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Claims  |
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What is claimed is:
1. An improved emergency communication system for summoning help in case of
an emergency comprising:
a portable transceiver having power level sequencing circuit means for
transmitting a coded message on a first radio link at incrementally
increasing power levels and for receiving a first acknowledge signal and a
second acknowledge signal, said coded message comprising a squelch code, a
transceiver identification code, a distress code and a directional code,
said portable transceiver including acknowledge decoder means responsive
to said first and said second acknowledge signals such that on receiving
said first acknowledge signal, said power sequencing circuitry means is
inhibited, thereby, stopping said portable transceiver from stepping into
a higher power level, and such that on receiving said second acknowledge
signal said portable transceiver is turned off;
a plurality of relay stations positioned along a highway for receiving said
coded message on said first radio link and for generating and transmitting
said first acknowledge signal on said first radio link, each of said
plurality of relay stations including electronic enabling means responsive
to said squelch code of said coded message such that a relay station
nearest to said portable transceiver is enabled allowing said coded
message to be received, and including relay receiving means responsive to
said coded message and relay transmission means responsive thereto,
thereby causing said portable transceiver to transmit at one of said
incrementally increasing power levels, said enabled relay station relaying
said distress code, said directional code and said portable transceiver
identification code via a second radio link, and generating and
transmitting a relay station identification code via said second radio
link; and
a terminal station for receiving said distress code, said directional code,
said portable transceiver identification code and said relay
identification code, said terminal station including display means for
displaying said distress code, said directional code, said portable
transceiver identification code and said relay identification code,
locating means for determining the location of said relay station nearest
to said portable transceiver by utilizing said relay identification code,
and electronic circuit means for generating and transmitting said second
acknowledge signal via said second radio link to said nearest relay
station, said nearest relay station retransmitting said second acknowledge
signal via said first radio link to said portable transceiver turning said
portable transceiver off.
2. The emergency communication system as recited in claim 1 wherein said
portable transceiver comprises:
input means for inputting distress and directional information;
storage means operably connected to said input means for receiving and
assemblying information for transmission, said information for
transmission being said coded message; and
circuit means operably connected to said storage means for controlling
automatic transmission of said coded message at predetermined intervals,
and enabling said portable transceiver to operate at said incrementally
increasing power levels until reception of said first acknowledge signal
and for enabling said portable transceiver to operate at a fixed power
level until reception of said second acknowledge signal turning said
portable transceiver off.
3. The emergency communication system recited in claim 2 wherein said
circuit means comprises:
gating means for controlling the transfer of signals generated by an
oscillator, with one input of said gating means operably connected to the
output of said oscillator and the other input operably connected to a
first control signal means activated by said first acknowledge signal;
counting means having a plurality of outputs operably connected to the
output of said gating means for generating a plurality of delay signals
and for generating variable power sequence control signals; and
system reset memory means operably connected through a second control
signal means to said counting means until said system reset memory means
is rendered inoperable by a push-to-send switch means.
4. The emergency communication system as recited in claim 2 wherein said
circuit means includes a power sequence control circuit means comprising:
a voltage supply means having positive and negative terminals with a first
terminal of a first coil connected to said positive terminal of said
voltage supply means;
a first capacitor having a first terminal connected to a second terminal of
said first coil and a second terminal connected to a first terminal of an
output resistor, a second terminal of said output resistor being connected
to said negative terminal of said voltage supply means;
a second capacitor having a first terminal connected to an RF driver means
and a second terminal connected to a first terminal of a first resistor, a
second terminal of said first resistor being connected to said first
terminal of said first capacitor;
a pair of switching means, each of said switching means having at least
three terminals, a first terminal of each switching means being operably
connected through a second coil and a third coil respectfully to gating
means for activating said switching means at predetermined intervals, a
second terminal of each switching means being connected to said negative
terminal of said voltage supply means and a third terminal of each
switching means being connected through second and third resistors to said
second terminal of said first coil and said first terminal of said first
capacitor said pair of switching means controlling the attenuation of the
output voltage of said power sequence control circuit across said output
resistor by changing the impedance thereof.
5. An emergency communication system having a terminal station with a relay
station identification decoder means for receiving and decoding a
plurality of identification signals transmitted thereto by respective ones
of a plurality of relay stations, said relay station identification
signals each being transmitted together with a relayed coded message from
a portable transceiver, said relay station identification decoder means
providing a plurality of output signals each indicative of reception of a
particular relay station identification signal, and said terminal station
further including display means and relay selection logic circuit means
for receiving said plurality of output signals to determine the location
of said portable transceiver, said relay selection logic circuit means
comprising:
event buffer means for storing and circulating said plurality of output
signals;
a plurality of logic circuits connected to said event buffer means for
determining the location of said relay station nearest to said portable
transceiver;
control means operably connected to each of said logic circuits for
sequentially enabling said logic circuits, one at a time, as said output
signals in said event buffer means are circulated, and for generating
timing control signals; and
memory means operably connected to said logic circuits for storing location
data obtained therefrom;
said display means being responsive to said timing control signals from
said control means and said location data from said memory means, thereby,
displaying the location of said relay station nearest to said portable
transceiver. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to communication systems which transmit information
via radio waves from one point to the next. Specifically, the system
comprises a transmitter or means which converts information signals such
as audio or coded signals for propagation through or along a transmission
medium. The transmitter is coupled to the medium and at least one receiver
is coupled to the medium such that the information or modulated signal
transmitted may be derived from the received modulated carrier wave
signals and converted into signals corresponding to the information
transmitted.
2. Description of the Prior Art.
In recent years, there has been a concerted effort on the part of federal
and state highway agencies to improve emergency communication systems used
by stranded motorists on rural and urban freeway systems, toll roads, and
other limited access highways. Generally, contemporary emergency
communication systems transmit coded information to a terminal station.
The coded information is decoded at the terminal station and help is
dispatched to the stranded motorist.
One type of contemporary emergency communication system consists of
roadside call boxes positioned at specified distances along the perimeter
of a limited access highway or toll road. A stranded motorist leaves his
car, walks to the nearest box and places his request. The call box has
means to encode the motorist's request and transmits the request in the
form of a coded radio signal to a remote terminal station. On receipt of
the coded radio signal, the terminal station decodes the signal and help
is dispatched to the stranded motorist. In this type of emergency
communication system, the terminal station does not generate an answer
back signal acknowledging the receipt of the motorist's message. In other
words, the motorist does not know whether or not his message has been
received.
Although the above-identified type of emergency communication system,
hereinafter called the fixed call box system, is accurate, if not precise,
in locating the location of a stranded motorist, it has several drawbacks.
One of the drawbacks is that the motorist has to walk across the highway
or along the shoulder of the highway to operate the call box. The practice
of crossing or walking along the shoulder of a highway in order to
activate the call box places the stranded motorist in danger, in that he
may be injured by automobiles traveling along the highway. In addition,
the fixed call box systems have no indicating means to warn a motorist of
failure in the system. The net result is that a motorist may be trying to
obtain help from an inoperative call box. During an emergency, the lack of
indicating means may be disastrous.
In addition, these systems are susceptible to pranksters. There have been
several occasions where emergency personnel have been dispatched to call
box locations only to find that there is no need for their services. The
reason is that mischievous youngsters, traveling along limited access
highways, generally stop their vehicle, activate the call boxes and then
move on. Since the system has no way of determining the caller, the guilty
party is not apprehended. Also, in a situation where emergency personnel
is limited, a genuine call may go unattended.
In another type of emergency system, referred to as a mobile communication
system, a call box is attached to a vehicle. In case of an emergency, the
stranded motorist manually activates the call box and a coded signal is
transmitted to a terminal station. In case of impact, some of these
systems will automatically transmit. On receipt of this signal by the
terminal station, an operator will determine the approximate location of
the stranded motorist. The terminal station then transmits an
"acknowledgement" to the stranded motorist informing him that his message
has been received. Although the mobile communication system has solved
some of the problems posed by the fixed call box communication system, the
mobile communication system has several problems of its own.
Perhaps one of the greatest problems with the mobile emergency
communication system is the inability of the system to determine the
location of the stranded motorist accurately. In this type of emergency
communication system, direction finding techniques are utilized to
determine the direction from which the coded signal is received from the
terminal station and hence the direction of the stranded motorist.
Specifically, most of these systems utilize a so-called "Adcock" type
antenna which operates on a nulling or peaking scheme to determine the
azimuth or direction from whence the coded signal comes. In these types of
systems, there is an ambiguity as to whether the signal is coming from the
back or the front along the line of the azimuth relative to the position
of a terminal station with an antenna. For example, suppose a terminal
station with an Adcock type antenna is located between two parallel
highways and a motorist is stranded on either of these highways. The
motorist will activate the call box and the box will transmit emergency
signals to the terminal station. On receipt of these signals by the Adcock
antenna, an operator will determine the aximuth or direction of the
stranded motorist, based upon the readings of the Adcock antenna relative
to the terminal station. However, it should be noted that the azimuth
crosses both highways and since the Adcock antenna is only capable of
determining the azimuth from whence the distress signals come, there is no
way for the operator to determine which highway the stranded motorist is
on. In addition, this type of system does not identify the direction of
travel of the motorist. It should also be noted that this type of system
requires an operator to locate the approximate position of the stranded
motorist.
Another drawback with the mobile emergency communication system is that
both the call box and the transmitting antenna have to be mounted on the
vehicle. In most cases, power for the system is obtained from the battery
of the vehicle. In a typical situation, a motorist on entering a limited
access road rents one of the systems and attaches it to his vehicle. On
leaving the limited access road the motorist has to remove the system. The
chore of attaching and removing the system is very cumbersome to
motorists. Due to the cumbersomeness of the system, its usefulness for
other purposes are rather limited, i.e., only motorists can use the system
since the system has to be hard mounted onto a vehicle. Other prospective
customers, for example, cyclist, and people who are hiking cannot use the
system since it is impractical to mount. In addition, the system is
expensive and consumes a relatively high amount of power.
Neither of the above described emergency communication systems meet the
present day needs of motorists since the described systems are plagued
with several inherent problems.
One of the pressing problems of the prior art emergency communication
systems is that coded signals or messages are transmitted at higher power
levels than is necessary to make contact with a terminal station and
obtain help. The net result of high power transmission is that it
aggravates the problem of electromagnetic interference within a shared
frequency band, i.e., a frequency band which is assigned to a plurality of
users. The problem of high power transmission stems from the fact that
designers of prior art communication systems design for "worse case"
conditions. The term "worse case" means that the designer will ascertain
the maximum power which is required under the worse atmospheric conditions
and will design the unit to radiate at fixed maximum power at all times to
ensure contact with a base station.
Another problem of the prior art emergency communication systems is path
loss. Path loss is the attenuation of a radio signal between finite points
due to changes in atmospheric conditions due to rain, snow, fog, icing,
time of day, month of the year, sun cycles, etc. The path losses also vary
due to topography, ground electrical characteristics and other
obstructions. Due to the uncertainty and unpredictability of path loss,
the range (i.e., location) of a radio transmitter can not be determined
accurately by the amplitude of the received signal. Instead of using
amplitude (power) to determine range, the prior art systems determine the
range (location) of a radio transmitter by measuring the time of arrival
of a signal between two known points, or as it is called the "hyperbolic
method". Another method is to measure "the round trip time" for a signal
to reach a target and return or as it is called "active ranging."
Notwithstanding the prior art ranging methods, the radio transmitter still
has to transmit the signal at a relatively high power level (i.e., the
maximum power required under the worse atmospheric conditions) to
circumvent the effects of path loss, and as noted above, this is not
desirable.
OBJECTS OF THE INVENTION
Therefore it is an object, according to the present invention, to transmit
coded signals at relatively lower power levels and to automatically locate
the position of a stranded motorist more accurately than has heretofore
been possible.
It is still a further object, according to the present invention, to
provide an emergency communication system which is relatively simple in
design, relatively easy to use, low in cost and reliable in operation.
It is still a further object, according to the present invention, to
discourage or minimize the tampering of emergency communication systems by
pranksters and therefore minimize false alarms at a terminal station.
SUMMARY OF THE INVENTION
The above-identified objects and features of the present invention are
accomplished by providing a selfpowered, hand-held hand-operated, portable
handset capable of automatically transmitting, at incrementally increasing
power levels one of a plurality of coded distress signals and one of a
plurality of coded directional signals via a plurality of roadside relay
stations to a remote terminal station.
When in use, distress and directional information are keyed into the
portable handset for transmission to the terminal station. The portable
handset outputs a a modulated RF signal, containing a squelch code and a
system identification number which activates and unlocks a roadside relay
station. The relay station then generates and transmits a signal
containing the original signal and its relay station identification number
to the terminal station.
On receipt of this signal, the terminal station transmits a control signal
back to the selected roadside relay station. The control signal from the
terminal station causes the roadside relay station to transmit a "first
acknowledge" signal to the portable handset and places the roadside relay
station in a transparent mode. When the roadside relay station is in the
transparent mode, it will accept all messages and retransmit the messages
to the terminal station without modification. The handset automatically
responds to the "first acknowledge" signal by transmitting the keyed-in
distress and directional messages which are relayed by the roadside relay
station to the terminal station.
The terminal station then decodes the message, determines the location of
the roadside relay station nearest to the transmitting portable handset
and displays the message and the identification number of the relay
station on a display means. The terminal station then generates a "second
acknowledge" signal which is relayed back to the portable handset by the
roadside relay station. This "second acknowledge" signal turns off the
portable handset and activates an indicator assuring the user that the
message has been received.
If, for any reason, the "first acknowledge signal" is not received at the
portable handset, within a predetermined time, the portable handset will
automatically repeat the transmission of the distress and directional
signal at a higher power level. If no "first acknowledge" is obtained, the
portable handset will automatically try once more at a third higher power
level. If the "second acknowledge" is still not obtained, the portable
handset will automatically recycle through the above outlined sequence of
transmissions beginning with the RF signal which activates and unlocks the
roadside relay station.
BRIEF DESCRIPTION OF THE DRAWINGS:
FIG. 1 is an overall perspective view of a highway in which a communication
system embodying the present invention may be employed.
FIG. 2 shows the portable handset of the present invention in block diagram
form.
FIGS. 3 and 3a shows the detailed embodiment of the portable handset of the
present invention.
FIG. 4 shows the roadside relay station of the present invention in block
diagram form.
FIG. 5 shows the terminal station of the present invention in block diagram
form.
FIG. 6 shows in detail the location logic in the terminal station for
locating the roadside relay station closest to the transmitting portable
handset.
FIG. 7 shows a transmission cycle of the portable handset.
FIG. 8 shows a truth table of the variable power control states.
FIG. 9 shows the attenuator circuitry of the portable handset.
FIG. 10 shows the roadside relay station timing diagram.
DESCRIPTION OF THE PREFERRED EMBODIMENT:
For simplicity of the description, the Emergency Communication System will
be divided into three subsystems, namely: the portable handset, the
roadside relay station, and the terminal station.
However, before describing the various subsystems in detail, an overview of
the entire system will be given. FIG. 1 depicts the overall system with
motorist traveling on the highway. Spaced, at strategic positions, along
the right-of-way of the highway are a plurality of roadside relay
stations. The fact that only two of the roadside relay stations 12A and
12B are shown in FIG. 1 should not be construed as a limitation since the
roadside relay stations are spaced at fixed distances throughout the
entire length of the highway. Portable handset 10 is shown interconnected
to the roadside relay stations 12A and 12B via radio frequency A link. The
A link can be one of the channels in the emergency band between 72 and 76
MHz. The A link message or signal is in coded tones squelched so that the
A link receivers of the roadside relay stations will reject or lock out
all traffic and ambient noise on the A link frequency. The receiver will
only open up, i.e., receiver a message after the proper coded tone
signature, from the portable handset 10 has passed through the receiver
detectors of the roadside relay stations.
Referring again to FIG. 1, the roadside relay stations 12A and 12B are
interconnected to terminal station 14 via radio frequency B link. The B
link is in a higher radio frequency channel than that of the A link, i.e.,
960 MHz. Inside the terminal station 14 is a dispatcher console 14A. This
dispatcher console monitors the highway and displays the position of the
roadside relay station nearest the stranded motorist and the type of
emergency services which the motorist requires.
Having described the overall structure of the emergency communication
system, the following is a brief description of the portable handset with
a more detailed description to follow. Referring now to FIG. 2, a block
diagram of the portable handset 10 is shown. Power on reset switch 15 is
interconnected to a battery 16. Activation of the power on switch 15 will
provide power to all portions of the portable handset except the RF
transmitter 21 which will be powered-up only during signal transmit times.
Distress select switch 18 is a four position switch interconnected to a 16
character data buffer 19 hereinafter referred to as character storage
element 19. The storage element 19 could be any type of storage element
which is used in contemporary computer systems, such as registers or delay
lines. Interconnected to the distress select switch 18 and the character
storage element 19 is a two position directional switch 17. Each position
of the distress switch 18 is used for inputting distress signals (police,
accident, towing, service, etc.) while each position of the directional
switch 17 is used for inputting direction of travel. The distress signals
specify the type of assistance which the motorist needs and the
directional signals specify the direction of travel. Although the distress
switch 18 and the direction switch 17 is shown as a four and two position
switch respectively, this should not be construed as a limitation since
the switches may have any desired number of positions.
The character storage element 19 receives data from the distress switch 18
and the directional switch 17 and transmits this data, along with fixed or
predetermined data, to the terminal station via the roadside relay
stations. Each of the characters in the storage element 19 is a numeric
0-9. The character breakdown of the storage element 19 is as follows:
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a) Squelch Code 1 character (FIXED)
b) System Entry ID (call no.)
6 characters (FIXED)
c) Unit Serial Number (ID)
8 characters (FIXED)
d) Distress Code 1 character (MANUAL
ENTRY)
TOTAL 16 characters
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It should be noted that the system entry ID (call no.) is identical for all
portable handsets, and the unit serial number (ID) is different for each
unit. The system entry ID, hereinafter called the system call number, is
decoded and checked by the roadside relay station to determine if the
coded signal should be accepted. If the system call number checks out,
i.e., the received system call number is equivalent to the valid system
call number, the coded signal will be accepted by the roadside relay
station. On the other hand, if the received system call number is not
valid, the coded signal will not be accepted. Likewise, the unit serial
number is recorded in the terminal station and is used to identify the
handset which transmits coded signals to the terminal station via the
roadside relay. With this scheme of recording the unit serial number it is
easy to determine the user of the handset at any point in time.
The character storage element 19 is interconnected to a data modulator 20
and the data modulator is interconnected to a transmitter 21. Push-to-send
switch 22 is a push button switch which is interconnected through control
logic 23 to timing generator 24. By activating the push-to-send switch 22,
the portable handset reverts into an automatic transmission mode and
transmits coded signals or information at programmed intervals. Timing
generator 24 is interconnected to the power sequence control 25. The power
sequence control 25 is controlled by the control logic 23 and the timing
generator 24. The timing generator 24 which, in turn, is controlled by
oscillator 53 determines the power levels at which coded information will
be transmitted through switch 26 to antenna 27. Switch 26 is also under
the control of the control logic 23 which determines whether the handset
is receiving or transmitting coded information. Receiver 28 is
interconnected through acknowledge decoder 29 to the control logic 23. As
will hereinafter be explained in more detail, at the end of each
transmission, the handset switches into a receiving mode and on receipt of
an answer back signal the acknowledge decoder 29 decodes the signal and
uses the signal to either retransmit the contents of the character storage
element 19 or turn off the portable handset.
Referring now to FIGS. 3 and 3a, a more detailed embodiment of the portable
handset is shown. As previously mentioned, character storage element 19
receives the distress and directional signals from the distress and the
directional switches for transmission to the terminal station via the
roadside relay station. The character storage element 19 comprises an
eight stage counter 100 with tone select gates 101, 102, 103, 104, 105,
106 and 107, and transistor switches 108 and 109. The output of the
counter is interconnected to the select gates via a plurality of inverters
and the output of the select gates are interconnected to transistor
switches 108 and 109 via resistors R7 through R13. Each resistor R7
through R13 has an approximate value of 24K. The outputs of the transistor
switches 108 and 109 are interconnected to the touch tone generator 110
which generates the coded tone for transmission. As can be seen from FIG.
3, the scheme used for generating the digital coded signal is dual tone
multifrequency modulation, also known as touch tone. This scheme is well
known in the art and will not be discussed any further. Of course, several
other well known modulation schemes may be used for generating the coded
signal, for example, frequency shift keying (FSK), pulse code modulation
(PCM), etc.
Still referring to FIGS. 3 and 3a, timing generator 24 comprises a binary
counter 115 and a control gate 116. Binary counter 115 generates the power
sequence control signals on terminal 32 and terminal 33, and delay A and
delay B signals on terminal 30 and terminal 31, respectively. As will be
described hereinafter, delay A determines the frequency of transmission
within a given transmission cycle while delay B determines the dwell time
between intermittent transmission cycles (i.e., delay B determines the
time between the end of one transmission cycle and the beginning of
another transmission cycle). Of course, binary counter 115 can generate a
plurality of delays and a plurality of power sequence control signals and
the fact that only two delays and only two power sequence control signals
are shown should not be construed as a limitation.
As previously mentioned, timing generator 24 is interconnected to power
sequence control 25. Power sequence control 25, in conjunction with the
power sequence control signals on terminal 32 and terminal 33, generates
the incrementally increasing power levels at which coded signals are
transmitted from the portable handset. Power sequence control 25 comprises
decoder gates A9A, A9B, A9D and an electronic attenuator circuit. The
attenuator circuit is shown in FIG. 9 and will be described hereinafter.
The output signals from decoder gates A9A and A9B are transmitted by power
control terminal 34 and power control terminal 35, respectively, to the
attenuator circuit. By varying the signals on terminals 35 and 34 in
accordance with the truth table in FIG. 8, the portable transceiver
transmits coded directional and distress signals at incrementally
increasing power levels. For example, during the first transmission from
the portable handset the 10% (50mw) power control terminal 34 is selected
via decoder gate A9A since terminals 32 and 33 of binary counter 115 is
logical 0. Similarly, when terminal 32 is logical 0 and terminal 33 is
logical 1 the 25% (250mw) power control terminal 35 is selected via
decoder gate A9B. It should be noted that decoder gate A9D which controls
the transmission cycle latch 48 does not reset the transmission cycle
latch until the portable handset cycles through an incrementally
increasing range of power transmission. Although FIG. 8 depicts a system
which transmits signals incrementally at one of three power levels, this
should not be construed as a limitation on the scope of the invention. It
would be obvious in light of the teachings herein to devise a system
having the capability to transmit signals incrementally at N power levels
where N is greater than or less than three.
Referring now to FIG. 9, a RF three-level electronic attenuator circuit
which is logic level compatible is disclosed. This attenuator circuit
comprises a voltage supply with a positive and negative terminal. The
positive terminal of the voltage supply is connected through coil 60 to
terminal 70 of the electronic attenuator circuit and the negative terminal
of the voltage supply is grounded. One terminal of capacitor 61 is
interconnected to terminal 70 and the other terminal of capacitor 61 is
interconnected to an output resistor 62 while the other terminal of output
resistor 62 is grounded. One terminal of another resistor 67 is connected
to terminal 70 of the attenuator circuit and the other terminal of
resistor 67 is interconnected to capacitor 68.
Switching means 66 has three terminals. The first terminal of switching
means 66 is interconnected through a coil 69 to terminal 34, the second
terminal of switching means 66 is interconnected through a resistor 65 to
terminal 70 and the third terminal of the switching means 66 is grounded.
Similarly, switching means 64 also has three terminals, one terminal being
interconnected through coil 71 to terminal 35, the second terminal being
interconnected through resistor 63 to terminal being 70, and the third
terminal grounded.
For illustration purposes, switching means 66 and 64 are depicted in FIG. 9
as NPN transistors having control terminals 34 and 35 interconnected
through coils 69 and 71 to their bases. If it is desired to use PNP
transistors for switching means 66 and 64, this could be accomplished by
reversing the polarity of the voltage supply. Of course, it is recognized
that switching means other than transistors could successfully be utilized
for switching means 66 and 64. For example, vacuum tubes, SCR's and the
many other substantially high speed switching means may be used. Following
is a list of approximate values of resistors and capacitors which are used
in the circuit of FIG. 9.
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62 500 ohms
63 120 ohms
65 620 ohms
67 510 ohms
61 100 pf
68 100 pf
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As previously stated, terminals 35 and 34 are the control terminals for the
electronic attenuator circuit. By varying the signals on terminals 35 and
34, at predetermined intervals, the output impedance due to the switching
action of switching means 66 and 64, varies across output resistor 62.
Since the impedance total including output resistor 62, is the input
impedance to the power amplifier of transmitter 21 (FIG. 2), the output
power of the portable handset will change depending on the input impedance
to the power amplifier.
Still referring to FIG. 9, a logical 1 at, for example, terminal 35 and a
logical 0 at terminal 34 causes switching means 64 to saturate, shunting
resistor 63 to ground. This forms a voltage divider action between series
resistor 67 and the parallel resistance of resistors 63 and 62. The effect
is to reduce by a fixed amount the power delivered to the power amplifier.
A similar action takes place when the signal on terminals 35 and 34 are
reversed. Switching means 66 will now be saturated shunting resistor 65 to
ground. A voltage divider action is then formed between resistor 67 and
the parallel resistance of resistors 65 and 62. Since the equivalent
resistance in the circuit is less than the previous amount, the power
delivered to the power amplifier will be higher. Maximum power is realized
when both switching means 64 and 66 are saturated thereby shunting
resistor 63 and resistor 65, respectively, to ground.
Now referring again to FIGS. 3 and 3a, acknowledge decoder 29 receives the
first and second acknowledge signals from the roadside relay station and
uses these signals to either stop the power sequencing or turn off the
handset. As previously mentioned, the portable handset transmits coded
signals at incrementally increasing power levels and cycles until
reception of an answer back signal at one of the power levels. On receipt
of the first acknowledge signal, acknowledge decoder 29 inhibits the power
sequencing circuitry of the portable handset from stepping into a higher
power level. Likewise, the second acknow | | |
