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Патент USA US3044065

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July 10, 1962
Filed May 9. 1.957
3 Sheets-Sheet l
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July 10, 1962
Filed May 9, 1957
5 Sheets-sheaf; 2
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July 10, 1962
Filed May 9, 1957
3 Sheets-Sheet 3
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United States Patent 0 " "ice
Patented July 10, ‘1962
range, and an optimum frequency can therefore be
3044 054
selected. In the second place, the transmitted‘ signals
may extend in‘ frequency beyond the audible range there
by increasing the number of available channels.
Oswald Barber Sneath, London, England, assignor to
Multitone Electric Company Limited, London, Eng
Thus, the receivers may each produce an audio-fre
quency note of, e.g. 1-2 kcs. and these notesare of sub
stantially constant intensity over a wide range of channel
Filed May 9, 1957, Ser. No. 658,148
Claims priority, application Great Britain May 16, 1956
4 Claims. (Cl. 340—-311)
frequencies and amplitude. If the range of channel fre
quencies emitted by the transmitter is from 2-15 kcs.,
This invention relates to receivers suitable for use with
call systems comprising a central transmitter capable of
transmitting signals at a number of di?erent frequencies
and a number of receivers each selective to a particular
frequency which is different from the others, so that a
desired receiver can be called by transmitting a signal at 15
the corresponding frequency. The invention is particu—
larly applicable to, but is not limited to, receivers which
operate by audio-frequency induction.
The practical requirements for systems as described
above are that a comparatively large number of channels
be available and this in turn requires that the receiver
operating on each channel should possess a high degree
about 50 to 60 individual channel frequencies and associ
ated receivers can conveniently be accommodated within
this range, the frequency spacing between individual chan
nels varying from about 150 c.p.s. at the lower end of
the range to about 300 c.p'.s. at the higher end.
As an alternative to a relaxation oscillator, acoustic
feed-back may be used in cases in which the receiver,
produces a sound signal. To this end, a microphone may
be incorporated in the input to the oscillator so as to
produce oscillation by acoustic feedback between the
sound producer and the microphone. The microphone
and the sound producer may with advantage have simi
lar acoustic characteristics and may, for example, con
of selectivity. Difficulties have however been experienced
sist of identical miniature earpieces although the electrical
in meeting these requirements, particularly where the
impedances need not necessarily be identical.
Conven- -
receivers are to be of a very small size as is the case 25 iently, the miniature earpieces may be situated in a
where the receivers are to be carried about the person.
chamber which is partially opened to the outside air and
One particular difficulty isto design the receiver so that
which acts as a resonator.
it can discriminate between a weak signal on its own
channel frequency and a strong signal on an adjacent
feedback is that oscillation is produced at a frequency at
which the sound producer, and its resonant chamber if
The advantage of acoustic
30 used, act efficiently.
channel frequency.
In accordance with one feature of this invention, im
proved frequency discrimination is achieved by employing
a receiver having an ampli?er/limiter circuit followed by
a frequency discriminating circuit the output from which
controls the operation of a signalling device such as a
sound-producing transducer. In this arrangement the
ampli?er/limiter circuit, which may comprise two or
more stages of transistor ampli?cation, serves to bring up
to substantially the same predetermined level of magni
tude a weak signal at the frequency at which the dis
crimination circuit is tuned and a strong signal at an ad
In a preferred miniature form, the receiver is relatively
long and of round oval or rounded rectangular cross
section, and the sound-producing transducer, which is situ
ated at the top, opens directly or through a sound rein
forcing resonant cavity upwards and thus towards an ear
of the person‘ in whose pocket the receiver is. To re
, duce trouble through feedback, the coil of the frequency
selective ?lter is close to the sound-producing transducer
and the pick-up coil is as far removed as possible from
both these, e.g. at the bottom of the casing, possibly at
the side of the cell. It is also an advantage if the coil
and transducer are in a metal tube, the metal being
jacent frequency. In this way, the output from the fre
quency discriminator circuit always shows a peak, more
either ferromagnetic or of high conductivity, for example
or less pronounced according to the selectivity of the
45 aluminium; While the casing of the bottomkpart of the
circuit, at the frequency of the signal to which it is in
receiver is a non-conductor so as not _to shield the pick
tended that the receiver should respond to give an audi
Yup coil. Alternatively, the lower part of the housing
ble or other indication. Coupled with the frequency dis
may be of metal but slotted to reduce screening due to
criminator circuit is any sound producing or other signal
circulating currents.
ling circuit arranged to operate only when the frequency
The frequency selective circuit of the receiver can, if
discriminator circuit receives a near peak signal of the fre
desired, be cut out by means of a switch so that the
quency to which it is tuned. Preferably the output from
receiver functions as an audio ampli?er although, owing
the frequency discriminator circuit triggers an audio-fre
to the limited power available, it may be necessary to
quency oscillator the output of which is applied to a sound
remove it from the pocket and raise it to the ear. The
cy oscillator the output of which is applied to a sound 55 switch may be worked by pressing the top end of a
producing devicesuch as a miniature earpiece of the type
pivoted clip, the bottom end of which normally clips it
used in deaf aid apparatus. This latter arrangement con
to the pocket. It will, in general, not be necessary to ob
stitutes a further feature of the invention now to be
tain full gain from all the stages when switched as a
speech ampli?er.
Preferably the oscillator employs a transistor connect 60
Embodiments of the invention will now be described
by way of example with reference to the accompanying
ed as a relaxation oscillator which operates only when
triggered by the output from the frequency discriminator
circuit and which, when such output ceases, becomes
quiescent after a short period of oscillation.
The ad
, drawings, in which:
FIG. 1 is a circuit diagram of a receiver for use in an
audio induction type of system described earlier; and
FIGS. 2-7 are circuit diagrams showing modi?cations
of parts of the circuit of FIG. 1.
Referring now to FIG. 1, the circuit there shown con
sists essentially of the following: an audio frequency in
duction pick-up coil L1; a three-stage ampli?er constituted
vantage of this arrangement is that the frequency of the
warning signal produced is largely independent of the
frequency of the received signal which triggers the oscil
lator. The advantage of an audible warning signal whose
frequency is substantially independent of that of the
by three junction transistors T1, T2 and T3 together with
received signal is twofold. In the ?rst place, neither 70 associated resistances R1—-—R7, condensers C1 and C2 and a
the human ear nor sound-producing transducers such
transformer L2, the third stage consisting‘ of'the transistor
as earpieces are uniformly e?icient over the whole audio
T3, resistors R5, R6, R7 and transformer'vLz constituting an
ampli?cation limiting stage; a frequency selective ?lter
constituted by transformer L3 and variable condenser C3;
on L3 to adjust the frequency of the selective ?lter. Sec
ondly, the emitter of the transistor T; has a resistor R10’
connected between its emitter and the tap on L4, and an
an oscillatory circuit constituted by a transistor T4 and as
sociated transformer L4 and‘resistance R9; and a sound
producing transducer TD. In addition, the circuit of FIG.
other resistor R11 is connected between ‘the emitter and
' 1 includes a two-way switch Sand associated'resistance R8
to convert the receiver into a straight audio frequency am
pli?er'for speech reproduction. ‘
which maybe connected at will to one of a number of taps
the negative of the supply. The effect of this latter ar
rangem'ent is to maintain the emitter at a'negative poten
tial relative to ‘base in the absence of signal, and this ren~
ders a bigger signal necessary to initiate oscillation, thus
i The pick-up coil L1 consists’ of a small bobbin contain
ing laminations about 1" long. This coil may be iden: 10 making the receiver less susceptible to changes in the
tical for all frequencies in the band covered by the trans
characteristics of T4 caused by changes of temperature or
mitter, but for the maximum sensitivity it is advantageous
replacement of T4 by a transistor of slightly different char
.‘to employ fewer turns as the frequency increases. Coil
acteristics. When employing this arrangement, a lower
Llwis connected to the transistor T1 in what is functionally
step-down ratio will be required as regards the secondary
a grounded emitter circuit. The resistances R1 and R2
of L3 than is the case with the circuit of FIG. 1. Either
‘control the potential of the base and hence the current
of the two modi?cations embodied in FIG. 3 may be uti
‘flowing through R3. The emitter of T1 is connected to
lised independently.
the "base of the transistor T2, the emitter of which is fed
FIG. 4 shows a modi?cation of the circuit of FIG. 1 in
through the resistance R; shunted by the by-pass con
which the secondary windingof transformer L3 is con- ,
denser C2. The collector of T2 is coupled through the step 20 uccted :with a recti?er K andcondenser. C3 and resistance
down transformer L2 to the base of T3. The ground side
R12 so as to apply on the arrival of a signal a negative bias
of the transformersecondary is connected to the junction
' to the base of T4ywith respect to positive. The emitter of
of the potential dividing resistances R5 and R6; The re
T4 is given a negative bias with respect to base by resistors
:sistor R6 may with advantage vbe of the type having a high
.1310 and R11 as in FIG. 3. ~' In this circuit L4 is a double
wound transformer and the connections thereto are mod
negativetemperature coe?icient as otherwise the current ‘
‘through T3 will increase rapidly withtemperature. By
i?ed as shown.
adjusting the values R5 and R6 so vthat approximately half ‘
FIG. 5 shows a modi?cation of the circuit of FIG. I 7
the ‘supply voltage is dropped, across the resistance R,
according to which‘ the transformer L4 is constituted by ‘a
when strong signals are being picked up the second har
second winding on the sound producing device TD. Bias
monic production in the collector circuit of T3 is mini 30 sing arrangements for Trare as in vFIGS. 3 and 4. _ ,
mized. The collector circuit of T3 contains the limiting
While with the arrangements hereinbefore described it
‘resistance R; in series with part of the primary windingof
possible to call all receivers at the same, time bytrans
the transformer L3 which has a “Ferroxcube” which is
mitting their frequencies in turn or by. scanning the fre
quency band, this has thelimitation, where a large num
tuned by the adjustable mica condenser C3 to constitute
the frequency selective ?lter.
ber of receivers are employed, that the call signal is of
short duration relative to the’ interval at which it can be
repeated; Accordingly, it may be desirable to make all
As shown, the base of the oscillatory transistor T4 can
be ‘switched by means of switch S through the resistance
R; to [the collector of T3 and in this case speech can be re
receivers responsive to a common frequency as well as tov
ceived the receiver willthen respond to all frequencies.
The selective effect of the tuned circuit, constituted by
‘transformer L3 and condenser-C3 is not then very notice
able as it is in series with the ohmic resistance R7. Alter
nativelyLthe‘basevcan ‘be switched through the secondary
their own sparticularrlcall frequencies, this common fre
quency being employed to transmit general calls intended
for ‘all receivers as opposed‘ to one individual receiver.
_An arrangement for this purpose is. shown inYFIG. 6
which is a modi?cation of FIG. 1, the modi?cation in
‘of L3 to a. tap on the winding of the auto-transformer L4,
which isialso connected to the emitter of T4 and to the pos
itive of the supply.
- .
_, _
volved being the inclusion of a second frequency selec-'
tive circuit composed of transformer L6 ‘and’ condenser
C9_ tuned to the common frequency._ This second fre
The transducer TD connected across part of the wind
quency selective circuit can be reduced in size if a some
ing of transformer L4 is conveniently a miniature ear
piece of medium impedance, say 300 ohms at 1000 cycles.
'I'heresistance R9 shunted across L4 has a negative tem:
perature coefficient to compensate wholly or partly for the
change of feedback due to temperature variation T4. The
circuit constituted by this resistance and transformer L4
is such that the transistor T4 oscillates when there is suit-_
able negative bias on the basewith‘respect to emitter; at
the same time this circuit is such that oscillations if started
what, broad frequency band at‘the high frequency end is
allocated to general calls and méycohsishforexample,
solely of an air-cored transformer relying on self-capacity
for tuning’.
FIG. 7 shows a ,modi?cationof the circuit illustrated
‘ in FIG.VV4 in which acoustic feedback isvemployed. A
' microphone M is arranged near the transducer ID so as
' arehnot maintained in the absence of ‘bias. The signal
voltage from th'e'secondaryiof transformer L3, however, ' ’
to pickv up soundvtherefrom and feedback electrical en
ergy into the base circuit of'the transistor T4 thus pro
ducing oscillation.
As a guide‘to the order of magnitude of the compo
causes oscillation to occur for so long ‘as the signal per
nents it may be mentioned that a coil L1. of 2,000 turns
sistsi The resistance R9 is connected in the particular" 60 of 48 S.W.G. on a core of approximately 1" long has
manner shown in order to achieve a suitable matching to
been found satisfactory. L2 may he a normal transistor
an available value of thermally variable resistor. '
interstage transformer.’ For L3 a transformer having
FIG. 2 shows a modi?cation of the pick-up circuit of
7 3175 turns of 45 S.W.G. tapped at 3069, and a secon_d~
FIG. 1, such modi?cation residing in the use of two pick
up transformersyLLand L5 set at an angle to each other. 65 ary of 80 turns of 45 S.W.G. has been found to be‘ suit
able for frequencies around 2,200 cycles per second. A
I45 has a condenser Cr connected in series with it, this con
denser being of suchvalue-as ._-to have approximately the
winding of 500 turns of 37 S.W.G. tapped at 480 and a
same impedance as L5 at the frequency to which the re
secondaryof 12 turns of 37 S.W.G. has been found to be
ceiver is tuned; hence, the impedance of L1 and L5 being
suitable for frequencies around 14,000 cycles per second.
largely inductive, the signal from _L1"is out of phase at the 70 Intermediate windings would ‘be used for intermediate
base-pr T1 with that'rof L5 and C4 in’ series‘.
FI ' .
L; may be wound on a core similar to an
hows two modi?cationspof thevcircuits, associe
interstage transformer and may consist of 1500 turns of
the with transistors T3 and T4 in FIG. 1. Firstly the 'vari- I 46 S.W.G. tapped at 400 and 1200, the outside end being
sue‘ condenser C3 is replaced by a hired condenser'C-IS but
two additional ?xed condensers C6 and C7 are employed
that which is shown in the drawing as connected only to
R9. In "conjunction with the values of L3 mentioned
above, R7 would be chosen to give the selectivity required
2. A small portable battery-operated selective receiver
for electromagnetic signals, said receiver comprising a
and would be of the order of 10,000 ohms.
While it is only necessary that the receivers for differ
ent frequencies should differ in the windings of the in
ductance and/or the capacity of the tuning condenser of
the selective circuit, greater sensitivity may be obtained
’ magnetic pick-up coil, an ampli?er arranged to amplify
signals picked up by said pick-up coil, said ampli?er ter
minating in a limiter stage, a frequency-selective ?lter
immediately coupled to the ampli?er, an oscillatory cir
cuit including a transistor having its base normally insuf?
and the coupling transformer also.
ciently negative with respect to its emitter to be capable
In a modi?ed construction of receiver according to the.
of building up spontaneous oscillation but arranged to be
invention, the parameters of the oscillator circuit are such 10 set in oscillation by an incoming signal, means directly
that oscillation, once started, will continue until it is in—
connecting said ?lter in triggering relation to said oscil
terrupted, for example by shorting the feedback from the
latory circuit, and a signal device arranged to be oper
sound producer either by a manual switch or by a switch
ated by the output of said oscillatory circuit.
operated by the vibration of the receiver when it is moved.
3. A receiver as claimed in claim 2, wherein the sig
by making changes to the windings of the pick-up coil
A modi?ed form of the invention may be constructed 15 nalling device consists of a sound-producing transducer.
employing a loudspeaker of normal type for placing in a
4. A small portable battery-operated selective receiver
room to call a person in such room in which case the cir
for electromagnetic signals, said receiver comprising a
cuit up to the frequency selective ?lter may be the same
magnetic pick-up coil, an ampli?er arranged to amplify
but the triggered oscillator will be of greater power or,
signals picked up by said pick-up coil, said ampli?er ter
alternatively, may be similar to the embodiment described 20 minating in a limiter stage, a frequency-selective ?lter im
but followed by a quiescent amplifying stage. In either
mediately coupled to the ampli?er, an oscillatory circuit
of these arrangements the no-signal current need be little
including a transistor and a transformer, means coupling
if any higher than in the embodiment more fully de- _
the output of the oscillatory circuit to the circuit between
scribed with reference to FIG. 1.
In another form suitable for use by the hard of hear
the base and emitter of said transistor in the sense to pro- 25 vide positive feedback, means normally biasing said base
ing the sound-producing device may be replaced by a
negatively with respect to the emitter to an extent insuf
transducer producing mechanical vibration which may
?cient to cause spontaneous oscillation but such that said
be appreciated either by bone conduction or by skin sen
oscillatory circuit is set in oscillation by an incoming sig—.
sation. Alternatively, the output voltage from the oscil
nal, means directly connecting said ?lter in triggering re
lator may be stepped up and applied to the skin between 30 lation to said oscillatory circuit, and a signal device ar
two small contact electrodes to produce electrical stimu
ranged to be operated by the output of said oscillatory
lation on receipt of a signal.
Although the invention has been particularly described
in its application to systems operating by audio frequency
induction, it will of course be appreciated that the inven
References ‘Cited in the ?le of this patent
tion also ?nds application to modulated or unmodulated ’
radio and higher frequency systems and to systems in
which the central transmitter station is connected by wires
to its associated receivers.
I claim:
1. A selective receiver for electromagnetic signals com
prising a magnetic pick-up coil, an ampli?er arranged to
amplify signals picked up by said pick-up coil, said am
pli?er terminating in a limiter stage, a frequency selec
tive ?lter immediately coupled to the ampli?er, an oscil 45 2,810,080
latory circuit capable of self-oscillation at a frequency
substantially independent of the signal frequency when
triggered by a signal from said frequency selective ?lter,
means directly connecting said ?lter in triggering relation
to said oscillatory circuit, and a signal device arranged to
be operated by the output of said oscillatory circuit.
Ware ________________ __ Dec. 6,
Nichols _____________ __ Apr. 4,
Wurmser ____________ __ Feb. 3,
Moore ______________ __ May 17,
Selinger ____________ __ Nov. 22,
Bostwick ____________ __ Mar. 3, 1953
Stanley ______________ __ July
Herzog _____________ _._ Aug.
Truesdale ___________ __ Oct.
Crow et al. __________ __ Aug.
Scantlin ____________ __ June 14, 1960
France ______________ __ Mar. 3, 1954
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