US3699477A - Signal generator - Google Patents

Signal generator Download PDF

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Publication number
US3699477A
US3699477A US178126A US3699477DA US3699477A US 3699477 A US3699477 A US 3699477A US 178126 A US178126 A US 178126A US 3699477D A US3699477D A US 3699477DA US 3699477 A US3699477 A US 3699477A
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Prior art keywords
frequency
series
resistive elements
oscillator
principal
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Expired - Lifetime
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US178126A
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English (en)
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Lynn J Mckell
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AT&T Corp
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Bell Telephone Laboratories Inc
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04M—TELEPHONIC COMMUNICATION
    • H04M1/00—Substation equipment, e.g. for use by subscribers
    • H04M1/26—Devices for calling a subscriber
    • H04M1/30—Devices which can set up and transmit only one digit at a time
    • H04M1/50—Devices which can set up and transmit only one digit at a time by generating or selecting currents of predetermined frequencies or combinations of frequencies

Definitions

  • ABSTRACT ries with two or more parallel resistive elements in at least one filter.
  • Each signal consists of one of a group of frequencies from a relatively low frequency band and one of a group of frequencies from a relatively high frequency band.
  • the frequencies in each group hereinafter referred to as the principal frequencies, are essentially uniformly separated from one another, and each dual frequency signal is indicative of an individual digit or other code symbol in accordance with a now standardized multifrequency code.
  • One approach to this problem is to have the signal generator transmit an invalid calling signal when two pushbuttons are simultaneously operated.
  • An invalid calling signal that is, a signal that differs sufficiently from a valid calling signal so as to be distinguishable by the central office receiver, is ignored. Therefore, if the user does not immediately recognize that he has operated two buttons simultaneously, the lack of response from'the central office after he has finished dialing causes him to redialthe telephone number. The user is slightly inconvenienced, but because dialing is fast and easy, little time is lost.
  • a significant advantage of this solution is that it offers the potential of utilizing the double push signals for transmitting data other than that used for calling.
  • the signal generator disclosed in US. Pat. No. RE 25,507 issued to L. A. Meacham on Feb. 7, 1964, and now in common use in the TOUGH-TONE dial, has these features. It comprises a single transistor oscillator having an inductively coupled feedback network in cluding a pair of tank circuits that are selectively tuned to the principal frequencies by the operation of individual pushbuttons of the dial. When, however, two buttons are actuated simultaneously, a single, rather than a dual, principal frequency signal is generated.
  • the single frequency signal is readily distinguishable from a dual frequency signal, but it is somewhat deficient insofar as serving as a data signal. This is because a single frequency signal is fairly susceptible to duplication by ordinary speech and background sounds.
  • Such a signal generator is disclosed in US. Pat. No. 3,424,870 issued to R. L. Breedien and R. M. Rickert on Jan. 28, 1969. It comprises a pair of interconnected transistor oscillators each having a twin'T notch filter feedback network.
  • One T of each notch filter consists of a pair of series resistances and a shunt capacitance while the other T consists of a pair of series capacitances and a shunt resistance.
  • the values of the resistances and capacitances are all fixed except for one series resistance in each notch filter, and both of these series resistances are selectively varied by the operation of the pushbutton dial to tune the oscillators to the two principal frequencies associated with the operated button.
  • each of the variable resistances consists of a plurality of parallel resistors, a single one of which is connected to the associated twin-T configuration responsive to the actuation of each individual pushbutton.
  • a pair of the resistors in one or both of the notch filters are connected in. parallel.
  • One or both the oscillators are thereby tuned to generate a nonprincipal frequency, resulting in an invalid calling signal.
  • the double push signal is again deficient insofar as serving as a data signal.
  • all of the double push signals are dual frequency signals, the dual frequencies of some of the signals are in essentially the same frequency band, and the frequenciesof one signal are so close that for all practical purposes they are indistinguishable.
  • some of the other signals have one frequency outside of the choice frequency portion of typical voice band facilities.
  • the signal generator of this invention is basically the same as the last-described signal generator and therefore it can be manufactured using integrated circuit techniques. However, it requires less resistance, and the double push signals are dual frequency signals that are readily distinguishable from valid calling signals and at the same time are readily usable as data signals.
  • the signal generator comprises a pair of interconnected transistor oscillators having twin-T notch filter feedback networks in which the variable resistance in each filter consists of a plurality of resistive elements arranged in a series parallel configuration.
  • This series parallel configuration is such that when a single button is actuated, at least two resistive elements are connected in series in each filter and when two buttons are actuated simultaneously, at least one resistive element is connected in series with two or more parallel resistive elements in at least one filter.
  • This arrangement provides the latitude to select the resistive elements so that when a single button is actuated, the oscillators are tuned to generate a pair of principal frequencies and when any two buttons are actuated simultaneously, at least one oscillator is tuned to generate a non-principal frequency that is separated from a principal frequency by less than the uniform separation between principal frequencies.
  • the resistive elements are selected so that when any two adjacent buttons are actuated simultaneously one or both oscillators are tuned to generate a non-principal frequency that is separated from a principal frequency by approximately half the uniform separation between principal frequencies.
  • FIG. 1 is a simplified circuit diagram of a key-controlled multifrequency signal generator in accordance with the present invention.
  • FIG. 2 is a table showing the 49 unique frequency combinations that can be generated by the signal generator of FIG. 1.
  • the key-controlled multifrequency signal generator of this invention while not limited thereto, is employed with particular advantage as a telephone call transmitter, and, for purposes of convenience, it will be described in terms of the disclosure in the above-mentioned Breeden-Rickert patent, identical reference numbers being used where reliance is placed on that disclosure for particulars.
  • the signal generator includes a dial DL having a plurality of pushbuttons arranged in a rectangular array of rows and columns.
  • the first three columns of pushbuttons carry the digits 1 through and the symbols and# and these pushbuttons, are now standard on all push button dials.
  • the fourth column of pushbuttons carry the letters A through D, and while these pushbuttons are not standard on pushbutton dials, they are available when needed to provide additional signalling beyond that available with the standard dial.
  • each row actuate one of a group of normally open low frequency selecting switches Ll through L4, while the pushbuttons in each column actuate one of a group of normally open high frequency selecting switches 'Hl through H4.
  • any pushbutton when depressed, it actuates one low frequency selecting switch and one high frequency selecting switch, and the pair of switches actuated by each pushbutton is unique.
  • each pushbutton when depressed actuates a common switch CS.
  • a pushbutton dial of this type is disclosed in U. S. Pat. No. 3,479,470, issued to .l. H. Ham, Jr., on Nov. 18, 1969.
  • the switches L1 through L4 serve to tune a low frequency oscillator OL comprising a multistage transistor amplifier 501 and a twin-T notch filter circuit connected between an intermediate output stage and the input of the amplifier.
  • the twin-T notch filter circuit includes a capacitance Cl and a capacitance C2 connected in series and shunted by a resistance R3 as the first T, and a resistance network RNl and a resistance R2 connected in series and shunted by a capacitance C3 as the second T.
  • the switches H1 through H4 serve to tune a high frequency oscillator OI-I comprising a multistage transistor amplifier 601 and a twin-T notch filter feedback circuit.
  • the notch filter circuit includes series capacitances C10 and C20 and a shunt resistance R30 as the first T, and a resistance network RN10 and a resistance R20 connected in series and a shunt capacitance C30 as the second T.
  • the amplifiers 501 and 601 are described in detail in the Breeden-Rickert patent, and, as set forth therein, the amplitude of the output of each amplifier is limited by a circuit in shunt with a series capacitance in each feedback path, the shunt circuit comprising a blocking capacitor connected in series with a pair of oppositely poled parallel diodes. Furthermore, the output of the amplifiers 501 and 601 is connected to tip lead T of a telephone line by way of a path that includes terminal 701 and normally open common switch contacts CS and a bias network 801 bridged across tip and ring leads T and R provides a common source of biasing potential for the transistors of both amplifiers.
  • a conventional voice network VN is also bridged across leads T and R, and the leads, respectively, include normally open switch hook contacts SH, and 8H that are closed when the telephone handset is removed from its cradle.
  • twin-T notch filters signals of a particular frequency are attenuated to a maximum degree while adjacent frequencies are attenuated to a lesser degree, resulting in a notch in the plot of the frequency spectrum.
  • a l phase shift occurs for a signal oscillating at the notch frequency, while signals only slightly displaced from the notch frequency depart markedly from this phase shift. Consequently, in the feedback loop of an oscillator whose amplifier has shifted the phase by l80 from the input to the output, an additional l80 phase shift produces regenerative feedback at the notch frequency, and the sharp departure at the other frequencies serves to suppress the non-notch frequencies.
  • Notch filters areparticularly suited for tuning oscillators that generate only one narrow frequency band.
  • an oscillator requires a circuit that exhibits a common phase departure effect at each of the particular frequencies. This requirement is met in accordance with this invention by means of the resistance networks RNl and RN10 which, respectively, incorporate the frequency selecting switches L1 through L4 and H1 through H4.
  • Resistance network RNl comprises resistive elements R101, R102, and R103 connected in series, a re sistive element R104 connected to the juncture of re sistive element R101 and R102, a resistive element R105 connected to the juncture of resistive elements R102 and R103, and resistive elements R106 and R107 both connected to the end of resistive element R103 opposite to its juncture with resistive element R102.
  • the resistive elements R104 through R107 are connected in parallel with one another, and each shunt path includes one of the normally open frequency selecting switches L1 through L4, each switch serving to connect selected resistive elements in the network into the feedback circuit of the low frequency oscillator DL.
  • the resistance network RN is the same as the resistance network RNl, comparable resistive elements being identified by the addition of one hundred to the reference numberQSuffice it to say that each of the normally open frequency selecting switches H1 through H4 is connected to an individual shunt resistive element in the networkRNlO and when actuated serves to connect selected ones of the resistive element R201 through 207 into the feedback circuit of the high frequency oscillator OH.
  • the signal generator is permitted a deviation of i 1.5 percent from these principal frequencies, this being an economically feasible tolerance that can be maintained in a telephone set, and to allow for the reception of these frequencies by the central office receiver, a recognition bandwidth of approximately t 2.5 percent is provided there.
  • a recognition bandwidth of approximately t 2.5 percent is provided there.
  • the resistance networks RNl and RN10 of this invention permit the resistive elements to be selected so that when any single frequency selecting switch is closed, a frequency deviating by less than 1.5 percent from a principal frequency is generated, and when any wo switches are closed, a frequency deviating by more than 2.5 percent but less than 7.5 percent from a principal frequency is generated. This is possible because whenever one frequency selecting switch is closed, at least two resistive elements are connected in series, and whenever two frequency selecting switches are closed, at least one resistive element is connected in series with at least two resistive elements in parallel. This provides the latitude to select the values of resistive elements so as to tune theoscillators to thedesired frequencies.
  • the resistive elements can be selected so that when two adjacent frequency selecting switches are closed, the frequency generated deviates by approximately 5 percent from a principal frequency, or, in other words, is a half-step removed from a principal frequency.
  • a frequency is capable of being used as a data signal, since it is separated far enough from the adjacent principal frequency so as to be recognized as a distinct signal by a receiver having a recognition band of i 2.5 percent for the principal and half-step frequencles.
  • 49 distinct dual frequency signals can be generated by the signal generator of this invention when tuned to generate such half-step frequencies.
  • the 33 data signals provided in addition to the sixteen calling signals are all generated by simultaneously depressing a pair of adjacent pushbuttons on the dial DL.
  • the dual frequency signal comprises a principal low frequency and a half-step high frequency.
  • the switch L1 is closed in the resistance network RN1 to connect the resistive elements R101, R102, R103, and R107 in series and tune the low frequency oscillator OL to the principal frequency of 697 Hertz.
  • switches H1 and H2 are closed in the resistance network RN10 to connect the resistive elements R201, R202, and R203 in series with the resistive elements R206 and R207 connected in parallel, tuning the high frequency oscillator OH to the half-step frequency of 1,405 Hertz.
  • the dual frequency signal comprises a half-step low frequency and a principal high frequency.
  • the switches L2 and L3 are closed in the resistance network RNl to connect the resistive elements R101 and R102 in series with resistive elements R103 and R106 connected in parallel with resistive element R105, whereby the low frequency oscillator OL is tuned to generate the half-step frequency of 897 Hertz.
  • the switch H3 is closed in the resistance network RN10 to connect resistive elements R201, R202, and R205 in series and tune the high frequency oscillator OH to the principal frequency of 1,477 Hertz.
  • the dual frequency signal comprises both a half-step low frequency and a halfstep high frequency.
  • the switches L3 and L4 are closed in the resistance network RNl to connect the resistive element R101 in series with the resistive elements R102 and R105 connected in parallel with the resistive element R104.
  • the low frequency oscillator 0L is thereby tuned to generate the half-step frequency of 990 Hertz.
  • the switches H1 and H2 are closed in the resistance network RN l0, tuning the high frequency oscillator OH to generate the half-step frequency of 1,405 Hertz in the same manner as described above.
  • these principal and halfstep frequencies are achieved with. the following values for the resistances and capacitances in the twin-T notch filter circuits:
  • each notch filter circuit is 281.7 kilohms.
  • the total resistance for each notch filter circuit in the Breeden-Rickert signal generator when it is tuned to generate the principal frequencies is 394.4 kilohms.
  • a signal generator comprising:
  • the circuit being coupled to the oscillator by each of the switches to produce a different principal frequency, the principal frequencies having a preselected maximum separation;
  • the circuit being coupled to the oscillator by any two of the switches to produce a frequency separated from the principal frequencies produced by either of the two switches, the separation from one of these two principal frequencies being less than the preselected maximum separation.
  • a signal generator as in claim 1 wherein the frequency determining circuit has only resistive and capacitive elements incorporated in a notch filter configuration, some of the resistive elements being arranged in a series parallel resistance network including the frequency selecting switches, the switches connecting selected ones of the resistive elements into the notch filter to provide frequency control.
  • the frequency determining circuit comprises a twin-T notch filter, one of the T configurations including a pair of series capacitances and a shunt resistance and the other.
  • T configuration including a pair of series re sistances and a shunt capacitance, one of the series resistances comprising a plurality of resistive elements arranged in a series parallel network including the frequency selecting switches, the switches connecting selected ones of the resistive elements to the other series resistance to provide frequency control.
  • a signal generator as in claim 5 wherein the series parallel network comprises three resistive elements connected in series, a first shunt resistive element connected to the junction of the first and second series resistive elements, a second shunt resistive element connected to the junction between the second and third resistive elements, and third and fourth shunt resistive elements connected to the other end of the third series resistive element, the frequency selecting switches respectively connecting each of the shunt resistive elements to the other series resistance.
  • a multifrequency signal generator comprising:
  • each oscillator including;
  • the feedback circuit connected between the output and the input point, the feedback circuit comprising a pair of T circuits connected in parallel, one T circuit consisting of a pair of series capacitances and a shunt resistance and the other T circuit consisting of a pair of series resistances and a shunt capacitance, one of the series resistances comprising a plurality of resistive elements arranged in a series parallel network;
  • keying means including a plurality of pushbuttons, each pushbutton when actuated connecting at least two resistive elements in series in each series parallel network to tune the first oscillator to one of a plurality of principal frequencies within the low frequency band and tune the second oscillator to one of a plurality of principal frequencies within the high frequency band, the principal frequencies in each frequency band being approximately uniformly separated from one another, and each pair of pushbuttons when actuated connecting at least one resistive element in series with'two parallel resistive elements in at least one series parallel network to tune at least one oscillator to a frequency separated from a principal frequency by less than the uniform separation between principal frequencies.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Push-Button Switches (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
  • Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)
  • Tone Control, Compression And Expansion, Limiting Amplitude (AREA)
  • Interface Circuits In Exchanges (AREA)
  • Electronic Switches (AREA)
  • Telephonic Communication Services (AREA)
US178126A 1971-09-07 1971-09-07 Signal generator Expired - Lifetime US3699477A (en)

Applications Claiming Priority (1)

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US17812671A 1971-09-07 1971-09-07

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US3699477A true US3699477A (en) 1972-10-17

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US178126A Expired - Lifetime US3699477A (en) 1971-09-07 1971-09-07 Signal generator

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US (1) US3699477A (fr)
JP (1) JPS5228522B2 (fr)
BE (1) BE788462A (fr)
CA (1) CA958083A (fr)
DE (1) DE2243625C3 (fr)
ES (1) ES406799A1 (fr)
FR (1) FR2152669B1 (fr)
GB (1) GB1411418A (fr)
IT (1) IT965227B (fr)
NL (1) NL7212067A (fr)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
USB392753I5 (fr) * 1972-09-15 1975-01-28
US4000379A (en) * 1974-06-14 1976-12-28 Mitel Canada Limited Tone generator
US4110568A (en) * 1976-10-08 1978-08-29 Gte Atea, S.A. Dual tone multifrequency generator
US6377128B1 (en) * 1999-08-26 2002-04-23 Cypress Semiconductor Corp. Circuitry, architecture and method (s) for phase matching and/or reducing load capacitance, current and/or power consumption in an oscillator
WO2013074737A1 (fr) * 2011-11-18 2013-05-23 Regan Edward D Procédé et dispositif de numérotation dans un système téléphonique

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5461804A (en) * 1977-10-26 1979-05-18 Mitsubishi Electric Corp Information transmitter

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3349350A (en) * 1964-12-04 1967-10-24 Otto E Rittenbach Frequency step filter
US3424870A (en) * 1965-09-14 1969-01-28 Bell Telephone Labor Inc Multifrequency signal generator for tone-dialed telephones

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3349350A (en) * 1964-12-04 1967-10-24 Otto E Rittenbach Frequency step filter
US3424870A (en) * 1965-09-14 1969-01-28 Bell Telephone Labor Inc Multifrequency signal generator for tone-dialed telephones

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
USB392753I5 (fr) * 1972-09-15 1975-01-28
US3916341A (en) * 1972-09-15 1975-10-28 Philips Corp R-c feedback oscillator with plural frequency selecting switches producing output signal only when a single switch is closed
US4000379A (en) * 1974-06-14 1976-12-28 Mitel Canada Limited Tone generator
US4110568A (en) * 1976-10-08 1978-08-29 Gte Atea, S.A. Dual tone multifrequency generator
US6377128B1 (en) * 1999-08-26 2002-04-23 Cypress Semiconductor Corp. Circuitry, architecture and method (s) for phase matching and/or reducing load capacitance, current and/or power consumption in an oscillator
WO2013074737A1 (fr) * 2011-11-18 2013-05-23 Regan Edward D Procédé et dispositif de numérotation dans un système téléphonique
US8605889B2 (en) 2011-11-18 2013-12-10 Edward D. REGAN Method and device for encoding characters and routing calls using expanded dual-tone multi-frequency (DTMF) encoding scheme

Also Published As

Publication number Publication date
NL7212067A (fr) 1973-03-09
CA958083A (en) 1974-11-19
DE2243625A1 (de) 1973-03-15
GB1411418A (en) 1975-10-22
ES406799A1 (es) 1975-10-16
JPS4837005A (fr) 1973-05-31
BE788462A (fr) 1973-01-02
DE2243625B2 (de) 1974-08-29
JPS5228522B2 (fr) 1977-07-27
DE2243625C3 (de) 1975-04-17
IT965227B (it) 1974-01-31
FR2152669A1 (fr) 1973-04-27
FR2152669B1 (fr) 1976-03-12

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