Note: Descriptions are shown in the official language in which they were submitted.
67~
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~
7~P~
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.
--4--
67'~
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.
7~
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.
~ 7~J~
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
--7--
7~
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.
--8--
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.
i7~
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.
-- 10 --
6~
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.
--11
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
-12-
~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
-13-
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.
:~`
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
-17-
~ 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
-18-
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~,
--19--
