JPH0370237A - Transmitter for digital data communication - Google Patents

Abstract

PURPOSE:To attain clear data communication by providing a means making the leading edge dull and trailing edge of a transmitter when a carrier for data transmission is intermitted. CONSTITUTION:A control signal from a data processing section 17 is fed to a gain control amplifier 13 via a low pass filter 16. The output waveform of the low pass filter 16 has no rapid change at the time of starting and stopping transmission, and is in dull waveform. Thus, the transmission output is controlled so that the level is gradually changed at the time of starting and stopping transmission, and the result is outputted. Then undesired spread spectrum is reduced. Thus, when the carrier for data transmission is intermitted to the transmitter, clear data communication is attained by making the leading edge and trailing edge dull.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides a digital data communication transmitter that is useful when adopted as a terminal transmitter of a cable television system (hereinafter referred to as a CATV system), for example. Regarding.

(Prior Art) A CATV system has a configuration in which a large number of terminal transmitters are connected to a central station via cables, and the terminal transmitters share one line. When polling from the central station is started, the called terminal transmitter modulates its transmission data with a carrier wave and sends it out to the line.

FIG. 5 shows the basic configuration of a terminal transmitter.

In response to polling, the data processing unit 1 controls the switch 2 to connect the oscillator 3 to the modulator 4 . Furthermore, the data to be transmitted from the data processing section 1 is sent to the signal conversion section 5.
The signal is outputted through the filter and input to the modulator 4. As a result, the output from the modulator 4 is sent to the cable via the excitation amplifier 6 and the power amplifier 7.

FIG. 6 shows a timing chart when the above transmitter performs data transmission.

(A) of the figure shows the output of the oscillator 3, (B) of the figure shows the control signal of the switch 2, and (C) of the figure shows the output of the transmitter.

Switch 2 introduces the output of oscillator 3 into modulator 4 when the control signal is at high level. From this point on, data modulation is performed in the modulator 4.

As the modulation method, for example, a modulation method such as an FSK method or a PSK method is adopted.

(Problems to be Solved by the Invention) According to the conventional system, a high level (constant level) transmission output is obtained immediately after the switch 2 is turned on, as shown in FIG. However, the phase relationship between the carrier wave and the switching time point is not necessarily the zero-crossing point of the carrier wave, as seen at time points tl and t2 in FIG.

Switching a carrier wave means performing 100% AM modulation on the carrier wave using a switching waveform, and the closer the switching waveform is to a rectangular wave, the wider the spectrum will be, and the more unnecessary spectrum will be present in the modulated output. Conventional systems are limited only by bandpass filters connected at the end of the transmitter to prevent spectrum spread from reaching the cable, but this is insufficient. In particular, when the transmission output is increased or when subscriber terminal transmitters are added, the influence of spectrum spread becomes large, causing problems in clear data communication.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a digital data communication transmitter that can prevent unnecessary spectrum from spreading at the start and stop points of data transmission and contributes to obtaining clear data communication. .

[Configuration of the Invention (Means for Solving the Problems) This invention provides a communication system in which a plurality of transmitters share the same line, in which a carrier wave for data transmission is intermittent with respect to the transmitter. , a means is provided to smooth the rise and fall.

(Function) Since the above means takes the form in which the rising and falling edges of the transmitted carrier wave gradually change in level, it is possible to significantly reduce the occurrence of unnecessary spectrum compared to sudden switching.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which the output of an oscillator 11 is supplied to a modulator 12 and subjected to PSK modulation, for example, using data from a signal converter 15.

Transmission data is output from the signal processing section 17 and provided through the signal conversion section 15.

The modulated output signal from modulator 12 is transmitted to gain control amplifier 13
The signal is amplified and supplied to the power amplifier 14. The output of power amplifier 14 is sent to the cable as a transmission signal.

For example, when polling is received from the central station, the data processing unit 17 analyzes the received data in a receiving system (not shown) and responds to the polling. At this time, the transmission data is output to the signal converter 15, and a control signal is applied to the gain control terminal of the gain control amplifier 13 via the low-pass filter 16.

FIG. 2 is a timing chart showing the operation when transmitting data. 3A shows the oscillation output of the oscillator 11, and FIG. 1B shows the control signal from the data processing section 17. This control signal is supplied to the gain control amplifier 13 via the low pass filter 16. The output waveform of the low-pass filter 16 is, as shown in FIG. 2(C), at the start of transmission and
When stopped, there is no sudden change and the waveform is dull. For this reason, the transmission output is also as shown in the same figure (D).
The output is controlled so that the level changes gradually between the start and stop of transmission.

As a result, unnecessary spectrum spread can be reduced compared to the conventional case where the transmission output changes rapidly.

FIG. 3 shows a comparison between the spectrum spread of a conventional transmitter and the spectrum spread of a transmitter according to this embodiment, which are analyzed using a spectrum analyzer. The center frequency fO is 48.00MHz, and fO
We were able to obtain an improvement of approximately 20 dB at ±500 KHz.

Furthermore, FIGS. 4(A) and 4(B) are waveform diagrams when observing the signal waveforms of each part in the conventional transmitter and the signal waveforms of each part in the transmitter of this embodiment using an oscilloscope. . As can be seen from FIG. 6B, it can be seen that the level of the transmission output (PSK modulation output) gradually changes between when transmission starts and when it stops.

In the above embodiment, in order to effectively utilize the control signal and the transmission data originally output from the data processing section 17 as they are, the control signal is passed through the low-pass filter 16 and the gain of the amplifier 13 is adjusted. It is used as a control signal. However, the present invention is not limited to this embodiment, and the control signal itself obtained from the data processing section 17 may not be a rectangular wave but may be set to a waveform that allows the gain to be gradually varied. Further, an attenuator may be provided at the input or output stage of the power amplifier 14 to control the attenuation rate.

[Effects of the Invention] As explained above, the present invention can prevent unnecessary spectrum from spreading at the time of starting and stopping data transmission, and can contribute to obtaining clear data communication.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a timing chart shown to explain the operation of the circuit of Fig. 1, and Fig. 3 is a conventional transmitter and a transmitter of the present invention. FIG. 4 is a diagram showing the signal waveforms of various parts of the conventional transmitter and the transmitter of the present invention,
FIG. 5 is a diagram showing the configuration of a conventional transmitter, and FIG. 6 is a timing chart shown to explain the operation of the circuit shown in FIG. DESCRIPTION OF SYMBOLS 11... Oscillator, 12... Modulator, 13... Gain control amplifier, 14... Power amplifier, 15... Signal converter, 16... Low pass filter, 17... Data processing Department.

Claims (1)

[Claims] In a communication system in which a plurality of transmitters share the same line, the transmitter has means for smoothing the rise and fall of a carrier wave for data transmission when the carrier wave is interrupted. A transmitter for digital data communication featuring: