US2282129A - Alternating current signaling system - Google Patents

Description

y 5, 1942- B. HADFIELD 2,282,129

ALTERNATING CURRENT SIGNALING SYSTEM 2 Filed April 22, 1939 I N V5 701?. BERT RAM MORTON HADFIELD A TTOR/VEY- Patented May 5,

UNITED T STATES PATENT OFFICE .2 2.120 ALTERNATING CURRENT SIGNALING I srsrm psi-mm ma... madam Alecia aesignor to The present invention concerns improvements I in or relating to alternating current signaling systems and more particularly to the reception of alternating current signals .in the form of pulses, whether of cyclic or non-cyclic character. and has for its object the translation of such signals into equivalent direct .current pulses with the minimum of distortion.

Such signaling systems are used where the transmission of direct current pulses is either impossible or only possible with serious transmission distortion.

According to the invention a frequency multiplying arrangement is provided for the purpose of increasing-the frequencyof the incoming signals, and thereby reducing the time of response of the tuned receiver.

According to a further feature of the inven- England,

ddlelex, ted Electric laboratories- Ina, M0, 111., a corporation of Delaware Application April :2, ms. Serial at. 209,524

In Great Britain May 1, ms

Claims. (01. 179-16) with the result that the incremental and decremental periods on the resonant circuits extend over a considerable portion of the pulse and can give rise to further distortion.

I It can be shown, that if distortionless reproduction is required from such an alternating voltage envelope whose incremental and decretion, use is also made of the frequency multiplier for reducing the time ofrelease, and thereby reducing distortion in the operation of the receiver.

The invention will be described with specific refer'erence to one such system, namely, the

transmission of signaling and dialing impulses over telephone lines. As is well known, it is common to transmit such impulses over a trunk line from one exchange to another to control automatic switching apparatus in the distant exchange to set up desired connections. They are employed also for various signaling and supervisory purposes. The choice of the frequencies used for these impulses in a telephone system is restricted to a band extending from some 500 cycles to 1000 cycles, partly on account of the use by other services of the ranges outside this band, and partly because the transmission characteristics of such lines are most favourable within this band. For example, frequencies of 600 or 750 cycles are commonly used for these purposes, the latter being considered more favourable for dialing.

Such frequencies are not, however, eminently satisfactory for dialing, since, in, the case of 750 mental periods are of the same nature but of inverse characteristics, the ensuing apparatus must effectively operate and release at points on these periods where the voltage has reached a value equal to one half the maximum. As the inception of the incremental period corresponds to the beginning of the applied pulse of the signaling current, then the transmission time due to the resonant circuit is the time required for the incremental period (or decremental period) to attain half its maximum value, Furthermore, it can be shown that for a resonant circuit of given magnification factor, and a given ratio between the desired operational bandwidth and the frequency, this transmission time is inversely proportional to the resonant frequency. -By magnification factor is meant the fraction where w=21r frequency, L the inductance in henries and r the effective series resistance when tuned with a capacity of C farads; this fraction being generally referred to as Q.

It will be seen therefore, that if the eifective response of the ensuing apparatus is acfiusted for distortionless reproduction at the resonant frequency in the above'manner, by means such as a direct current bias of half the maximum peak response at the resonant frequency, than as the input frequency is altered from resonance the reproduction will be maintained until the response of the resonant circuit has fallen by one half. Over this range of frequencies however, the reproduction will be progressively distorted by an amount depending to a large extent on the initial transmission time. Hence, if the response frequency of the resonant circuit can be increased, the distortion with frequency variations within the desired bandwith can be progressively reduced.

It will be appreciated that the line frequency may be multiplied at the input of the receiver by any well-known system such as a frequency doubier or tripler, used singly or in series with or without thermionic valve amplifiers, until a frequency is attained where the transmission'time and the associated distortion are conveniently time,

' direct current bias, about one-half the peak tfrequencyresponse of the, resonant cir-.

W vcuit,rlserivedfi'ornthleline frequencyandthereof value proportional to the line level and "the with'the resonant circuit voltage to the ensuing t circuit response, is applied in series apparatus of any well-known type which eflectively operates and releases at the same voltage, due allowance having been made for the operating level of such apparatus.

In this manner, multiplying m -mm ming frequency also multiplies the bandwidth by the same amount, so that the postulated constancy of the ratio between the bandwidth and frequency is maintained. likewise, variations in input level, above the initial response required on the resonant circuit to produce distortionless :re-

'production. are automatically com for by the adiusted ratio of resonant circuit voltage 'to'direct current bias.

Takingthespecificcaseofalinecurrentof 750 cycles per second, this applied by a suitable transformer to the first frequency multiplying stage. This may consist of a frequency doubler. such as the well-known full wave rectifier using copper oxide-elements or a push-pull valve rectifier stage whose anodes are strapped together, or a frequency tripler, such as an inductance with an easily saturatable magnetic core. The output at twice or three times the input frequency is utilised by a resonant circuit at this frequency to feed another frequency doubler or tripler stage, or to operate the ensuing apparatus. It may be arranged to tune each multiplying stage in order to reduce the input frequency component to the negligible quantities. In this manner the input of 750 cycles canrbe successively converted to 1500, 3000, 6000 cycles, etc.

The direct current component is preferably derived from a point in the frequency multiplyingstages where a reasonably uniform frequency response is obtainable. This may readily be effected in the case of a rectifier frequency doubier, by making use of the direct current component on a series resistance. This component is in the form of full wave rectified pulses and must be smoothed before being applied for the purpose of biasing. a It will be realised from the above, that the ditothiafrequencyandrespmdsthereto,

' latter will discharge slowly the back rerect current bias must be present to its full extent at or before the transmission time of the resonant circuit and must remain until this time has elapsed at the cessation of the input signal pulse. As the bias is generated by the signal pulse it must have a small charging time constant and a longer decay time constant. In addition, the eifective bias applied to the ensuing apparatus must be less than that generated by an amount equal to the minimum operating level of the ensuing apparatus, as otherwise the postulated operation at the half maximum amplitude of the resonant circuit will not be maintained at all input levels. In order to achieve both of these characteristics the generated bias (at half maximum resonant circuit amplitude) is fed through a rectifier in the forward direction and via an opposing bias equal to the minimum operating level of the ensuing apparatus to a condenser across whose terminal is connected a further rectifier in the reverse direction. Thus the generated bias will not be effective until it assault a wed! the pposin bias. whilst the latter is Y prevented from appearing on the condenser when no signal is applied to the receiver, by the combined action of the two rectifiers: likewise when the generated bias exceeds the opposing bias it will charge the condenser rapidly, and the sistances of the rectifier-s when the si nal ceases, providing an eifective smoothing action in addition.

The drawing illustrates by way of example a specific embodiment by which the foregoing result may be achieved when the input signaling frequency is of the order of 500 to 1009 cycles such as is used for telephone systems.

Inthedrawingthesignaiingcurrentsarerevceived over the conductors ii and '12. These may be connected to a telephone line over which is fed the signaling current only or over which speech frequency may also be fed. TI, TI and TI represent transformers appropriately designed for the relative signal frequencies. T3 is tuned by the condenser C3 to the multiplied signal frequency. VI represents a frequency doubling arrangement employing a'pair of valves. RD represents a second frequency doubling arrangement'employing metal rectifiers. R represents a resistance in the output circuit of the frequency doubler RD. RI and R2 represent rectifiers for controling the potential across the condenser C and bias for the valve V2. P represents a potentiometer and VB. represents the connection to whatever responding device or circuit it is desired to control over the plate of valve V2. This device or circuit forms no part of the present invention, and may be any suitable valve relay arrangement adapted to be operated by the impulses received over the conductors ii and 12.

The operation is as follows:

The frequency of all currents received over lines ll and Z2 is doubled by the valve doubler VI so that a signaling frequency of say 750 cycles becomes 1500 cycles. These currents are again doubled in frequency by the metal rectifier doubier RD thus bringing a line signaling frequency at, say 750 cycles, up to 3000 cycles, the latter frequency then being used to energise the resonant circuit T3CI. The metal rectifier doubler RD also creates a potential across the resistance R which serves to provide a negative D. C. bias in opposition to the effect on the grid of the valve VI of the positive half-cycles of the alternating voltage on T3C3.

The tapped portion of the D. C. voltage on R is adjusted to be equal to one half the maximum resonant circuit amplitude in order to satisfy the before-mentioned relationship for distortionless reproduction, but as also mentioned before must not appear on the grid of V2 until it exceeds the operating level of valve V2, in order that thereafter operation and release of valve VI may continue to take place at the half peak resonant response of a resonant circuit TSCI. In addition this D. C. bias voltage must appear on the grid of valve V2 at or before the attainment of the half peak resonant response to an input signal and be maintained until at least this time has elapsed after the cessation of the input signal pulse. This time interval has been previously referred to as the transmission time of the circuit.

The application and control of the required signal bias voltage on resistance R to the grid of valve V2 in the manner described above can This bias is such as normally to prevent Operation of valve V2 (and consequently of the subse- 'quent valve/relay circuit), and exceeds the bias at which valve V2 would normally Just operate by an; amount pl. Consequently pi can be termed the operating level of valve V2. The rev quired portion of the signal bias voltage on resistance R is denoted by P2 and tends rapidly to charge condenser-C via the forward. direction of R2 so that a negative signal bias appears on the grid of valve V2. in so doing the signal bias must first overcome the opposing bias voltage at pi and if the latter is made equal to the operating level of V2, the desired condition for maintenance of action at the half peak resonant response of resonant circuit TSCI is attained.

Furthermore at the cessation of the input signal pulse the net signal bias on condenser C will tend to remain at substantially the same value owing to'the high discharge path resistance presented by the back resistance of rectifier R2. The second requirement of rapid application and slow removal of the net signal bias voltage to the grid of valve V2 is therefore met.

In practice it"is desirable to place a second rectifier RI across condenser C, in the manner shown. The back resistance of the rectifier R2 will not in general (if of the copper oxide type) be comparable with the insulation resistance of condenser C and therefore in the non-signal condition the potentiometer bias pl will appear as a positive grid bias on condenser 0, thus diminishing the postulated static bias on valve V2. By connecting rectifier RI in the manner shown no such positive grid bias can appear on condenser C and little effect is produced onthe negative signal bias desired on condenser C. In this manner the rectifier RI effectively shunts condenser C to any voltage other than the desired net signal bias which can be built up rapidly through R2 but will decay slowly via the back resistances of rectifiers RI and R2. Moreover, it should be noted that this controlling circuit also provides adequate smoothing of the fullwave rectifier potential on resistance R, as applied to the grid of valve V2.

It will be seen that as each multiplying stage is made most responsive to the multiplied frequency,- then the receiver will only effectively respond to the given line signaling frequency. In addition, since the bias is generated from the line frequency, or its equivalent, then the system described automatically becomes commerprimary and secondary load impedances.

Since the eifect of the system described is to tend to give an operating alternating current envelope closely approximating to the input frequency pulse envelope. by making the transmission time small, then it is apparent that high speed repetition and short break periods can be utilised. The system is therefore found capable of providing dialling impulses with little distortion.

The system described is not limited to the use of one signaling frequency but can be used with a plurality of frequencies provided each frequency. is treated in the manner described. Similarly, the system described is applicable to other forms of signaling using alternating ourcially immune from operation by speech, on i the well-known principle that the latter contains only a small percentage of the signaling frequency, and a large percentage of other frequencies. Thus all speech frequencies will produce a biasing effect which will substantially overcome the response of the ultimate resonant circuit to the smaller percentage of signaling frequency in speech.

In applying the system described, it is found that each frequency multiplying, stage introduces some considerable loss of energy as between the input and the multiplied frequencies. This results effectively in a loss of voltage, which may be overcome by the employment of a transformer or auto-transformer winding on the resonant circuit or circuits, to overcome, this rents such as single or multi-channel voice frequency telegraphs with corresponding improvements in the response of the receiver.

I claim:

1. In a grid control arrangement for a. thermionic valve, a signal receiving circuit connected to said grid. a resistor in said circuit, a source of biasing potential having one side connected to the cathode of said valve, a bridge across said source comprising, in series, a unidirectional conductive device, said resistor and a second unidirectional conductive device, the first of said devices being connected in said bridge in a conductive direction with respect to said source and the second of said devices being connected in said bridge in a non-conductive direction with respect to said source, and a condenser bridging said first device and charged only when the 2. In a grid control arrangement for a ther mionic valve, a circuit over which signals having an alternating current component are received including, in series, a resistor and the primary winding of a transformer, a biasing circuit connected to the grid of said valve including, in series, a first source of biasing potential, a second source of biasing potential and a secondary winding of said transformer, means for connecting said resistor in shunt with one of said sources, said means including a unidirectional current carrying device poled to prevent current from said one source from traversing said resistor.

3. In a grid control arrangement for a thermionic valve, a source of fixed direct current potential, means connecting said source to the grid of said valve to bias said grid negatively, a resistor, means connecting said resistor in shunt to said source, said last means including a unidirectional current carrying device poled to prevent current from said source from traversing said resistor, a circuit over which signals having an alternating current component are received, and means for causing said signals to traverse said resistor to produce therein a varying potential, said varying potential effective to vary the bias placed upon said grid by said source.

4. In a grid control arrangement for a thermionic valve, a source of constant direct current potential, a source of intermittent undulating direct current potential, a condenser, means permanently connecting said condenser and said two sources in a series circuit, said sources being poled in apposition to one another in said circuit, means always eflective to prevent said first source from charging said condenser, said second source effective to charge said condenser over said with any desired de ree. depending upon the.

cordance with variations in the charle upon the other o'isaid sources. to prevent said otiiersource from charsing'said condenser. said-other source eilective, whenever its potential is creator tha'n thepotentiaioi saidone source. to charse,

said condenser varialny over-said circuit in ac cordance with the amount by which the potential or said other source exceeds that oi said one source. means always eiifectiv'e to prevent said one source from charsina .said'co'ndenser, and means connecting said condenser to the arid oi saidvalvetovarythebiasonsaidlridinacoordancflwithvariations in the charse upon said condenser.

tial is equal to or greater than the potential oi BER'I'RAM MORTON HADFIELD.

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