GB2114843A - Digital transmission system - Google Patents

Abstract

A digital regenerator for a submerged optical transmission system utilises a resonator (10) as part of the clock extraction circuit and which comprises a semi-rigid cable as shown in the various embodiments of Figs. 2 to 5.

Description

SPECIFICATION Digital transmission system This invention relates to digital transmission systems particularly but not exclusively submerged systems.

In some transmission systems, particularly submerged systems, reliability is of paramount importance. If a failure occurs causing the system to be put out of service until a repair is made, the cost of loss of revenue during down time added to the cost of raising the cable from the ocean floor, is enormous.

In this context phase locked loop oscillators and injection locked oscillators may be considered too unreliable to put into a submerged digital system regenerator and it is an object of the present invention to provide a clock extraction circuit which is simple and reliable in operation.

Land line systems often use an open coil and capacitor parallel resonant circuit. It has the following drawbacks:- (1) The open coil has inductance and resistance dependant on its surroundings, and cross talk to other parts of the circuit limit the "safe" 0 which can be used.

(2) The available 0 is almost proportional to size and so high Q's lead to rather large components.

The use of a screening can make the structure even bigger.- (3) Stray capacity reduces the available stability.

(4) Commercial capacitors tend to have rather a poor Q at frequencies over 100 Mc/s.

According to the present invention there is provided an electric resonator which includes a semi-rigid cable yielding the major part of the inductance and capacitance of the resonant circuit, the cable being housed in a conductive box.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Fig. 1 is a block circuit diagram of a digital regenerator embodying the invention; Figs. 1 A to 1 G show various waveforms which appear in the block circuit diagram of Fig.

1; Fig. 2 shows schematically the resonator of Fig. 1 according to an embodiment of the invention; Fig. 2A shows the lumped equivalent circuit of Fig. 2; Fig. 3 shows schematically the resonator of Fig. 1 according to another embodiment.

Fig. 3A is an approximate equivalent circuit of Fig. 3 embodiment.

Fig. 4 shows schematically the resonator of Fig. 1 according to a third embodiment; Fig. 4A shows the lumped equivalent circuit of Fig. 4; and Fig. 5 shows how a resonator could be constructed without using coaxial cable.

Referring to Fig. 1 there is shown in block schematic form a regenerator for a digital repeater in an optical digital submerged transmission system. The signal from the optical fibre is fed to a receiver 1 which transduces the optical signal to an electrical signal and this signal is amplified in an amplier 2 having an adjustable gain control circuit 3. In some systems'it is arranged for the gain control 3 to be remotely controllable.

The amplified data signal is fed to a decision and timing circuit 4 which decides if the data is valid and regenerates the bit stream in synchronism with the incoming data, by means of a clock signal 5. The regenerated signal is then transmitted by the transmitter 6 as an optical signal to the outgoing fibre 7.

The clock signal from the incoming data is detected by a wave shaping circuit 8, amplified in amplifier 9 and applied to a passive resonator 10.

The resonator can take any of the forms shown in Figs. 2 to 5.

The operation of the decision and timing circuit 4 using the clock signal 5 can best be seen from the waveforms shown in Figs. 1 A to 1 G. Fig. 1 A shows a typical data stream 1110100 in ideal form (non return to zero-RNZ). In practice the signal would appear more like Fig. 1 B and is the kind of signal which might be expected in a submerged digital signal after travelling along the fibre for say 20 km from the previous regenerator.

This would be fed to the circuit 8. This circuit is effective to differentiate the signal, as shown in Fig. 1 C and rectify it Fig. 1 D. The clock information is present in the signal, but not all the time and npise is present.

Fig. 1 E shows how the resonator can reduce the noise and "fill in" the missing waves. Fig. 1 E shows the in phase resonator signal limited and Fig. 1 F represents the resonator signal phase shifted. Thus the extracted clock signal can by feeding it to the resonator, be used to trigger gates to regenerate the original signal, Fig. 1 G, which as been retimed NRZ. The amplitudes in Figs. 1 A to 1 G are not to scale.

Figs. 2 to 5 show various constructions for the resonator.

Referring to Fig. 2 a semi-rigid copper coaxial cable 21 is coiled as shown. It is a 1 60 mbit resonator; a 325 mbit resonator has only one turn. The copper outer conductor at 22 is soldered to a saddle 23 which earths it at one point to a copper box 24. The coaxial cable is terminated by connecting the inner conductor 25 at D to the outer 22 at E. Energy is introduced to the tuned circuit between C and A via input terminal 26. The input impedance increases as CA increases.

Energy is taken from the tuned circuit between B and A via output terminal 27. The output impedance increases as BA increases.

At the resonant frequency insertion loss and 0 are interchangeable by changing AC and AB (i.e.

input and output) for any given impedance circuit.

A copper screw (shown in later examples) can be put down the centre of the coil to adjust the resonant frequency. The equivalent circuit of Fig. 2 is shown in Fig. 2A and is similar to the embodiment of Fig. 4.

Fig. 3 shows an alternative embodiment. The copper box 34 houses a coiled semi-rigid cable 31 consisting of about 3/4 of a turn and yielding inductance L and capacitance C and mounted on copper pillars 32, 33 which yield inductance I.

Input and output terminals 35 and 36 are connected respectively to the outer conductor of the cable at 37 and the inner conductor of the cable at 38. A copper turning set screw 39 is provided to set the resonant frequency.

Fig. 3A shows the crude lumped equivalent circuit of the arrangement of Fig. 3. The structure shown in Fig. 3 has yielded a resonator with 0 of 1 50 and an insertion loss of about 7 dB in a 50 ohm circuit. A maximum Q of 200 was obtained with 20 dB of insertion loss. Adjusting 1, the stud length, makes it possible to interchange Q and loss. In timing circuit applications low impedance filters are connected between the transistor emitters and so circuit strays tend to be less troublesome and fewer transistors are required compared with a high impedance parallel resonant circuit. Putting the structure inside the box defines the earth circuit limits stray fields and enables the set screw to be used to tune the circuit.

Fig. 4 shows another embodiment for the resonator 10. This has a copper box 44 housing two part-circular pieces of semi-rigid cable 41 and 42. The outer conductors of pieces 41, 42 are earthed by soldered connection to the box 44 at G and H respectively. The other ends of the pieces, which are of equal length, are connected together. The inner conductor of piece 41 being connected to the outer conductor of piece 42, and the inner conductor of piece 42 to the outer conductor of piece 41, at J. Energy is introduced via input 45 between the outer conductor of cable piece 42 at K and earth, and energy is extracted at output 46 between the outer conductor of cable piece 41 and at M and earth. Turning screw 47 adjusts the resonant frequency. The embodiment described in Fig. 4 is a smalle; and symmetrical structure.The first model gave a O of 146 with 4.3 dB of loss.

Fig. 5 shows another embodiment for the resonator 10. Here semi-rigid conductors 51 and 52 are part circular in shape and constructed from copper tape, each being 3.5 cm long and 1.5 cm wide. They are connected to earth at N and P, to respectively an output and an input 53 and 54 at R and S, and to opposite sides 55 and 56 of a metallised alumina substrate 57. The whole is housed in a copper box 58.

The crude lumped equivalent circuit is shown in Fig. 5A in which the copper tapes 51 and 52 provide inductances (L2+12) and (L,+l,) respectively and the metallised substrate 57 provides capacitance C,. The embodiment of Fig.

5 is an alternative construction to Fig. 4. The alumina substrate 57 normally used for thick film circuits and metallised on each face, produces a high Q, high value capacitor. We have obtained a O of 200 with insertion loss of about 10 dB for this resonator.

As previously a tuning stud 59 enables adjustment of the resonant frequency.

The general parameters of the resonators in Figs. 3, 4 and 5 are affected, adjustable and controllable in the same way as discussed with reference to Fig. 2.

Claims (9)

Claims

1. An electric resonator which includes a semirigid cable yielding the major part of the inductance and capacitance of the resonant circuit, the cable being housed in a conductive box.

2. A resonator as claimed in claim 1, wherein the cable also yields the major part of the inductance and capacitance of the resonant circuit, the cable being housed in a conductive box.

3. A resonator as claimed in claim 1 or claim 2, wherein the cable comprises a curval semi-rigid coaxial cable having an inner conductor and an outer conductor coextensive therewith are being connected to an input of the resonator and the other being connected to an output of the resonator.

4. A resonator as claimed in claim 3, comprising a second semi-rigid coaxial cable connected between one end of the firstmentioned coaxial cable and the input or output.

The inner and outer conductors of the second cable being connected to respectively the outer and inner conductors of the first-mentioned cable.

5. A resonator as claimed in claim 1, wherein the cable is made of curved metal tapes and capacitance is yielded by an insulating substrate metallised on opposite sides which are connected to opposing ends of the curved metal tapes.

6. A resonator as claimed in claim 1 or claim 2.

wherein the cable comprises a coiled semi-rigid coaxial cable terminated by having its inner conductor at one end connected to its outer conductor at the opposite end, and input and output connections are made to spaced-apart points on the outer conductor.

7. An electric resonator substantially as hereinbefore described with reference to and as illustrated in Fig. 2, Fig. 3, Fig. 4 or Fig. 5 of the accompanying drawings.

8. A regenerator having a clock extraction circuit comprising a resonator as claimed in any preceding claim.

9. A regenerator substantially as hereinbefore described with reference to all the Figures of the accompanying drawings.