DIRECT CURRENT MAGNETIZED HYBRID TRANSFORMER

Abstract: A direct current magnetized hybrid transformed in a telephone line repeater has both its primary windings (l1, l2 and l1', l2' respectively) magnetically coupled to two pairs of secondary windings, one pair (l3, l4) creating the four-wire path in the repeater and the second pair is connected to the balance impedance (Zb). According to the invention the primary windings are each divided in two parts (l1, l1' and l2, l2' respectively) with, for exemple, the same number of turns (n1 and n2 respectively). The feeding voltage source (Eo, R1b, R2b) included in the repeater is connected between the connection point common for the winding parts.

Description: DIRECT CURRENT MAGNETIZED HYBRID TRANSFORMER TECHNICAL FIELD The present invention relates to a direct current magnetized hybrid transformer included in a telephone repeater and connected between a two-wire line and the repeater to provide a transfer between the two- wire and four-wire circuits in the repeater. More fully, the invention relates to a hybrid transformer in repeaters, used to transform the analogue telephone signal ' s coming.from the line, into PCM-coded signals and vice versa. BACKGROUND ART When converting analogue telephone signals from a certain subscriber across a certain line, repeaters are used to provide two different sig- nal paths. In one of these paths the analogue signals are coded to the PCM-coded signals and in the other a decoding of the digital PCM-coded signal flow is carried out. Furthermore, amplifiers are connected to compensate for the attenuation of the line. It is previously known to use a direct current magnetized hybrid transformer when dividing the input of the repeater towards the two-wire line, the transformer giving the desired current feeding to the subscriber's set connected to the line. At the same time, however, a DC-magnetizing of the transformer core is obtained. DISCLOSURE OF THE INVENTION When constructing transformer hybrids included in. repeaters, certain demands are put regarding the design of the hybrid and consideration must be taken to the DC-magnetizing of the transformer. Firstly the input terminals towards the two-wire side should be balanced to an earth point, since otherwise the so called longitudinal currents arise across the line. Secondly a balance impedance should be connected to the trans- former so that the impedance as seen out from the hybrid towards the two-wire side can be balanced out, i e impedance match is to be achieved to the incoming line. Finally the echo attenuation on the two-wire side has to be high for impedances i e line and subscriber's set impedances which are connected to the two-wire side of the hybrid. Otherwise re- flections arise across the line between the hybrid and the subscriber's set. Besides there will be problems with a DC-magnetized hybrid transformer to prevent incoming speech voltages being short-circuited via the curren loop through which direct current is supplied from the battery towards the line. Current supply has previously been carried out via current transformers with high output impedance or a choke has been connected in series with the direct current source, see the attached Figures 2 and 3. Especially for a DC-magnetized hybrid transformer certain demands have been put for the current durability of the core material, i e in order to obtain the desired inductance, the direct current should have a cer-r * tain value for obtaining the transformer function. In accordance with the invention the above mentioned problem is solved by dividing the two primary windings at the line side of the hybrid into two winding parts each, and by connecting the feeding voltage source which primarily will supply the subscriber with direct current, to the common connection point between the two winding parts. The invention is then characterized as it appears from the characterizing part of the claim. BRIEF DESCRIPTION OF DRAWINGS The invention will be more fully described in. the light of prior art with reference to the accompanying drawings, where Figure ' 1 shows a bloc diagram of a telephone repeater and its connection to.a subscriber via a two-wire line, Figures 2 and 3 show two different embodiments of a hybrid transformer of known kind, Figure 4 shows a circuit diagram of a hybrid transformer according to the invention, Figure " 5 shows a simpli¬ fied circuit diagram of one input side of the transformer according to Figure 5, Figure 6 shows an impedance diagram. MODE FOR CARRYING OUT THE INVENTION The block diagram according to Figure 1 shows an over-all diagram of an repeater in which the hybrid transformer according to the invention is used. A subscriber unit Ab is via a line connected to a hybrid circuit • G1, the function of which, is to couple the incoming tworw re line L to a four-wire line. To the hybrid connection G1 a balance impedance Z is connected in order to simulate the impedance of the subscriber unit Ab and the line L seen from the two-wire side of the hybrid circuit. The four-wire Line is divided into a first signal path, which contains a bandpass f lter F1 for the speech -band (for example 0.2-3.4 kHz) and a PCM-coder. K for converting the analogue signals, from the subscriber unit Ab to a PCM-bit flow which is transmitted across the output of. the re- peater via the hybrid circuit G2. Incoming PCM-coded signals to the re¬ peater are converted in , the decoder DK in the second signal path to corresponding analogue signals and transmitted via a lowpass filter and eventually via an amplifier, not shown, ' to the hybrid circuit G1. Figures 2 and 3 show a circuit diagram of previously known constructions of the hybrid circuit G1, in Figure 1. In the construction according to Figure 2, the two windings I and I' of the hybrid transformer are fed • from a voltage source E connected to each of the winding end points via two current transformers I and I so that a direct current I is A B flowing through each of the windings ' I, l'. Hereby .it is primarily ob- tained current feeding, to the subscriber unit Ab and simultaneously the windings I and l* are premagnetized to a certain value. One pole of the feeding voltage source is earthed, the two-wire input to the line L then being balanced relative earth. In a. second alternative according to Fig¬ ure 3, the feeding voltage source E is connected in series between two similar windings of a. choke D. Two current limited resistances are de¬ signated R, . The circuit according to Figure 2 is considered to be ex¬ pensive, especially when high demands of balance towards earth is re¬ quired. The circuit according to Figure 3 is more advantageous and simp¬ ler to dimension. However, the choke D requires here a large volume when being designed with passive components. The embodiment according to Fig¬ ure 3 cannot at all be used when the mechanical construction of the printed card requires, limited height (usually ca 13.5 mm) just depending on the choke D. In Figure 4 an embodiment is shown of the hybrid transformer according to the invention. The winding I according to Figures 2 and 3 connected URE to the two-wire line L with one end point has been divided into two parts I * and L 2 each with the winding turn n.. and n 2 - In the same way the winding I * , according to Figures 2 and 3 has been divided in two * parts I.. ' and L^' with the same number of turns n.. and n ? . The connect- ing points between the two.winding parts I.., I2 and * l 2 ', respective¬ ly are connected to the feeding voltage source E , the inner resistance of which has been symbolized by the resistances R... and R->., since its positive pole is earthed according to the Figure. The resistance R... should be = R , to fulfil the requirement of. symmetry relative earth in the two-wire input " terminals to the Line. The end points of the two windings \.^, l^' t as .well as of the known hybrid circuits according to Figures 2 and 3, are connected to a common coupling capacitor C . The feeding direct current I will then according to Figure 4 flow from the feeding voltage source E through the resistance R... , one winding U, the line L, the subscriber.unit Ab, the Line L, the second winding I ' and through the resistance R-,, back to the voltage source E . The inner resistance R... connected to one terminal a of the Line side of the hybrid transformer will be transformed to a. higher value through the dividing of the primary winding into the part windings L 1 and L- as is shown in- Figure 5. The influence of the coupling capacitor C on the speech currents w.ill be neglected. According to this Figure, the primary winding will form an autotransformer on account of the two wind¬ ing halves L^ and l 2 being magnetically coupled. The earth point 0 in Figure 5 corresponds -to the earth point 0 in. Figure 4. According to known principles for an autotransformer the following ' is valid for the resistance „. as seen from the terminals a-0 I D R„. ' = R„ , ( V n 2 )2 according to above. D D n^ The same is valid for the two winding parts I,, and l^ at the input ter¬ minal b relative earth 0. The inner resistance 1h y R-> of the feeding voltage source, as seen f rom the two input termina ls a and b has thus been t ransformed by a factor * Should, for example, n^n- be chosen a four-folding of the resistance values R^. and R,, respectively will be obtained. As a consequence the choke D according .to Figure 3 can be saved. According to the circuit diagram in Figure 4, the premagnetizing current I only flows through.part of the primary winding-< and L * respective ly) of the hybrid transformer. Therefore, the direct.current pre ag- netization is lowered with a factor C 1/n^+n-), since the inductance of the primary winding is proportional to the. square of the number of turns n. Due to this the cross sectional area of the transformer core, for example, can be reduced, since the pcemagnetization of the core is inversely proportional to the cross sectional area of the core (premagnetizatioπ ■■ ' ' ' ) fe Alternatively a core material, with Lower current endurance can be used i e a material which maintains a higher inductance for a smaller pre¬ magnetization current. An additional advantage is that an improved filtering of noise tones from the voltage source can be obtained by the proposed circuit. As seen from the feeding voltage source the respective resistance R,., , R^, lies in series with the coupling capacitor C . In the diagram according to Figure 6, the impedance 1 * of the filter created by the respective re¬ sistance R ΛI _, R-,, and the capacitor C is shown. The dashed Line shows 1b' 2b o the impedance characteristic as the sum of the resistance value, for example R„, , and the reactance 1/Ω of the capacity C . The impedance ID o Z, for this lower value R 1h has the zero crossing (is resistive ) at a frequency f which generally is considerably higher than the speech o band Δ. Thus the filter R 1bΛ winding I-,, capacitor C gives a relatively poor filtering of the noise tones from the voltage source E Lying withir the speech band. When transforming the resistance R... to a higher value. the filter impedance Z ' will instead gets its zero crossing within the speech band for which reason the noise tones cane be coupled by the filter and do not reach the line or any of the four wire paths of the repeater via the windings I, and I,. In the above embodiment of the invention it is assumed that the line impedance has .a susceptive character, the balance impedance Z, having likewise .a susceptive character.. The proposed hybrid can naturally be used for lines with inductive or resistive character.

Claims: WHAT WE CLAIM IS : A direct current magnetized hybrid transformer Included, for example, in a telephone line repeater to provide a two -wire-four wire coupling in the repeater, containing a first primary winding (L, l_) magnetical ly coupled to a first pair of secondary windings (I, l_) and a second primary winding Cl * 1 * l- ? ') magnetically coupled to a second pair of secondary -windings Cl,, I,), the secondary winding pairs then on the one hand creating a four-wire path and on the other hand being connecte to balance impedance (Z, ) matched to the line impedance, characterized in that the said first and second primary windings (.1 * . , l 2 and l..', I-' respectively) are divided in two winding parts so that the number of turns (n-) of one winding part (I.., I * ' .is in a determined relation to the number of turns Cn~) of the second winding part (L , I and that a feeding source (E ) is connected to the connection point common for the winding parts, to magnetize only one winding part (L, I,. 1 ) of the first and second primary winding.

Inventors: JAKI IVAN
Applicants: ERICSSON TELEFON AB L M, JAKI IVAN
Application year: 1982
Main classification: H04B1
IPC codes: H04B1