JP2504581B2 - Digital signal transmission system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A digital signal transmission system having a roll-off filter in each of a transmission unit that modulates a carrier wave with transmission data and outputs the carrier wave, and a reception unit that demodulates the reception signal to obtain reception data, A transmitter for transmitting a digital signal modulated with transmission data through a first roll-off filter for the purpose of improving high-frequency response, and receiving data through a second roll-off filter after receiving and demodulating the digital signal. In a digital signal transmission system including a receiver, the first roll-off filter is
A first low-pass filter for oscillating a low-frequency component of the transmission data, and first and second delay equalizers having predetermined amplitude and group delay characteristics, respectively, are cascade-connected, The second roll-off filter is composed of only the second low-pass filter having the same amplitude characteristic as the first low-pass filter.

[Industrial applications]

The present invention relates to a digital signal transmission system, and more particularly to a digital signal transmission system having a roll-off filter in each of a transmission unit that modulates a carrier wave with transmission data and outputs the carrier wave, and a reception unit that demodulates a reception signal to obtain reception data. .

When a digital signal is transmitted using an analog line such as a telephone switching network, the transmitting unit modulates the transmission data (for example, phase modulation) and sends it to the analog line, and the receiving unit demodulates it to receive data. To get Here, the analog line has a predetermined transmission band and does not transmit frequency components outside the transmission band. On the other hand, since the digital signal (transmission data) is a square wave and has frequency components from OHz to infinity, it is difficult to transmit the digital signal as it is using an analog line.

Therefore, generally, after converting a digital signal into an impulse, data is transmitted by transmitting it every Nyquist interval τ (= 1 / 2f ₀ : f ₀ is the Nyquist frequency) seconds according to the amplitude of the impulse. Digital signals are transmitted according to Nyquist's theorem.
Here, the roll-off filter on the transmission side transmits the impulse at the Nyquist time interval, and the roll-off filter on the reception unit is used to read the amplitude of the impulse at the accurate time interval.

FIG. 4 shows a block diagram of an example of a transmission section of a digital signal transmission system to which the present invention is applied. In the figure, the input terminal
The first and second transmission data input to 11a and 11b are supplied to D / A converters 12a and 12b. The first and second transmission data are, for example, pulse code modulation (PCM) digital signals. The D / A converters 12a and 12b perform digital / analog conversion on the input transmission data to generate data shifted so that the center level becomes OV, and supply the data to the roll-off filters 13a and 13b.

The roll-off filters 13a and 13b have a kind of low-pass filter characteristic for transmitting the input impulse-shaped data at every Nyquist time interval τ seconds. The data band-limited by the roll-off filters 13a and 13b is the mixer 1
It is supplied to 4a and 14b, where the carrier wave input from the carrier wave generator 15 via the hybrid circuit 16 is modulated. The carrier waves input to the mixers 14a and 14b have the same frequency, but their phases differ by 90 °.

The modulated signals extracted from the mixers 14a and 14b are combined by a hybrid circuit 17 into a transmission signal which is, for example, four-phase phase modulated (PSK), and is sent to an analog line (not shown) via an output terminal 18. To be done.

FIG. 5 shows a block diagram of an example of a receiving section of a digital signal transmission system to which the present invention is applied. In the figure, the input terminal
The transmission signals from an analog line (not shown) are input to the mixers 23a and 23b via the hybrid circuit 22 and the hybrid circuit 22, respectively. The mixers 23a and 23b are hybrid circuits 30 to be described later.
The data obtained by multiplying the signal from the input signal and the input transmission signal are supplied to the roll-off filters 24a and 24b and the clock recovery circuit 25, respectively. The roll-off filters 24a and 24b are a kind of low-pass filter, and apply a predetermined filter characteristic to the input data to flatten the group delay time characteristic and then the A / D converter 2
Supply to 6a and 26b.

The operation clocks of the A / D converters 26a and 26b are supplied from a clock recovery circuit (BTR: Bit Timing Recovery) 25, and the roll-off filters 24a and 24b are based on the operation clocks.
The data sent from the device is converted from analog to digital, and the received data (PCM)
Signal) and outputs it to the output terminals 31a and 31b.

The A / D converters 26a and 26b supply error signals (ε _I and ε _Q ) to the signal processing unit 27 together with the above received data (D _I and D _Q ). The signal processing unit 27 generates an error signal based on these input signals, applies it as a control voltage to the voltage controlled oscillator (VCO) 29 through the loop filter 28, and controls it so that its output oscillation frequency has the correct phase. To do. VCO2
The output oscillation frequency of 9 is supplied as a demodulation carrier wave to the mixers 23a and 23b through the hybrid circuit 30. Here, the input demodulation carrier waves of the mixers 23a and 23b have the same frequency as the output carrier wave of the carrier wave generator 15, and the phases thereof are different from each other by 90 °.

In the digital signal transmission system including the transmitter and the receiver described above, improvement of high frequency response is required as one means of the code error low band.

[Conventional technology]

In the conventional digital signal transmission system, the roll-off filters 13a and 13b of the transmission unit and the roll-off filters 24a and 24b of the reception unit have the same configuration from the idea of so-called equal transmission and reception, and the amplitude characteristics are equal to the transmission and reception. I am trying to become.

That is, the roll-off filters 13a of the transmission section have the same structure, and the low-pass filter 35 and the delay equalizer 36 are connected in cascade as shown in FIG. 6 (A). On the other hand, the roll-off filters 24a and 24b of the receiving section have the same configuration, and each has a configuration in which a low-pass filter 38 and a delay equalizer 39 are cascade-connected as shown in FIG. 6 (B). Therefore, as can be seen by comparing FIGS. 6A and 6B, the roll-off filters 13a and 13b of the transmission unit and the roll-off filters 24a and 24b of the reception unit have the same configuration of transmission and reception. . As a result, the required cosine roll-off characteristic is obtained in the entire transmission / reception system.

[Problems to be Solved by the Invention]

However, the roll-off filters 24a and 24b of the receiving unit are
Mixers 23a, 23b → roll-off filters 24a, 24b → A / D converters 26a, 26b → signal processing unit 27 → loop filter 28 → VCO 29 →
Since it is provided in the phase-locked loop (PLL) circuit made up of the hybrid circuit 30 and the mixers 23a and 23b, the PLL circuit is applied to the high frequency input due to the time delay of each delay equalizer 39 in the roll-off filters 24a and 24b. Could not follow,
There is a problem that the code error increases with respect to the high frequency input and the high frequency response is poor.

The present invention has been made in view of the above points, and an object thereof is to provide a digital signal transmission system capable of improving high frequency response.

[Means for solving the problem]

FIGS. 1 (A) and 1 (B) respectively show a principle configuration diagram of a roll-off filter which is a main part of the method of the present invention. The present invention is the first
In a digital signal transmission system comprising a transmitter for transmitting a digital signal modulated by the transmission data passed through the roll-off filter, and a receiver for receiving the digital signal and demodulating it to obtain the received data through the second roll-off filter. The first roll-off filter has the structure shown in FIG. 1 (A), and the second roll-off filter has the structure shown in FIG. 1 (B).

That is, the first roll-off filter includes a first low-pass filter 101 that waves low-frequency components of transmission data and a first and a second delay equalizer 1 as shown in FIG.
02 and 103 are connected in cascade. However, in the present invention, the order of connection is not limited to the order shown in FIG. 1 (A), and 101-103 may be connected in any order.

On the other hand, as shown in FIG. 1 (B), the second roll-off filter is composed of only the second low-pass filter 104, and the second low-pass filter 104 is the first low-pass filter 101. It has the same characteristics as.

[Action]

In the digital signal transmission system, the required cosine roll-off characteristic may be obtained in the entire transmission system from the transmitter to the receiver.

Further, as shown in FIG. 4, the roll-off filters 13a and 13b are not included in the feedback loop in the transmitter, so that there is no influence of the time delay due to the delay equalizer.

Therefore, the present invention pays attention to the above points, and the first roll-off filter of the transmitting unit is further provided in the first delay equalizer 102 in the conventional transmitting unit as shown in FIG. 1 (A). While the second delay equalizer 103 in the conventional transmission unit is connected, the roll-off filter of the reception unit is only the second minimum low-pass filter 104 necessary for a predetermined low frequency component wave. It is composed of.

According to the present invention, since there is no delay equalizer in the second roll-off filter 12 that enters the feedback loop of the receiver, the time delay due to the delay equalizer can be eliminated. However, although the second low-pass filter 104 also has a time delay, the time delay is extremely small, and there is practically no problem.

[Embodiment] Next, one embodiment of the present invention will be described. In this embodiment, the roll-off filter 13a of the transmitter shown in FIG. 1 (A) and the first roll-off filter in FIG. , 13b, and the second roll-off filter shown in FIG. 1B is used as the roll-off filter 24 shown in FIG.
It was used as a, 24b.

FIG. 2 shows the group delay characteristics of the second roll-off filter in this embodiment and the group delay characteristics of the roll-off filter of the conventional receiving section in comparison. In the figure, I indicates the group delay characteristic of this embodiment, and II indicates the conventional group delay characteristic. Second
As can be seen from the figure, according to the present embodiment, the frequency around 2 MHz is about 190 ns shorter than that of the conventional method.

In the present embodiment, the group delay time I of the second roll-off filter in the PLL circuit of the receiving unit is the group delay time of the conventional method.
Since it is shorter than II, the response to high frequency input can be improved compared to the conventional method.

On the other hand, the characteristic of the first roll-off filter in the transmitter is as shown in FIG. In the figure, III is the first
The roll-off filter's group delay characteristic, IV shows the amplitude characteristic. The group delay time at 17.15725 MHz marked with a circle in Fig. 3 is 284.05 nsec, and the amplitude is -7.224 dB.

The time delay due to this first roll-off filter is considerably increased compared to the conventional method, but since this first roll-off filter (13a, 13b in FIG. 4) is not included in the PLL circuit, There is no adverse effect on the response characteristics due to the increase in delay. The amplitude characteristic of the second roll-off filter is substantially the same as the amplitude characteristic of the roll-off filter of the transmitter shown by IV in FIG.

〔The invention's effect〕

As described above, according to the present invention, since the delay equalizer of the roll-off filter that enters the feedback loop of the receiving unit is eliminated and the time delay due to the delay equalizer is removed, the reception equals to the conventional method. It is possible to improve the high-frequency response in the unit, and therefore, it is possible to perform demodulation with less code errors even for a high-frequency input digital signal as compared with the related art.

[Brief description of drawings]

FIG. 1 is a diagram showing the principle configuration of the essential part of the system of the present invention, FIG. 2 is a diagram showing one embodiment of the present invention in comparison with the group delay characteristics of the conventional system, and FIG. 3 is one embodiment of the present invention. FIG. 4 is a block diagram of an example of a transmitter, FIG. 4 is a block diagram of an example of a receiver, FIG. 6 is a block diagram of an example of a receiver, and FIG. is there. In the figure, 101 is a first low pass filter, 102 is a second delay equalizer, and 10
3 is a second delay equalizer, 104 is a second low pass filter, 13a,
Reference numeral 13b indicates a roll-off filter of the transmission unit, and reference numerals 24a and 24b indicate roll-off filters of the reception unit.

Claims (1)

(57) [Claims] 1. A transmission section for transmitting a digital signal modulated with transmission data through a first roll-off filter, and a reception section for receiving the digital signal and demodulating it to obtain reception data through a second roll-off filter. In the digital signal transmission system, the first roll-off filter is a first low-pass filter (101) that waves low-frequency components of the transmission data,
The first and second delay equalizers (102, 103) each having a predetermined amplitude characteristic and group delay characteristic are connected in cascade, and the second roll-off filter is the first low-pass filter ( The second low-pass filter (10
4) A digital signal transmission system characterized by being composed of only 4).