JPH033016Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a detection circuit that detects the presence or absence of an MSK (Minimum Shift Keying) signal.

(Prior art) MSK (Minimum Shift Keying) signals are phase continuous FSK (Frequency Shift Keying) signals with a modulation index of 0.5.
Keying) signal, and its phase (t) is expressed as follows.

Here, n is a positive integer, and a _o is the n-th transmission code and takes a value of +1 or -1 depending on the code.
Further, the variable t represents time, and the constant T represents the period of the transmission code.

As a method for detecting the presence or absence of an MSK signal having such a phase, there is a method using a circuit as shown in FIG. 3, for example. In the figure, 1 is a signal input terminal, 2 is a bandpass filter, and 3 is a signal detection circuit, which is a delay detection circuit or a synchronous detection circuit. 4 is a detection output terminal, 5 is an envelope detection circuit, 6 is a low frequency wave filter, 7 is a comparison circuit, and 8 is an output terminal.

The operation of this circuit is as follows. In other words, when an MSK signal is applied to the signal input terminal 1, the bandpass filter 2 limits the band of the signal, the envelope detection circuit 5 detects it and converts it into a DC signal, and the low-pass filter 6 removes the high frequency component. Thereafter, the comparison circuit 7 compares the magnitude with a preset value and outputs the result to the output terminal 8 as the presence or absence of the MSK signal.

(Problems to be Solved by the Invention) However, the above-mentioned system has the following drawbacks. That is, when noise is added to the input terminal 1 instead of the MSK signal, there is a drawback that when the level is large, a result of the presence of the MSK signal appears even though there is no MSK signal. In addition, when transmitting an audio signal by switching between an audio signal and an MSK signal using an MSK signal in the audio band, if an audio signal is accidentally applied to input terminal 1, the output terminal will be sent to the output terminal even though there is no MSK signal. There was also a problem in that the result was that there was no result.

(Means for solving the problem) In order to solve the above problems, the present invention has the following features: (a) Bandwidth _B i connected in series that outputs the MSK signal with constant energy of the input clock _frequency b (b) a multi-stage shift register 10 having a clock oscillator 11 and giving the constant energy MSK signal a delay time of 1/4 of the period of its center frequency _c ; and (c) the output of the multi-stage shift register 10 and the
(a) An exclusive OR circuit 12 to which the MSK signal with constant energy is input; (d) connected to the output of the exclusive OR circuit 12, with a bandwidth of B _p (B _p << B _i ) and a ( _e ) the exclusive OR circuit ₁₂ ; (e) the exclusive OR circuit 12; is connected to the output of the first detection section, and has the same configuration as the first detection section, that is, a band wave detector 17, an envelope detection circuit 18,
It has a configuration in which a low frequency wave generator 19 and a comparator circuit 20 are connected in series, and the bandwidth of the band wave generator 17 is
a second detection section whose center frequency is 2 _c − _b /2 at B _p ; and (f) an OR circuit 21 whose inputs are the outputs of the first detection section and the second detection section. This is the MSK signal detection circuit.

(Operation) The operation of the present invention will be explained. As previously mentioned,
The MSK signal is a phase continuous FSK signal with a modulation index of 0.5. The double wave is a phase continuous FSK with a modulation index of 1.0.
The signal is the so-called Sunde FSK signal, for example, as described in Japanese Patent Publication No. 59-25502,
An emission line spectrum is generated at a frequency that is detuned by _b /2 ( _b : clock frequency) in the positive and negative directions from the center frequency.

The present invention makes use of the properties of the MSK signal described above, passes the input MSK signal through a bandpass filter 2 and an amplitude limiter 9 to keep its energy constant, and further includes a clock oscillator 11 to convert the input MSK signal to the constant energy MSK signal. , through a multistage shift register 10 that provides a delay time of 1/4 of the period of its center frequency _c , and by passing the output thereof and the MSK signal with constant energy through an exclusive OR circuit 12, the original MSK signal is converted into 2. Obtain the multiplied signal,
The emission line spectra are each transmitted by a bandpass filter 13.
and a first detection section consisting of an envelope detection circuit 14, a low frequency waveform generator 15, and a comparison circuit 16, and a band waveform waveform generator 17.
and a second detection section consisting of an envelope detection circuit 18, a low frequency filter 19, and a comparison circuit 20,
Furthermore, the circuit detects the MSK signal by inputting the output of the first detection section and the output of the second detection section to the OR circuit 21, taking into consideration the space ratio of the mark of the MSK signal. Hereinafter, it will be explained in detail using examples.

(Embodiment) FIG. 1 is a block diagram showing an embodiment of the present invention, and the same numbers as in FIG. 3 represent the same elements as in FIG. 3. Reference numeral 9 denotes an amplitude limiter that converts the input waveform into a rectangular wave in order to perform subsequent processing in a digital circuit. 10 is a shift register connected in cascade in multiple stages, and a clock oscillator 11
The output of the amplitude limiter 9 is sequentially shifted in accordance with the clock of the amplitude limiter 9, and has the function of delaying the output of the amplitude limiter 9 by a certain period of time. 12 is an exclusive OR circuit, 13 and 17 are band wavers,
14 and 18 are envelope detection circuits, 15 and 19 are low frequency filters, 16 and 20 are comparison circuits, and 21 is an OR circuit.

Now, let _c be the center frequency of the input MSK signal,
Let _b be the clock frequency. At this time, the delay time in the multi-stage shift register 10 is set at the center frequency _c.
1/4 of the period of , that is, 1/4 _c . This can be realized by setting n/F _c =1/4 _c (2) where the number of stages of the multi-stage shift register 10 is n and the frequency of the clock oscillator 11 is F _c . If the delay time is selected as described above, the output of the exclusive OR circuit 12 can be obtained by doubling the output of the amplitude limiter 9. This situation is shown in FIG.

In FIG. 2, (a) shows the output of the amplitude limiter 9, (b) shows the output of the multistage shift register 10 whose output is delayed by 1/4 _c , and (c) shows the output of the exclusive OR circuit 12. The output shows that the output in (b) above has been doubled. As mentioned above, when the MSK signal is multiplied by 2, it increases by _b /2 in the positive and negative directions from the center frequency.
Since the center frequency of the MSK signal in this case is _c , one bright line spectrum after doubling is 2 _c +
_b /2, and the center frequency of the bandpass converter 13 is assumed to be 2 _c + _b /2. Then, the other bright line spectrum 2 _c − _b /2 is selected as the center frequency of the band wave generator 17 .

At the output point of the amplitude limiter 9, the input terminal 1
No matter what signal or noise is added to it, its output level will remain constant, and the sum of its total energies will remain constant. Therefore, in the case of noise, the energy is dispersed over a certain frequency band, whereas in the case of a signal with a certain frequency, the energy is concentrated at that frequency, making it possible to identify the signal.

This will be explained in more detail. First, let the bandwidth of the band waver 2 be B _i . If the input signal is only noise, the output of the exclusive OR circuit 12 is doubled, so its bandwidth is 2B _i ,
Let that energy be E _2Bi .

Next, when the input signal is an MSK signal, the output energy E of the exclusive OR circuit 12 is expressed as E=E ₁ +E ₂ (3). However, E ₁ is the energy of the emission line spectrum whose frequency is 2 _c + _b /2, and E ₂ is the energy of the emission line spectrum whose frequency is 2 _c + _b /2. Assuming that the marks and spaces of the digital code of this MSK signal are balanced, E ₁ =E ₂ =E/2 (4). As mentioned above, the sum of the entire energy is constant at the output point of the amplitude limiter 9, so
E _2Bi = E holds, and E _2Bi = 2E ₁ = 2E ₂ ...(5) holds. That is, the energy when there is only noise is twice the energy of each emission line spectrum when there is only MSK signal.

Furthermore, when the bright line spectrum is passed through a narrow band waver, if the frequency of the bright line spectrum and the frequency of the band wave waver match, there will be no attenuation, but in the case of noise, the band of the band wave waver is The narrower the area, the greater the attenuation.

Here, the bandwidth of the band waver 13 or 17 is B _p
(B _p ≪ B _i , B _i is the bandwidth of band wave generator 2), and when noise is added to input terminal 1, band wave wave generator 13
Or, if the energy output by 17 is E _o ,
Since the energy entering the band wave generator 13 or 17 is expressed by equation (5), E _o =E _2Bi ×B _p /2B _i (6) holds true. On the other hand, if the output energy of the bandpass filter 13 or 17 when the MSK signal is input to the input terminal 1 is E _n , then from equations (4) and (5), E _n = E/2 = E _2Bi /2 …(7) becomes. Therefore, from equations (6) and (7), E _o /E _n = E _2Bi ×B _p /2B _i /E ₂ B _i /2 = B _p /B _i ...(8), and the bandwidth B _i and B If we take a large ratio of _p ,
It is possible to clearly distinguish between MSK signals and noise. Furthermore, even if the signal applied to the input terminal 1 is an audio signal, it can be treated similarly to noise, so that it can be similarly distinguished from an MSK signal.

Note that since the mark-to-space ratio of the MSK signal code is generally not balanced, E ₁ or E ₂ , whichever is larger, is between E/2 and E according to equation (3). Therefore, the OR logic circuit 21 in FIG. 1 may be set to detect either E ₁ or E ₂ , whichever is larger.

(Effects of the invention) As explained above, according to the invention, it is possible to realize an MSK signal detection circuit that can clearly distinguish MSK signals from noise and voice signals, and malfunctions of communication equipment using MSK signals. It can be used for prevention, etc.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a diagram illustrating doubling of an MSK signal, and FIG. 3 is a block diagram illustrating a conventional MSK signal detection circuit. 1... Signal input terminal, 2, 13, 17... Bandwidth wave generator, 3... Detection circuit, 4... Detection output terminal,
8...Output terminal, 10...Multi-stage shift register,
11... Clock oscillator, 12... Exclusive OR circuit, 14, 18... Envelope detection circuit,
15, 19...Low frequency generator, 16, 20...Comparison circuit, 21...OR circuit.

Claims (1)

[Claims for Utility Model Registration] (a) A bandpass filter with a bandwidth _{B i and} an amplitude limiter connected in series, which output the MSK signal with a constant energy of an input clock frequency b, and ( b) has a clock oscillator, and applies 1/4 of the period of its center frequency _c to the MSK signal with constant energy.
(c) an exclusive OR circuit to which the output of the multistage shift register and the constant energy MSK signal of item (a) are input; (d) the exclusive OR circuit output, with a bandwidth of B _p (B _p ≪ _{B i} ) and a center frequency of
2 _c + _b / 2 band wave detector, envelope detection circuit, low frequency wave detector, and comparator circuit connected in series; (e) connected to the output of the exclusive OR circuit; The width is B _p (B _p ≪ _{B i} ) and the center frequency is
(f) a second detection section in which a 2 _c - _b /2 band wave detector, an envelope detection circuit, a low frequency wave detector, and a comparison circuit are connected in series; (f) an output of the first detection section; An MSK signal detection circuit comprising: an OR circuit inputting the output of a second detection section;