JPS62110346A - Carrier detection system - Google Patents

Abstract

PURPOSE:To exclude the effect of out-band noise by connecting an input of a carrier detection circuit to the output of a roll-off filter. CONSTITUTION:An input of a carrier detection circuit is extracted from the output side of a roll-off filter 3 via a demodulator 2 to exclude the invasion of out-band noise into the carrier detection circuit. The operation is executed at a frequency three times the symbol rate at the output of the roll-off filter 3 and a limiter 5 applies amplitude limit, then the result is squared by a multiplier 6 to calculate the power, the mean value between the symbol rates is obtained by arithmetic units 11, 12 and delay circuits 13, 14, outputted to circuits after an arithmetic unit 15 to detect the presence of carrier. Thus, the effect of out-band noise is eliminated to avoid malfunction and the fluctuation of the detection level due to the signal pattern is avoided.

Description

[Detailed Description of the Invention] [Summary] In modem carrier detection, in order to remove the influence of out-of-band noise, the carrier detection circuit input is taken from the output side of the roll-off filter, and furthermore, the carrier detection circuit input is taken from the output side of the roll-off filter, and the level is not changed due to the signal pattern. I made it.

[Industrial application field]

The present invention relates to a carrier detection method in data communication,
In particular, the present invention relates to a method for detecting the presence or absence of a carrier by receiving a CCITT Recommendation V, 27bis/ler training signal.

Conventional carrier detection methods have the disadvantage that malfunctions may occur due to the presence of out-of-band noise, and improvements have been sought.

[Conventional technology]

FIG. 4 is an explanatory diagram of an example of a conventional gear rear detection method.

In the figure, 1 is an AGC amplifier (AGC), 2 is a demodulator, and 3 is a demodulator.
is the roll-off filter (ROF), and 4 is the equalization layer (EQ
L), 5 is a limiter, 6 is a multiplier, 7 is an averaging circuit,
8 is an arithmetic unit, 9 is a sign determiner, and 10 is a threshold switch (TH). The same symbols represent the same objects throughout the following discussion.

As shown in Figure 4, the conventional gear detection method first inputs the received signal to a limiter 5 to limit the amplitude, then squares it in a multiplier 6 to calculate the power, and then uses a low-pass filter, etc. The averaged signal (g No.
This was done by comparing the results using the following methods, and determining the result using a sign determiner 9.

In addition, AGC amplifier 1, demodulator 2, roll-off filter 3,
The circuit consisting of the flower vase 4 and the like is a signal receiving circuit, and has no direct relation to the present invention.

In the carrier detection method described above, the threshold switch 10 switches and outputs the threshold levels Th, and ThL to the arithmetic unit 8. That is, the sign determiner 9 is ・1−
The threshold level 1 switch 10 sets the threshold level ThH when it is determined that there is no carrier, and sets the threshold level T when it is determined that there is a carrier.
h L wo calculation RW 8 Switch to output.

The arithmetic unit 8 calculates the difference between the threshold level and the signal X.

Therefore, when there is no carrier, the signal X output from the averaging circuit 7 (signal X is the average value of the power of the carrier signal as described above) increases from O, and T h n
When ≦0, the sign determiner 9 determines that a carrier is present.

Also, when there is a carrier, the signal X decreases and becomes X-
When ThL≦0, the sign determiner 9 determines that there is no carrier.

Conventionally, the presence or absence of a carrier was detected in this way.

[Problem that the invention seeks to solve]

However, in the conventional method described above, out-of-band noise is
Since the signal enters the subsequent circuits, the output level of the averaging circuit 7 fluctuates, causing a problem in that the carrier detection signal malfunctions.

An object of the present invention is to provide a carrier detection scheme that is not affected by out-of-band noise.

[Means for solving problems]

The above problem is caused by V, 27bis/le as shown in Figure 1.
The received signal in the R-type modem is demodulated by a demodulator 2, and after removing out-of-band noise by a roll-off filter 3, it is input to a carrier detection circuit.In the carrier detection circuit, the output of the roll-off filter 3 is converted into a symbol. The output amplitude of the roll-off filter 3 is limited by the limiter 5, then squared by the multiplier 6 to obtain the average value between the symbol rates, and the average value is calculated from the output of the multiplier 6. The value obtained by subtracting, taking the absolute value, smoothing, and adding the value multiplied by a coefficient α to the average value is compared with the threshold level to generate on/off of the CDi signal, and the received signal is For cases other than the /π signal, the problem can be solved by setting the coefficient α to 0.

[Effect]

According to the present invention, since the input of the carrier detection circuit is connected to the output side of the roll-off filter 3, it is possible to remove out-of-band noise, and therefore, there is no possibility of erroneously transmitting ON/OFF of the CDi signal due to out-of-band noise. Also, by setting the coefficient α, there is an advantage that the detection level can be prevented from changing.

〔Example〕

FIG. 1 is a diagram showing an embodiment of a carrier detection method according to the present invention.

Figure 2(a) shows the training signal, Figure 2(b)
is the frequency spectrum, and FIG. 2(C) is the output characteristic of the roll-off filter.

FIG. 3 is an explanatory diagram of the circuit operation of the present invention.

In the figure, 11.12.15.16.18 and 21 are arithmetic units, 13.14 are delay circuits, 17 is a determination circuit, and 19
is the absolute value circuit (ABS), and 20 is the average value circuit (MEAN).
).

In the present invention, as shown in FIG. 1, the input to the carrier detection circuit is taken from the output side of the roll-off filter 3 via the demodulator 2, thereby eliminating out-of-band noise from entering the carrier detection circuit.

At the output of the roll-off filter 3, calculation is performed at a frequency three times the symbol rate, which is the same as in the conventional example (after the amplitude is limited by the limiter 5, the power is calculated by squaring by the multiplier 6, 11°12, and delay circuit 13.
14 calculates the average value between symbol rates (in this example, the sum of three sample values), and outputs it to the circuits below the arithmetic unit 15 to detect the presence or absence of a carrier.

However, when training signals such as V, 27bis/ler, etc. are manually generated, the following problems occur.

Figure 2 (al indicates training signals such as V, 27bis/ler, etc. As is well known, the training signals are first a π/π signal, then an O/π signal, then a 5CRZ signal, and then a data signal. ing.

In this way, the π/π training signal first comes in. The frequency spectrum in this case is shown in Figure 2 (as shown by diagonal lines in bl), but when this signal is demodulated and passed through the roll-off filter 3, the characteristics of the roll-off filter 3 are as shown in Figure 2 (as shown in C1). Attenuation (approximately 6 db) is caused by

However, in the case of the next O/π signal, 5CRZ signal, and data signal, the frequency spectrum is spread within the entire band of the output characteristic of the roll-off filter 3 shown in FIG. 2 (C1), and the degree of attenuation is reduced. Therefore, the power level of the multiplier 6 output is amplified.

When taking input from the output side of the roll-off filter 3 like this, it is impossible to use the same detection method as in the past, and in order to correct this drawback, arithmetic units 15 and 18, an absolute value circuit 19, and an average value circuit are used. 20 was added.

In a V.27 bis/ler type modem, when changing from a carrier off state to an on state, the incoming signal is usually the training signal π7118 or an unmodulated carrier.

Therefore, the response of the circuit of FIG. 1 to the π/π signal and the non-modulated gear will be explained below with reference to FIG.

(i) In the case of a π/π signal, or when the input is a π/π signal, the waveform at the output side ■ of the limiter 5 in FIG. 1 is attenuated by the roll-off filter 3, and the waveform shown in ■ in FIG. It becomes a waveform. Therefore, the waveform at the output side (2) of the multiplier 6 becomes a waveform in which a sine waveform is superimposed on the DC component shown by the dotted line, as shown in (2) in FIG.

This DC component is output to the output side ■ of the calculator 11, and since the calculator 18 subtracts the waveform ■ from the waveform ■, the waveform shown in ■ in Figure 3 is output to the output side ■ of the calculator 18. Output. Therefore, the waveform shown in (2) in FIG. 3 is output to the output side (2) of the absolute value circuit 19. This is averaged in the average value circuit 20, multiplied by the coefficient α in the arithmetic unit 21, and the third
A waveform shown in ■ in the figure is output and input to the arithmetic unit 15.

In the arithmetic unit 15, the output waveform (2) of the arithmetic unit 11 and the output waveform (2) of the second arithmetic unit are added, and the waveform (2) on the output side of the arithmetic unit 15 becomes the DC waveform shown in (3) in FIG.

The coefficient α is set so that the DC component at this time has a unit value of 1. Note that the coefficient α is set to 0 when the signal is not a π/π signal.

(ii) In the case of an unmodulated gearbox, or when the input is an unmodulated carrier, the carrier is not attenuated by the roll-off filter 3, and the arithmetic unit 1
The output waveform of 1 is 1 as shown in ■ in Figure 3, and the output of the calculator 21 is 0 as shown in ■ in Figure 3, so the magnitude of the output side of the calculator 15 is equal to the level The value becomes 1.

Therefore, whether the input is an unmodulated carrier or a π/π signal, the output of the arithmetic unit 15 becomes a unit value 1 and does not fluctuate.

Therefore, if the output of the arithmetic unit 15 and the threshold level Th1I or ThL, which is the output of the threshold switch 10, are input to the arithmetic unit 16, the judgment circuit 17 judges the presence or absence of a carrier and turns on the CDi signal, as in the case of the conventional example. /Can be turned off.

〔Effect of the invention〕

As explained in detail above, according to the present invention, by taking the input of the carrier detection circuit from the output side of the roll-off filter 3, the influence of out-of-band noise is removed and malfunctions are eliminated.
Moreover, it has the great effect of preventing the detection level from varying depending on the signal pattern.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of a carrier detection method according to the present invention. Figure 2(a) shows the training signal, Figure 2(b)
is the frequency spectrum, and FIG. 2 (TC) is the output characteristic of the roll-off filter. FIG. 3 is an explanatory diagram of the circuit operation of the present invention. FIG. 4 is an explanatory diagram of an example of a conventional carrier detection method. In the figure, 1 is an AGC amplifier (AGC), 2 is a demodulator, and 3 is a demodulator.
is a roll-off filter (ROF), 4 is an isometric vase (E
QL), 5 is a limiter, 6 is a multiplier, 7 is an averaging circuit, 8 is an arithmetic unit, 9 is a sign judger, 10 is a threshold switch (TH), 11.12.15.16.18, and 21 13 and 14 are delay circuits, 17 is a determination circuit, 19 is an absolute value circuit (ABS), and 20 is an average value circuit (MEAN). Figure 3

Claims (1)

[Claims] A received signal in a data transmission modem is demodulated by a demodulator (2), and after removing out-of-band noise by a roll-off filter (3), the signal is input to a carrier detection circuit. In this case, the output of the roll-off filter (3) is calculated at a frequency multiple times the symbol rate, and after the output amplitude of the roll-off filter (3) is limited by a limiter (5), it is squared by a multiplier (6). Then, find the average value between the symbol rates, subtract the average value from the output of the multiplier (6), take the absolute value, smooth it, and add the value multiplied by a coefficient α to the average value to obtain the average value. a carrier detection signal (CDi) is generated by comparing the received signal with a threshold level, and the coefficient α is set to 0 when the phase of the received signal is other than a signal rotating by 180 degrees. method.