CH684224A5 - Device for DC isolation of coaxial cables - Google Patents

Abstract

If an isolating transformer (T2) is used for DC isolation of coaxial cables (K) in the case of radio-frequency transmission installations using electrical conductors, then the screens of the two cable sections must be connected to one another in RF terms, via a capacitor, in order to maintain the good screening effect of these cables. Since the voltage drop which is produced across the capacitor in the event of a stray current (iS) flowing in the cable screen is coupled into the signal path via the winding capacitances of the isolating transformer, this capacitor would have to have an extremely low impedance, which could scarcely be implemented. In order to solve this problem, the voltage difference (uC1) which exists anyway between the screens across a first capacitor (C1) is divided down to a considerably lower voltage (uC2) by means of a voltage divider which consists of a lossy supplementary transformer (T1) and a second capacitor (C2), one of the winding ends of the isolating transformer being connected to the second capacitor. The effect of the stray current on the signal path is greatly reduced by the considerably lower voltage across the second capacitor. In a preferred embodiment, the supplementary transformer consists of a coaxial cable section over which ferrite-ring cores are pushed. <IMAGE>

Description

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The present invention relates to a device for the galvanic separation of coaxial cables according to the preamble of the first claim.

In the case of wire-bound broadband RF transmission systems, the transmitting and receiving sides are often connected to one another via coaxial cables. In order to interrupt disturbing or even harmful equalizing currents on the coaxial cables, caused by potential differences between transmitting and receiving systems, by atmospheric discharges or by insulation defects in a power supply unit, a galvanic isolation between the transmitting and receiving side is required. This separation should take place on the cable side and must not significantly impair the transmission properties, but also the shielding quality of the coaxial cable used.

1 shows a known solution using an HF broadband transformer T with a corresponding dielectric strength and a transmission ratio of 1: 1. In order not to significantly degrade the shielding effect of the cable K against interference fields, the two shield-side transformer connections must be connected to a high-voltage capacitor C. This connection must have a very low impedance in the entire frequency range of interest, since the voltage drop uc across the capacitor, generated by an external current is in the cable shield, is coupled into the signal path via the winding capacitances of the transformer, as a result of which the shielding quality deteriorates considerably. Since broadband RF transformers must have a very tight magnetic coupling and therefore have a relatively high capacitance between the windings, a reduction in the winding capacitance is not a viable way to reduce the coupling into the signal path.

In this known solution, the high shielding quality of good coaxial cables requires such a low capacitor impedance that it can hardly be achieved even by connecting several high-voltage capacitors in parallel.

It is an object of the present invention to provide a new solution for the galvanic separation of coaxial cables by means of a broadband HF transformer, in which solution the interference voltage - uc the known solution - between the two transformer windings can be made small, so that the necessary HF - moderate connection of the shields of the two coaxial cables results in a reasonably realizable impedance.

This object is achieved by the features mentioned in the characterizing part of the first claim. An embodiment of the invention will now be explained in more detail with reference to the drawing, for example.

The drawing shows:

2 shows a circuit diagram of the device according to the invention;

3 shows an equivalent circuit diagram of the device according to FIG. 2: and

Fig. 4 shows a preferred embodiment of the separating device.

The device according to FIG. 2, like the known solution, has an HF broadband transformer T2, hereinafter called an isolating transformer, with a correspondingly high dielectric strength and a transmission ratio of 1: 1, which is used for the electrical isolation of the two coaxial cables. Furthermore, a further transformer Ti, hereinafter referred to as additional transformer, also with a ratio of 1: 1, as well as two capacitors Ci and C2 and a resistor R are present. The high-voltage capacitor Ci connects like the capacitor C of FIG. 1 the two cable shields. When an external current is flowing, a voltage uci is applied to this capacitor in the cable shield, which corresponds to the voltage uc of FIG. 1. In contrast to the circuit according to FIG. 1, however, this voltage is not present at the shield-side ends of the isolating transformer T2, from where it would be coupled into the signal path via the winding capacitances of this transformer T2, but rather a voltage uc2 which is formed by a voltage divider Additional transformer Ti with its load resistor R and the capacitor C2, is divided down to a fraction of uci. The useful signal is not influenced by the additional transformer Ti, since its magnetic flux is canceled by the same signal current in both windings directed against each other.

3, Ri and R3 are the terminating resistors of the coaxial cable and the capacitor Ci is shown as a voltage source with the interference voltage uci. The load resistor R2 is necessary in order to vaporize the series resonance circuit, which is formed from the series connection of the high-voltage capacitors Ci and C2 and the shunt inductance of the additional transformer T-i. 3 shows that the additional transformer Ti is absolutely necessary. The voltage uc2 could indeed be made small by uci by suitable choice of R2 and C2, but the voltage across R2 would be superimposed directly on the useful signal without this additional transformer Ti. By means of the additional transformer Ti with the transmission ratio 1: 1, the voltage ur2 in the loop formed by the coaxial cable with the two terminating resistors Ri and R3 is coupled into both branches, so that ur2 = ur2 ', the winding sense indicated by the dots is that these two tensions cancel each other out in the loop.

The additional transformer Ti must be a broadband HF transformer, the bandwidth of which should cover the entire interference frequency spectrum. In principle, it can have the same structure as the isolating transformer.

4 shows a particularly simple embodiment of the additional transformer T1. It consists here of a coaxial cable section 1, over which one or more ferrite ring cores 2 are pushed. The windings of the isolating transformer T2 are again between the inner and outer conductors

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of the neighboring cable ends connected. The capacitor C2 is connected between the shield-side winding ends as in FIG. 2, while the capacitor Ci is connected between the shield of the incoming cable and the shield of the outgoing cable. Because the ferrite ring cores are subject to HF losses, these cores take over the damping of the series resonance circuit which is caused by R and R2 in FIGS. 2 and 3. In the form shown, the two windings of the additional transformer Ti each have one turn, but with a suitable choice of the ferrite cores, it would also be possible to route the coaxial cable several times through the opening of the ring cores, so that a higher number of turns could be achieved.

The bandwidth of the additional transformer in the embodiment according to FIG. 4 can be influenced by a suitable choice of the ring cores, optionally by a combination of ring cores with different properties.

By inserting the additional transformer with damping of the series resonance circuit, a galvanic separation of coaxial cables can be achieved, in which the shielding effect of the coaxial cable is not significantly deteriorated without simultaneously using an unrealistically high number of high-voltage capacitors connected in parallel for the HF connection of the shields of the coaxial cables have to.

In the preferred embodiment of the additional transformer according to FIG. 4, the use of a coaxial cable section with ferrite ring cores pushed over it results in only a slight additional outlay compared to the solution according to the prior art.

Claims (5)

Claims 1. Device for the galvanic separation of coaxial cables in broadband transmission systems by means of an HF isolating transformer with two windings, in which device the shields of the coaxial cable to be separated (K) are connected through HF to maintain the good shielding effect of the coaxial cable by means of a capacitor (C) , characterized in that the one winding of the isolating transformer (T2) is connected between the inner conductor and shield of the one coaxial cable, that the shield-side end of said one winding of the isolating transformer is connected to the shield of the other coaxial cable via a first capacitor (Ci) that the same shield-side end is connected via a second capacitor (C2) both to the shield-side end of the other winding of the isolating transformer and via a winding of an additional transformer (Ti) with two windings with the shielding of the other coaxial cable anden is that the other winding of the additional transformer is connected to the inner conductor of the other coaxial cable and is connected in series with said other winding of the isolating transformer, and that means are provided for damping the series resonant circuit consisting of the series connection of the two capacitors and the transverse inductance of the additional transformer are. 2. Device according to claim 1, characterized in that both the isolating transformer (T2) and the additional transformer (T1) have a transmission ratio of 1: 1, that the windings of the additional transformer are connected such that a useful signal is not influenced by this transformer , since its magnetic flux is canceled by the same signal current in both oppositely directed windings, and the voltage drop (UR2) caused by a disturbance current (is) in said one winding causes a compensation voltage (ur2 ') in the other winding, and that the Damping of the series resonant circuit is brought about by a resistor (R) connected in parallel with the one winding of the additional transformer. 3. Device according to claim 1, characterized in that said additional transformer consists of a section of a coaxial cable (1) with one or more ferrite cores (2) pushed over the outer jacket, that said winding of this transformer consists of the shielding of the coaxial cable section and the said other winding consists of the inner conductor of this section, and that the damping of the series resonant circuit is caused by the RF losses of the ring cores. 4. The device according to claim 3, characterized in that said ferrite ring cores are dimensioned such that a coaxial cable section can be threaded several times, so that an additional transformer is formed, the windings of which have a plurality of turns. 5. The device according to claim 3 or 4, characterized in that a plurality of ferrite core rings (2) with different characteristics are pushed onto the coaxial cable section in order to achieve the desired bandwidth of the additional transformer (T1). 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 3rd