JP2531377B2 - Modulation method identification circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulation system identification circuit, and more particularly to an identification circuit which identifies whether the modulation system of a received signal is a spread spectrum modulation system or a residual carrier modulation system.

In a communication system in which signals modulated by mutually different modulation systems are transmitted from two types of transmission sources, the receiving device receives the signal by appropriately switching the receiving circuit section according to the modulation system of the received signal. The circuit parts are made common as much as possible. For example, the transmission / reception wave in the inter-satellite communication is the spread spectrum modulation method, while the transmission / reception wave in the communication between the earth station and the satellite is the residual carrier wave modulation method. In order to selectively receive both transmission waves with the satellite-mounted transponder, a modulation method identification circuit is required to identify the modulation method of the received signal and automatically perform reception by the detection method corresponding to the modulation method. It is said that

[0003]

2. Description of the Related Art Conventionally, a circuit for discriminating an amplitude modulation (AM) wave and a frequency modulation (FM) wave has been known as a modulation system discrimination circuit in a receiving device for receiving transmission waves modulated by different modulation systems. (Japanese Patent Laid-Open No. 4-227125). FIG. 4 shows a block diagram of an example of this conventional modulation method identification circuit. In the figure, the received signal input to the input terminal 1 is input to the mixer 2 and frequency-converted with the local oscillation signal input via the input terminal 3 to form an intermediate frequency signal (IF signal). To be converted.

This IF signal is a temporal amplitude fluctuation extractor (F
FT) 20 and the temporal amplitude fluctuation of the carrier wave is detected. When the received signal is an AM wave, the temporal amplitude of the carrier of the IF signal is constant, whereas the received signal is an FM wave.
In the case of a wave, the temporal amplitude of the carrier of the IF signal changes.

The discrimination processor 21 time-differentiates the output signal of the FFT 20, squares it, and further integrates it to obtain
In the case of AM waves, a signal with a constant amplitude is obtained regardless of the passage of time, and in the case of FM waves, a signal of which the amplitude increases with the passage of time is obtained, and then the amplitude value of the signal is compared with the reference signal in magnitude. By doing so, it is determined whether the received signal is the AM modulation method or the FM modulation method, and the identification signal is output to the output terminal 10.

[0006]

However, since the above-mentioned conventional modulation system identification circuit identifies the modulation system of the received signal based on the presence or absence of temporal fluctuation of the carrier wave, it is modulated by the spread spectrum modulation system. There is a problem that the modulation system cannot be accurately discriminated in the case of signals having constant energy in the vicinity of the carrier, such as a signal and a signal modulated by a residual carrier modulation system such as a phase modulation system.

[0007] The present invention has been made in view of the above points,
An object of the present invention is to provide a modulation method identification circuit capable of identifying a spread spectrum modulation method and a residual carrier modulation method.

[0008]

In order to achieve the above object, the present invention has a frequency conversion means for frequency-converting a received signal into a low frequency signal, and a first detection for band-limiting the low frequency signal after envelope detection. The amplitude of each output detection signal of the first detection circuit and the second detection circuit of the first detection circuit and the second detection circuit that performs envelope detection after the low frequency signal is band-limited to a narrow band are compared, and the amplitude ratio is almost equal to the threshold value. An amplitude comparator that generates and outputs an identification signal that identifies the modulation scheme of the received signal depending on whether they are the same or significantly different.

[0009]

With the spread spectrum modulation method, the occupied bandwidth is sufficiently wider than that of a narrow band modulation method such as an amplitude modulation (ASK) method for the purpose of obtaining a modulated wave excellent in noise resistance and interference. Therefore, when the modulated wave modulated by the spread spectrum modulation method is converted into a low frequency signal after reception and detected by the first detection circuit,
While the detection signal of a sufficient level can be obtained, the second
When the signal is detected by the detection circuit of (1), it is detected after being band-limited to a narrow band, so that the detection signal whose level is significantly lower than that of the output detection signal of the first detection circuit is taken out.

On the other hand, the residual carrier modulation method is, for example, a phase modulation (PM) method or an amplitude modulation (AM) method, and is a modulation method in which the carrier wave is not suppressed and remains with sufficient energy. The occupied frequency bandwidth of the modulated wave is a relatively narrow band, and even if the second detection circuit limits the band to a narrow band, the carrier wave is not suppressed and remains with sufficient energy. Is converted into a low-frequency signal after reception and detected by the first detection circuit, and the level difference between the detection signal obtained by detection by the second detection circuit is small.

Therefore, in the present invention, the power of the entire signal band of the low frequency signal is detected by the wideband first detection circuit, and the power in the vicinity of the carrier is detected by the narrowband second detection circuit.
By comparing the levels of the output detection signals of both detection circuits with the amplitude comparator, it is possible to determine the magnitude of the occupied frequency bandwidth of the reception signal. Here, the amplitude comparator is configured to generate a modulation method identification signal by comparing the ratio A / B between the output signal A of the first detection circuit and the output signal B of the second detection circuit and the threshold value. can do. Further, both the first and second detection circuits can use a low-pass filter for band limitation before or after the detector.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a block diagram of an embodiment of the present invention. In the figure, the same components as those in FIG. 4 are denoted by the same reference numerals. In FIG. 1, a high frequency signal received by an antenna (not shown) is input to the input terminal 1. In this embodiment, this high-frequency signal is a modulated wave obtained by modulating with a spread spectrum modulation method or a modulated wave obtained by modulating with a residual carrier modulation method.

Here, as shown in FIG. 2, the frequency spectrum of the modulated wave (spread spectrum modulated wave) obtained by modulating with the spread spectrum modulation method has a center frequency f _0.
The spectrum is distributed almost uniformly over a wide band centering around. On the other hand, the frequency spectrum of the modulated wave (residual carrier modulated wave) obtained by the modulation by the residual carrier modulation method falls in the low frequency region around the carrier I of the frequency f _C as shown in FIG. The sideband II is distributed and the upper sideband III is distributed in the high frequency region. Although each of the lower sideband II and the upper sideband III is a relatively narrow band, the carrier I It remains with a large amount of energy.

Returning to FIG. 1 again, the low-pass filter 4 is a filter circuit for band-limiting the received signal bandwidth, and its cut-off frequency is set to 3 MHz, for example. The first detection circuit 5 is provided on the output side of the low-pass filter 4.
And the second detection circuit 6 are provided in parallel. The first detection circuit 5 includes a first detector 7 and a first low pass filter 8. The second detection circuit 6 includes a second low pass filter 9 and a second detector 10. That is, the first detection circuit 5 and the second detection circuit 6 each include one detector 7, 10 and one low-pass filter 8, 9, but the first detection circuit 5 uses the low-pass filter 8 Is provided after the wave detector 7, whereas the second detection circuit 6 is different in that the low-pass filter 9 is provided before the wave detector 10.

The detectors 7 and 10 are envelope detectors, respectively. Further, the low-pass filters 8 and 9 are filter circuits for band-limiting the input signal to a narrow band of, for example, 1/1000 times, and the cutoff frequency thereof is, for example, 3 kHz.
Is set to The amplitude comparator 11 receives the first and second output detection signals as input signals, and generates a modulation method identification signal based on the determination result of whether the input detection signal levels are greatly different from each other or not. Circuit.

Next, the operation of this embodiment will be described.
The received signal input to the input terminal 1 is supplied to the mixer 2, where it is frequency-converted with the local oscillation signal from the input terminal 3 to be a low frequency signal, for example, an intermediate frequency signal (IF signal).
It is said. An unnecessary high frequency component is removed from the intermediate frequency signal by the low pass filter 4, and the band is limited to the bandwidth of the received signal of about 3 MHz.

The output signal of the low-pass filter 4 has a substantially uniform spectrum in the reception band when the reception signal is a spread spectrum modulation wave, and in the reception band when the reception signal is a residual carrier modulation wave. There is a large spectrum of energy at the frequency of the difference between the local oscillation frequency and the carrier frequency f _C.

The output signal of the low-pass filter 4 is envelope-detected by the first detector 7 and then input to the first low-pass filter 8. At the same time, the output signal of the low-pass filter 4 has an upper limit frequency of 3 kHz by the second low-pass filter 9.
After being band-limited to a signal in a narrow band of about z, the signal is input to the second detector 10 where envelope detection is performed. The output detection signal of the first detection circuit 5 extracted through the low-pass filter 8 and the output detection signal of the second detection circuit 6 that is envelope-detected by the detector 10 and extracted are output to the amplitude comparator 11, respectively. Is entered.

Here, at the time of reception of the spread spectrum modulated wave in which the spectrum exists almost uniformly in the reception band, the second detection circuit 6 limits the band to a narrow band by the low-pass filter 9, and therefore the input of the detector 7 The input signal energy of the detector 10 is greatly reduced as compared with the signal energy, and therefore the level of the first detection signal from the first detection circuit 5 obtained through the low pass filter 8 after being detected by the detector 7 Then
The level of the second detection signal from the second detection circuit 6 is significantly reduced.

On the other hand, when the residual carrier modulated wave is received, a frequency component of the difference between the local oscillation frequency of large energy and the carrier frequency f _C exists in the low frequency region of the output signal of the low pass filter 4, and this Passes through the low-pass filter 9, the level of the second detection signal extracted from the second detection circuit 6 is slightly lower than that of the first detection signal extracted from the first detection circuit 5, and a large difference is produced. Absent.

The amplitude comparator 11 has a level A of the first detection signal input from the first detection circuit 5 and a level B of the second detection signal input from the second detection circuit 6.
Then, after calculating the ratio A / B of both, a predetermined threshold value C
Compare with the size. Then, when A / B> C, it is determined that the second detected signal level B is significantly lower than the first detected signal level A, and the received signal is identified as a spread spectrum modulated wave, On the other hand, when A / B ≦ C, it is determined that there is no great difference between the first detected signal level A and the second detected signal level B, and the received signal is identified as the residual carrier modulated wave. The amplitude comparator 11 outputs an identification signal indicating this identification result to the output terminal 12.

The present invention is not limited to the above-mentioned embodiment, but it is possible to perform band limitation by a band filter, and the amplitude comparator 11 uses the first detection signal and the second detection signal. It is also possible in principle to identify whether the received signal is a spread spectrum modulated wave or a residual carrier modulated wave based on the result of comparison between the difference from the detected signal and the threshold value. However,
When the present invention is applied to a satellite communication system or the like in which the distance of the radio wave propagation furnace changes or the received signal level greatly changes due to the gain of the antenna or the like, the first and second detected signal levels as in the embodiment. Discrimination based on the ratio is preferable because it is not affected by input level fluctuations.

[0023]

As described above, according to the present invention,
The received signal is converted into a low-frequency signal, the power of the entire signal band of this low-frequency signal is detected by the wideband first detection circuit, and the power in the vicinity of the carrier is detected by the narrowband second detection circuit. Since the level of the occupied frequency bandwidth of the received signal is determined by comparing the levels of the output detection signals of both detection circuits, the received signal is modulated by the spread spectrum modulation method or the residual carrier modulation method. It is possible to correctly identify what has been done. Further, according to the present invention, since the band limitation is performed by the low-pass filter, it is easy to realize the filter.

[Brief description of drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is a diagram showing an example of a frequency spectrum of a spread spectrum modulated wave.

FIG. 3 is a diagram showing an example of a frequency spectrum of a residual carrier modulation wave.

FIG. 4 is a block diagram of a conventional example.

[Explanation of symbols]

 1 Received Signal Input Terminal 2 Mixer 3 Local Oscillation Signal Input Terminal 4 Low-pass Filter for Intermediate Frequency Signal Filtering 5 First Detection Circuit 6 Second Detection Circuit 7, 10 Detector 8, 9 Low-pass Filter 11 Amplitude Comparator

Claims (3)

(57) [Claims] 1. A frequency conversion means for frequency-converting a received signal into a low-frequency signal, a first detection circuit for band-limiting the low-frequency signal after envelope detection, and the low-frequency signal being band-limited to a narrow band. The second detection circuit for post-envelope detection is compared with the amplitudes of the output detection signals of the first and second detection circuits, and the received signal modulation method is determined depending on whether the amplitude ratio is substantially the same as or greatly different from the threshold value. And an amplitude comparator that generates and outputs an identification signal that identifies the modulation method identification circuit. 2. The first detection circuit includes a first detector that envelope-detects the low-frequency signal, and a first low-pass filter that filters a low-frequency component of an output signal of the first detector. The second detection circuit includes a second low-pass filter that filters low-frequency components of the output low-frequency signal of the frequency conversion unit, and envelope detection of the output signal of the second low-pass filter. The modulation system identification circuit according to claim 1, comprising a second wave detector. 3. The amplitude comparator modulates the received signal when a ratio A / B between an output signal A of the first detection circuit and an output signal B of the second detection circuit is larger than the threshold value. It is determined that the method is a spread spectrum modulation method, and the ratio A
3. The modulation method identification circuit according to claim 1, wherein when / B is less than or equal to the threshold value, an identification signal which determines that the modulation method of the received signal is the residual carrier modulation method is output.