CA1116700A - Narrow band communication system - Google Patents

Abstract

A B S T R A C T

Ultra-narrow-band system for communication between a transmitter device and a receiver device provided by locking both to a carrier signal from a radio broadcast station and synthesizing a precise local frequency at each device as a preset multiple of the frequency of the carrier. Such a system is usable in a multi-station alarm and status communica-tion system including a multiplicity of independent radio alarm transmitters whose various carrier frequencies are different from but phased locked to a local radio broadcast station, and a central alarm receiving station which employs a corresponding multiplicity of synchronous detectors. The detectors each have associated with them a synthesized local oscillator source which is also phase-locked to the same radio broadcast station used by the alarm transmitters. Both the alarm transmitters and the central receiver are thereby accurately referenced to a readily available local frequency source (the broadcast station). As a consequence, very narrow-band radio circuits may be employed and the receiver and transmitter bandwidth may be accurately matched to the information content of the alarm and status signal source to achieve high signal-to-noise ratio reliable transmissions.
The disclosed apparatus is relatively immune to intentional or unintentional interference and will burn-through most conventional transmissions which may be transmitting on the same radio channel at the same time, without unduly bothering these other unrelated transmissions.

Description

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This invention relates to narrow-band, relatively ultra-stable radio apparatus for communicating signals from protected premises or other locations from which mess~ges must be transmitted to a central monitoring point, such as a police station or maintenance center.

The transmission of priority signals such as burglary, fire, emergency medical, ana other signals represent an important segment of the communication art wherein high reliability and immediacy of transmi.sion are important~
The alarm industry has customarily employed telephone lines for this purpose and in particular has used a so-called DC circuit wherein a continuous direct current E~ath is established bet~een a sender and a receiver to provide a channel of commu-nication and to detect the presence of tampering or inter-ference, for example the cutting of a telephone line. Such DC telephone paths are becoming obsolete and are being replaced by more expensive multiplexed telephone circuits employing subcarriers. A strong need has arisen for alternative means of alarm communication~

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Radio alarm communication using conventional techniques has not been very reliable because of the rel~atively poor signal-to--noise ratio that results from interference from both natural and man-made signals and because of interference from legitimate signals sharing the channel whlch may be S using the channel when an alarm condition occurs.
The scarcity of radio-frequency channels ~urther aggravates this conaition.
Radio alarm transmitters in ~he prior art generally incorporate hoth a receiver and a transmitter so that they may monitor for the presence of radio transmission on their channel prior to transmitting an alarm signal in order to avoid interference or masking of signals which could negate both the alarm transmission and the intelligibility of the interfering signal.
Prior-art devices have not been able to economically er.ploy narrow-band transmission techniques because the available frequency determining sources, for e~ample crystal oscillators, have not been sufficiently stable to permit very narrow-band transmissions that are commensurate with the bandwidth of the alarm and status signal information content (e.g. under 100 Hz bandwidth~ and acceptable transmission time.
Prior-art devices are also relatively complicat:ed when designed to use synchronous detection techniques in the receiving circuits since no local frequency re~erence for the synchronous process is available and it must be generated from the usually ~eak incoming alarm signal.

S~anson (~.S. Patent 3~883,874) has disclosed an apparatus which provides a source of standard frequency for radio communication and navigation. This apparatus is phase locked to commuted signals at the same frequency using a multiplexed antenna. S~anson employs very-low frequency (VLF) radio signal sources, specifically highly stable OMEG~
navigation signals, or possibly other highly stable VLF
transmissions such as standard radio transmission from the l~ational Bureau of Standard ~N stations, in order to, yenerate absolute standard frequencies. The aetection and use of such standard frequency transmissions is-unnecessarily complicated because special radio receiver circuits are needed to detect VLF signals and because of the attempt to generate an absolute standard frequency.

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SU~ARY OF THE INYENTION
The disclosed apparatus emp]oys a local radio broadcast station as a relative frequency reference to precisely establish the frequency of independent alarm transmitters and to precisely establish the frequency of a muliplicity of local oscillators in the alarm receiver so as to permit synchronous detection of the alarm signals.
Each independent alarm transmitter receives signals from the local broadcast station and incorporates a frequency synthes-izer which is phase locked to this broadcast station signalto synthesize the carrier frequency of the alarm transmitter.
I'he frequency of the synthesized signals varies in proportion to the variations in frequency of the carrier of the local broadcast station, but the synthesized signals at the trans-mitter and receiver are locked to the same carrier and thusallow very narrow selection of received frequencies to eliminate noise. Transmitted and received signals are ultra-stable relative to each other.
The alarm transmitter frequency may be selected from a multiplicity of closely spaced alarm channels, which channels may be only about 100 Hz wide. Therefore, upwards of 100 or more separate alarm transmitters may operate on one conventional radio voice channel of 10 KHz bandwidth.
S2veral alarm transmitters may also operate on the same alarm channel by providing suitable time synchronization, using, for example, a polling method hereinafter described.

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The central alarm receiver also emplo~s the same radio broadcast station used by the alarm transmitterS
to phase lock a multiplicity of frequency synthesi~ers which are used as local oscillators. The central receiver synchronously detects the alarm transmissions in one of a multiplicity of independent parallel channels, ~hich channels correspond to the multiplicity of requencies used by the independent alarm transmitters. The alarm signals detected by the central receiver are subsequently sent to a response agent, such as a police station~ On-off alarm sisnals may be com~unicated using this apparatus or status signals comprising a digital coded message may be transmitted.
The alarm transmitters need not monitor their radio channel prior to signal transmission because t~eir narrow transmission bandwidth results in a sufficiently intense concentration of energy in a very small speetrum space so that they can burn through or override most other signal transmissions that may also occupy the same channel. The alarm transmission will not unduly interfere with other signal sources which they override. Though a brief background chirp may be heard in the audio channels of these other transmissions, this ~ill not usually in~er~ere ~ith their intelligibility. The alarm signal may also be transmitted in the so-c~lled "guard band" of frequencies between assigned channels of~ for example~ voice transmitters.

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Any of a ~umber of di~feren~ broadcast stations, such as commercial AM broadcast stations, television stations, radio navigation stations, and other similar transln;tters may be used in the practice of this invention, or a special transmitter may be erected for this purpose.
Conventional AM broadcast stations are preferred since they are readily available and are relatively po~erful and stable. ~ stations that are designated "clear channel"
operate 24 hours per day and provide back-up transmitters in the event of failure o~ the main ~ransmitter, and thereore, are especially attxactive in my invention. Such clear channel AM stations provide strong si~nals in most metropolitan areas and are readily adapted as reference signals in the practice of the invention.
~ M stations will sometime over-modulate and in such instances the carrier o the station is cut-off for time durations as long as 50 milliseconds or longer, and this can sometime interfere with the stability of the frequency synthesizers empl~yed in the alarm transmitters and xeceivers using my invention. A technique is disclosed herein which uses both a voltage-controlled crys~al oscillator and a simpler RC oscillator in a combination which provides a frequency inertia capability which readily smooths out these AM broadcast station carrier drop-outs. The latter -combination of two oscillators employs two frequency dividers and associated phase detectors in a phase locked loop arrangement which permit the selection of any one of a number of local broadcast station requencies for reference, and the selection of any one ~f a multipllcity of alarm carrier channels, at the choice of the userO

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A met]-~od is disclosed for modulating the local radio ~roa~cast sta~ion using techniques which do not interfere ~ith the normal signal transmission of said brbadcast station in order to provide means to individually poli each of the many independent alarm transmitters so that they may report their status to the central alarm receiving station in se~uential or random order.
The invention provides a very-narrow-band radio con~munication apparatus to achieve high signal-to-noise ratio transmissions. The alarm transmission apparatus can burn throush most natural and man-made interfering signals. The radio alarm transmission apparatus need not monitor its radio channel prior to transmission and does not unreasonably interfere with other users of the same local radio channel, even if they transmit simultaneously. Independent alarm transmitters and a central receiiving station are tightly and precisely synchronized both at the radio carrier frequency and at digital data clock frequencies associated with the digital status messages.
The invention pro~ides a high capacity alarm transmission system wherein 100 or more alarm transmitter channels can be compressed within one conventional radio voice channel, thereby substantially conserving the radio - spectrum. ~5eans are provided for polling and initiating status reporting transmission of independent alarm transmitters using a local radio broadcast station modified to transmit polling signals, which polling signals do not interere with the normal signal transmissions of said broadcast station~
Such an alarm cor~unication apparatus is relatively i~une to jamming and intentional interference by intruders.

7~3 BXIE~ D~SC~IP'rION OF q~llE DR~1INGS
. Figure 1 is a simplified overall block diagram - of;one alarm transmitter and one alarm receiver channel and a local broadcast station.
- Figure 2 is a simplified diagram of a fre~e~ncy synthesizer that phase locks to the broadcast station siynal and synthesizes the alarm radio carrier frequency.
Figure 3 is a block diagram of a circuit for detecting broadcast station carrier drop-outs and holding the synthesized frequency at the last value which ~as locked to the carrier.
Figure 4 is a block diagram illustrating the modifications necessary to a conventional ~1 broadcast station so that it ~nay generate polling signals.
Figure 5 is a bloc~ diagram of the alarm transmitter shown in Figure l but modified to detect polling signals.

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Figure 1 graphieally portrays broadcast station

2 wnieh may be, for example, a eonventional commercial AM
broadcast station operating on a ciear ehannel. Reeeiving antennas 4 and 6 detect the signal from broadcast station 2. This signal is amplified and li~ited by tuned amplifiers 8 and 10 so as to remove most of the amplitude modulation on the broadcast station signal, Limiter-amplifiers 8 and 10 are designed to provide symmetrical amplitude limiting in both the positive and negative excursions of the ~roadeast sianal and to provide a symmetrical bandnass characteristic so as to minimize undesirable amplitude modulation (AM) to phase modulation (PM) translation which can occur in unsymmetrical ehannels. Tnis AM to PM translation appears as phase jittex in the output of limiter-amplifiers 8 and 10 and can cause instability in frequency synthesiæers 12 and 14. Frequeney synthesizers 12 and 14 phase loc~ to the output signals from limiter-amplifiers 8 and 10 and synthesize a frequency fi, which is usually higher than the frequency of the broadcast station. In the transmitter, a modulator 16 accepts the output fi of frequeney sysnthesizer 12 and the output of alarm and status signal source 18 and modulates fi with the signal from source 18. This modulated carrier is amplified by an amplifier 20 and the resulting signal is radiated by an antenna 22.

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Tlle front end of the receiver at t~e central receiving station, including elements 6, 10 and 14, is almost identical to the front end of the alarm transmitter.
AnLenna 6 receives signals rom broadcast station ~ and these signals are amplified and limited by amplifer 10 and fed to frequency syn~hesi~er 14 ~hich synthesizes a Irequency fi identical to the carrier frequency radiated by antenna 22. Antenna 24 detects this transmi~ted frequency radiated from antenna 22 and it is amplified in amplifer ~6 to a level necessary to drive synchronous detccior 28.
This receiver operates in a mallner analogous to so-called zero-IF receivers wherein the local oscillator signal from frequency synthesizer 14 is at the same ~requency as the incoming signal detected by antenna 24 so that the mixture of these two signals results in a zero intermediate frequency, except for the thus detected alarm signal which continues - through low-pass filter 30. The output of low-pass ilter 30 is essentially identical to the transmitted alarm signal generated in souxce 18~
A frequency synthesizer ~hich may be used in the alarm transmitters or receivers is illustrated in Figure 2 which provides a "flywheel" or inertial smoothing action using two separate voltage-controlled oscillators. Conventional AM broadcast stations often over-modulate their radio carriers and this results in an interrrupted signal that may result in the loss of a re~erence carrier for up to 50 milliseconds or perhaps longer. Frequency synthesizers which are phase-locXed to such signals may suffer from these carrier interruptions, since they cause signi~icant frequency-synthesizer excursions which are undesirable.

The frequency synthesizer illustrated in Figure 2 overcomes these problems by providing an ~nertial smvothiny action which will now be descxibed. The signal from limiter-amplifier 8 in Figure 2 is connected to one input of a phase comparator 3~. A second input to phase comparator 32 is ta~en from the output of a voltage controlled oscillator (VCO) 34. The output of phase comparator 32 is a voltage which is proportional to the difference in phase between the signal from limiter-amplifier 8 and VCO 34. This difference signal from phase comparator 32 is smoothed by a low-pass filter 36-and applied to control the frequency of a voltage controlled crystal oscillator (VCXO) 38. The frequency of VCXO 38 is chosen to be equal to the desired alarm transmitter radio carrier frequency, or a submultiple of it.
The frequency from VCXO 38 is divided by frequency divider 40, which divides by the integer M. The outpllt of VCO 34 is divided by the integer N using a divider 44. The output of divider 44 and output of divider 40 are both sent to a phase comparator 46, which provides an output voltage proportional to the difference in phase betwen the signals from divider 40 and divider 44. The output of phase detector 46 is smoothed in a low-pass filter 48 and applied to control the frequency of VCO 34. The result of .his combined action is that VCO 34 is effectively phase loc~ed to VCXO 38 t and any var.iations in VCXO 38 will be followed by corresponding frequency variations in VCO 34. On the other hand, any variation in VCO 34, when compared to the output of limiter-amplifier 8 in phase detector 32, serve to correct the frequency of VCXO 38, which then corrects the fr~quency of VCO 34 in such a manner so as to minimi~e the output from phase detector 32. If a signal from limiter-amplifier 8 is momentarily lost ~67~

due, for example, to over-modulatioll of the ~roadcast station carrier, then ~CXO 38 will coast and maintain its frequency until the output of limiter-amplifiex 8 again appears, at which time the output of VCO 34 will be only slightly out of phase with the output bf limiter-amplifier 8. This slight phase error will be im~ediately detected and will serve to correct the VCXO 38 frequency, consequently bringing VCO 34 into phase ~ith the output of amplifier 8.
Terminals 50 pL-ovide an instantaneous indication of the phase Aifference bet~een limiter-amplifier 8 and VCO 34, and these terminals are used in an identification code comparator and to detect special broadcast signals which will be described in a later section of this specification.
The detection is made possible by the smoothing action of VCXO 3~. ' The frequency synthesizer arrangement in Figure 2 serves another important function in that the frcquency divider ratios N and ~l of frequency dividers 44 and 40 provide a means of selecting the desired broadcast station frequency and also the desired alarm transmitter frequency fi, respectively. This operation may best be described by a specific example. If we assume that broadcast station 2 is transmitting at an assigned frequency of 640 KHz, and that each alarm transmitter carrier frequency is separated from other alarm transmitter carrier frequencies by an interval of 100 Hz, than the following divider integers N and M are obtained. The integer N is selected to be 6400 so that the output of frequency divider 44 is 100 Hz when VCO 34 oscillates at 640 ~Hz. This ~ill matcn the output of limiter-amplifier 8 if the sy~tem is properly lvcked.
If we assume that ~e desire an alarm carrier frequency of 7~

27.065000 MHz, then divider 40 should be set out to divide by the integer M = 270,~50 so that the output of divider 40 will also be 100 Hz. Under these circumstances, VCXO 38 will oscillate at a frequency of 27,065,000 Hz and VCO 34 will oscillate at a frequency of 640,000 Hz when the incoming broadcast station frequency is also 640,000 Hz and the fre~uency synthesizer is properly phase locked. As a further example, assume that frequency divider 40 is now set to divide by the ratio M = 270,651, the frequency of VCXO 38 would now appear 100 Hz higher in frequency than the example previously described. That is, the frequency of the alaxm transmitter would now appear as 27,065, 100 Hz.
It should be pointed out that VCXO 38, being a crystal controlled oscillator, is expected to maintain a short term stability on the order of one part in 10-8, which is readily achieved in reasonabl~ priced crystal oscillators.
This means that if VCXO 38 is operating at a frequency of 27,065 MEz, then even if the output of limiter-amplifier 8 suddenly disappears for one second, VCXO 38 would drift in frequency by no more than 0.27 Hz, or approximately 90 degrees in phase from its desired plane. As another, more realistic example, if the broadcast station carrier disappears for 100 milliseconds, then VCXO 38 would not slip out of phase from its desired relationship to the broadcast station by an amount greater than about 10 electrical degrees at 27 MHz. Such a phase error is easily corrected by the disclosed circuit when the broadcast station carrier reappears.

If on the other hand a voltage controlled oscillator of much lower stability than a crystal oscillator were used in place of VCXO 38, the amount of dri~t occuring duriny ~roadcast station carri~r drop-outs could be substantially greater than 360 and this could result in the loss of several ~F carrier cycles. This would appear as undesirable frequency and phase perturbations on the alarm carrier frequency and might result in unlocking of the alarm transmitter-receiver link.
Thus the frequency synthesizer in Figure 2 not only provides an inertial smoothing action but also provides a means of preselecting the broadcast station carrier frequency and the alarm carrier frequency. Frequency-divider chains fabricated from large-scale integrated circuits can be readily designed with the necessary number of divider stages and selectable frequency-division ratios.
~lso, the tuning range of VCXO 38 can be readily designed to cover several thousand ~ertz so that one crystal can be made to operate over many alarm carrier frequencies, i these frequencies are spaced at intervals on the order of 100 Hz.
Fiyure 3 illustrates ~ circuit in which a carrier drop-out detector 52 detects any ~ station carrier drop outs and develops a gate voltage Vg which causes a sample-and-hold circuit 54 to freeze the output of phase comparator 34 at the voltage immediately preceeding a carrier drop-out and thereby further to minimize undesirable excursions of oscillator 38 during these carrier drop-outs. When the AM carrier reappears, gate voltage Vg is removed and the output of comparator 3~ is effectively reconnected to filter 36.

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Identification code comparator and special signal detector 35 detects special information and code transmissions from the broadcast station to control actions at the trans-mitter site such as status signal transmissions and power-up sequences. Polling code demultîplexers 37 may be used at the central receiver stations to generate time gating signals to sort out and identify multiplexed alarm signals and thereby further to increase the number of alarm transmitters sharing one conventional radio channel.
Figure 4 shows a modification to a conventional AM broadcast station so that this station will be capable of transmitting digitally coded polling code signals without interfering with the transmission of conventional audio program material. A conventional ~M broadcast station usually incorporates a master oscillator 56 which establishes the radio carrier frequency of the broadcast station, and a buffer amplifier 58 following the master oscillator. This is followed by an amplitude modulator 60 which receives input signals from audio signal source 62 and provides at its output a modulated carrier to power amplifier 64, which subsequently radiates this modulated carrier through an antenna 66. Polling interrupt circuit 71provides a means for inter-rupting the normal polling sequence to query a specific alarm transmitter.
In one variation of the iNvention, this arrangement is modified by inserting a phase modulator 68 between the master oscillator 56 and the buffer amplifier 58, and a polling code generator 70 is included. Polling code generator 70 incorporates a stored sequence of digital-code 67~

signals which correspond to the identification code assigned to the multiplicity of alarm transmitters which will be phase locked to the broadcast station. These coded identification signals may be generated by the polling-code generator in a sequential order or in any arbitrary order. For example, polling code generator 70 can generate a speciic identification code to interroyate a specific alarm transmitter at any time, thereby interrupting the normal sequence.~
Polling code generator 70 phase modulates the ou~put of master oscillator 56 in phase modulator 68.
The amount of phase modulation, that is the phase-devlation ratio, is set so that the carrier ~requency of the A~ broadcast station will not effectively deviate beyond the legally assigned requency tolerance limits FOI ~ brOaaCaSt stations in the United States, this tolerance in carrier frequency set-on accuracy is establishecl by the Federal Communications Commission and is + 20 Hz at the present time. In other words, the result o~ phase modulation caused by modulator 68 must not result in significant side bands which have the effect of de~iating the median frequency of master oscillator 56 beyond 20 Hz from the normally assigned frequency of the broadcast station. This criteria can be readily established by appropriately selecting the amount of phase deviation and the rate at which this phase is deviated based on well-known modulation theory. For e~ample, one may use an effective polliny modulation rate of 18 Hz and deviate the AM carrier by + lSD in phase. This would be readily detectable by a code comparator and would not ~ 3L67~

disturb VCXO oscillator 38 in the alArm transJnitters and receivers, or the regular AM audio program material.
Audio signal source 62 comprises the conventional ~audio-proyram material and results in AM side bands which normally exist at frequencies greater than 20 l~z away from the nominal radio carrier frequncy. On the other hand if the modulation due to phase modulator 68 is kept t~ithin 20 Hz of the nominal carrier requency, then the modula~ion due to phase modulator 68 will not interfere with modulation due to amplitude modulator 60. Therefore both signals can be transmitted in a compatible mode so that neither signal inter-feres wi~h ~he other.
Figure 5 is a block diagram of the alarm trans-mitter shown in Fiyure 1 but modified to include an identification code comparator 72 which is designed to detect the coded polling signals transmitted from the modified ~M broadcast sta~ion shown in Figure 4.
The operation of the polled alarm txansmitter is as follows~ The terminal 50 shown in Figure 2 and Figures 5 provides a voltage which represents the instantaneous phase difference between the output of VCO 34 and limiter-amplilier 8. Therefore r if any phase modulation exists on the incoming AM broadcast station, this phase deviation will appear on terminal 50. When the modified A*$ broadcast station shown in Figure 4 is modulated with polling-code signals, these signals will appear on terminal 50~of the frequency synthesizer in Figure 2.
Thus, identification co~e comparator 72 will see these digital-coded polling signals and can compare these with pre-selected 7~

code stored wit]lin co~nparatox 72. ~ cn the i.ncoming identification code matches the code stored in comparator 72, ~ trigger voltage pulse Vc is sent to gated power amplifier 20 so as to cause a status-signal translnission. Thus, the polled alarm transmitter in Figure 5 will transmit wllenever it is polled by the modiied AM broadcast station shown in Figure 4 or when an alarm-siynal condition exists in source 18. In the latter case, source 18 will cause a gate voltaye Va which turns on gated transmitter 20 and causes an alarm transmission.
Obviously many modifications and variations of the present i.nvention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than is specifically described~,

Claims (2)

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. A frequency-synthesizer responsive to a carrier frequency provided in an amplitude-limited signal from a limiter-amplifier to provide a synthesized signal having a frequency phase-locked thereto, comprising:
(A) voltage-controlled first oscillator means, (B) first phase-comparator means having a first input connected to an output of the amplifier-limiter means and a second input connected to an output of the voltage-controlled first oscillator for providing an output signal proportional to the difference in phase between the signals of the first and second inputs of the phase-comparator means, (C) first low pass filter means for smoothing the signal from the output of said first: phase comparator, (D) voltage-controlled crystal oscillator means responsive to an output of said first low-pass filter means for providing an output signal with a frequency which is variable as a function of the phase difference between the signals at the first and second inputs of the first phase comparator, (E) first frequency-divider means responsive to the output signal of the crystal oscillator to divide the frequency of this output signal from the crystal oscillator by the integer M, (F) second frequency-divider means responsive to the output signal of the first oscillator to divide fre-quency of the output signal from the first oscillator by the integer N, (G) second phase-comparator means having a first input responsive to the output of the first frequency di-vider and a second input responsive to the output of the second frequency divider for providing an output signal proportional to the phase difference between the first and second inputs, and (H) second low-pass filter means responsive to the output of the second phase-comparator means and providing an output connected to an input of the voltage controlled first oscillator means to control the frequency of said first oscillator in a manner which minimizes the phase difference between the first and second inputs to the second phase comparator, whereby the first oscillator frequency is phase-locked to the crystal oscillator frequency and the crystal oscillator is phase-locked to the incoming carrier frequency, the crystal oscillator providing a carrier frequency which can be predetermined by selecting the integer M, while the incoming frequency carrier may be predetermined by selecting the integer N.

2. A frequency-synthesizer in accordance with claim 1 wherein the frequency synthesizer further comprises:
(A) broadcast-station carrier dropout detector means for detecting over-modulation of the incoming carrier and providing an output gate voltage during said over-modulation, and (B) sample-and-hold circuit means responsive to the gate voltage from the drop-out detector and connected between the output of the first phase comparator and the input to the first low-pass filter to freeze the output voltage of the first phase comparator at the voltage exist-ing immediately preceding said over-modulation of the carrier and thereby to minimize phase-lock errors in the voltage-controlled crystal oscillator during such over-modulation.