Patent
1983-11-08
1986-05-20
Dwyer, James L.
H04B 330, H04B 152
Patent
active
045903344
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a supply circuit included in a telephone repeater 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 supply circuit in such repeaters, used to transform the analogue telephone signals coming from the line, into PCM-coded signals and vice versa.
BACKGROUND ART
When converting analogue telephone signals from a certain subscriber via a certain line, repeaters are used to provide two different signal 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-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 made 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. Furthermore the transformer hybrid should fulfill the acoustic feedback attenuation demands on the four-wire side for the line impedance of the actual two-wire side. Operation attenuation distortion in the speech band should be low (for example, Ad<0.5 dB). 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 reflections arise across the line between the hybrid and the subscriber's set.
In addition there will be problems with a DC-magnetized hybrid transformer to prevent incoming speech voltage from being short-circuited via the current loop through which direct current is supplied from the battery towards the line. Previously current supply has been carried out across a mid point capacitor C.sub.o or a choke D has been connected in series with the DC-voltage source, see appended FIGS. 2 and 3, respectively.
The DC-premagnetizing of some of the windings 1,1' is defined according to
According to equation (1), previously known current feed alternatives demand according to FIGS. 2 and 3 large core volume of the hybrid transformer and the choke, respectively, especially if the feed current I.sub.o is high. Higher feed current also imply several other problems:
(a) The temperature in the transformer windings increases according to
(b) In particular a temperature increase T.sub.M is obtained in the box where the feed resistors R.sub.1b and R.sub.2b are placed according to ageing of the current feed resistors R.sub.1b and R.sub.2b is critical.
(c) At a low-ohmic current feed alternative the current feed resistors R.sub.1b and R.sub.2b are low. At overvoltages, for example, upon lightning, the energy of the thunder increases according to
(d) In the known current feed principles according to FIGS. 2, 3 and 4 it is particularly difficult to attain a high echo attenuation A.sub.e for low frequencies if the current feed resistors R.sub.1b,R.sub.2b are low.
According to the invention the above mentioned problems are solved, on the one hand by transforming the feed resistance R.sub.1b,R.sub.2b and, on the other hand by a special current limitation by means of PTC-resistors.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be more fully described in the light of prior art with reference to the accompanying drawings, where
FIG. 1 shows a block diagram of a telephone repeater and its connection to a subscriber via a two-wire line,
FIGS. 2, 3 and 4 show three different embodimen
REFERENCES:
patent: 3778563 (1973-12-01), Bise et al.
patent: 3875350 (1975-04-01), Fischer
patent: 3911372 (1975-10-01), Seidel
patent: 3987257 (1976-10-01), Place
patent: 4297536 (1981-10-01), Buchner
patent: 4336423 (1982-06-01), Morlec et al.
Dwyer James L.
Telefonaktiebolaget LM Ericsson
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