Note: Descriptions are shown in the official language in which they were submitted.
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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 messages
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, and other signals represents
an important segment of the communication art wherein high
reliability and immediacy of tran~mission 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 path is established
between 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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~ adio alarm co~nunication using co~ventional t~chniques
has not been very reliable because of the relatively 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 which may be using the channel when an
alarm condition occurs. The scarcity of radio-fre~uency channels
further aggravates this condition.
Radio alarm transmitters in the prior art generally in-
corporate both 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 mask-
ing 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 employ
15 narrow-~and transmission techniques because the available frequency
determining sources,for example 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 lO0 Hz handwidth) and acceptable
transmission time. Prior-art devices are also relatively complicated
when designed to use synchronous detection techniques in the receiv-
ing circuits since no local frequency reference for the synchronous
process is~yailable and it must be generated from the usually weak
incoming alarm signal.
Swanson (U.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 frequenc~ using a multiplexed antenna. Swanson employs
very-low frequency (VLF) radio signal sources, specifically highly
30 stahle O~GA navigation signals, or possibly othex highly sta~le VLF
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~ transmissions such as standard radio transmission from the National
Bureau of Standard WWV stations, in order to generate absolute.
standard frequencies. The detection and use of such standard
frequency transmisslons 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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SUMI~RY OF TI~E Il'~VENTION
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The disclosed apparatus employs 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 multiplicity of local oscillators in
the alarm receiver so as to permit synchronous detection of the
alarm signals. Each independent alarm trar,smitter receives signals
from the local broadcast station and incorporates a frequency
synthesizer which is phase locked to this hroadcast s~ation signal
to synthesize the carrier frequency of the alarm transmitter.
The 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 transmitter and re-
ceiver are locked to the same carrier and thus allow very narrow
sel~ction of received frequencies to eliminate n~ise. Trans-
mitted 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. rrherefore upwards of 100 or more
separate alarm transmitters m~y operate on one conventional radio
voice channel of 10 KHz bandwidth. Several alarm transmitters
may also operate on the same alarm channel by providing suitable
time synchronization, using, for example, a polling method herein-
after described.
The central alarm receiver also employs the same~ radio broad-
cast station used by the alarm tran3mitters to phase lock a
multiplicity of frequency synthe~izers which are used as local
oscillators. The central ~eceiver synchronously detects the
alarm transmissions in one of a multiplicity of independent
parallel channels , which channels correspond to the multiplicity
of frequencies used by the independent alarm transmitters. The
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alarm signals detected by the central receiver are subsequently
sent to a response agent, such as a police statio~, On-off
alarm signalsmay be communicated using this apparatus or status
- signals comprising a digital coded message may be transmitted.
- 5 The alarm transmitters need not monitor their radio
`, channel prior to signal transmission because their narrow trans-
; mission bandwidth results in a sufficiently intense concentra-
tio~ of energy in a very small sp~,ctrum 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 will not usually interfere
with their intelligibility. The alarm signal may also be
transmitted in the so-called "guard band" of frequencies between
- assigned channels of, for example voice transmitters.
Any of a number of different broadcast stations, such as
; commercial AM broadcast stations, television stations, radio
,~ navigation stations, and other similar transmitters may be used
in the practice of this invention, or a spPcial transmitter may
; be erected for this purpose. Conventional ~M broadcast stations
are preferred since they are readily available and are relatively
powerful and stable. AM stations that are designated "clear
channel" operate 24 hours per day and provide ~ack-up transmitters
in the event of f~ilure of the main transmitter, and therefore,
are especially attractive in my invention. Such clear channel
A~S stations provide strong signals in most metropolitan areas -
and are readily adapted as reference signals in the practice of the
invention.
AM Stations will sometime over-modulate and in such instances
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the carrier of 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 freq~ency synthesizers employed in the alarm
tra~mitters and receivers using my invention. A technique is
disclosed herein which uses both a voltage- controlled crystal
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 com-
bination 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 frequencies for reference, and the selection
of any one of a multiplicity of alarm carrier channels, at the
choice of the user.
A method is disclosed for modulating the local radio
broadcast station using teclmiques which do not interfere with
the normal signal transmission of said broadcast station in
order to provide means to individually poll each of the many
independent alarm transmitters so that they may report their
status to the central alarm receiving station in sequential or
random order.
The invention provides a very-narrow-band radio communication
apparatus ~o achieve high signal-to-noise ratio transmissions.
The alarm transmission apparatus can burn through 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 receiving station
are tightly and precisely synchronized both at the radio carrier
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frequency and at digital data clock frequencies associated with
the digital status messages.
The invention provides a high capacity alarm transmission
system wherein 100 or more alarm transmitter channels can be
compressed within one conventional radio voice channel, there,by
substantially conserving the radio spectrum. Means 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 sig-
nals do not interfere with the normal signal transmissions ofsaid broadcast station. Such an alarm communîcation apparatus
is relatively immune to iamming and intentional interference by
intruders,
BRIEF DESCRIPTION OF THE DRAWINGS
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 frequency synthesizer
that phase locks to the broadcast station signal 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 ~alue which was locked to 'he carrier.
Figure 4 is a block diagram illustrating the modifications
necessary to a conventional AM broadcast station so that it may
generate polling signals.
Figure 5 is a block diagram of the alarm transmitter shown
in Figure 1 but modified to detect polling signals.
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Figure 1 graphically portrays broadcast station
2 which may be, for example, a conventional commercial AM
broadcast station operating on a clear channel. Receiving
antennas 4 and 6 detect the signal from broadcast station
2. This signal is amplified and limited 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 broadcast
10 signal and to provide a symmetrical bandpass characteristic `~
so as to minimize undesirable amplitude modulation (AM)
to phase modulation (PM) translation which can occur in
~;~ unsymmetrical channels. This AM to PM translation appears
as phase jitter in the output of limiter-amplifiers 8 and
10 and can cause instability in frequency synthesizers 12 and
14. Frequency synthesizers 12 and 14 phase lock 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 frequency 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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The front end of the receiver at the central receiving
station, including elements 6, 10 and 14, is almost identical to
the front end of the alarm transmitter. Antenna 6 receives signals
from broadcast station 2 and these signals are amplified and
limited by amplifier 10 and fed to frequency synthesizer 14 which
synthesiæes a frequency fi identical to the carrier~frequency
radiated by antenna 22. Antenna 24 detect~ this transmitted
frequency radiated from antenna 22 and it is amplified in amplifier
26 to a level necessary to drive synchronous detector 28. This
receiver operates in a manner analogous to so-called zero-IF receivers
wherein the local oscillator signal from frequency synthesizer
14 is at the same frequency as the incoming signal detected by
antenna 24 so that ~he 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 filter 30 is essentially identical to the transmitter
alarm signal generated in source 18.
A frequency synthesizer which may be used in the ala~m trans-
mitters or receivers is illustrated in Figure 2 which provide~
a "flywheel" or inertial smoothing action using two separate
voltage-controlled oscillators. Conventional ~M broadcast stations
often over-modulate their radio carriers and this results in an
interrupted signal that may result in the loss of a reference
carrier for up to 50 milliseconds or perhaps lonyer. Frequency
synthesizers which are phase-locked to such signals may suffer
from these carrier interruptions, since they cause significant
frequency-synthesiæer excursions which are undesirable.
The frequency synthesizer illustrated in Figure 2 overcomes
these problems by providing an inertial smoothing action which
will now be described. The signal from limiter~amplifier 8 in
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Figure 2 ic connected to one input of a phase comparator 32. A
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second input to phase comparator 32 is taken from the output of a
~oltage 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 su~multiple of it. The frequency
from VCXO 38 is divided by frequency divider 40, which divides by
the integer M. The output 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 p~portional to the difference in phase between
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 this combined ac-
tion isthat YCO 34 is effectively phase locked to VCXO 38, and
any variations in VCXO 38 will be followed by corresponding
, 20 frequency variat~ons in YCO 34 . On the other hand, any variation
in VCO 34, when compared to the output o limiter-amplifier 8 in
phase detector 32, serve to correct the frequency of VCXO 38, which
then corrects the frequency of VCO 34 in such a manner so as to
minimize the output from phase detector 32. If a signal from
limiter-amplirer 8 is mom~ntarily lost due, for example, to over-
modulation of the broadcast station carrier, then VCXO 38 will
` coast and maintain its frequency until the output of limiter-
amplifier 8 again app~ars, at which time the output of VCO 34 will
be only slightly out of phase with the output of limiter~amplifier 8.
This slight phase error will be immediately detected and will serve
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t correct the VCXO 38 frequency~ consequently bringing VCO 34
into phase with the output of amplifier 8.
Terminals 50 provide an instantaneous indication of the
phase difference between limiter-amplifier 8 and VCO 34, and
5% these terminals are used,in an identification code comparator
and to detect special broadcast signals which will be described
in a later section of ~hisspecification. The detection is made
possible by the smoothing action of VCXO 38.
The frequency synthesizer arrangement in Figure 2 serves
another important function in that the frequency divider ratios N
and M of frequency dividers 44 and 40 provide a means of select-
ing 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
lS 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 lO0 Hz, than the following divider integers N and M
are obtained. The integer N is selected to be 6400 so that the
~utput of frequency divider ~4 is lO0 Hz when VCO 34 oscillates
at 640 KHz. This will match the output of limiter-amplifier 8 if
~ the system is prop~rly locked. If we assume t.hat we desire an ~
; alarm carrier frequency of 27.065000 MHz, then divider 40 should
be set to divide by the integer M = 270,650 so that the output of
divider 40 will also be 100 Hz. Under these circumstances VC~O
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 frequency synthesizer
is properly phase locked. As a further example, assume that
frequency divider 40 is now set to divide by the radio M ~ 270,651,
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the frequency of V~XO 38 would 130W appear 100 Hz higher in
frequency than the example previously described. That is, the
frequency of the alarm transmitter would now appear as 27,065,100
Hz. -
I~ 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 reasonably priced crystal oscillators. This means
that if VCXO 38 is operating at a frequency of 27.065 MHz, then
even if the output of limiter-amplifier 8 suddenly disappears for
one second, VCXO 38 wou]d drift in frequency by no more than
0.27 Hz, or approximately 90 de~rees in phase from its desired
phase. As anothe~,more realistic example, if the broadcast
station carrier dis~ppears 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
i
' 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 us~d-in place
of VCXO 38, the amount of drift occuring during broadcast station
carrier drop-outs could be substantially greater than 360 and this
could result in the loss of several RF carrier cycles. This
would appear as undesirable frequency and phase perturbations on
the alarm carrier frequency and miqht 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 pre- ¦
selecting the broadcast station carrier frequency and the alarm
carrier frequency. Frequency-divider chainsfabricated frcm large-
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scale integrated circuits can be readily designed with the necessary
number of divider stages and selectable frequency-division ratios
Also, the tuning range of VCXO 38 can be readily designed to
cover several thousand Hertz so tl-zt one crystal can be made to
operate over many alarm carrier freyuencies, if these frequencies
are spaced at intervals on the order of 100 Hz.
Figure 3 illustrates a circuit in which a carrier drop-
out detector 52 detects any ~M station carrier drop outs and
develops a gate voltage Vg which causes a sample-and-hold circuit
. 10 54 to freeze the output of phase comparator 34 at the voltage
immediately preceding a carrier clrop 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 32 is effectively re-
connected 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 transmitter
site such as status signal transmissions and power-up sequences.
Polling code demultiplexers 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 AM broadcast station usually
5''. incorporates a master oscillator 56 which establishes the
radio carrier frequency of the broadcast station transmitting
on an assigned frequency in the band 550-1600 KHz, and a
r~' buffer amplifier 58 following the master oscillator. This
,' is followed by an amplitude modulator 60 which receives input
signals from audion 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 71 provides means
, for interrupting the normal polling sequence to query a
specific a ~ m~ 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
` 30 70 incorporates a stored sequence of digital-code
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siynals which correspond to the identification code assigned to
the multiplicitY f ;alarm transmitters which will be phase
locked to the broadcast station. These coded identification~sig-
nals 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 specific identification code to
interrogate a specific alarm transmitter at any time, thereby
interrupting the normal sequence.
Polling code generator 70 p~ase modulates the output of
master oscillator 56 in phase modulator 68. The arnount of phase
modulation, that is the phase-deviation ratio, is set so'that
the carrier frequency of the AM broadcast station will not
effectively deviate beyond the legally assigned'requency tolerance
limits f~r AM broadcast stations in the United States, this
tolerance in carrier frequency set-on accuracy is 'established
by the Federal Comrnunications Comrnission and is ~ 20 Hz at the
present time. In other words, the result of phase modulation caused
by modulator 68 must not result in significant side bands which
have the effect of deviating the m~dian frequency of master
oscillator 56 beyond 20Hz 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 example, one may use an effective polling modulation
rate of 18 ~z and d~via'e the AM carrier by ~ 15 in phase- Thiswould
~O~ be readily detectable by a code comparator and would not
disturb VCXO oscillator 38 in the alarm transmitters and
receivers, or the regular AM audio program material.
Audio signal source 62 comprises the conve~tional autio-
prograrn material and results in AM side bands which n3rmally exist
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at frequencies grea1.er than 20 1~z away from the nominal radio
carrier frequency. On the other hand if the modulation d~e to
phase modulator 68 is kept within 20 ~Iz of the nominal carrier
frequency, then the modulation due to phase modulator 68 will
not interfere with modulation due to amplitude modulator 60-.
Thereore both signals can be transmitted in a compa~ible mode
so that neither signal interferes with the other.
Figure S is a block diagram of the alarm transmitter
shown in Figure l but modified to include an identification
code comparator 72 which is designed to detect the coded poll-
ing signals transmitted from the modified AM broadcast station
shown in Figure 4.
The operation of the polled alarm transmitter is as
follows. TIle terminal 50 shown in Figure 2 and Figures 5 pro-
lS vides a voltage which represents the instantaneous phase differ-
ence between the output of VCO 34 a~d limit~er-amplifier 8.
Therefore, if any phase modulation exists on the incoming A~l
broadcast station, this phase deviation will appear on terminal
50. When the modified AM 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 code comparator 72 will see these digital-coded
polling signals and can compare these with pre-selected code
stored within comparator 72. When the inco ~ng identification
code matches the code stored in comparator 72, a trigger voltage
pulse Vc is sent to gated power amplifier 20 so as to cause a
status-signal transmission. Thus, the polled alarm transmitter
in Figure 5 will transmit whenever it is polled by the modified
AM broadcast station shown in Figure 4 or when an alarm-signal
condition exists in s~urce 18. In the latter case, source 18
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will cause a gate voltage Va which turns on gated transmitter 20
and causes an alarm transmission.
Obviously many modifications and variations of the
present invention 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.
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