JPS6342885B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an identification circuit for identifying digital signals consisting of codes without DC balance.

When transmitting a code without DC balance, such as a simple binary code, in a digital transmission device,
It is necessary to also transmit the DC component and amplify the transmission so that the DC levels of "1" and "0" do not change. If the DC component is not transmitted, each DC level will change when the ratio of "1" and "0" (mark rate) changes, making it impossible to identify the code. However, in an actual circuit, the circuit that generates this signal and the identification circuit must be temporarily disconnected from each other due to the power supply. Therefore, it becomes necessary to regenerate the blocked DC component later. Normally, when dealing with such codes, the direct current is blocked by using a capacitor or transformer, or by using a cheap and highly stable AC amplifier as the first stage and using a capacitor or transformer in this amplifier. For example, a diode connected to a reference voltage is used as a DC regeneration circuit. However, in such a circuit configuration, a DC reproduction error occurs due to the forward resistance of the diode. This has the disadvantage that the margin of the identification circuit is reduced accordingly.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a digital signal discrimination circuit in which the above-mentioned DC reproduction error is reduced.

According to the present invention, there is provided a DC cutoff circuit for cutting off the DC component of a digital signal consisting of a code without DC balance, a DC regeneration circuit for regenerating the blocked DC component, and a DC regeneration circuit for regenerating the blocked DC component. and a discriminating section for discriminating the signal from the DC regenerating circuit, wherein a filter circuit is configured between the DC cutoff circuit and the discriminating section, the gain at high frequencies being smaller than the gain at low frequencies; A digital signal identification circuit is obtained which is characterized by reducing reproduction errors due to forward resistance.

Next, a detailed description will be given with reference to the drawings.

FIG. 1 is a block diagram showing an example of the configuration of a conventional identification circuit. The DC component of the signal input from the input terminal 1 is cut off by AC coupling by the capacitor 2, and the original DC component of the cut-off DC component is regenerated by the DC current supplied from the diode 3 connected to the reference voltage V _REF . The DC component of the regenerated signal is thus identified by the identification circuit 4. However, in this circuit configuration, when the mark rate of the input code changes, the current value supplied through the diode changes, and the voltage drop across the diode changes due to the forward resistance. As a result, a so-called DC reproduction error occurs in which the DC level of the DC reproduced pulse changes somewhat. As mentioned above, this reduces the margin of the identification circuit.

FIG. 2 is a diagram showing the configuration of an embodiment of the present invention. The signal that has passed through input terminal 11 and coupling capacitor 12 is transferred to resistors 13 and 14 and capacitor 15.
After passing through a filter 16 consisting of a filter 16 , the DC signal is regenerated by a diode 17 and is identified by an identification section 18 . As mentioned at the beginning, the coupling capacitor 12 may be the capacitor used in the preceding AC amplifier.

FIG. 3 is a diagram showing an example of the transfer characteristic in the identification circuit of FIG. 2. Curve B in FIG. 3a shows the characteristics of the coupling capacitor 12 (including the characteristics of the amplifier (not shown) connected to the previous stage),
When the frequency is high, the gain shows a high value, and the low cutoff frequency is set to a value low enough that "sag" that occurs in the pulse does not become a problem. Curve C in Figure 3b
shows the characteristics of the filter 16, in which the gain in the high frequency region is smaller than the gain in the low frequency region. Returning to FIG. 3a, curve D shows coupling capacitor 12, filter 16, and diode 17.
This shows the integrated characteristics of a circuit consisting of .

Next, the operation of the identification circuit according to the present invention will be explained in comparison with the operation of a circuit according to the prior art.

7 and 8 show examples of input signals, in which a indicates a pulse waveform and b indicates its frequency component. FIG. 7 shows a case where the pulse density is high, and its frequency components include many high frequency components as shown in b. On the other hand, FIG. 8 shows an example where the pulse density is sparse, where a is the waveform component and b is the frequency component. When the pulses are sparse, there are fewer high frequency components than when the pulses are dense.

First, consider the case where the two waveforms shown in Fig. 7 and Fig. 8 are input to the conventional DC regeneration circuit shown in Fig. 1. If the dense signal shown in Fig. 7 is input, pulse 7
Since the repetition frequency is large, the current flowing through the diode 3 becomes larger than when the signal shown in FIG. 8 is input. That is, the resistance value of the diode when the signal shown in FIG. 7 is inputted is smaller than that when the signal shown in FIG. 8 is inputted. If the repetition frequency of the waveform differs due to this difference in resistance value, the DC regeneration rate will also differ, as described above.
At this time, the reference voltage is represented by V′ _REF indicated by a broken line.

Next, the operation of the identification circuit according to the present invention will be explained. When the two waveforms shown in Fig. 7 and 8 are input to the DC regeneration circuit shown in Fig. 2, by inserting the filter circuit 16 having the transmission characteristic shown in Fig. 3b, the repetition frequency becomes When the number of high-frequency components becomes large and the number of high-frequency components increases, the pulse repetition frequency can be similar to the case where the pulse repetition frequency is sparse as shown in FIG. The reference voltage in this case of the present invention is at the position indicated by V _REF in a of FIGS. 7 and 8. Therefore, the direct current regeneration rate can be made to hardly change depending on the density of the pulses.

The waveform formed as described above is identified by the identification circuit 18, and the waveform has the shape of curve B in FIG. 3a. However, at this time, the time constant of the filter 16 is set so large that it does not affect the characteristics of B. It goes without saying that the reproduction error does not necessarily have to be equal to zero, and there is no practical problem even if it remains to some extent. The key is to create a safe margin for the entire circuit.

To take an example of the numerical values of the components of the filter 16 mentioned above, by selecting 0.01 μF as the coupling capacitor 12, 20 Ω as the resistor 13, 250 Ω as the resistor 14, and 1 μF as the capacitor 15, a characteristic curve as shown in FIG. 3b is obtained. can be drawn. Of course, these values are just examples and can vary over a wide range, but the capacitance of capacitor 14 needs to be at least an order of magnitude larger than the capacitance of the coupling capacitor to accommodate the aforementioned circuit time constant constraints. .

In the embodiments described above, although not specified otherwise, the impedance of the preceding stage signal output circuit (not shown) is treated as zero. However, when the impedance of this signal output circuit is large to a certain extent, this impedance is used to connect the resistor 1 in Figure 1.
As an alternative to 3, a filter can be constructed.

FIG. 4 is a diagram showing the structure of another embodiment of the present invention having the above-mentioned features, in which the filter 21 consists only of a capacitor 22 and a resistor 23. In this case, the gain characteristics of the filter in a broad sense, including the impedance of the preceding stage output circuit, remain to some extent as reproduction errors, but as long as they are within a certain range, they can be used practically without any problem.

FIG. 5 is a diagram showing the configuration of still another embodiment of the present invention. In this embodiment, the filter 31 is constructed using resistors 32 and 33 and a coil 34, and its characteristics as a filter are the same as those of the filter 16 shown in FIG. 2, and the obtained effects are also the same. It's the same.

FIG. 6 is a diagram showing the configuration of yet another embodiment of the present invention. In this embodiment, the filter 41 consists of a resistor 42 and a coil 43, and the resistor 33 in the embodiment of FIG.
6 is substituted with an impedance, and its characteristics are the same as those of the embodiment shown in FIG.

In the above embodiments, a diode is used in the DC regeneration circuit, but it is clear that similar effects can be obtained with a circuit that utilizes the base and emitter characteristics of a transistor or other DC regeneration circuits. Further, although a capacitor is used as the DC cutoff circuit, it may be something like a transformer. Furthermore, it is clear that the same effect can be obtained even if the positions of the filter and the diode are reversed.

As explained above, by cascade-connecting filters with different losses in the DC and signal passbands to the DC regeneration circuit, the DC regeneration error can be reduced to almost zero.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of a conventional identification circuit, FIG. 2 is a diagram showing the configuration of an embodiment of the present invention, and FIG.
The figure shows an example of the transfer characteristic in the identification circuit of the present invention, Figure 4 shows the configuration of another embodiment of the invention, and Figure 5 shows the configuration of still another embodiment of the invention. 6 is a diagram showing the configuration of still another embodiment of the present invention, and FIGS. 7 and 8 are diagrams showing the relationship between the input signal and the pulse repetition frequency. Explanation of symbols: 12 represents a coupling capacitor, 16 represents a filter, 17 represents a DC regeneration diode, and 18 represents an identification circuit.

Claims (1)

[Claims] 1. A DC cutoff circuit for cutting off the DC component of a digital signal consisting of codes without DC balance;
In an identification circuit having a DC regeneration circuit for regenerating the blocked DC component and an identification section for identifying the signal from which the DC component has been regenerated, the 1. A digital signal identification circuit comprising a filter circuit having a gain smaller than that at low frequencies, and reducing a reproduction error due to a forward resistance component of the DC reproduction circuit.