing the desired signals
`from the self-oscillations present in low level signals. The
`inventor herein has offered architecture in frequency
`counters for differentiating self-oscillations from valid input
`signals. In inventor’s application Ser. No. 08/310,228,filed
`Sep. 21, 1994, the frequency counter employsa statistical
`comparison circuit which discriminates the main signal from
`the self-oscillation by comparing successive measurements,
`looking for consistency, so that only valid counts are dis-
`played to the user. While the statistical comparison circuit
`can compare and correlate successive measurementsto find
`consistency in results, eventually a series of random counts
`could appear to be correlated and passed as valid. Thatis. if
`an inaccurate reading is duplicated by the multiple reoccur-
`rence of similar self-oscillating signals and/or noise, they
`could be recorded incorrectly as valid frequency measure-
`ments if repeated enough. It has been noted, however, that
`self-oscillation frequency counts typically center around
`some nominal frequency value, as determined by the factors
`described that lead to its generation, with some variation
`from count to count. If invalid signal samples could be
`shifted upon command,then self-oscillation counts may be
`shifted and compared to unshifted counts to reduce false
`correlations and readings. So, although statistical compari-
`son circuitry can discriminate uncorrelated signals such as
`noise from the desired signal, a frequency counter further
`able to separate the desired signal from the self-oscillations
`would further discriminate the signals and reduce false
`correlations. Such a frequency counter would represent a
`further improvement in frequency counters and would be
`well received.
`Accordingly, the present invention provides an improved
`frequency counter that provides increased efficiency over
`known frequency counters bydistinguishing a desired signal
`from self-oscillations through frequency modulation
`techniques, which may or may not be used together with the
`statistical comparison circuit disclosed in application Ser.
`No. 08/310,228. The instant invention differentiates valid
`signals from self-oscillating signals by attempting to shift
`the average frequency of the self-oscillation and correlating
`variations seen in the resulting counts.
`
`

`

`5,710,710
`
`3
`SUMMARY OF THE INVENTION
`
`The present invention represents an improvementin fre-
`quency counter technology, whereby it incorporates and
`integrates signal
`input circuitry, a modulator circuit, a
`prescaler, a frequency or pulse counter, and a correlator, to
`reduce and/or prevent false correlations between readings by
`shifting self-oscillation counts inherent in amplified low
`level signals to distinguish them from the main signal
`counts. The concept of the instant invention is based on
`shifting the probability distribution of counts taken from the
`input signals so that it becomesless likely for successive
`counts of self-oscillation signals to match. As is known in
`the art, a signal is received, amplified, and converted to
`pulses by a circuit such as the Schmitt trigger. In correlating
`readings, a number of counts are taken and are usually found
`to center around a particular frequency, The instant inven-
`tion is an improvement in frequency counter technology,
`whereby it
`includes a frequency modulator circuit
`that
`reduces false correlations by broadening the count distribu-
`tion of the self-oscillating signals and further differentiating
`them from a desired signal. Signal shift is accomplished by
`modulating the self-oscillation counts and forcing said count
`distributions to center on a different frequency value. In
`essence,
`the instant
`invention bifurcates the probability
`density function so as to make the probability of matching
`inaccurate successive counts smaller, thereby reducing the
`likelihood of producing false correlations.
`In accordance with these and other objects which will
`become apparent hereinafter, the instant invention will now
`be described with particular reference to the accompanying
`drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG.1is a graphical illustration of the probability density
`functions of successive signal frequency counts at a first
`frequency and a shifted frequency.
`FIG. 2 is an electrical block diagram of the frequency
`counter of the instant invention.
`FIG. 3 is an electrical circuit diagram of the instant
`invention.
`FIG.4 is a flow chart of operationof the instant invention.
`
`10
`
`20
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`25
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`35
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`45
`
`With reference to the drawings, FIGS. 1-4 illustrate the
`instant invention. With reference to FIG. 1, probability
`density functions of representative count occurrences are
`monitored and plotted, whereby theleft distribution function
`plot represents the determinedself-oscillation counts. It can
`be seen from the left curve that most measurements of a
`self-oscillation signal may center around one frequency,
`such as F1 asillustrated by way of example in FIG. 1. For
`example, if two successive counts are taken from the left
`distribution or curve, compared and tested to be within an
`arbitrary amount of each other, it can be assumed that
`eventually two random counts will be sufficiently close and
`be accepted as valid and correct readings. Tf, on the other
`hand,
`the second of these two counts was taken from
`distribution F2, the probability of correlation is much less,
`depending on the amountof overlap with the two probability
`functions.
`With reference to FIG. 1, the probability density functions
`plotted represent arbitrary functions that may be produced
`by successive readings from the frequency counter. The
`Gaussian function graph serves to illustrate the principles
`
`35
`
`65
`
`4
`involvedin the instant invention, which include distinguish-
`ing a valid signal from error producing self-oscillations.
`Since the center of a probability density function of counted
`frequency occurrencesis easily influenced,it can be shifted
`around by frequency modulation means. With reference to
`FIG. 1. if the density function can be movedsufficiently so
`that there is little overlap between the two functions,it is
`unlikely that a count taken from each of the two displaced
`functions will match within the acceptable threshold count
`differentials. The count differential is the maximum differ-
`ence allowed between successive counts by the correlator
`62. In accordance with the instant invention, the modulator
`applies a dither signal to the counter input circuitry 20 that
`displaces the density function of the input signal so that
`counts taken from the original and displaced frequency
`distribution F1 and F2 will center around different frequen-
`cies and will be less likely to fall within a preset tolerance
`that is used to accept readings as valid. The amount of
`frequency offset required is directly related to the degree of
`false count immunity required. Thatis, the smaller the shift,
`the more likely false readings will occur. Since these dis-
`tribution functions essentially tail off infinitely in each
`direction, it is impossible to guarantee zero false counts.
`However, in practice, it has been foundthat false counts may
`be reduced from several per hour without the use of the
`modulator circuit 30 to less than one per day whenutilizing
`the modulator circuit 30.
`Accordingly, since the counter is intended to measure
`frequency,
`the instant
`invention attempts to frequency
`modulate the self-oscillations. If the self-oscillation counts
`are modulated by the modulator 30, they will not pass the
`correlator test and as a result, they will be rejected. Finally.
`it should be noted that any form of phase or frequency
`modulation may be used. The purpose of the dither or
`modulation achieved by the modulator 30 is to center
`frequency counts around separated frequencies. In the
`instant invention, the modulation chosenis binary, thatis,
`the modulator circuit 30 is either turned on or off. However,
`any form of modulation ofthe self-oscillations can be used,
`such as a sine wave, whichlinearly varies the self-oscillation
`frequency over a range of values. As long as the modulator
`30 can effectively and sufficiently displace the probability
`density function so that there is lite overlap at the extreme
`ends of the modulation applied,
`the modulator 30 will
`achieve the purposes of the instant invention.
`Several methods may be used to modulate the self-
`oscillation. Since the self-oscillation frequency is not con-
`trolled by any particular high Q resonance,it is easy to move
`the frequency within a small range. All that is required is
`moving the frequency enough so that the overlap between
`probability density functions is small. Modulation can be
`linear over a range of values or binary in the simple
`modulated or un-modulated case. With reference to FIG.3,
`the actual method used to effect the frequency modulation
`mayvary, butit typically consists of a varactor or PIN diode
`CRI and switched capacitor C3 used to changetherelative
`phase of a signal passing through the counter input circuitry.
`The preferred location for this PIN diode CR1 and switch
`capacitor C3 is at the junction of the input amplifier and
`prescaler input since there it can changetherelative phase of
`both the overall antenna to prescaler loop and the prescaler
`input reflection phase. It is desirable that the method of
`modulation chosen not impair the sensitivity of the counter
`under any condition.
`Regardless of the modulation technique employed, the
`modulation shifts and effectively broadens the distribution
`of counts to greatly reduce the probability of false correla-
`
`

`

`5.710,710
`
`5
`tions. The modulation drive signal originates in the micro-
`controller 60 and effects the modulation on alternate counts.
`The desired signals will be measured and correlated nor-
`mally since their frequency is independent of the self-
`oscillation frequency and not affected by the modulation.
`The preferred block diagram of the instant invention is
`shown in FIG. 2. Input signals are fed through at least a
`single stage amplifier 14 the output of whichis electrically
`associated with the modulator 30. The prescaler 40 is driven
`by the modulator 30 output and the prescaler 40 outputis fed
`into the counter 50. The counter 50 is a frequency measure-
`ment means and provides a measurementof the signal after
`it has been amplified, modulated and prescaled. If the
`modulator 30 is activated, then it experiences a phase prior
`to being measured. The signal is then passed through the
`correlator 62 and fed to an output for further processing by
`the microcontroller 60 and possible display. Except for the
`correlator 62 and modulator 3@ sections, the block diagram
`is that of a typical high frequency counter. The modulator 30
`is used to shift the self-oscillation frequency slightly by
`varying the phase of the self-oscillation loop. If a valid
`signal is applied to the input, then the modulator 30 has no
`effect and the signal is counted as normal.
`With reference to FIG. 4 of the instant invention, the
`correlator 62 comprises a statistical comparison means,
`similar to that described in the parent case, for correlating
`pulse counts. It may comprise a set of instructions execut-
`able by the microcontroller 60. The modulator 30 is trig-
`gered by the microcontroller 60 for shifting the self-
`oscillating frequency. The amplifier 20, prescaler 40, and
`counter 50 may comprise known components. The counter
`50, correlator 62, and output have electrical connections
`which comprise digital data paths.
`A typical low amplitude input signal is received through
`an antenna 12 and, thus, requires amplification. Generally,
`since a frequency counter operates by counting zero cross-
`ings of the input signal, proper operation will only occur
`with a single dominant signal since multiple simultaneous
`signals will confuse the zero crossing detector. Input signal
`samples are amplified by the input circuits 20 to a level
`sufficient for driving the zero crossing detector present in the
`prescaler 40 and counter 50. The modulator 30 changes the
`relative phase shift of the self-oscillation loop and hence
`alters the frequency when it is activated to produce a
`difference, as seen in FIG. 1 and discussed above, so as to
`generate a shifted count which is compared with subsequent
`counts. The modulator 30 is activated and deactivated by the
`microcontroller 60 which drives a control signal output to
`the modulator 30, The modulator 30 is designed so as to not
`substantially affect the amplitude of the input signals.
`Since the counter 50 cannot count signals of high fre-
`quency efficiently, the prescaler 40 is used to reduce the
`frequency applied to the counter 50 by delivering one output
`pulse for each N number of input pulses. The number N can
`be any number, but is typically 4, 8, 16, 32, or 64. In the
`specific application of the instant invention, an N number of
`8 is preferred. Thus, barring other restrictions, the maximum
`input frequency measurement capability is eight times that
`of the counter section alone. The counter 50 counts the
`number of prescaler 40 output pulses that occur in a given
`gate time. The gate time of the counter $0 may be operator
`selected to yield the desired resolution, with resolution and
`gate time being directly proportional.
`The countdata from the counter 5@ is fed to the correlator
`62, which is implemented as part of the microprocessor
`controller. The correlator 62 compares the counter frequency
`
`6
`data with previous readings, as discussed with reference to
`FIG. 4. The modulator 3@ is triggered by the correlator 62
`through a control signal generated by the microcontroller 60
`and fed through a transistor Q1 so as to provide a digital
`signal
`to activate the PIN diode CR1. The diode CRI
`switches a small capacitor C3 in and out of the circuit to
`activate and deactivate the modulator circuit 30. The counter
`50 takes one count with the modulator 30 in the off state and
`another with the modulator 30 in the active state, and if the
`counts are foundto be different by a predetermined amount,
`the count is deemedinvalid and the result of self-oscillation.
`As noted above, when the modulator 30 is on, if only self
`oscillation signals are present the countis shifted from the
`count achieved when the modulatoris off. Consequently,it
`is the object of the instant invention to distinguish the main
`signal from self-oscillations, whether the self-oscillations
`are the result of regeneration from the amplifier circuit,
`noise, or other signals, so that a valid frequency count can
`be obtained from the input signal of interest.
`With reference to FIG. 3, the receiving antenna 12, the
`amplifier 20, the modulator 30, and the prescaler 40 are
`shown in greater detail. The amplifier 20 may comprise a
`two-stage amplifier, including RF amplifiers 14 and 16 with
`nominal 50-ohm input and output
`impedances. The RF
`amplifier 16 represents the second stage of input amplifica-
`tion. Signal conditioning elements C1, C2, Cb1, R2, Cb2,
`and C4, together with amplifiers 14 and 16 produce ampli-
`fied trigger pulses, one pulse per cycle of the signal. The
`prescaler 40 is shown with voltage feeding and impedance
`loading elements R6, CR2, CB3, R7, and CB4, and its
`output signal drives the frequency/pulse counter 50. The
`prescaler 40 and counter 50 may comprise components
`knownto a practiced artisan. The modulator 30 is outlined
`in FIG. 3 and preferably comprises resistance R4, R5, R9,
`capacitance C3-C5, PIN diode CR1,andtransistor QI. The
`transistor Q1 receives a modulator control signal from the
`microcontroller 60 to begin the modulation of the self-
`oscillating signal. The microcontroller 60 includes the cor-
`relator 62. The diode CR1 comprises a PIN diode which
`receives its required voltage to activate the modulator 30
`from the microcontroller 60. The PIN diode CRI acts like an
`RF switch, wherein it assumes a low RF impedance when a
`DC bias is applied. When the diode CRI is activated, a
`capacitor C3, shown in series with the diode CRI.
`is
`switched in shunt with the RF path and the self-oscillation
`frequency is shifted slightly. The capacitor C3 preferably
`comprises approximately 2.7 pF.
`In the fully developed productof the instant invention,the
`signal input is received through an antenna port 12, passed
`to the two-stage amplifier 20, and then to the modulator
`circuit 30. The signal is shifted, as shown in FIG. 1, when
`the correlator 62 provides the required bias voltage to the
`PIN diode CR1 through the transistor switch Q1. The
`counter 50 and microcontroller 60 are conventionally known
`and their particular implementation is used in combination
`with the modulator 30 in accordance with the instant inven-
`tion.
`FIG. 3 provides a system level schematic of the instant
`invention. With reference to FIG. 3, amplifiers 14 and 16
`comprise RF amplifiers having nominally 50-ohm input and
`output impedance. The prescaler 40 preferably comprises a
`divide-by-cight prescaler integrated circuit and resistors
`R6-R8,zener diode CR2, capacitors C4, Cb3, and Cb4, and
`a 5-volt supply. The output of the prescaler 40 is received by
`the counter 50. The frequency of the inputsignal is divided
`by the division ratio of the prescaler 40, such as eight. so as
`to allow the counter to more easily read or count pulses. The
`
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`

`

`5,710,710
`
`7
`prescaler IC 40 may comprise other division ratios, such as
`2, 4, 16, 32. etc. The PIN diode CR1, shown in modulator
`circuit 30, is used to switch the series capacitor C3 in shunt
`with the RF signal line. This allows the self-oscillation
`frequency to be shifted by the modulatorcircuit. The capaci-
`tors referenced by “Cb” refer to bypass capacitors used for
`establishing an AC and RF ground andfor allowing DC
`voltage biasing. Capacitors C1, C2, and C4 comprise cou-
`pling capacitors between the various stages of the circuit,
`that is, the amplifier 20, the modulator 30, and prescaler 40.
`The capacitor C5 andresistor R5 together provide a low pass
`filter to eliminate any noise presentat the digital output from
`the microcontroller so as to preventit from reaching the RF
`stages and interfering with input signals. The resistors
`R1-R3 and R10 areballasting resistors used for dividing the
`5-volt voltage supply to lower values in the amplifiers 14,
`16. Theresistor R4 in addition to the resistor RS, collectively
`limit the drive current to the PIN diode CR1. Theresistor R6
`and diode CR2 provide a simple zener shunt regulator for
`dropping the voltage supply to the required level needed by
`the prescaler IC. Preferably, the voltage supply is dropped
`from 5 volts to 3 volts. The resistor R7 establishes a DC bias
`on each of the prescaler ICs 40 differential inputs. Finally,
`resistor R9 discharges capacitor C5 when the microcontrol-
`ler output goes low.
`With reference to FIG.4, a flow chart of the process used
`by the microcontroller 6@ for determining valid measure-
`ments is shown and referenced by numeral 100. In block
`102, the microcontroller 60 moves the dither signal to the
`opposite state from the present state. Next, the microcon-
`troller 60 takes a frequency measurement and comparesit to
`the previous measurementresult stored in the stack. If the
`result is within the predetermined tolerance of previous
`results, then the frequency is considered valid and stored in
`memory. If the comparison is not within the predetermined
`tolerance is not valid,
`then the count’s measurement is
`pushed into a stack location for comparing with subsequent
`measurements. Accordingly, the oldest count result is dis-
`carded so as to make room for the latest result. The loopis
`then repeated as long as the frequency counter is active. The
`valid result stored in memory may be displayed to the
`operator or used in variety of ways.
`The instant invention has been shown and described
`herein in what is considered to be the most practical and
`preferred embodiment.It is recognized, however, that depar-
`tures may be made therefrom within the scope of the
`invention and that obvious modifications will occur to a
`person skilled in the art.
`Whatis claimed is:
`1. A frequency counter for accurately determining the
`frequency of a valid periodic RF input signal of interest, said
`frequency counter distinguishing the frequency countof said
`valid periodic RF input signal
`from undesired self-
`oscillations, said frequency counter comprising:
`(a) signal input means for receiving successive signal
`samples,the signal samples including the valid periodic
`RF input signal and the undesired self-oscillations;
`(b) frequency measuring means, electrically associated
`with said signal input means, for measuring the fre-
`quency of the signal samples over a predetermined
`period of time so as to obtain a plurality of frequency
`counts;
`(c) means for separating the signal samples to obtain
`successive frequency counts andto facilitate isolation
`of the valid periodic RF inputsignal from the undesired
`self-oscillations, said separating means being in elec-
`
`20
`
`35
`
`45
`
`50
`
`55
`
`65
`
`8
`trical communication with said signal input means and
`said frequency measuring means,;
`(d) correlating means, electrically communicated with
`said frequency measuring means and said separating
`means, for calculating a difference in frequency mea-
`surements between said successive frequency counts
`and for comparing said difference in said frequency
`counts to a preselected value, wherein a valid periodic
`RF input signal is deemed measured when said prese-
`lected value exceeds said difference; and
`(c) frequency display means,electrically associated with
`said correlating means, for displaying said frequency
`measurement of said valid periodic RF input signal.
`2. The frequency counter of claim 1, wherein said sepa-
`tating means comprises:
`modulator means, in electrical communication with said
`correlating means and said signal input means, for
`selectively shifting the phase of signal samples selected
`from said successive signal samples, wherein the fre-
`quency countof the shifted signal sample is compared
`to the frequency count of a non-shifted signal sample
`by said correlating means.
`3. The frequency counter of claim 2, further comprising
`control signal generating means for providing an enabling
`signal to said modulator means so as to enable said modu-
`lator meansto shift the phase of said selected signal samples.
`4. The frequency counter of claim 3, wherein said corre-
`lating means and said control signal generating means are
`defined by a microcontroller having a set of executable
`instructions for comparing the successive frequency counts
`and for selectively enabling said modulator means.
`5. The frequency counter of claim 3 wherein said modu-
`lator means includes:
`a low passfilter in electrical communication with said
`signal input means; and
`an enabling means, in electrical communication with said
`control signal generating means, for enabling said
`modulator when receiving said enabling signal.
`6. The frequency counter of claim 1, further comprising
`amplifying means electrically coupled to said input means
`for amplifying said signal samples to improvetheefficiency
`of said frequency measuring means in detecting threshold
`crossings.
`7. The frequency counter of claim 6, further comprising
`signal conditioning means electrically associated with said
`amplifying means for converting said signal samples into a
`series of electronic trigger pulses, each said trigger pulse
`representing a cycle of said signal and having an amplitude
`sufficient for detection by said frequency measuring means.
`8. The frequency counter of claim 7. further comprising:
`a microcontroller meansin electrical communication with
`said modulator means and said frequency measuring
`means for selectively providing an enabling signal to
`said modulator means, said correlating means and said
`control signal generating means being defined by said
`microcontroller, said microcontroller storing said suc-
`cessive frequency counts for comparing andselectively
`displaying individual frequency counts when a valid
`periodic RF input signal is determined.
`9. The frequency counter of claim 1 wherein said corre-
`lating means includes a microcontroller for storing said
`frequency counts of said successive signal samples and for
`thereafter arithmetically comparing said successive fre-
`quency counts.
`10. The frequency counter of claim 9 wherein said micro-
`controller includes a storing means for storing said fre-
`
`

`

`5,710,710
`
`9
`quency countsof said signal samples and for comparing said
`frequency counts to arrive at said difference.
`11. A frequency counter for determining the frequency of
`a valid low-level periodic RF input signal of interest having
`self-oscillations, said frequency counter distinguishing the
`frequency count of said main per