JPS60130950A - Signal demodulating system - Google Patents

Abstract

PURPOSE:To decrease the margin of a signal at the demodulation of a reception signal by extracting a DC component of a reception signal, and changing the threshold voltage of an identification circuit in response to the fluctuation of a DC level. CONSTITUTION:When the DC balance of an optical modulation signal (c) is unbalanced and a signal (e) at the output side of a coupling capacitor 10 goes to small, this signal (e) passes through a diode 12, a capacitor 13 and an amplifier 14 and an analog voltage in response to the change in the peak voltage is detected at points D and E. This voltage is set to a prescribed value by resistors 15, 16 and 17 and formed into the threshold voltage of identification circuits 11, 11'. When the DC level of the signal (e) is lowered, the threshold voltage is also lowered.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a signal demodulation method, and more specifically, in optical communication, etc., when a DMI code is used as a transmission code and a received signal is demodulated by a duobinary shaping method, A method for automatically adjusting the threshold voltage for signal identification [Background of the invention] As is well known, optical communication involves converting an electrical signal into an optical signal on the transmitting side and transmitting it, and converting the optical signal into an electrical signal on the receiving side. Send and receive data. There are many types of modulation and demodulation methods used in optical communications, but they are easy to extract timing signals, have good DC balance, and require relatively small scale modulation and demodulation circuits. from.

The DMI duobinary molding method is attracting attention.

1st. The figure shows a block diagram of an optical transmitter/receiver circuit using the conventional DM, I duobinary forming method. In FIG. 1, the final output stage circuit following the modulation circuit 1 on the transmitting side is composed of a light emitting element 2 and a resistor 3.

On the other hand, the receiving side includes a light receiving element 4, a preamplifier circuit 5, a main amplifying circuit 7, a duobinary forming circuit 9, and an identification circuit 11.1.
1' and a coupling capacitor 6.8.10. FIG. 2 shows signal waveforms at various parts in FIG. 1.

Now, assume that the NRZ format signal a shown in FIG. 2(a) is input to the A terminal. In Figure 2, tr Hu is logic #l
II, II LII are areas of logic "0". The modulation circuit 1 converts the NRZ signal into a DMI signal as shown in FIG. 2(b).
Modulates the signal. The output signal of this modulation circuit 1 is converted into an optical signal C by a light emitting element 2. FIG. 2(e) shows an optical modulation signal sent out from the light emitting element 2. On the receiving side,
After the optical modulation signal C is converted into an electrical signal by the light receiving element 4, it is amplified by the preamplifier 5, the coupling capacitor 6, and the main amplifier 7, and the signal d shown in FIG. 2(d) is sent to the output side of the coupling capacitor 8. get. This signal d is converted into a duobinary forming circuit 9
AMI signal e shown in FIG. 2(e) appears on the output side of the coupling capacitor 10. This AMI
By comparing the signal e with a constant threshold voltage ■9° in the discrimination circuit 11, a signal f shown in FIG.
By comparing the signal g shown in FIG. 2(g) with C
Get to the terminal. Although omitted in FIG. 1, the signal shown in FIG. Signal a shown in a) is reproduced.

By the way, in the configuration shown in FIG. 1, if the DC balance of the optical modulation signal C is disrupted due to the response characteristics of one light-emitting element 2 or the characteristics of the transmission path, a slight DC level will appear in the output of the preamplifier 5 on the receiving side. The fluctuation of is cursed. This DC level fluctuation results in a large level fluctuation on the output side of the duobinary forming circuit 9 due to the transient response caused by the coupling capacitor 6.8, the characteristics of the main amplifier 7, and the like. In such a case, the first
An example of the signal waveform of each part in the figure is shown in FIG.

Figures 3(a) and (b) are the same as the waveforms in Figures 2(a) and (b), but the optical modulation signal C shown in Figure 3(c) has a decreased positive direction component. There is. In this case, the waveforms on the output side of the coupling capacitors 8 and 10 are shown in Fig. 3(d) and Fig. 3(e).
become that way. That is, the DC level fluctuates greatly on the output side of the duobinary forming circuit 9, which causes the discrimination circuit 11.
This reduces the margin of signal identification in 11' and causes errors in signal identification. Furthermore, if fluctuations in the DC level on the output side of the coupling capacitor 6 are repeated over a long period due to the state of the code string of the optical modulation signal input, it will be affected by the low-frequency characteristics of the amplifier circuit system, resulting in duobinary formation. DC level fluctuations on the output side of the circuit 9 become complicated, and normal signal identification becomes difficult.

Further, such fluctuations in the DC level also become a problem when an optical modulation signal is input from a state where no optical modulation signal is present. FIG. 4 is a diagram explaining this. When an optical modulation signal is input from a state where no optical modulation signal is input as shown in FIG. 4(a), the output side of the coupling capacitor 6 is )
As shown in , immediately after receiving the optical modulation signal, the maximum
2V, and thereafter, through a transient response of the coupling capacitor 6, the upper end voltage of the signal becomes +V and the lower end voltage becomes 1.5V, and a steady state is reached. That is, the DC level fluctuation of the optical modulation signal appears transiently, and this becomes a large DC level fluctuation on the output side of the duobinary shaping circuit 9 due to the main amplifier 7, coupling capacitor 8, etc. For this reason, normal signal transmission is impossible until a predetermined period of time has elapsed from the start of signal reception.

[Purpose of the invention]

An object of the present invention is to add DMI to a transmission code in optical communications, etc.
The purpose of this invention is to prevent a reduction in signal identification margin and occurrence of signal identification errors when demodulating a received signal using a duobinary shaping method using a code.

[Summary of the invention]

The gist of the present invention is to extract the DC component of the received signal and change the threshold voltage of the identification circuit in accordance with fluctuations in the DC level of the received signal.6 [Embodiment of the Invention] Figure 5 shows This is an embodiment of the present invention, and the differences from FIG. 1 are as follows.
A diode 12.12' and a capacitor 13.13' are connected between the duobinary forming circuit 9 and the identification circuit 11.11'.
, an amplifier 14.14', and an automatic threshold value adjustment circuit 18 consisting of resistors 15 and 16.17.

After the DMI optical variable modulation signal is converted into an electrical signal by the optical element 4, it is amplified by the preamplifier 5, the coupling capacitor 6, and the main amplifier 7. By passing this amplified DM■ modulation signal through a coupling capacitor 8 and a duobinary forming circuit 9, an AMI signal e is obtained at the output side of the coupling capacitor 10. The operation up to this point is the same as in FIG.

Now, suppose that the DC balance of the optical modulation signal C is disrupted and the signal e on the output side of the coupling capacitor 10 becomes as shown in FIG. 3 (,e). This signal e is connected to diode 12 and capacitor 1.
3. V of the waveform of FIG. 3 by passing through the amplifier 14.

An analog voltage corresponding to a change in the peak voltage V+ on one side centered on is detected at point D. Similarly, the signal e passes through the diode 12', the capacitor 13', and the amplifier 14', so that the voltage is increased by the resistors 15, 16, and 17, respectively, to v10 V, /2. V-+V, /2, and identify these 51
114811, 11' (7) Threshold voltage V* z , V
It is compared with signal e as R2.

FIG. 6 shows the relationship between the signal e and the threshold voltages VR2 and vn2. That is, when the DC level of the signal e decreases, v:z, v;z also decrease accordingly. Therefore, it is possible to prevent a reduction in the signal identification margin in the identification circuits 11, 11' and to prevent signal identification errors from occurring.

Although optical communication has been described above, it goes without saying that the present invention can be applied to other applications besides optical communication as long as the DMI code is used as the transmission code and the received signal is demodulated by the duobinary shaping method. do not have.

[Effects of the Invention] As described above, according to the present invention, even if the DC level of the received signal changes, the threshold voltage changes accordingly, so the margin for signal identification does not decrease. .

Further, even if a modulated signal is input from a state where there is no modulated signal, there is an effect that the signal can be received normally immediately after the start of signal reception without being affected by transient fluctuations in the DC level.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a configuration example of a conventional signal demodulation method. Figure 2 is a signal waveform diagram of each part in Figure 1 when there is no DC level fluctuation, Figure 3 is a signal waveform diagram of each part in Figure 1 when there is DC level fluctuation, and Figure 4 is the input of the received signal. FIG. 5 is a diagram illustrating an embodiment of the present invention, and FIG. 6 is a diagram illustrating the relationship between the output signal of the duobinary shaping circuit and the dark value voltage in FIG. 5. 1... Binary AMI modulation circuit, 2... Light emitting element, 4
...Photodetector, 5.Preamplifier, 7.Main amplifier, 9.Duobinary forming circuit, 11゜1
1'...Identification circuit, 18...Automatic threshold adjustment circuit.

Claims (1)

[Claims] (1) In a method in which a DMI code is used as a transmission code and a received signal is demodulated by a duobinary shaping method, a DC component included in the received signal is extracted and a voltage that changes according to this DC component is obtained, A signal demodulation method characterized in that a duobinary shaped signal is identified using the voltage as a threshold.