AU539338B2 - A method and apparatus for synchronizing a binary data signal - Google Patents

Description

A METHOD AND APPARATUS FOR SYNCHRONIZING A BINARY DATA SIGNAL

TECHNICAL FIELD

The present invention relates to a method and apparatus for synchroniz¬ ing a binary data signal coming to a receiver with a clock signal locally available in the receiver.

The binary data signal can be of the so-called RZ-type (return to zero⁾ or of the so-called NRZ-type (non-return to zero).

BACKGROUND ART

The synchronizing problem is always present in all data transmission, and is solved with respect to application, demand on accuracy etc. in different ways. For example, if the Transmitter and Receiver sides clocks are synchronized, possible against a common reference, the detection of data on the Receiver side does not cause a^*ny^* problems of course. Synchronizing of a Receiver clock can be in a mode such that the timing information is extracted from the transmitted data signal, e. g. by time determination of its zero passes, subsequent to which a signal corresponding to the timing information is allowed to actuate a controllable local clock signal generator. Requirements in respect to transient time and permitted error in the data transmission naturally affects the selection of the synchronizing method also.

DISCLOSURE OF INVENTION

The technical problem in the present case lies in correctly detecting, with the aid of a signal which is asynchroneous to the data signal, of a message sent to the receiver, with the condition that the addition or fall-away of a binary character in the message has no effect. This condition is fulfilled in a redundant system, for example, in which the same message of a fixed number of bits is sent repeatedly in succession and the receiver accepts the message on condition that it can detect the same message a given number of times during a given time. If the addition or fall-away of a binary character in the data signal occurs relatively rarely, such an occassional happening would thus not effect the receivers' correct detection of the message. The clock signal being asynchronous must naturally not signify that there is a too great frequency deviation from the correct value. A frequency deviation in the order or magnetude of one per thousand gives rise, in accordance with the invention, to addition or fall-aw of information in approximately every thousandth bit position, which can be accepted in many applications.

The solution of this problem proposed by-the present invention is characterized in the appended patent claims. The greatest advantage with an apparatus in accordance with the invention is its extreme simplicity and meagre power requirement.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described with the aid of some embodiments with reference to the appended drawing on which Figure 1 is a block diagram of an apparatus in accordance with the invention.

Figure 2 is a phase-reversing circuit incorporated in the apparatus according to Figure 1,

Figure 3 is a first sampling circuit incorporated in the apparatus according to Figure 1,

Figure 4 illustrates the time sequence for a plurality of signals in the apparatus according to Figure 1, and Figure 5 illustrates the same signals as in Figure 4, for another embodiment.

BEST MODE OF CARRYING OUT THE INVENTION

Figure 1 is a block diagram of an apparatus in accordance with the invention. Between a data input 4 and a data output 6 there is a fir sampling circuit 1 and a second sampling circuit 2 coupled in series A phase reversing circuit 3 is connected between a clock input 5 and the clock signal input on the first sampling circuit 1. The second sampling circuit 2 is clocked directly from the clock input 5.

The data signal transmitted from the transmitter side is applied to

CM data input 4, and this signal is assumed to be a binary coded signal of the RZ or NRZ type, according to what has been mentioned above. The data signal is sampled with the aid of the clock signal on the clock input 5, the clock signal being asynchronous relative the data signal, according to the assumptions, to give on the data output a signal synchronous with the clock signal and corresponding to the input data signal.

The phase-reversing circuit 3, which will be-described more closely below, reverses the phase of the clock signal to the first sampling circuit when, as a result of the frequency difference between data signal and clock signal, positive or negative flanks in the respective signal tend to coincide.

In the preferred embodiment the sampling circuits 1 and 2 are realized with the aid of ordinary positively flank-triggered D fLip flops. The clock signal is assumed to have 50% pulse rate and the input data to be a signal of the NRZ type. To these assumptions, Figure 4 illustrates the time sequence for a plurality of signals in the apparatus of Figure 1.

Figure 2 illustrates an embodiment of the phase reversing circuit 3. The circuit has a data input 12, a clock signal input 11 and a clock signal output 13. The EXCLUSIVE-OR-circuit 10 can be regarded as a controlled inverting circuit. If the input 20 is namely zeroed, the clock signal from the input 11 passes unaffected with the exception of a lag of no interest in this connection. Two pulse-shaping circuits 7 are connected with their inputs to the EXCLUSIVE-OR-circuit output and the data input 12. These are so formed that on their respective output they deliver square pulse of given duration when the input signal has a positive flank. The pulse length is short in relation to the digit time slot of the data signal in question. A coincidence detector in the form of an AND circuit 8 with its inputs connected to the outputs of the pulse shaping circuits sends a pulse on its output when the output signals from the circuits 7 overlap each other. Such a coincidence-making pulse is allowed to clock a positive flank triggered D-flip flop 9, connected as a binary counter. The Q output from the flip flop is connected to the input 20 on the EXCLUSIVE-OR-circuit. This arrangement thus signifies that a small distance between a positive data flank and a positive clock signal flank reverses the phase in the signal on output 13 by 180 .

In Figure 4 the signal A shows an input data flow of the NRZ type. This signal w ll.thus be detected in the receiver with the aid of the local asynchronous clock signal B. The bit rate for data is illustrat as being constant, as well as the frequency of the clock signal. In general, these quantities can however be permitted to drift in relati to each other. -

It will be seen immediately that it would not be possible to directly synchronize input data with the clock signal B. In accordance with th inventive idea as described above -in connection with Figure 2, there are now created positive pulses C when positive flanks in the input data A are detected. Positive pulses D are^" formed in the same way whe positive flanks are detected in the possibly phase-shifted clock sign on the output from the EXCLUSIVE-OR-circuit 10 according to Figure 2. For the positive flank occurring first in the input data there is obtained an overlap, coincidence, between the positive pulses thus formed. This is indicated by the pulse in the signal E. According to the above, this pulse controls the phase reversal of the incoming clo signal applied to the input 11 according to Figure 2. Two further coincidences are marked in the signal E. The clock signal corrected , by the phase reversal is illustrated by the signal F. In accordance with what has been said hereinbefore, this signal constitutes the clo signal to the first sampling circuit 1. The output signal from this sampling circuit has been denoted by the Letter G.

In accordance with the inventive idea, this signal G will now be cloc in a second sampling circuit 2 with the unaffected clock signal in th receiver. The resulting output signal from the second sampling circui which thus constitutes the synchronized data signal has been given th denotation H. It will be observed that the second phase reversal of t clock signal resuLted in a distortion of the data signal in the form an addition of one bit, marked by dashes in the figure. In actual fac such distortion is obtained in every second phase shift of the clock signal. The asynchronous relationship has been exaggerated to illustr the modus operandi of the invention, which has resulted in th occurring phase reversals. In an actual application of the invention, these phase reversals occur with a time spacing which is greater by several powers of 10, as mentioned above.

In a second embodiment of the invention adapted for input data in the form of a RZ-type signal, the first sampling circuit 1, in the form of a simple D flip flop has been replaced with the circuit according to Figure 3. This circuit is provided with a data input 17, a clock signal input 18 and an output 19 to the subsequent sampling circuit 2. The circuit function will be described below while referring to Figure 5, showing simultaneous values for a plurality of signals in the appara¬ tus.

The signal A in Figure 5, showing the input signal to the first sampling circuit 1, is thus RZ-type input data. This signal will be detected in the receiver with the aid of the local asynchronous clock .signal B and- converted to an NRZ signal synchronous with B.

The upper pulse-forming circuit 7 in Figure 2 has, as before, the task of generating a short positive pulse at the positive flank of the signal A. When the input signal is of the RZ type, one could possibly . imagine that this pulse-shaping circuit is unnecessary since the RZ signal consists of short pulses. In the cases where the pulse rate of the incoming RZ signal is extremely small, or very large, this pulse- shaping circuit is necessary, however. In the case described in Figure 5, we assume for the sake of simplicity that the output signal from said pulse-shaping circuit is the same as the signal A, i. e. in this particu- lar case the circuit is superflous.

A delaying circuit 14 is connected to the data input 17 on the circuit according to Figure 3. The delay in this is great in comparison with the delay in the D flip flop 15, but small in comparison to the duration of the pulses from the circuits 7 in Figure 2. The delayed signal is denoted by A'^*. In the sequence illustrated in Figure 5, the delay has no importance, and therefore its function will be explained later.

The function block in Figure 2 has already been described. The signals D, E and F in Figure 5 correspond to signals with the same denotatio in Figure 4.

When RZ pulses A' arive at the clock input on the D flip flop 15 in Figure 3, the flip flop is set in the one state, and the output sign from the Q output corresponds to the signal I in Figure 5. A fixed voltage corresponding to a logical one is applied to the data input the flip flop the whole time. After ^'some^'time, the flip flop is rese by a signal on its R input coming from the output 13 in Figure 2. Th signal is denoted F in Figure 5. Detection and phase reversal on coincidence between A and D prvent the flip flop 15 from being reset immediately after or simultaneously with it being clocked. Coinciden occurs in good time before the front flanks of the signals D and A coincide, and coincidence generates the signal E which in turn gener phase shift in the signal D.

The signal D also has the task of clocking the flip flop 16 in Figur Since the time delay between the signals R and Q .at the flip flop 15 is much greater than the time which data must remain stable on the D input of the flip flop 16, there is no problem in clocking in data i the flip flop 16 before it disappears. This is providing that the fl flops 15 and 16 are in the same circuit family.

If no pulse occurs in the signal A, i. e. a Logical zero has been se the Q input on the flip flop 16 will remain zeroed, which results in that a zero is clocked in on this flip flop. Signal G is on the outp 19 of the flip flop 16 and on the output from flip flop 2 in Figure there is the signal H.

The indications P in the signal H in Figure 5 denote data errors. Da should have been 1 for both indications. Irrespective of what values the signal A has for corresponding times, a zero is obtained in the signal H for alternate pulses in the signal E.

The denotation R in the signal H denotes an extra bit analogous with the dashed extra bit according to Figure 4. The necessity of using a delaying circuit 14 is not directly apparent from Figure 5. The delaying function is only required when the frequency of the clock signal B is lower than the bit frequency of the data signal, i. e. for the opposite situation compared with Figure 5.

Assume that the clock frequency B is lower than the bit frequency. Assume further that the delaying circuit 14 is bypassed to start with. The rear flank on the pulses in the -signal D will then successively approach the front flank of the pulses in signal A. For the signal D to shift phase, the signals A and D must slightly overlap each other so that the pulses E will be sufficiently Long. This situation imme¬ diately before phase reversal causes problems, since the D flip flop 15 is clocked simultaneously as the signal on the R input is high.

By introducing the delaying circuit 14, this problem does not occur since we obtain phase reversal before the positive flank in the signal A' and the negative flank in the signal D coincide. A prerequisite of this is naturally that the delay is sufficiently long for the signal E to have time to cause phase shift before the flanks coincide.

Claims

WHAT WE CLAIM IS :

1 A method in a receiver for binary coded data signals for syn¬ chronizing an incoming data signal (A) with a. Local clock signal CB) available in the receiver, characterized in that it comprises the following steps: a) the data signal CA) is clocked with a phase Corrected clock sign (F) whereafter ^'* - b) the signal thus resulting is once again clocked with the uncorre clock signal CB).

2 A method as claimed in claim 1, characterized in that the loca clock signal CB) is phase-shifted 180 to form the phase corrected clock signal CF) when positive flanks in* the local clock signal CB⁾ and the incoming data signal CA) tend to coincide due to mutual dri

3 A method as claimed in claim 1, characterized in that the loca clock signal CB) is phase-shifted 180 for forming the phase-correc clock signal CF) when negative flanks in the local clock signal CB) and the incoming data signal CA) tend to coincide due to mutual dri

4 An apparatus in a receiver for binary coded data signals for synchronizing an incoming data signal CA) with a local clock signal CB) available in the receiver, characterized in that it comprises: a) a first and a second sampling circuit C1, 2) each provided with data input, a data output and a clock input, b) a phase-reversing circuit C3) provided with a first and a second input and an output, whereby the data input of said sampling circuit CD constitutes the input C of the apparatus and is adapted to receive the data signal arriving at the receiver and whereby its data output is connected to the dat input of the sampling circuit C2) the data output of said sampling circuit C2) constituting the output C6) of the apparatus, and where its clock input is adapted to receive the local clock signal CB) an the first input of said phase-reversing circuit C3) is adapted to receive the clock signal CB), its second input being adapted to receive the data signal CA) and whereby its output is conn the clock input of the sampling circuit CD for applying thereto a corrected clock signal CF) in response to the relative position between the flanks of pulses in the data signal CA) and clock signal CB).

5 Apparatus as claimed in claim 4, characterized in that said first and said second sampling circuits. constitute positively flank-triggered D flip flops.