Receiver with feedback filter, and eye monitor for the feedback filter

An optical receiver with an electronic filter is described, the parameters of the filter being set by means of high-speed eye monitors. Also described is a high-speed eye monitor with threshold-value decision elements which are set close to the vertices of the eye of an eye diagram, the eye monitor being optimized through connection of a pseudo-error generator and comparison with setpoint values and outputting the eye opening and the Q-factor.

A receiver 1 for optical signals is shown schematically in FIG. 2 . The receiver 1 is connected to an optical transmission link 2 . In the receiver 1 there is an opto-electric converter 4 which is connected to a high-speed eye monitor 5 . The high-speed eye monitor 5 is connected, in turn, to a filter 6 . The output of the filter 6 is connected to an electrical output line 3 . FIG. 3 shows an exemplary embodiment of a receiver 1 for optical signals, The electronic filter 5 —in this special case a DFE (distributed feedback equalizer)—is aconnected to the optical transmission link 2 and to an opto-electronic converter, not represented here. The electronic filter most commonly consists of two threshold-value decision elements connected in parallel. The outputs of the threshold-value decision elements are connected to a switch, so that the signal is sampled by either the first threshold-value decision element of the DFE or the second threshold-value decision element. The thresholds of the threshold-value decision elements can be set. However, any other adaptive system (optical PMD compensator, electronic filter) whose parameters can be set through measurement of the quality of the channel is suitable for realization of the invention. An example of a DFE is also known from DE 10015115.9, which we hereby consider as belonging to the disclosure of this application. The DFE 5 is connected to the signal output line 3 . In addition, there is a connection between the DFE 5 and a first eye monitor 61 and a second eye monitor 62 . The DFE 5 additionally has a control line S 1 and S 2 to each eye monitor respectively. The better the quality of the transmission line can be represented in the eye monitor, the better the signals decided by the DFE 5 can be measured and made available as parameters. The threshold values of the DFE can thus be set via the two eye monitors. The eye monitors each provide a threshold value V eye — lower• and V eye — upper• . These measured quantities are determined by the eye monitors. In this case, the eye monitors measure the edges of the eye opening of the signal. The parameters of the decision element in the electronic filter DFE 5 are determined through measurement of the two extreme values. Measurement at the extreme points of the eye opening improves the determination for the signal in the centre of the eye opening. Not only does such an arrangement take account of high-probability signals, but the method is also based on low-probability signals. The bit error rate is substantially improved as a result. The DFE 5 has control outputs S 1 and S 2 which are activated when the DFE effects the decision through Vth 1 or Vth 2 respectively. The eye monitors operate following activation through the control signals S 1 and S 2 . The eye monitors supply information on optimum threshold values and return it to the DFE 5 . An embodiment for a high-speed eye monitor is represented in FIG. 4 . FIG. 4 shows the high-speed eye monitor 5 . The data input 7 is connected to three threshold-value decision elements S 0 , S 1 and S 2 . The output of the threshold-value decision element S 0 is the data signal line 8 . The outputs of the threshold-value decision elements S 1 and S 2 are each connected to an EXOR circuit E 1 and E 2 respectively. The second input of each of the EXOR circuits E 1 and E 2 has a connection to the data signal line 8 . The output of each of the EXOR circuits E 1 and E 2 is connected to an integrator I 1 and I 2 respectively. The outputs of the integrators are in turn each connected to an adder A 1 and A 2 respectively, the second input of which is connected to a line for setting a threshold value. On the output side, the adders A 1 and A 2 are connected to regulators RI and R 2 . The outputs of the regulators are connected both to a further adder A 3 and to the threshold-value decision elements S 1 and S 2 , whose threshold value they set. The output of the adder A 3 is connected to a data line for the eye height. The high-speed eye monitor 5 receives the opto-electrically converted data of the converter 4 on its input signal side 7 . The received data has been garbled and blurred by non-linear effects on the transmission link. This garbled data is distributed to the three threshold-value decision elements, where it is compared with a threshold value. The threshold-value decision element S 0 compares the received garbled data with a reference value V 0 . The comparison in the threshold-value decision element S 0 is influenced by a parameter C 0 which is obtained from the result of the measurement of the eye height. The result at the threshold-value decision element S 0 is “determined” data which, in the ideal case, corresponds to the transmitted data. The eye monitor comprises two further threshold-value decision elements S 1 and S 2 . Applied respectively to them are the garbled input signal and a threshold value V 1 and V 2 . These threshold values are set so that V 1 and V 2 are located at the lower and upper edge vertex of the eye. The thus respectively determined signals are applied to EXOR circuits E 1 and E 2 , in which they are compared with the determined signals of the data channel. This comparison is used to determine the respective errors in the monitor channels. The errors are then respectively integrated in the integrators I 1 and I 2 . The result for S 1 , E 1 and I 1 is an internal voltage V int — upper which represents a control variable for the upper vertex of the eye opening. The control variable V int — lower which represents the lower vertex of the eye, is obtained from the monitor channels S 2 , E 2 and I 2 . The internal control variables are compared, in the adders A 1 and A 2 , with a preset setpoint value V 1target and V 2targer . The deviation of the internal quantities from the setpoint values is used for adjusting the regulators R 1 and R 2 . Their output voltage, added at the adder A 3 , provides a value for the eye opening. This value is to have an optimum value. Consequently, in the event of deviations from the control variables, the regulators readjust the threshold values for the decision elements S 1 and S 2 and output these as eye edges. FIG. 5 shows a result of a measurement with the high-speed eye monitor. The figure shows the internal control variable V int over the difference V eye upper and V eye — lower . Shown within the figure is an eye diagram with an eye opening which is equal to the quantity V eye — upper• -V eye — lower• . The result of the internal control variables V int — lower and V int — upper is shown. It can be seen that there is a deviation of the control variables from the setpoint value V target . In such a case, the regulators readjust the threshold values of the decision elements so that the resulting internal control variables approximate to the setpoint value. In order to measure the sharpness of the eye edges, a small signal is superimposed on the setpoint value V target as shown in FIG. 6 . This sinusoidal signal is detected as a response in the internal control variables and evaluated. This small-signal response and the eye opening are used to determine the Q-factor of the transmission link. This can then be used in an equalizer, as an active parameter of the transmission link, and the signal is thereby optimized. The method according to the invention for recovering signals by means of a DFE and parameters determined through measurements of the signal quality in an eye diagram permits rapid and optimum recovery of garbled optical signals. In this case, the connected eye monitors supply not only the feedback signals for the threshold values of the DFE 5 , but additionally supply valuable information for optimization of the transmission link. The measurement of the Q-factor and of the magnitude of the eye opening serves to optimize the entire transmission link and the entire transmission system.