IE48004B1

IE48004B1 - A device for monitoring the quality of a synchronous digital transmission signal

Info

Publication number: IE48004B1
Application number: IE599/79A
Authority: IE
Original assignee: Cit Alcatel
Priority date: 1978-03-10
Filing date: 1979-08-08
Publication date: 1984-08-22
Also published as: FR2419622A1; IT7967507A0; IE790599L; DE2908587A1; BE874261A; LU81021A1; FR2419622B1; NL7901889A; GB2016246B; JPS54126407A; CA1126867A; GB2016246A; IT1118413B

Abstract

A circuit for monitoring the quality of a digital base band transmission signal having a constant unit time interval, includes a sign coincidence auto- correlator (4,5), which correlates the polarities of two versions of the received signal delayed in relation to each other by (3), an integer multiple of the unit time interval T. The input signal is limited at 2 to provide logic levels 1 or 0 and the combining or correlating device may be an Exclusive OR circuit or a multiplier. The combined output is integrated at 6 with a time constant long compared to the time interval T. The circuit qualitatively measures the differences between a reference time interval and the time interval which separates the consecutive zeros of the input signal and provides an indication analogous to that provided by the eye pattern.

Description

The invention comes within the field of digital transmission. It relates more particularly to monitoring the quality of a synchronous digital transmission. The monitoring is performed on data received in the base band. It allows the degree of distortion of the transmitted signals and hence the error rate which can result therefrom to be estimated.

The quality of a base band digital transmission signal is generally judged by observing the apperture of the eye pattern which is the figure formed by superposing all the possible configurations -over a unit time interval of the transmitted signal. The apperture of the eye pattern is generally observed by means of an oscilloscope and requires an operator.

Preferred embodiments of the present invention provide a device which gives a quantitative indication of the quality of a synchronous digital transmission link.

The present invention provides a device for monitoring the quality of a digital base band transmission signal having a constant unit time interval, wherein the device includes a sign coincidence autocorrelator which correlates the polarities of two versions of the received signal, one of the versions being delayed in relation to the other version by an integer multiple of the unit, time interval.

According to a first embodiment, the monitor device includes: - an absolute limiter connected to its input, the output of said limiter delivering a logic signal whose level is +1 if its input signal is positive and 0 in the contrary case; - an exclusive OR logic gate with two inputs each connected to the output of the absolute limiter, one directly, the other via a delay circuit; - said delay circuit introducing a delay equal to an integer multiple of a unit time interval; and - an integrator disposed at the output and connected to the output of the exclusive OR logic gate.

According to a second embodiment, the monitor device includes: - an absolute limiter connected to its input, the output of said limiter delivering a binary signal whose level is +1 if its input signal is positive and -1 in the contrary case; - a multiplier with two inputs each connected to the output of the absolute limiter, one directly, the other via a delay circuit; - said delay circuit introducing a delay equal to an integer multiple of a unit time interval; and - an integrator disposed at the output and connected to the output of the multiplier.

Two embodiments of the invention are described by way of example, with reference to the accompanying drawing, in which : - figures 1 and 2 are block diagrams of two monitor devices in accordance with the invention; and - figures 3 and 4 are graphs which illustrate the operation of the devices illustrated in figures 1 and 2.

Two examples of sign coincidence autocorrelators will be described hereinbelow. It will then be shown that they qualitatively measure the differences between a reference time interval and the time interval which separates the consecutive zeros of their input signals, and that the measurement expresses the quality of a synchronous digital transmission when the input signal is the signal which comes from this transmission and the reference time interval is taken to be equal to an integer multiple of the unit time interval of the above-mentioned transmission.

The sign coincidence autocorrelator illustrated in figure 1 includes: - an absolute limiter 2 disposed at its input; - an adder 4 with two inputs each connected to the output of the absolute limiter 2, one directly and the other via a delay circuit 3; ~ the delay circuit 3; and - an integrator 6 connected to the output of the adder 4.

An input signal s(t) applied to the input 1 is received by the absolute limiter 2 whose output delivers a logic signal u^t). The level of the logic signal u1(t) is, by definition, 1 if the input signal s(t) is positive and 0 in the contrary case. The delay circuit 3 receives the signal u^t) which comes from the absolute limiter 2 and delays it by one period τ. The adder performs the exclusive OR function. One of its inputs receives the signal u^(t) which comes from the absolute limiter 2 and its other input receives the same signal delayed by one period τ by the delay circuit 3. Its output 5 delivers a signal q(t) applied to the integrator 6 whose output signal Q(t) , available at 7, is related to the signal q(t) by the equation: to+ti Q(t) = jq(t) dt (q(t) = 0 or 1) fco The sign coincidence autocorrelator illustrated in figure 2 includes: - an absolute limiter 12 disposed at its input; - a multiplier 14 with two inputs each connected to the output of the absolute limiter 12, one directly and the other via a delay circuit 13; - the delay circuit 12; and - an integrator 16 connected to the output of the multiplier 14.

An input signal s(t) applied to the input 10 is received by the absolute limiter 12 which, by definition, delivers an output signal u^(t) equal to +1 when s(t) is positive and in the contrary case to -1. The signal u2(t) is applied without delay to one input of the multiplier 14 and to the other input of the multiplier with a delay of τ. This generates, at the output 15, a signal p(t) which is related to the signal q(t) of the exclusive OR logic gate 4 of the preceding circuit by the equation: p(t) = 1 - 2 q(t) The output 17 of the integrator 16 delivers a signal P(t) related to the signal p(t) by the equation: to+ti P(t) = F jp(t) dt ( p(t) = +1 ) to The signals Q(t) and P(t) are related by the same equation as the signals p(t) and q(t).

The delay circuits 3 and 13 which process only binary signals can be formed by means of shift registers which have n stages and operate at a frequency of η/τ, n being an integer chosen so as to obtain an acceptable compromise between the cost of the registers and the precision of the autocorrelators. In pratice, the number n is chosen between 10 and 100.

The integrators 6 and 16 can be formed by means of low-pass filters with a time constant of t^.

During experiments, it has been observed that the devices described with reference to figures 1 and 2 give an indication concerning the quality of a synchronous digital signal analogous to the one given by an eye pattern when the delay τ of their delay circuits 3, 13 is equal to an integer multiple of the unit time interval T of the connection and when the integration period is long in comparison with the unit time interval T.

To explain this phenomenon, it can be shown that, like the width of the eye pattern, the output signals of the devices described with reference to figures 1 and 2 are sensitive to the differences in absolute value between the unit time interval T and the time intervals Τ' which separate the consecutive zero passes of the synchronous digital transmission signal.

In order to do this, let us consider a signal s(t) which passes through zero at intervals of I'. If the origin of a period is a zero pass of the signal, an example of this type of signal applied to the input 10, figure 2, can be: s(t) = siniri The signal u2 (fc) at the output of the absolute limiter is expressed as: u2(t) = sgn s(t) = sgn 3ΐηπψ, Taking the delay τ caused by the circuit 13 as equal to T, the two signals applied to the multiplier 14 are: t t - T sgn simr^, and sgn sinir—— The output signal p(t) of the multiplier 14 is therefore t t - T p(t) = sgn βίηπψ, . sgn sin*—p— t t - T = sgn (sir,π ψ, . sinir—^r-) , T 2 t - T. = sgn (cos π - cosir-^-) On examining the preceding expression, it will be seen that the parenthesis remains negative when T is equal to Τ' 2t - T except for particular values of t such as cosrc-= 1 where it is zero.

Besides these particular values of t, we have: p(t) = -1 therefore the average value p'(t) of the signal p(t) over any period is equal to -1.

A similar reasoning to that used for T/T' = 1 shows that where T/T' = 0 and T/T' = 2, the average value p'(t) of the signal p(t) is equal to +1.

Where T/T is not an integer, p(t) is a periodic function of Τ', and the average value p'(t) of p(t) can. therefore be calculated over a period which is an integer multiple of Τ' and in particular over a period Τ'. For values of T/T' which are not whole values and lie between the intervals (0,1) and (1,2), the parenthesis of the expression of p(t) is positive for a part of the time and p'(t) is greater than -1. It can be shown that p'(t) varies linearly from -1 to +1 when the ratio T/Τ' varies from 1 to 0 and from 1 to 2.

Figure 3 is a graph which shows the variation of p'(t) as a function of the ratio T/T. It will be observed, according to the graph, that the average value p'(t) is a minimum value only when T is equal to Τ' and that it is independent from the sign of the difference between T and Τ'. Since the integration period t^ is long with respect to T, 8 0 0 4 firstly, the signal P(t) is at its minimum value only when the time intervals Τ' between the consecutive zeros of the input signal s(t) are each strictly equal to T, and secondly the difference between the signal P(t) and its minimum value represents the average value of the differences, taken in absolute value, between the time intervals Τ' in relation to the unit time interval T, Since the latter property is also true for the width of the eye pattern, it is deduced that the signal P(t) represents the width of the eye pattern and therefore the quality of the transmission.

Figure 4 is a graph which shows the average value q'(t) of the signal q(t) over a period Τ' for a signal s(t) analogous to the one considered previously. The curve is drawn as a function of T/Τ' and is deduced from the preceding curve by the equation p' (t) =1-2 q' (t) already mentioned for the functions p(t) and q(t).

The curve shows that the average value q'(t) varies between 0 and 1 and is at its maximum only when T is equal to Τ'; it is independent from the sign of the difference between T and Τ'. As in the preceding case, since the integration period is long in relation to T, the signal Q(t) is at its maximum value only when the time intervals T’ between the consecutive zeros of the input signal s(t) are each strictly equal to T and the difference between the signal Q(t) and its maximum value represents the average value of the differences, taken in absolute value, between the time intervals Τ' and the unit time interval T. It is also deduced therefrom that the signal Q(t) represents the width of the eye pattern and therefore the quality of the transmission.

The two devices which have just been described provide information concerning the quality of a digital transmission in the same way as does the eye pattern. But since they deliver signals which can be used directly, they have a much wider field of application.

Without going beyond the scope of the invention, some dispositions can be modified or some means can be replaced by equivalent means.

Claims

1./ A device for monitoring the quality of a digital base band transmission signal having a constant unit time interval, wherein the device includes a sign coincidence autocorrelator which correlates the polarities of two versions of the received signal, one of the versions being delayed in relation to the other version by an integer multiple of the unit time interval.

2. / A monitor device according to claim 1, wherein that the sign coincidence autocorrelator correlates the polarities of two versions of the received signal which are delayed in relation to each other by one unit time interval.

3. / A monitor device according to claim 1, including : - an absolute limiter connected to its input, the output of said limiter delivering a logic signal whose level is +1 if its input signal is positive and 0 in the contrary case; - an exclusive OR logic gate with two inputs each connected to the output of the absolute limiter, one directly, the other via a delay circuit; - said delay circuit introducing a delay equal to an integer multiple of a unit time interval; and - an integrator disposed at the output and connected to the output of the exclusive OR logic gate.

4. / A monitor device according to claim 1, including : - an absolute limiter connected to its input, the output of said limiter delivering a binary signal whose level is +1 if its input signal is positive and -1 in the contrary case; - a multiplier with two inputs each connected to the output of the absolute limiter, one directly, the other via a delay circuit; - said delay circuit introducing a delay equal to an integer multiple of a unit time interval; and - an integrator disposed at the output and connected to the output of the multiplier.

5. / A monitor device according to claim 3 or 4, wherein 5 said delay circuit is a shift register.

6. / A monitor device according to claim 3 or 4, wherein said integrator is a low-pass filter.

7. / A monitor device substantially as herein described with reference to figures 1 and 3 or figures 2 and 4 of

8. 10 the accompanying drawings.