IL49139A

IL49139A - Method and apparatus for the regeneration of carrier-frequency digital signals

Info

Publication number: IL49139A
Application number: IL49139A
Authority: IL
Original assignee: Siemens Ag
Priority date: 1975-03-14
Filing date: 1976-03-03
Publication date: 1977-12-30
Also published as: JPS5542794B2; IT1056853B; LU74551A1; NO760643L; DE2511253B1; IL49139A0; DE2511253C2; CH607481A5; JPS51116608A; DK72176A; SE7601322L; BE839509A; AU1163076A; NL7602661A; FR2304232A1; BR7601500A

Description

mniK ©ιτπ^ jpnm nt>»w Method and apparatus for the regeneration of carrier-frequency digital signals SIEMENS AKTIENGESELLSCHAFT This invention relates to the regeneration of the sideband, employed for transmission purposes, of carrier M frequency digital signals.

In transmission technology there is a growing need to be able to transmit digital signals 'from a transmitting station to a receiving station which stations are not interconnected by a digital transmission link. In many cases such transmitting and receiving stations are however already interconnected by some other form of transmission link, for example cables for low-frequency or carrier-frequency signals. It has proved possible additionally to employ carrier frequency cables to transmit digital signals, if, in order to avoid disturbances in the low-frequency and carrier-frequency transmission, the digital signals are amplitude-modulated with a carrier of suitable frequency. For full exploitation of the transmission capacitance, only a single sideband of the carrier frequency digital signal is transmitted.

Digital signals possess the property, which is exploited on a large scale, that they are regenerable, provided the signal-to-noise ratio does not fall below a specific minimum. This property is also" retained when the digital signals are amplitude-modulated with an additional carrier and both sidebands are transmitted. If, however, only one sideband is transmitted, direct regeneration in the transmission state is not possible. Regeneration can be carried out by reconverting the carrier frequenc digital signal into its base state, i.e. by demodulating it.

This reconversion can be effected by a relatively complicated conversion in two stages or in one stage with the same carrier frequency as at the transmitter. The carrier frequencies required for this purpose must be synchronised by a regenerator pulse train and must exhibit a determinate phase state in relation to the received signal. However, in-phase derivation of the pulse train and carrier from the signal reconverted into the base state is not possible for specific pulse sequences.

This invention seeks to provide a simplified way of enabling regeneration of a received single sideband of a carrier frequency digital signal.

According to this invention there is provided a method of converting, for the purpose of regenerating a received single sideband of a carrier frequency digital signal in the form of a pseudo-ternary code into the carrier frequency digital signal, the carrier frequency digital signal being produced using a carrier frequency which is a whole-numbered multiple of half the bit repetition frequency of the digital signal and being synchronised with the digital signal in the transmission state with a phase which is displaced by 90° in relation to the phase of an oscillation, whose frequency is equal to the bit repetition frequency, of the upper sideband of the carrier frequency digital signal, comprising the steps of amplitude-modulating the received sideband with a signal whose frequency is equal to the bit repetition frequency thereby to produce the other non-received, sideband of the carrier frequency digital signal, and combining the received sideband and said other sideband to form the carrier frequency digital signal. The method preferably ' further includes the step of regenerating the formed carrier frequency digital signal in terms of amplitude and time to produce a regenerated signal.

The method of the invention is based on the recognition that, in order to achieve a clearly defined synchronisation of the regenerator pulse train and the demodulation carrier, it is necessary to synchronise the carrier frequency and the regenerator pulse train of the digital signal in the transmission state, and that under specific conditions it is also possible to regenerate the carrier- frequency digital signal in the transmission state. This regeneration in the transmission state allows the previously required multiple conversion to be dispensed with. This not only simplifies the regeneration process, but also eliminates additional interference sources and thus provides a higher degree of reliability in the signal transmission and a reduction in the outlay for stabilisation and monitoring means.

Advantageously the method further includes either the step of deriving a single sideband, which may be further transmitted, from the regenerated signal, or the step of demodulating the regenerated signal.

Preferably said received and said other sidebands are respectively the upper and lower sidebands of the carrier frequency digital signal. In this case the lower sideband is suppressed at the transmitting end, and this produces particularly low cross-talk disturbances in the carrier frequency connections which are simultaneously transmitted via the cable.

The sideband which is suppressed at the transmitting end is regained at the receiving end by modulation with the bit repetition frequency from the transmitted sideband. The additional sideband which is formed as a result of the receiving end modulation is suppressed by a filter.

The invention also extends to apparatus, for use in carrying out the method recited above, comprising a first bandpass filter, a first distortion corrector, a sideband restoration circuit, a digital signal regenerator, and two amplifiers, the arrangement being such that in operation the received sideband is conducted via the first band-pass filter and the first distortion corrector to the sideband restoration circuit which produces the formed carrier frequency digital signal at its output, the latter signal being conducted via one amplifier to the regenerator, wherein the regenerator comprises an amplitude regenerator, a time regenerator and a controlled pulse train supply, wherein the signal whose frequency is equal to the bit refE ition frequency is derived from the controlled pulse train supply, and wherein the first amplifier has an output connected to a control input of the first distortion to a further transmission link to the next intermediate corrector and an input connected to the output of either the first distortion corrector or the sideband restoration circuit or th second amplifier.

In one embodiment of this apparatus the sideband restoration circuit comprises a combining circuit having two inputs and an output which- cons titutes the output of the sideband restoration circuit; a first phase shift device having an input connected to an input of the sideband restoration circuit and an output connected to one input of the combining circuit; a modulator having a first input connected to said input of the sideband restoration circuit; a filter having a pass-band corresponding to said other sideband, an input connected to an output of the modulator, and an output connected to the other input of the combining circuit; and a second phase shift device having an input connected to receive the signal whose frequency is equal to the bit repetition frequency and an output connected to a second input of the modulator; wherein the modulator comprises a double push-pull modulator which suppresses the signals supplied to its inputs and wherein the first phase shift device includes an amplifier or an attenuator which serves to adjust the relative levels of the signals supplied to the combining circuit.

In another, particularly low-cost, embodiment of the apparatus the sideband restoration circuit comprises a push-pull modulator having a first input which constitutes the input of the sideband restoration circuit and a second input the signal from which the modulator suppresses, low-pass filter having an input connected to an output of the modulator and having a pass-band for both of said sidebands, a second distortion corrector, having an input connected to an output of the low-pass filter and an output which constitutes the output of the sideband restoration circuit, and a phase ~~~"~" shift device having an input connected to receive the signal whose frequency is equal to the bit repetition frequency an'd an output connected to the second input of the modulator.

The reduction in outlay in the intermediate regenerators of the invention is achieved as a result of- the regeneration in the transmission state, in which the double conversion previously proposed for use in intermediate regenerators, comprising two double push-pull modulators, an interposed band-pass filter and a following low-pass filter, is replaced by a single push-pull modulator with a following low-pass filter and possibly an additional distortion corrector. Thus the transmitting-end converter is dispensed with in the intermediate regenerators. The carrier supply for the preliminary converter is also considerably simplified so that approximately 30 to 40 oscillating circuits are saved in respect of each intermediate regenerator.

The invention will be further understood from the following description by way of example of embodiments thereof with reference to the accompanying drawings, in which:- Fig. 1 illustrates the spectral distribution of a carrier frequency digital signal at the output of a transmitting modulator; Fig. 2 illustrates the construction of an intermediate regenerator or an end regenerator constituting apparatus in accordance with one embodiment off the invention; and Fig. 3 illustrates a modification of the apparatus constituted by a simplified sideband restoration circuit.

Fig. 1 shows a graph in which the standardized frequency which relates to the frequency of the carrier oscillation is plotted on the abscissa and the level in dB is plotted on the ordinate, the level occurring on the transmission of a sequence of logic ones having been selected as a reference level.

For the carrier frequency transmission of a digital signal a bipolar signal in the form of a pseudo-ternary code is modulated with a carrier oscillation of half the bit repetition frequency, and in order to avoid a d.c. component the phase state of the carrier is displaced by 90° in relation to the phase state of the bipolar signal.

The produced carrier frequency digital signal has the spectral distribution shown in Fig. 1 with a lower sideband 0 The intermediate regenerator or end regenerator shown in Fig. 2 basically consists of a sideband restoration circuit, a known form of regenerator for digital signals, and in terminal devices provided at the input and the output. For the use of an intermediate regenerator, the output is terminated with a band-pass filter which possesses a pass band of 0.5<-Ω-< 1.5, whereby the upper side band is filtered out and made available for further signal transmission. For use as an end regenerator, the output is connected to a modulator which, by means of further modulation, produces the base band, which can be conducted to further parts of a line terminal device.

An input 1 of the intermediate regenerator or end regenerator illustrated in Fig. 2 obtains from the transmission link the upper sideband which is used for transmission purposes and which covers the frequency range 0.5 < -Ω.<£ 1.5. This input 1 is connected to a first band-pass filter BP1, whose pass band is identical to the frequency range of the upper sideband and which conducts the latter to a first automatic distortion corrector El. The automatic distortion corrector El emits a corrected transmission signal to the input A of a sideband restoration circuit SBR. In the form illustrated in Fig. 2, the circuit SBR contains two signal paths which are separate from one another and both of which commence from the input and lead to separate inputs of a combining circuit Z whose output constitutes an output B of the circuit SBR. One signal path contains a first transit time element or phase shift device T 1 which additionally comprises an amplifier or an attenuator so that not only the transit time, but also. the amplitude of the upper sideband transmitted via this signal path, can be set to match the other (lower) sideband. The other signal path contains a modulator Ml which has an input connected via a second transit time element or phas.e shift device t2 to an input C to which is applied a signal, hereinafter referred to as the carrier oscillation, which is required for the modulation and whose frequency Si is identical to the bit repetition " frequency of the digital signals. The modulator Ml is in the form of a double push-pull modulator exhibiting carrier and input signal suppression and in addition to other modulation products produces at its output a lower sideband which is folded in on itself and which covers the standardized frequency range of 0<Ώ-<0.5. This lower sideband is conducted via a low-pass filter TP1 to an input of the combining circuit Z. The low-pass filter TP1 serves to suppress an upper sideband in the frequency range 1.5<Λ 2.5 likewise emitted by the modulator Ml. The combining circuit Z consists of a known hybrid circuit; to prevent reactions occurring bteween the two signals paths, in place of this hybrid circuit, the two signal paths can be combined ohmically.

From the output B of the circuit SBR the signal is conducted to a second amplifier V2 which produces a signal level required for the signal regeneration, and the amplified signal is conducted to the input of the actual regenerator R. The output of the second amplifier V2 is also connected to the input of a first amplifier VI which supplies the control signal for the automatic distortion corrector E .

In a simplified embodiment of the invention the input of this first amplifier VI can instead be connected to the output of the automatic distortion corrector El, and therefore the first amplifier VI and the first automatic distortion corrector El can be combined to form a common assembly.

Alternatively, the input of the amplifier VI could be connected directly to the output of the circuit SBR.

The input of the actual regenerator R, which is in the form of a regenerator for pseudo-ternary signals of the bit reptition frequency 2 SI, is directly connected to the input of an amplitude regenerator AR. The amplitude regenerator AR possesses two signal outputs via which regenerated pulses each of a respective polarity are conducted to respective inputs of a time regenerator ZR which serves to effect a time regeneration; from the time regenerator ZR the regenerated pulses are separately emitted to signal outputs 2, 3.

The regenerator R also contains a controlled pulse train supply which is constructed in the form of a phase-locked loop and contains an oscillator G, a rectangular pulse shaper RF, a phase discriminator PD, and a frequency divider FT. The input of the phase discriminator PD is connected preferably to an additional output of the amplitude regenerator AR but alternatively to the output of the first automatic distortion corrector E . The phase discriminator PD emits, at its output, a control voltage which is fed to a control input of the oscillator G in which a control d.c. voltage serves to alter the capacitance of capacitance diodes and thus the oscillation frequency. The signal produced by the oscillator G possesses a frequency of L =2, i.e. double the carrier frequency, and this signal is converted into a timing signal composed of approximately rectangular pulses by the pulse shaper RF. This timing signal is conducted to pulse train inputs of the phase discriminator PD and the time regenerator ZR and to the input of the frequency divider FT. The frequency divider FT possesses a divider ratio of 2:1 and produces the carrier oscillation from the timing signal. The carrier oscillation is conducted to the carrier input C of the circuit SBR and to a carrier output 4.

As already mentioned, the connection of a second bandpass filter BP2 with a pass band of 0.5 SX(.l .5 , in fact to the signal outputs 2 and 3, results in an intermediate regenerator which supplies a regenerated upper side band to a following transmission link ZWR.

The connection of a modulator M3 to the two signal outputs 2 and 3 and to the carrier output 4 produces an end regenerator which, as a result of renewed modulation with a carrier oscillation of the frequency - . , produces a base band, i.e. a demodulated, digital signal which can be conducted to other parts of a line terminal device LE.

Fig. 3 illustrates a simplified circuit for side band restoration, whose terminals A, B and C are identical to those of the circuit SBR shown in Fig. 2. The input A is connected to the input of a modulator M2, which is constructed as a push-pull modulator exhibiting carrier suppression and obtains the carrier oscillation with the frequency -Ω. = via a transit time element or phase shift device T3 from the carrier input C. In addition to the sidebands, the input signal also occurs at the output of the push-pull modulator M2, so that it is possible to dispense with means for splitting the signal path into two parallel paths and the combining circuit. The output of the modulator M2 is connected to a low-pass filter TP2, with the transition frequency 2.5--Ω-, which emits the carrier frequency digital signal via a second distortion corrector E2 serving to set up a uniform output level, across the entire frequency range, to the output B. The output B and the carrier input C are adjointed by the circuit components already described with reference to Fig. 2. \

Claims

WHAT WE CLAIM IS :-

1. A method of converting, for the purpose of regenerat ing, a received single sideband of a carrier frequency digita signal in the form of a pseudo-ternary code into the carrier frequency digital signal, the carrier frequency digital signa being produced using a carrier frequency which is a whole- numbered multiple of half the bit repetition frequency of the digital signal and being synchronised with the digital signal in the transmission state with a phase which is displaced by 90° in relation to the phase of an oscillation, whose frequen cy is equal to the bit repetition frequency, of the upper sideband of the carrier frequency digital signal, comprising the steps of amplitude-modulating the received sideband with signal whose frequency is equal to the bit repetition frequen cy thereby to produce the other, non-received sideband of the carrier frequency digital signal, and combining the received sideband and said other sideband to form the carrier frequenc digital signal.

2. A method as claimed in Claim 1, wherein said receive and said other sidebands are respectively the upper and lower sidebands of the carrier frequency digital signal.

3. A method as claimed in Claim 1 or Claim 2, and furth er including the step of regenerating the formed carrier freq uency digital signal in terms of amplitude and time to produce a regenerated signal.

4. A method as claimed in Claim 3 and further includin the step of deriving a single sideband from the regenerated s ignal .

5. A method as claimed in Claim.3 and further includin the step of demodulating the regenerated signal.

6. Apparatus for use in carrying out the method of Cla 3 comprising a first bandpass filter, a first distortion corrector, a sideband restoration circuit, a digital signal rege nerator, and two amplifiers, the arrangement being such that in operation the received sideband is conducted via the first bandpass filter and the first distortion corrector to the sid band restoration circuit which produces the formed carrier frequency digital signal at its output, the latter signal being conducted via one amplifier to the regenerator, wherein the regenerator comprises an amplitude regenerator, a time regenerator, and a controlled pulse train supply, wherein the signal whose frequency is equal to the bit repetition frequency is derived from the controlled pulse train supply, and wherein the first amplifier has an output connected to a control input of the first distortion to a further transmission link to the next intermediate corrector and an input connected to the output of either the first distortion corrector or the sideband restoration circuit or the second amplifier.

7. Apparatus as claimed in Claim 6, wherein the sideb; restoration circuit comprises a combining circuit having two inputs and an output which constitutes the output of the sideband restoration circuit; a first phase shift device having an input connected to an input of the sideband restoration circuit, an output connected to one input of the combining circuit; a modulator having a first input connected to said input of the sideband restoration circuit; a filter having a pass-band corresponding to said other sideband, an input connected to an output of the modulator, and an output connected to the other input of the combining circuit; and a second phase shift device having an input connected to receive the signal whose frequency is. equal to the bit repetition frequenc and an output connected to a second input of the modulator; wherein the modulator comprises a double push-pull modulator which suppresses the signals supplied to its input and wherein the first phase shift device includes an amplifier or an attenuator, which serves to adjust the relative levels of the signals supplied to the combining circuit.

8. Apparatus as claimed in Claim 6 wherein the sideband restoration circuit comprises a push-pull modulator having a first input which constitutes the input of the sideband restor tion circuit and a second input the signal from which the modulator suppresses, a low-pass filter having an input connec ted to an output of the modulator and having a pass-band for both of said sidebands, a second distortion corrector, having an input connected to an output of the low-pass filter and an output which constitutes the output of the sideband restoration circuit, and a phase shift device having an input connected to receive the signal whose frequency is equal to the bit repetition frequency and an output connected to the second input of the modulator.

9. Apparatus as claimed in any of Claims 6 to 8, wherei the controlled pulse train supply is in the form of a phase-locked loop and comprises a controlled oscillator, a rectangular pulse shaper, a phase discriminator, and a 2 frequency divider, wherein the phase discriminator which is selectively connected has a first input connected to the output of either the amplitude regenerator or the first distortion corrector, a second input connected to an output of the pulse shaper, an an output connected to a control input of the oscillator whic serves to produce a regenerator pulse train having a frequenc of twice the bit repetition frequency, wherein the pulse shap er has an input connected to an output of the oscillator, and wherein the time regenerator and the frequency divider have inputs connected to an input of the pulse shaper, the signal whose frequency is equal to the bit repetition frequency bein produced et an output of the frequency divider.

10. Apparatus as claimed in any of Claims 6 to 9 , and in cluding a second band-pass filter, whose passband corresponds to the received sideband, having inputs connected to outputs of the regenerator.

11. Apparatus as claimed in any of Claims 6 to 9 , and including a further modulator having inputs connected to out— puts of the regenerator and serving to demodulate the carrier frequency digital signal.

12. Apparatus substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

13. Apparatus as claimed in Claim 12, and modified substantially as herein described with reference to Figure 3 of the accompanying drawings.