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Patent 1149918 Summary

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(12) Patent: (11) CA 1149918
(21) Application Number: 342152
(54) English Title: FREQUENCY MODULATED RAILROAD TRACK CIRCUIT
(54) French Title: CIRCUITS A MODULATION DE FREQUENCE POUR VOIE FERREE
Status: Expired
Bibliographic Data
(52) Canadian Patent Classification (CPC):
  • 346/32
(51) International Patent Classification (IPC):
  • B61L 21/00 (2006.01)
  • B61L 23/00 (2006.01)
(72) Inventors :
  • POGGIO, MARIO (Italy)
(73) Owners :
  • WABCO WESTINGHOUSE COMPAGNIA ITALIANA SEGNALI S.P.A. (Not Available)
(71) Applicants :
(74) Agent: FETHERSTONHAUGH & CO.
(74) Associate agent:
(45) Issued: 1983-07-12
(22) Filed Date: 1979-12-18
Availability of licence: N/A
(25) Language of filing: English

Patent Cooperation Treaty (PCT): No

(30) Application Priority Data: None

Abstracts

English Abstract



(Case No. 7033)

ABSTRACT OF THE DISCLOSURE
A frequency modulated track circuit arrangement in which
a transmitter supplies an FSK signal to the rails, with side
frequencies deviating by equal amounts from a central carrier
frequency and alternating at a preselected modulation rate.
The receiver apparatus includes first and second verification
circuit networks coupled to the rails through a band pass
filter network which passes only the correct side band fre-
quencies based on the central frequency of the transmitter.
The first verification network demodulates only the correct
side frequencies and decodes the resulting signal to produce
an output only if the modulation rate is correct. The second
verification network comprises an amplifier with a predeter-
mined threshold level based on the known attenuation of the
transmitted signal by the track circuit and produces an output
only if the track signal equals or exceeds this threshold
level. The two network outputs, only if both are present,
activate an AND gate which supplies an output signal to a
delay circuit element which energizes the associated track
relay only if the duration of the verification outputs exceeds
the preset delay period which eliminates very short noise
pulses possibly having true characteristics.


Claims

Note: Claims are shown in the official language in which they were submitted.




Having now described the invention, what I claim as new
and desire to secure by Letters Patent, is:

1. A frequency modulated track circuit for a section of
railroad track, to detect the presence or absence of a train
within that track section, comprising,
(a) transmitter means coupled to the track rails at one end
of said section and operable for transmitting a fre-
quency modulated alternating current signal with
first and second frequencies alternating at a pre-
selected modulation rate,
(b) a receiver means coupled to said rails at the other
end of said section for receiving signals through
said rails, and
(c) a track relay normally occupying a first position
and operable at times to a second position for
indicating said track section occupied or unoccupied,
respectively, by a train,
(d) said receiver means further coupled to said track
relay and responsive to received signals for opera-
ting said relay to its second position only when the
received signal includes said first and second fre-
quencies alternating at said preselected modulation
rate, is of at least a predetermined threshold level
selected in accordance with rail circuit attenuation
and known transmitted level, and maintains the modu-
lation and level characteristics for greater than a
preselected delay period.

- 13 -



2. A frequency modulated track circuit as defined in
claim 1, in which,
(a) said frequency modulated signal transmitted by said
transmitter means comprises a central frequency
signal, frequency shift keyed to produce said
alternate first and second frequencies at said
preselected modulation rate,
and in which said receiver means includes,
(b) a first verification network coupled to said rails
and responsive to the frequency characteristics of
the received track signal for producing an output
signal only when the received signal consists of
alternate first and second frequency signals shifted
from said central frequency and alternating at said
preselected modulation rate,
(c) a second verification network coupled to said rails
and responsive to the level of said received track
signal for producing an output signal only when the
level of said received signal is equal to or greater
than a threshold level predetermined in accordance
with the transmitted signal level and the attenua-
tion characteristics of the track rails circuit path,
and
(d) a logic means coupled to said first and second veri-
fication networks and responsive only to the recep-
tion of both output signals for operating said track

- 14 -




relay to its second position after said preselected
delay period.

3. A track circuit as defined in claim 2 in which said
receiver means further includes,
(a) an input filter network coupling said first and
second verification networks to said rails and tuned
for passing from said rails to said networks only a
received signal of said first and second frequencies
shifted from said central frequency.
4. A track circuit arrangement as defined in claim 3,
in which said first verification network comprises,
(a) signal shaper means coupled to said input filter
network for squaring the wave shape of the filter
network output signal and producing a constant
amplitude output,
(b) a demodulator means coupled to said signal shaper
means and responsive to an output signal therefrom
for producing a code rate signal equivalent to the
modulation rate of said first and second frequency
signals received only if such signals are shifted
from said central frequency, and
(c) a decoding means coupled to receive the code rate
signal from said demodulator means and responsive
for supplying a signal to said logic means only if
the demodulated code rate is identical with said
preselected modulation rate.

- 15 -




5. A track circuit arrangement as defined in claim 4,
in which said second verification network comprises,
(a) a linear amplifier element having a threshold level
predetermined in accordance with the level of the
transmitted rail signal and the attenuation charac-
teristics of said track rails,
(b) said amplifier element responsive to a signal re-
ceived from said input filter network for sup-
plying an output signal to said logic means only
when the received signal level is equal to or
greater than said predetermined threshold level.

6. A track circuit arrangement as defined in claim 5
in which said logic means includes,
(a) an AND gate coupled to said first and second veri-
fication networks and responsive for producing an
output signal only when output signals are re-
ceived simultaneously from both verification net-
works, and
(b) a delay circuit means with said preselected delay
time coupled between said AND gate and said track
relay and responsive for operating said track re-
lay to its second position only when said AND gate
output signal has a duration greater than said
preselected delay time.

- 16 -



7. A track circuit arrangement as defined in claim 6,
in which said input filter network comprises,
(a) a first band pass filter means coupled to the
section rails and tuned to said central frequency
for passing a band of frequencies including said
first and second frequencies,
(b) a second band pass filter means coupled to said
first band pass filter means and tuned for passing
said first and second frequencies, and
(c) an amplifier means coupling said second band pass
filter means to said first and second verification
networks.

- 17 -


Description

Note: Descriptions are shown in the official language in which they were submitted.


(Case No. 7033)
~1~99~8
FREQUE~CY MODULATED RAILROAD TE~ACK CIRCUIT

BACKGROIJND OF THE INVENTION
My invention pertains to a frequency modulated railroad
track circuit. More particularly, the invention pertains to
an audio frequency track circuit in which an audio frequency
signal is modulated by frequency shift keying to provide a
modulated track signal whose accurate reception may be veri-
fied at the receiver to increase the safety of the track
circuit operation.
All railroad track circuits, and particularly audio
frequency overlay circuits, are subject to extraneous noise
signals induced from sources other than and external to the
track circuit source. This noise may interfere with the
operation of the track circuit or even cause the registration
of incorrect indications as to track occupancy conditions.
Such interference i8 particularly prevalent in electrified

railroads where propulsion current flows in the rails. ~u~
F~e7~e hC~/
e~ret~ Y~ e~ modulation (FM) techniques to modulate the
transmitted track circuit signals in order to provide greater
distinctiveness from the noise signals for the track circuit
receiver have been used, frequently in the form of Frequency
Shift Keying (FSK). However, frequency modulation itself is
not the complete answer to improved reliability and safety.
The conventional FM receiver apparatus used may not detect
the absence of one of the side band frequencies due to an
extremely low level of the received track signal resulting
from an apparatus fault or a broken rail. In addition, the




-- 1 --

~149~i8
external noise may match one of the side band frequencies,
although at a lower level. Therefore, a track circuit receiver
arrangement is desirable which will check that the signal
received from the rails has the proper characteristics and ``
conditions. For example, one of three very desirable checks
is to determine that both side band frequencies are present
and alternating at the initial modulation frequency or code
rate. Further, the received signal should have the proper
level determined in accordance with predetermined track
section attenuation characteristics and, even if it has these
proper characteristics, should also have a time duration of
at least a predetermined minimum length which will eliminate
very short induced pulses which may have equivalent charac-
teristics.
Accordingly, an object of the invention is a frequency
modulated railroad track circuit arrangement which verifies
the reception of a proper frequency modulated track signal of
predetermined sufficient signal level at the receiver end of
the section prlor to enabling the registry of an unoccupied
track section indication.
Another object of the invention is receiver apparatus,
for a frequency modulated audio frequency railroad track
circuit, which includes verification means to aseure that
the received track signal correctly contains both side band
frequencies of the central carrier signal and that the
received signal is of at least a preselected minimum level
and duration.




., -

~149918

Still another object of the invention is a frequency
shift keyed overlay track circuit which includes verification
networks at the receiver location, to check the reception of
a track signal having identical frequency characteristics to
that transmitted and a preselected level in accordance with
the known attenuation characteristics of the track section
prior to enabling the registry of an unoccupied section indi-
cation.
Also an object of the invention is a receiver arrangement,
for a frequency modulated track circuit, which includes a
first verification network, responsive only to a received
track signal containing both side frequencies of a frequency
shift keyed carrier signal and which occur at the correct
modulation rate, and a second verification network, respon-

sive only to a received track signal with at least a predeter-
mined minimum level, and which registers an unoccupied track
section indication only when both networks re~pond to the
received signal for a predetermined minimum time duration.
Yet another object of the invention is an alternating
current railroad track circuit arrangement in whieh a fre-
quency shift keyed carrier signal i~ transmitted through the
rails from one end of the railroad traek seetion to reeeiver
apparatus at the other end whieh ineludes a first channel
verifying that both side band frequencies are received at the
correct modulation rate, a second channel verifying that at
least a minimum signal level of modulated carrier is received,
and logic registry means to indicate an unoccupied track


~1499i8

section condition only when the existence of the corresponding
distinctive conditions are verified by both channels over a
predetermined time period.
Other objects, features, and advantages of the invention
will become apparent from the following specification and
appended claims when taken in connection with the accompanying
drawing.

Sl~MMARY OF THE INVENTIOl~ .
According to the invention, track circuit transmitter
apparatus, connected to the rails at one end of a railroad
track section, generates and transmits through the rails an
alternating current signal having a basic or central carrier
frequency which may be in the audio range. This central fre-
guency i8 modulated by the FSK method so that the actual
track signal has two ~ide band frequencies which deviate from
the central carrier in opposite directions by the same amount
and which alternate at the selected modulation rate. At the
other end of the section, track circuit receiver apparatus is
connected to the rails to receive the track signal and energize
a track relay only when the received signal has frequency and
modulation characteristics corresponding to the transmitted
signal, has a signal level above a threshold predetermined in
accordance with the transmitted signal level and track section
attenuation characteristics, and has a duration in excess o a
predetermined ~afe delay period.
In the specific showing, the track circuit transmitter
comprises a relatively conventional combination of elements




_ 4 --

1149918

to generate an FSK signal which is filtered and amplified
for transmission through the track section rails. It
provides a frequency modulated signal with two side fre-
quencies spaced about a central carrier frequency and
deviating an equal amount on both sides at a selected
modulation rate. The receiver apparatus includes two veri-
fication channels or circuit networks, each verifying the
presence, in the received signal, of predetermined charac-
teristics of the transmitted signal. Each of these networks
is fed from the rails through a common input network including
two filters and an amplifier coupled in series. The filters
are both of the band-pass type, the first passing a frequency
band centered about the modulated central frequency, and the
second passing the two side freguencies of the FSK signal.
The filtered signal is amplified to a sufficient operating
level for verification. The fir~t verification network
receives the filtered signal with the side band frequencies
and demodulates thi~ signal to determine the presence of
both frequencies at the proper spacing from the central
carrier frequency. The demodulated signal is then decoded
to as~ure that the modulation rate of the received signal is
the same as that of the transmitted signal. If these condi-
tions are met, the first network supplie~ an output signal
to one input of a logic AND element. The second verification
channel or network receives the filtered and amplified signal
from the input network and, by a conventional linear amplifier,
determines whether the received signal has at leat a minimum




-- 5 --


,:

~1499~8

level, that is, equals or exceeds a preset threshold level of
this amplifier. The selected threshold level is determined
by subtracting the predetermined attenuation caused by the
characteristics of the track rail circuit from the known
signal level transmitted from the other end of the section.
If this threshold is at least equaled, an output from the
linear amplifier is supplied to the second input of the logic
AND element. If both verification or concurrence signals are
present, the output of the AND gate is supplied through a
delay circuit to energize the track relay. This delay element
requires that the output of the AND gate be of at least a pre-
determined duration to eliminate very short, extraneous noise
signals induced into the track circuit which might meet the
other verification tests. The track relay is energized then
only if the received signal is verified as having the proper
characteristics, that i8, an unoccupied track ~ection indica-
tion i8 registered only by the reception of a correct signal.
DESCRIPTION OF THE PREFER~ED EMBODIMENT
Prior to defining the invention in the appended claims,
I shall describe a preferred embodiment of the invention as
illu~trated in the accompanying drawing which shows schemati-
cally, and principally by block diagram, the transmitter and
receiver apparatus for a frequency modulated railroad track
circuit embodying the invention.
Referring now to the drawing, across the top are conven-
tionally shown the two rails 1 and 2 of a section T of a
stretch of railroad track. Connected to the rails at the

~9918

right end of the section is a track circuit transmitter,
diagrammatically shown below the horizontal dashed line
separating the upper and lower parts of the drawing. Con-
nected to the rails at the left end of the section is the
track circuit receiver which is similarly illustrated above
the dashed line. The rails of the section T may or may not
be insulated from the adjoining rails of the stretch by
insulated joints although none are here shown. The track
circuit of the invention is preferably used in an overlay
fashion without insulated joints, e. g., for a highway
crossing warning installation. However, as is well known
in such track circuits, the receiver apparatus is tuned to
respond only to the signal from the corresponding track
circuit transmitter so that no interference with other track
clrcuits occurs, for example, such as used for traffic control
signaling.
The transmitter include9 a modulation signal or code
generator 6 which produces the modulation signal having a
frequency Fm which may be on the order of 7.5 to 20.0 Hz.
Any known arrangement, preferably using solid-state elements,
may be used to produce this code or modulation frequency
signal. A signal generator and modulator unit 8 produces a
signal, having a central carrier frequency Fc, which is then
modulated, preferably by the FSK method previously discussed.
Generator 8 is driven by the signal from generator 6 to
produce the FSK modulated output which is alternately, at
rate Fm~ of frequencies Fl and F2. Preferably, frequencies


1~49918

Fl and F2 deviate from the basic or central frequency Fc by

the same amount, that is, by a frequency shift on the order
e4r~d
A f, for example, 25 Hz above the below, respectively, the
central frequency. If the central frequency Fc is selected
to be within the audio range, then the side band frequencies
Fl and F2 are also audio frequencies.
m e output of generator 8 is passed through a band pass
filter 10 and then, for safety, shaped, e. g., squared, and
limited in the shaper and limiter unit 11 to provide a sub-

stantially constant amplitude signal as the supply voltagefor the generator varies. This squared wave form is then
applied to pre-amplifier 12 and power amplifier 13, which are
coupled by interstage transformer 14, to furnish sufficient
signal level for transmission through the rails, to which the
amplifiers are coupled by output transformer 16. A final
series LC band pas~ filter 17, tuned to the central frequency
Fc~ iB inserted in the rail connections, to block interfering
signals in the rails from the transmitter network. The trans-
mitter arrangement thus applies to the rails 1 and 2 of the
track circuit a signal characterized by having two side fre-
quencies Fl and F2, which alternate in time at the modulation
frequency Fm as driven by the modulation generator 6. This
signal has substantially a constant amplitude, even though
the supply voltage may vary, to permit a stability in the
track circuit operation, that is, in the shunt condition which
it will detect.


9918

The track circuit receiver apparatus illustrated above
the dashed divider line receives the track signal from the
rails at the other end of section T through a band pass
filter 19 which, for example, may be a series LC circuit
tuned to a narrow frequency band, centered on the base fre-
quency Fc but wide enough to include side frequencies Fl and
F2. A second band pass filter 20 coupled by input transformer
21 is more selective than the initial filter 19, being tuned
to pass the side frequencies Fl and F2. The output of filter
20 is coupled by the impedance amplifier unit 22 to a first
and second verification circuit network outlined by the dashed
blocks 23 and 24, respectively.
The first verification network includes a signal shaper
26, a demodulator 27, and a decoder element 28. Unit 26
shapes the signal received from filter 20 through the impe-
dance amplifier into a square-wave form of constant amplitude
and supplies this output to the demodulator 27. This latter
unit demodulates the received signal only if the frequency
deviation i6 identical to the predetermined shift in the
transmitter apparatus, for example, + 25 Hz from the central
carrier frequency Fc. If this condition exists, the output
signal at the demodulation rate is applied to decoder unit 28,
which inc1ude-q a resonant LC circuit. Unit 28 generates an
output, that is, a concurrence or true signal, only if the
frequency or code rate of the demodulated signal coincides
with the modulation frequency or code rate Fm of the signal
supplied by the transmitter apparatus. Thus, the first


~149918

verification network 23 produces a true or concurrence output
signal only when the input from band pass filter 20 is an
FSK signal, including frequencies Fl and F2, alternating
at the modulation frequency Fm and based on, and with equal
deviation from the central carrier frequency Fc as transmitted
from the other end of the track circuit by the transmitter
apparatus. In other words, a true signal output by network
23 designates that the received rail signal has frequency and
dulation characteristics identical to the signal transmitted
into the rails at the other end of the section.
The second verification circuit network 24 consists
solely of a linear amplifier 32 with a preselected threshold
level. Amplifier 32 produces an output signal only when the
input received from band pass filter 20 through amplifier 22
equals or exceeds this threshold. This level is preselected
in accordance with the calculated or known attenuation of the
tran~mitted signal by the associated track circuit which, in
this case, includes rails 1 and 2 of section T. Thus, an
output signal from this amplifier signifies that the track
signal received i8 from the transmitter apparatus at the
other end because it has at least the expected minimum level.
The true or concurrence signal outputs of networks 23
and 24, specifically from decoder 28 and amplifier 32, re~pec-
tively, are applied to the ~wo inputs of the logic D gate
29. As is conventional, element 29 produces an output signal
only when both input signals are present. An output from
element 29 is applied to a delay circuit 30 which has a




-- 10 -

~1499~-8

predetermined delay time, for example, on the order of two to
three seconds. Unit 30 produces sufficient energy to energize
track relay TR but only when the input signal from AND gate
29 is of a longer duration than this predetermined delay time.
When energized, relay TR picks up, closing its front contact to
indicate an unoccupied condition for section T. However, if
relay TR becomes de-energized, for example, when the AND
element output ceases, it releases immediately to register an
occupied track section condition. The delay time of unit 3~0
assures that relay TR does not pick up in response to input
pulses of very short duration from an interference source
which may, however, have frequency and amplitude character-
istics equivalent to proper track signal. In other words,
this delay time inhibits the response of relay TR to a
variable noise source which might activate simultaneous
re~ponse, for a very short interval, by verification networks
23 and 24.
The receiver apparatus, therefore, energizes track relay
TR, to register an unoccupied track circuit condition, only
when the signals received through the track circuit rails
from the tran~mitter apparatus have the correct or true
characteristics. These characteristics include being an FSK
modulated signal with the same side frequencies Fl and F2,
deviating from a central carrier frequency Fc by the same
frequency shift amount, having the same dulation frequency
Fm as transmitted, an amplitude equal to or greater than a
predetermined threshold level selected in accordance with the




-- 11 --

~4~918

the known track circuit attenuation characteristics, and a ~-
duration greater than a predetermined safety time period.
The satisfying of these conditions, without which track relay
TR is not energized, renders the receiver apparatus very
selective with regard to the signals received, thereby
increasing its degree of nonresponse to interferin~ or noise
signals present in the rails 1 and 2 of the track circuit.
Although I have herein shown and described but a single
preferred form of a frequency modulated track circuit arrange-

ment embodying the invention, it is to be understood thatvarious changes and modifications within the scope of the
appended claims may be made without departing from the spirit
and scope of my invention.




- 12 -

Representative Drawing

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Administrative Status

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Administrative Status

Title Date
Forecasted Issue Date 1983-07-12
(22) Filed 1979-12-18
(45) Issued 1983-07-12
Expired 2000-07-12

Abandonment History

There is no abandonment history.

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Application Fee $0.00 1979-12-18
Owners on Record

Note: Records showing the ownership history in alphabetical order.

Current Owners on Record
WABCO WESTINGHOUSE COMPAGNIA ITALIANA SEGNALI S.P.A.
Past Owners on Record
None
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.
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Document
Description 
Date
(yyyy-mm-dd) 
Number of pages   Size of Image (KB) 
Drawings 1994-01-14 1 22
Claims 1994-01-14 5 142
Abstract 1994-01-14 1 34
Cover Page 1994-01-14 1 14
Description 1994-01-14 12 470