GB2094104A - Measuring the eye height of a data-waveform - Google Patents

Abstract

An apparatus and method for measuring the eye height of a data- waveform. The voltage of the waveform is sampled at successive sampling intervals, signals of opposite polarity to the intersymbol interference at the respective sampling interval are added to each sampled voltage, whereby the intersymbol interference is minimised, and the moduli of the magnitudes of the added signals are summed to provide a signal representative of the data- waveform eye height.

Description

SPECIFICATION Measuring the eye height of a data-waveform The present invention relates to an apparatus and method for measuring the eye height of a data-waveform.

In a digital data transmission system degradations occur which increase the probability of symbols being wrongly interpreted at the receiver. As described below, a well known measure of the performance of such a transmission system is the eye height. In practice this is a difficu!t parameter to quantify because of the necessity of making a large number of observations.

It is known that the eye height is a function of the sum of the moduli of the amplitudes of the intersymbol interference measured at the correct sampling intervals when a single pulse is applied to the system. It is an object of the present invention to provide a means of quantifying this sum.

According to to the present invention there is provided an apparatus for measuring the eye height of a data-waveform, characterised by means for sampling the voltage of the waveform at successive sampling intervals, means for adding to each sampled voltage a signal of opposite polarity to the intersymbol interference at the respective sampling interval, whereby the intersymbol interference is minimised, and means for summing the moduli of the magnitudes of the added signals to provide a signal representative of the data-waveform eye height.

The invention also provides a method for measuring the eye height of a data-waveform, characterised in that the voltage of the waveform is sampled at successive sampling intervals, each sampled voltage is modified to minimise intersymbol interference by adding to it signals of opposite polarity to the intersymbol interference at the respective sampling interval, and the moduli of the magnitudes of the added signals are summed to provide a signal representative of the data-waveform eye height.

Preferably the sampling means comprises a plurality of delay means connected in series and each providing a delay equal to the bit period of the data-waveform. The signals at the terminals of each delay means are each modified by a variable gain/loss circuit and then aggregated.in a summing amplifier.

Circuit means are connected to the output of the summing amplifier for providing a data output, a data clock output and an error signal. A strobed slicer providing one more data bit level than the transmission system is connected to the output of the summing amplifier to provide at its most significant output level said data output, circuit means are connected to receive said data output and to derive a clock output therefrom, and at its last significant bit output level a shift register is connected to provide said error signal, the slicer and the shift register being clocked by said clock output.

A correlator circuit is provided in respect of each variable gain/loss circuit, the correlator circuit being arranged to correlate the said error signal with said data output delayed by a number of sampling intervals appropriate to the respective variable gain/loss device. The delayed data output may be provided by a shift register through which the said data output is clocked by said clock output.

Preferably the correlator circuit comprises an exclusive or gate to the inputs of which the said delayed data output and the said error signal are applied, a counter circuit connected to the output of the exclusive or gate and clocked by said clock output, and a digital to analogue converter connected between the counter circuit and the variable gain/loss device to be controlled.

The counter circuit may comprise a first up/down counter which when it reaches a terminal count is reset to half its maximum count and a second up/down counter clocked by the terminal count pulse of the first and connected to the output of the exclusive or gate, the second counter counting up or down in dependence upon the output ofthe gate.

Preferably the output of the counter circuit and a signal identical to the midcount output of the counter circuit are applied to an adder which provides an output equal to the modulus of the difference between its inputs, the outputs of the adders of all the correlator circuits being applied to a summing circuit which provides an eye height representative output.

An embodiment of the pesent invention as applied to the measurement of teletext eye height will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an ideal teletext data-waveform; Figure 2 shows superimposed oscilloscope traces for successive sample periods of a teletext signal; and Figure 3 is a block schematic diagram of an embodiment of the present invention.

Eye height is the parameter which is usually taken as the figure of merit for data-waveforms. For the ideal data wave of Figure 1 the decoder decision level can be varied through the full amplitude range, in this case from 0 to V, without causing errors. Eye height is determined by normalising the possible range of decision levels and expressing it as a fraction of the data amplitude, i.e. the voltage difference between the steady state values for '0' and 'I'. Hence for an ideal data wave the eye height is unity, or 100 percent.

The value of an ideal data wave at any pulse-sampling instant is independent of surrounding pulses. When data is passed through a linear network having arbitrary characteristics however this independence is lost and the signal suffers intersymbol interference. Where this occurs there will always be some combination of surrounding sample values which can maximally reduce the value of any logic 'I' sample. In consequence, intersymbol interference always causes the eye height (and the noise margin) of the data to be less than unity.

If the voltage waveforms for successive sample intervals were superimposed on the display of an oscilloscope, the result might appear as shown in Figure 2. The minimum separation between the '0' level and the 'I' level assumes the characteristic eye shape which gives rise to the term "eye height".

It is known that the response of a network to a single teletext pulse is such that the reduction of eye height due to the presence of intersymbol interference contributions f(kT) is equal to the sum of their moduli L:'1 f(kT) I. Thus the eye height h is given by:

where f (0) is the maximum value of the pulse, L:' denotes summation excluding k = 0, and the optimum sampling time occurs att = 0. The present invention is based on this knowledge.

Referring now to Figure 3, a teletext adaptive equalizer embodying the invention will be described. An incoming teletext signal, 1, is applied to an even number of delay lines 2, connected in tandem. Each delay line has a delay equal to the bit period, which in the case of teletext is 144ns. The signal at the terminals of each delay line 2 is passed through a respective variable gain/loss device 3 to a summing amplifier 4. The summed output is applied to a two bit strobed slicer 5. The most significant output bit of the slicer comprises the data output D, which is applied to a video processor circuit 6 which extracts the required data clock signal C, which strobes the slicer 5. Both outputs from the slicer 5 are combined in an exclusive or gate 7, the output of which is delayed one bit period by means of shift register 8.The resultant output comprises an error signal E, associated with the antecedent data bit D.

Each variable gain/loss device 3 is controlled by a respective circuit, only one of which is shown in Figure 3, where the error signal E is correlated with a suitably delayed data signal Dn produced by using the clock signal C to clock the data signal D through a shift register 9. The signals E and Dn are correlated by an exclusive or gate 10 the output of which determines the direction of count of an up/down counter 11 clocked by the data clock signal C. Counter 11 is arranged such that when it achieves a terminal count it is reset to half its maximum count while the terminal count pulse clocks a second up/down counter 12 in the direction determined by gate 10.

The outputs of counter 12 are applied to a digital to analogue converter 13, producing a d.c. output used to control the gain or loss of the respective variable device 3. This is a double balanced amplifier arranged such that when counter 12 is at midcount there is no output from the amplifier 3. When however the count exceeds this value the amplifier 3 produces an output in antiphase with its input of a magnitude proportional to the excess. When the count is inferior the amplifier 3 produces an output in phase with its input of a magnitude proportional to the difference. The modulus of the diference between the actual count output and the midcount output of digital counter 12 is obtained by means of an adder 14 which is connected to receive the output of the counter 12 and an input 15 representative of the midcount output of the counter 12.

After a short period of time, when the equalizer has settled at its optimum, the output 16 of each adder 14 is proportional to the intersymbol interference which would be derived from a data signal comprising a single pulse delayed or anticipated by the appropriate number of bit periods.

All the outputs 16 are combined in a summer 17 to produce an output corresponding with the signal eye height. This is processed in a display unit 18 to show a numerical percentage readout.

In a particular embodiment of the invention the circuit elements employed to carry out the functions of the arrangements in Figure 3 were as follows: Suitable devices Block 2 Matthey Delay Unit Type UN097 3 3 Motorola Integrated Circuit Type MC1595L 4 4 Signetics Integrated Circuit Type NE592 " 5 Signetics Integrated Circuit Type NE521 6 6 Mullard Integrated Circuit Type SAA5030 7, 7, 10 Signetics Integrated Circuit Type 74LS86 8 8 Signetics Integrated Circuit Type 74LS74 9 9 Signetics Integrated Circuit Type 74LS164 n 12 Signetics Integrated Circuit Type 74LS191 13 13 Signetics Integrated Circuit Type NE5007 14 14 Signetics Integrated Circuit Type 74LS83 17 Texas Instruments Integrated Circuit Type 745281 18 18 Texas Instruments Integrated Circuit Type T1L311

Claims (12)

1. An apparatus for measuring the eye height of a data-waveform, characterised by means for sampling the voltage of the waveform at successive sampling intervals, means for adding to each sampled voltage signals of opposite polarity to the intersymbol interference at the respective sampling interval, whereby the intersymbol interference is minimised, and means for summing the moduli of the magnitudes of the added signals to provide a signal representative of the data-waveform eye height.

2. An apparatus according to claim 1, characterised in that the sampling means comprises a plurality of delay lines connected in series and each providing a delay equal to the bit period of the data-waveform, and the adding means comprises a plurality of variable gain/loss devices connected to the outputs of respective delay lines, the outputs of the variable gain/loss devices being connected to a suming amplifier.

3. An apparatus according to claim 2, characterised by circuit means connected to the output of the summing amplifier for providing a data output, a data clock output and an error signal.

4. An apparatus according to claim 3, characterised by a two bit strobed slicer connected to the output of the summing amplifier to provide at its most significant output said data output, a text acquisition circuit connected to receive said data output and to provide a clock output, an exclusive or gate connected to the two outputs of the slicer, and a shift register connected to the output of the gate to provide said error signal, the slicer and the shift register being clocked by said clock output.

5. An apparatus according to claim 3 or 4, characterised by a correlator circuit in respect of each variable gain/loss circuit, the correlator circuit being arranged to correlate the said error signal with said data output delayed by a number of sampling intervals appropriate to the respective variable gain/loss device.

6. An apparatus according to claim 5, characterised in that the said delayed data output is provided by a shift register through which the said data output is clocked by said clock output.

7. An apparatus according to claim 5 or 6, characterised in that the correlator circuit comprises an exclusive or gate to inputs of which the said delayed data output and the said error signal are applied, a counter circuit connected to the output of the exclusive or gate and clocked by said clock output, and a digital to analogue converter connected between the counter circuit and the variable gain/loss device to be controlled.

8. An apparatus according to claim 7, characterised in that the counter circuit comprises a first up/down counter which when it reaches a terminal count is reset to half its maximum count and a second up/down counter clocked by the terminal count pulse of the first and connected to the output of the exclusive or gate, the second counter counting up or down in dependence upon the output of the gate.

9. An apparatus according to claim 7 or 8, characterised in that the output of the counter circuit and a signal identical to the midcount output of the counter circuit are applied to an adder which provides an output equal to the modulus of the difference between its inputs, the outputs of the adders of all the correlator circuits being applied to a summing circuit which provides an eye height representative output.

10. A method for measuring the eye height of a data-waveform, characterised in that the voltage of the waveform is sampled at successive sampling intervals, each sampled voltage is modified to minimise intersymbol interference by adding to it signals of opposite polarity to the intersymbol interference at the respective sampling interval, and the moduli of the magnitudes of the added signals are summed to provide a signal representative of the data-waveform eye height.

11 ^ An apparatus for measuring the eye height of a data-waveform substantially as hereinbefore described with reference to the accompanying drawings.

12. A method for measuring the eye height of a data-waveform substantially as hereinbefore described with reference to the accompanying drawings.