JPH04180438A - Data transmission system - Google Patents

Abstract

PURPOSE:To simplify the constitution of the entire transmitter by using a data subject to a Manchester coding processing for a data frequency-modulated by an FSK modulator. CONSTITUTION:In the case of sending a digital data of a Manchester code used for a data source, an FSK modulator modulates the data at a frequency f1 when the data is at a high level and modulates the data at a frequency f2 when the data is at a low level (f1>f2) respectively. After the frequency- modulated FSK signal is converted into an optical signal by a light emitting section 6, the optical signal is sent to a transmission line 5. Then the FSK signal converted into an electric signal by a light receiving section 6 is subject to envelope detection by an envelope detector 7a of an AGC circuit 7. The envelope detection output is not smooth, but the output integrated by an integration device 7b is smoothed completely and constant. That is, a gain control signal outputted to an amplifier 7c to be controlled is proportional to only the attenuation of a transmission line 5. Thus, the control signal is used for the gain control of the AGC circuit 7 on the receiver side.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a data transmission system in which digital data is frequency modulated and transmitted via a transmission path such as an optical fiber.

<Conventional technology> Generally, when transmitting digital data, the so-called baseband transmission method, in which data is transmitted by directly sending a DC data signal to a transmission line, has a simple configuration of the transmission equipment, but it uses DC. Because of this, it is susceptible to attenuation and noise, making it not very suitable for long-distance transmission.

On the other hand, band transmission methods, in which digital data is first modulated into an AC signal and then sent out, and then demodulated on the receiving side and extracted as digital data, are more suitable for long-distance transmission than baseband transmission methods. There is. There are various methods for modulating digital data into alternating current signals, such as amplitude modulation, frequency modulation, and phase modulation, but frequency modulation (FSK) methods are particularly resistant to the effects of noise and level fluctuations. It is widely used because the structure of the modulation and demodulation circuits is relatively simple.

Even when data is transmitted based on this frequency modulation method, the amount of signal attenuation varies depending on factors such as the distance and temperature of the transmission path.
The gain for the FSK signal input via the transmission path is controlled by the AGC circuit to compensate for signal attenuation.

By the way, digital data is usually encoded using the code "l".
The so-called NRZ (non-return zero) method is used, in which "" is a high level and the code "0" is a low level (or vice versa, the code "l" is a low level and the code "0° is a high level). If such NRZ digital data is frequency modulated and transmitted as is, the following problem occurs.

Generally, signals transmitted on a transmission path have different transfer characteristics depending on their frequency components, and normally, the higher the frequency component, the greater the attenuation. Therefore, now “l
data of “0” at frequency f, data of “0” at frequency f
, (However, here f.

> L), the attenuation of the signal component of the high frequency f1 is large on the receiving side,
The amount of attenuation of the signal component of the low frequency f becomes small. For this reason, as shown in Figure 4, if the content "110111" (where the code "I" occurs more often than the code "0") is frequency modulated and transmitted as digital data in the NRZ system (the number of occurrences of the code "I" is higher than the code "0"), (a)), this FSK signal is sent to the AG on the receiving side.
After envelope detection is performed in the C circuit ((C) in the same figure), the level of the gain control signal obtained by integrating the detection output is not flat but uneven ((d) in the same figure). In other words, it will also be modulated in the amplitude direction. Similarly, as shown in Fig. 5, if the content "100000" in which the code "0" occurs more often than the code "l" is frequency modulated and transmitted (Fig. 5(a)), this FSK signal is transmitted. A on the receiving side
After envelope detection in the GC circuit ((C) in the same figure), the level of the gain control signal obtained by integrating the detection output is also not flat and increases in the middle ((D) in the same figure)
.

In other words, when NRZ digital data is simply frequency modulated and transmitted, the gain control signal of the AGC circuit on the receiving side depends not only on the transfer characteristics of the transmission path, but also on the content of the transmitted data (i.e., high level The level also changes depending on the low level occurrence ratio = mark space ratio). Therefore, it is not possible to perform gain control by detecting only the influence of fluctuations in signal attenuation due to the transmission path.

Therefore, in the conventional technology, on the transmitting side, for an FSK signal obtained by frequency modulating NRZ digital data,
While a pilot signal with a single frequency f3 (rs < L, f,) is superimposed as a carrier, the receiving side detects this pilot signal and converts it into AGC.
By using it as a gain control signal for the circuit, the amount of signal attenuation due to the transmission path is compensated for without being affected by the content of the data.

<Problem to be solved by the invention> However, in the conventional method of superimposing a pilot signal on an FSK signal obtained by frequency modulating digital data, the pilot signal is superimposed on the pilot signal generator or the FSK signal. Since a separate mixer is required, there is a problem that the configuration of the entire transmission device becomes complicated.

<Means for Solving the Problems> The present invention has been made in view of the above-mentioned circumstances, and is designed to prevent the level of the gain control signal of the AGC circuit from varying depending on the content of data to be transmitted. hand,
This makes it possible to compensate for the amount of signal attenuation caused by the transmission path without particularly superimposing a pilot signal on the FSK signal.

Therefore, in the present invention, on the transmitting side, digital data is
The frequency is modulated by the SK modulator and sent to the transmission line as an FSK signal, while on the receiving side, the gain for the FSK signal input via the transmission line is controlled by the AGC circuit. In a data transmission system in which an FSK signal is demodulated into digital data using a data demodulator, Manchester encoded data is used as the digital data that is frequency modulated by the FSK modulator.

<Operation> Since the Manchester code is a perfectly balanced code with a mark-space ratio of 50%, its average level is not affected by the data content and is proportional to the amount of attenuation of the transmission path. Therefore, the modulated carrier of frequency modulation can be used as a pilot signal for gain control of the AGC circuit on the receiving side.

<Embodiment> FIG. 1 is a block diagram of a data transmission device to which the data transmission method is applied. In the same figure, the symbol l is the data source,
The data handled by this data source is Manchester encoded in this example. In other words, as shown in Fig. 2, this Manchester code corresponds to the rising edge of a signal to the code "l" and the falling edge of the signal to the code "0" in a certain sampling period t, and the mark space is It is a perfectly balanced code with a ratio of 50%,
There are advantages such as clock reproduction can be performed reliably.

2 is F that frequency modulates this Manchester code data.
SK modulator, 3 is a light emitting unit that converts the FSK signal obtained by frequency modulation into an optical signal, 5 is an optical fiber transmission line, 6
7 is a light receiving unit that converts the optical signal transmitted through the transmission line 5 into an electrical signal, and 7 is an FS3S2O output from the light receiving unit 6.
AGC circuit that controls gain, 8 is gain controlled F
This is a data demodulator that demodulates dental data from the SK signal.

The AGC circuit 7 described above includes an envelope detector 7a that envelope-detects the modulated carrier of the FSK signal, an integrator 7b that integrates and outputs the envelope-detected signal, and outputs the output of the integrator 7b as a gain control signal. and a controlled amplifier 7c that controls the gain of the FSK signal.

In the above configuration, when transmitting Manchester code digital data used in the data source, when the data is at noise level by the FSK modulator 2, the frequency f,
When the signal is at a low level, it is modulated at a frequency f (here, ft>ft). In this example, as shown in FIG. 3(a), it is assumed that data with content "11011 bits" is frequency-modulated and transmitted using Manchester code.Then, this frequency-modulated F
The SK signal is converted into an optical signal by the light emitting section, and then transmitted through the transmission line 5.
will be sent to. Signals transmitted on a transmission path have different transfer characteristics depending on their frequency components, and usually have a high frequency f.
The amount of attenuation of the signal component of , is large, and the amount of attenuation of the signal component of low frequency f, is small. In other words, it is also modulated in the amplitude direction. For this reason, the F
The SK signal becomes as shown in FIG. 4(b). This FSK
The signal is input to the envelope detector 7a via the controlled amplifier 7C and subjected to envelope detection. The envelope detection output is
As shown in FIG. 6(c), it is not flat and has irregularities, but its shape follows the waveform of the transmitted digital data. As mentioned above, since the Manchester code is a completely balanced code, the mark-space ratio is 50%. Therefore, the output obtained by integrating the envelope detection output by the integrator 7b has the unevenness completely flattened and is shown in the same figure. It becomes constant as shown in (d). That is, the gain control signal output from the integrator 7b to the controlled amplifier 7C does not vary depending on the content of the data, and is proportional only to the amount of attenuation of the transmission line 5.

Therefore, there is no need to separately provide a pilot signal generator as in the prior art, and the modulated carrier of frequency modulation can be used as it is as a pilot signal for gain control of the AGC circuit 7 on the receiving side.

<Effects of the Invention> According to the present invention, since the Manchester code, which is a perfectly balanced code with a mark-space ratio of 50%, is used, the average value level is not affected by the data content and is dependent on the attenuation amount of the transmission path. Proportional. Therefore, the modulated carrier of frequency modulation can be used as a pilot signal for gain control of the AGC circuit on the receiving side. Therefore, as before, FS
Since signal attenuation due to the transmission path can be compensated for without separately superimposing a pilot signal on the K signal, the overall configuration of the transmission device can be simplified.

[Brief explanation of drawings]

1 to 3 show embodiments of the present invention, in which FIG. 1 is a configuration diagram of a data transmission device to which the data transmission method is applied, FIG. 2 is an explanatory diagram of Manchester code, and FIG. 3 is a diagram of Manchester code. FIG. 4 is a waveform diagram for explaining a case where a control signal of an AGC circuit is extracted after frequency modulating code data. FIGS. 4 and 5 are waveform diagrams for explaining the case where a control signal for an AGC circuit is extracted after frequency modulating NRZ code data. DESCRIPTION OF SYMBOLS 1... Data source, 2... FSK modulator, 5... Transmission line, 7... AGC circuit, 8... Data demodulator.

Claims (1)

[Claims] (1) On the transmitting side, the digital data is frequency-modulated by an FSK modulator and sent out as an FSK signal to the transmission path, while on the receiving side, the FS signal input via the transmission path is
The gain for the K signal is controlled by the AGC circuit, and this AG
A data transmission method in which an FSK signal whose gain is controlled by a C circuit is demodulated into digital data by a data demodulator, characterized in that Manchester encoded data is used as the digital data frequency modulated by the FSK modulator. Transmission method.