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Патент USA US3098946

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July 23, 1963
Filed Dec. 30, 1959
lnven ior
By ?GEA/f
United States Patent 0 " r'ce
Patented July 23, 1963
tioned above, plus a margin including all possible varia
tions in the sender output level and in the receiver
Jean Victor Martens, Antwerp, Belgium, assignor to inter
Assuming for example a bidirectional multifrequency
signalling transmission scheme permitting to transmit
national Standard Electric (Iorporation, New York,
?fteen signals in either direction by the use of a two out
of six code, twelve distinct frequencies must therefore be
N.Y., a corporation of Delaware
Filed Dec. 30, 1959, Ser. No. 862,318
Claims priority, application Netherlands 3am. 19, 1959
5 (Ilaims. (Cl. 397-385)
provided if simultaneous signalling in the two directions
and on the same two-wire line is contemplated. Using
10 voice frequencies, a reasonable overall bandwidth would
be that between 500 and 2000 cycles per second. For
such an example, with the twelve frequencies forming an
The invention relates to ‘a signal receiver and more par
ticularly to a receiver suitable for the so-called multi
arithmetic progression, the largest possible frequency
?requency systems.
Multifrequency signalling systems are generally under
spacing between two adjacent frequencies would be
about 136 cycles per second. If closer spacing is desired,
stood to consist in the transmission of signals each of
the cost of the receiver ?lter will increase, and the re
which corresponds with a particular combination of two
lation between cost and frequency spacing is evidently a
or more signals of distinct frequencies, taken out of a
discontinuous one since below certain frequencies it is
group 'of such ‘frequencies. Any combination of fre
necessary to add elements to the ?lter. A particularly
quencies consists in a constant number thereof and this is
a useful safeguard since one may readily ‘differentiate 20 sharp increase in the cost of the ?lter would be required
if the frequency spacing grew below 100 cycles per sec
between a true signal and a false one consisting of less
ond. Therefore, a reasonable spacing might be 120
or more than the predetermined number of frequencies
cycles per second corresponding to that used for multi
simultaneously transmitted. Thus, if there are m fre
telegraph systems. In such a case, an example
quencies Which may be used, and if each signal consists
in the transmission of n such frequencies at a time, the 25 of frequency allocation might be the six frequencies from
540 to 1140 with steps of 120 cycles per second in the
total number of distinct signals is equal to
backward direction, ‘and the six frequencies from 1380 to
1980 with steps of ‘120 cycles per second in the forward
Even then, the design of the receiver bandpass ?lter
leads to a rather complicated and costly ?lter, with the
result that the economical value of such a system as
envisaged above might be doubtful.
Such multifrequency signalling systems are already
An object of the invention is to realize a signal receiver
well known and one may ‘for instance refer to the U.S.
35 for a multifrequency signalling system in which relative“
Patent No. 2,826,638.
ly wide level variations may occur, with a reasonably
Such multifrequency signalling systems generally use
simple bandpass ?lter due to the introduction of a limiter
voice frequencies and possess various advantages.
circuit associated with said ?lter.
Nevertheless, multi-frequency signalling systems raise a
It is to be noted that the use of a limiter ampli?er
certain number of problems. This will be particularly the
e.g. 10, 15 and 56 distinct signals for two out of ?ve, two
out of six and three out of eight codes.
telephone systems. Therein, the system might be used
to provide signalling means ‘between various exchanges
which is used in common for the various individual tuned
receivers is already contemplated in the U.S. patent re
ferred to above in order that a weak incoming signal may
which may either be toll centres, zone centres or local
be ampli?ed to a value satisfactory for ‘the operation for
case if it should be ‘desired to use such an arrangement in
the individual channel receiver, While a strong signal is
The transmission equipment will generally
be associated with a register which may have to exchange 45 predetermined to a maximum value. The present inven
information with registers distributed over several more
or less distinct stations. This means that the transmis
sion losses between a multifrequency centre and a multi
?requency receiver over a two-wire connection may vary 50
over a rather wide range, which may be as high as 30
tion however envisages the use of a relatively simple
limiter circuit individually associated to each signal re
ceiver and particularly cooperating with the bandpass
?lter thereof so as to considerably simplify the design of
the actual ?lter.
While a limiter circuit will be able to cater for the
largest part of the expected level variations, the introduc
Particularly in view of the wide level variations which
tion of such a nonlinear circuit in the signal receiver
may be expected between the signal transmitter and the
signal receiver, the latter can probably be considered as 55 gives rise to phenomena which are characteristic of non
linear circuits handling rnore than one ‘frequency simul
the most di?’icult part of the apparatus to be designed.
taneously. In this respect, one may refer to the Belgian
The wide level variations imply that for a signal arriv
Patent No. 510,949 where such effects were already
ing at the lowest level admitted, and even under adverse
conditions of power supply, temperature, noise, etc., the
Brie?y, the ?rst effect is an intermodulation of two free
receivers tuned to the corresponding frequency constitut
quencies which may simultaneously be present at the
ing the signal should respond correctly. On the other
input of the receiver. A consequence of the intermodu
hand, any signal arriving at the highest possible level
lation is the production of sum and difference freqpencies
should only act on the receivers tuned to the correspond—
of the fundamental waves and of their harmonics. These
ing frequencies, without producing any response for the
intermodulation products may coincide with one of the
other receivers.
These conditions which are particularly severe when
wide‘ level di?erences must be taken into consideration,
could theoretically be satis?ed by a receiver incorporat
ing a very selective bandpass ?lter. The discrimination
of this‘ ?lter between the frequency range of a particular
receiver and the frequency ranges of the other receivers
should be equal to the level variation of 30 decibels men
signalling frequencies and give rise to false operations of
the corresponding receivers.
For example, if the frequencies used form'an arithmetic
progression, the second harmonic of any frequency minus‘
the next higher or lower frequency is respectively equal to
the next lower or higher frequency. ‘Moreover, it may
be desirable to choose the frequencies so that they all‘
are odd harmonics of half the frequency spacing so that
the even harmonic and particularly the second harmonic
of the lower frequencies will never coincide with any of
the higher frequencies. Then, further undesirable inter
modulation products may arise, since any frequency is
equal to the di?erence between the rth harmonic of the
next higher frequency and the (r—|-l)th harmonic of the
next lower frequency, 2r+1 representing the ratio be
tween the imitated frequency and half the frequency spac
Saturation is a second result of the introduction of non
linear circuits.
When two waves of distinct frequencies
ceiver would be the rather awkward one of passing the
pulses at the sending end through ?lters providing rough
ly the same discrimination as the receiver ?lters.
However, the introduction of a limiter in the manner
proposed above modi?es the transfer characteristic of the
receiver ?lters for short bursts of energy at a frequency
within the passband. At the ?lter input, these frequencies
may be present in pulses to be received by receivers tuned
10 to adjacent frequencies, at the beginning and at the
end of these pulses. The corresponding energy is con
centrated in short time intervals and may occur at a high
power level.
are applied simultaneously to the input of a limiter, the
output energy of this output limiter will be distributed
over several frequencies, namely the fundamental fre
quencies and all the intermodulation products. The out
put level for each fundamental frequency will depend on
the relative levels of the two frequencies at the limiter
input. When these levels are very different, the fre
quency arriving at the lowest level will have a tendency
to vanish at the output of the limiter.
Thus, if the limiter could be-located right at the ?lter
output, it would absorb these undesirable bursts of fre
quency with an ideal e?iciency. Further away from the
?lter input, the limiter becomes less e?icient because the
energy to be absorbed spreads over a longer time interval
and this passes the limiter more completely.
Since on the other hand an input limiter offers the
Thus, the recep
tion of an incoming frequency may be hampered by any
other frequency arriving, at a sufficient level, in the lim
iter circuit of its particular receiver.
In accordance with a ?rst characteristic of the inven
tion, a signal receiver adapted to react to a particular
frequency or to a relatively narrow range of frequencies
and to be unresponsive to other frequencies, said receiver
comprising an ampli?er, a limiter and a bandpass ?lter
and being adapted to relatively wide level differences of
input signals, is characterised in that said bandpass ?lter
only solution in the case of purely linear ?lters in the r -
drawbacks of saturation and intermodulation effects men
tioned above, the optimum solution now proposed con
sists in having an input part of the ?lter which is just
sut?cient to avoid the troubles due to these effects.
The above mentioned and other objects and character
istics of the invention will be better understood from the
following detailed description of an embodiment of the
invention to be read in conjunction with the accompany
ing drawing which represents one of a series of similar
30 tuned signal receivers using transistors for a multifre
quency signalling scheme,
is split into a ?rst part cascaded with a second part and
As shown on the ?gure, the signal receiver consists
with said limiter at the junction of said two parts.
essentially in three parts: an input bandpass ?lter using
The insertion of the limiter inside the ?lter permits to
coils and condensers and also incorporating a limiter
reduce the saturation effect mentioned above and due to 35 circuit, a class A transistor ampli?er using the transistor
that part of the ?lter which precedes the limiter. On the
other hand, the remaining part of the ?lter following the
limiter permits to render the limiter really effective. In
other words, the ?rst part of the ?lter permits to avoid
the undesirable intermodulation effects mentioned above. 40
In accordance with a further characteristic of the in
vention, each of said parts of the bandpass ?lters are in
themselves frequency selective, i.e. they each include a
tuned circuit.
The purpose of the ?rst tuned circuit part of the over
all bandpass ?lter will be to prevent the frequencies for
eign to the particular receiver from reaching the limiter
T1, and ?nally an output ampli?er stage using the tran
sistor T2.
As shown, the bandpass ?lter which may for instance
be designed to operate between impedances of 600 ohms
is the symmetrical but unbalanced type. The ungrounded
input terminal P1 of the ?lter and of the signal receivers
is connected to the output terminal P2 of the ?lter which
corresponds to the input terminal of the Class A am
pli?ers through the impedance L, the condenser C2, the
45 condenser C2’, the inductance L’, all in series and in that
order. The junction point of the inductance L with the
condenser C2 is connected to ground at terminal P3
circuit at a level su?‘icient to produce intermodulation
through the shunt condenser C1. Likewise, the shunt con
[or to hamper the transmission of the wanted frequency.
denser Cl' is connected between ground and the junction
The second tuned circuit following the limiter is how 50 point of condenser C2’ within inductance L’. A further
ever essential since the combination of the ?rst tuned
shunt condenser C3 is connected between ground and the
circuit with the limiter alone would generally not give
junction point of condensers C2 and C2’. To this last
suf?cient discrimination. This is because the advantage
junction point are also connected the recti?ers W1 and
of the limiter circuit resides in the fact that, at its output,
W2, the cathode of W1 being grounded while the anode of
the level di?erences are attenuated to such an extent that
W2 is also grounded.
the required frequency discrimination between the wanted
frequency and the undesirable ones, is easily obtained with
the help of a simple selective circuit following the limiter.
As noted, the band pass ?lter is symmetrical and the
elements indicated with primes have the same values as
the corresponding unprimed elements. Without con
With a distribution of the total amount of linear ?lter
sidering the limiter, the bandpass ?lter shown could be
ing circuits respectively before and after the limiter cir 60 reduced to a simpler circuit in which there would be a
cuit, conditions arising from the transient response of the
single series condenser of value
complete ?lter may have to be taken into account, espe
cially when the latter is used in a multi-frequency signal
202+ C3
receiver since voltage surges occurring on the transmis
sion line should not give rise to false-operations of the re 65 and two shunt condensers corresponding to C1 and C1’
ceivers. Signal pulses containing essentially another fre
but having values of
quency than the intrinsic frequency of the receiver should
not give rise to any response. These pulses will however
1+ 02+03
generally contain a small percentage of energy falling
within the frequency bandwith of the receiver considered. 70 Then, these equivalent shunt condensers in conjunction
If these pulses are applied at a high energy level, this
with the inductance L determine the frequency to which
small percentage of energy would however generally be
the receiver is tuned. Thus, there are an input and an
sui?cient to operate a receiver equipped with a mere
output tuned circuit, tuned to the same frequency and
linear band ?lter. ‘In this respect, increasing the amount
capacitively coupled. The coupling factor k may be
of ?ltering in the receiver would be useless. Then, the 75 taken as the ratio between the equivalent series condenser
It is also desirable that for all the signal receiver-s, the
product kQ should be constant for all the signal receivers,
k ‘being the coupling factor of the ?lter. Since it has been
mentioned above that the Q factor should be proportional
mentioned above and the sum of this equivalent series
condenser plus the value of one of the equivalent shunt
condensers also de?ned immediately above.
The limiter shown to be connected across the shunt con
denser C3 acts therefore on the coe?icient of coupling,
without any appreciable reduction of the Q factor of the
to the frequency, k should therefore be inversely pro-'
portional to the frequency, and hence should be smaller
‘for the signal receivers tuned to the higher frequencies;
The ratioxbetween the voltage across condenser C3 and
and that across condenser C1 can‘ be reckoned approxi
resonant circuits.
The threshold above which the limiter operates is de
termined by the characteristics of W1 and W2, which may
be embodied by silicon recti?ers requiring a certain bias 10 mately by considering only the network of the ?ve con
to become conductive.
densers C1, C2, C3, C2’ and C1’. Then, this voltage ratio
As soon as the voltage across C3 is well beyond the
will be equal to
threshold voltage, the output from the ?lter depends on
the transmission characteristic of ‘C2’, C1’ and L’, i.e. on
the frequency of the incoming signal. Variations of the 15
where .5‘ represents the value of the ratio for the signal re
input level have in this condition but little in?uence on the
ceiver tuned to the lowest frequency and x is a dimen—'
output. The only secondary e?ect of the voltage limita
sionless parameter directly proportional to the frequency
tion is a shift of the passband to the lower frequencies
and equal to unity ‘for the lowest frequency to which a
due to the increase of the effective tuning condenser. In
deed, since condenser C3 is then short-circuited, the e?ec 20 signal receiver is tuned.
Likewise, the coupling factor which should also be in
tivetuning condenser is now C1+C2 instead of
versely proportional to the frequency may readily be
computed as previously explained by considering va pi
condenser network (not shown) equivalent to the ?ve
This frequency shift should be taken into account when 25 condenser network of the ?gure. Then, this coupling
factor de?ned by
tuning the resonant circuits.
At ?rst sight, condenser C3 could be left out of the cir
cuit shown, with a corresponding change in the values of
the other condenser and particularly C2 and C2’ which
would have to be reduced. However, the impedance 30 where k is the coupling factor corresponding to the low
est frequency to which a signal receiver is tuned.
between which the limiter operates for various receivers
From the relations (1). and (2) one ‘may readily ob
tuned to the various individual frequencies, would then
depend on the particular individual frequency and it
would be di?icult to choose a single type of reciti?er
which could be used for the complete series of signal re 35
ceivers. The threshold voltages for the limiters used in
the various signal receivers may be the same provided
that the impedance seen at the junction point of the two
recti?ers is independent of the frequency to be trans
Thus, since there is another relation determining the ‘fre
quency to which the signal receiver is tuned/the values
This may be shown as follows: First of all, the absolute
of all the condensers are now determined. Adjustment
bandwidth of the ?lter may be independent of frequency
to the ?nal exact values may then be secured experi
when equal spacings between the frequencies are used.
To take a practical example, with a frequency spacing of
Since C3 has some in?uence on- the determination of
120 cycles per second, and taking into account all pos—
the, tuned. frequency, as it is included in the effective
sible tolerances, both at the receiving and at the sending
timing condenser previously mentioned, and since this
end, a bandwidth of 48 cycles per second may be used for
effective tuning condenser is merely equal to Ci+C2
each receiver. Then, from 24 cycles per second above or
the limiter exerts its short circuiting action, it might’
below the centre frequency, the attenuation should rise
Ibe- dcsira-ble that the value of C3 should not be too low
and reach a su?icient value from 120—24=96 cycles per 50 in order to limit- the frequency shift. vHowever this may
second away from the centre frequency, when the band
in fact be a secondary effect and where space is at a
width of the adjacent signal receiver is reached.
premium, it will be ‘found 1more advantageous instead
With such an absolute bandwidth of the ?lter inde
to limit the size of the condensers to a minimum. ‘The
pendent of frequency, the Q factors of the resonant cir
cuits should be proportional to the frequency. In other 55 value of C3 will be the determining factor and the smallest
values ‘for the condensers will be obtained when C3 is
words, the inductance of the coils can be the same foran
equal to zero and hence can be omitted, ‘for the lowest
frequencies if their series- resistance is constant. This
signalling frequency, i.e. when x is equal to unity. From
very useful condition which permits to standardize the
(4) one thus obtains the following condition between s
coils for all the voice frequency receivers may be satis
?ed by using ferrite'coils.
With L’ being a. constant inductance, irrespective of
the frequency to which the receiver is tuned, and with
the effective series resistance of that coil independent of
and (3) and (4) respectively become
frequency, the-ratio between thevoltage across condenser
C1 and the input voltage between input terminals P1 and 65
P3 will be proportional to the frequency to which the
signal receiver is tuned. vIf, as stated above, the limiter
circuit is to be the same for all the signal receivers ir
respective of the frequency to which they are tuned, the
ratio between the voltage across condenser C3 and that at
C2- 1+k
the input of the receiver between terminals P1 and P3
‘It will be observed from y(6) ‘and (7) as well as from
should be the same for all the signal receivers. This
means therefore that the ratio between the voltage across
condenser C3 and that across condenser C1 should be
inversely proportional to the frequency.
(3) and (4) that
is constant or substantially so, While
relay operates when the collector current of T2 reaches
4 milliamperes, whereas it remains unoperated as long
as this collector current does not reach 2 milliamperes.
In this manner, the operating level at the signal re
is a linear function of the signalling frequency.
It should be observed that the recti?ers W1 and‘ W2
ceiver input may thus 'be practically independent of the
relay sensitivity. The low Ico value will be particularly
constituting the limiter are connected with opposite p0
useful if a relay With a low release current is used.
ilarities directly across the condenser C3, due to these
While the principles of the invention ‘have been de
recti?ers, i.e. silicon diodes, necessitating a small posi
scribed above in connection with speci?c apparatus, it is
tive bias to make them conductive. With other types 10 to be clearly understood that this description is made
of diodes, e.g. germanium, some external biasing would
only by Way of example and not as a limitation on the
be required for the recti?ers. In this case, the cathode
scope of the invention.
of W1 and the anode of W2 would not be directly con
I claim:
nected to ground but might be biased to some suitable
1. In a signal receiver for receiving signals lying within
potential, preferably derived from the emitter circuit to 15 a predetermined range of frequencies and having vary
the class A ampli?er comprising the transistor T1. Some
ing amplitudes, a symmetrical bandpass ?lter of the ,un
degree of asymmetry in the back biasing of the limiter
balanced type having an input section and an output
recti?ers might in fact be tolerable.
Actually, even with the circuit shown, it might be ‘found
section capacitively coupled for passing signals lying with
in said range, each of said sections of said ?lter com
of some advantage to disconnect the cathode of the recti 20 prising a tuned circuit, thereby rendering each section
?er W1 from ground and to bias it to a potential which
frequency selective, said ?lter further comprising a ?rst
would become negative as a result of spurious input sig
‘and second circuit branch ‘with a ?rst inductance, a ?rst
nals such as inductive kicks and such like. In this case,
and second condenser and a second inductance in series
such spurious signals could not cause an undesired opera
in said ?rst circuit branch, a third, fourth and ?fth con
tion of the signal receiver, as this would lead to both 25 denser connected between the second circuit branch and
recti?ers W2 and W1 becoming conductive.
Terminal P2 constituting the output of the ?lter also
corresponds to the input of the class A ampli?er stage
the junction points between said ?rst inductance and
said ?rst condenser, between the said ?rst and second
condensers, and between the said second condenser ‘and.
vand it is connected to the base of the PNP transistor T1
said second inductance, respectively, and a signal ampli
and also to the negative battery potential of —48 volts 30, tude limiter circuit being connected between the second
through resistor R1, and ?nally to ground at terminal
?lter circuit ‘branch and the junction point of said ?rst
P3 through resistor R2. Transistor T1 is operated in
grounded emitter fashion, and the emitter is connected to
ground through resistor R3 which is of a relatively high
value, and shunted by decoupling condenser C; also of
suitably high value. Thus, the ‘base of transistor T1 is
biased by the potentiometer termed by resistors R1 and
and second condensers.
2. A signal receiver as claimed in claim 1 wherein
said ?rst and second inductances have a ?xed inductance
value irrespective of the individual frequency to which
the signal receiver is tuned and the Q factor of the said
inductances increases substantially linearly with the fre
R2, and a transistor arrangement with closely controlled
and stabilized current gain is obtained. A 2N524 tran
sistor may be usedvlfor T1 whose collector is biased to 40 fourth condenser has a capacitance value such that the
the negative battery potential through resistor R4.
ratio between the voltage across said fourth condenser
The collector of transistor T1 is directly connected at
and the input voltage of said bandpass ?lter remains sub
terminal P4 to the base of transistor T2 which is a high
stantially constant regardless of the frequency to which
current ‘gain transistor, e.g. 0076 and which gives the
the receiver is tuned.
output signal at its collector connected to terminal P5,
' 4. A signal receiver as claimed in claim 1, wherein said
its emitter being biased to a potential of -—28 volts
limiter comprises two oppositely poled recti?ers, each
through resistor R5. Terminal P5 is connected to nega
connected between the said second circuit branch and the
tive battery through the Winding of the output relay Tr
junction of said second and third condensers.
which is shunted by a bypass condenser C5. Finally, 50
5. A signal receiver as claimed in claim 4 wherein
the bias potential of ~28 volts is obtained as shown by a
each of said recti?ers are connected in shunt with said
potentiometer constituted by the resistorsRs and R7 be
fourth condenser and wherein said recti?ers are of the
tween negative battery and ground, these resistors being
respectively shunted by the bypass condensers C6 and 0;.
When no signal is received, transistor T2 is blocked 55
as the base voltage is at lowest equal to about —24 Volts
whereas the emitter of this PNP transistor is biased to
‘_28 volts. When a signal is received and is of su?i
cient strength to counteract the reverse bias voltage, col
lector current starts to flow in T2 and the relay Tr will
be operated. A sharp increase in the D.C. output cur
rent of transistor T2 may be obtained when the input
signal reaches a predetermined value and this output cur
, rent may become practically independent of the signal
level. The arrangement may be designed so that the 65
type requiring forward biasing for conductivity.
References Cited in the ?le of this patent
Travis ______________ __ Feb. \13,
Gruen _______________ __ Oct. 25,
Buekeman ____________ __ ‘Nov. 4,
Chauvin et a1. ________ __ June 23,
Mitchell __' __________ __ Nov. 10,
Tautner _____________ __ Mar. 22,
Peterson et a1. _________ __ Dec. 5,
Waters _______________ __ Dec. 5,
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