FSK Signal demodulator

Abstract: An FSK signal demodulator to demodulate digital data from a frequency shift keying signal comprises a circuit (5) to extract a timing clock of the digital data from the FSK signal; and a sampling circuit (4) to sample the FSK signal by a clock having a frequency which is almost equal to one frequency of the FSK signal or which is almost equal to a multiple of this frequency.

Description: BACKGROUND OF THE INVENTION The present invention relates to a demodulator to demodulate a data signal from an FSK ( F requency S hift K eying) signal in a transmission system for FSK modulating the data signal and transmitting and, more particularly, to an FSK signal demodulator suitable for use in realization of a digital integrated circuit. Such a kind of technique is disclosed in, for example, "Analog and Communications Products Data Book", pp. 9·22 to 9·42, Advanced Micro Devices, Inc., 1983. In addition, as an apparatus for realizing a digital integrated circuit of an FSK signal demodulator, an FSK signal demodulating apparatus is disclosed in JP-A-54-87465. Fig. 2 shows main sections of this apparatus and Fig. 3 shows a waveform in each section. The signal with FSK modulation of such a data signal as shown at a waveform 3a in Fig. 3 is converted into a pulse train such as a waveform 2a by a band pass filter 1 and a slicer 2. This pulse train is converted into a pulse train of a constant pulse width such as a waveform 2b by a pulse width adjusting circuit 21. This pulse train is set to a reset pulse of a counter 22. A pulse train such as a sufficiently higher-speed waveform 2c than the waveform 2b is input from an oscillator 23 as a clock of the counter 22. In Fig. 3, numeral 3b denotes the operation of the counter 22. This diagram shows a change in count number as an axis of ordinate. It is decided by a decision circuit 24 whether the counter output is "1" or "0" using the count number, as a reference value, indicated by an alternate long and short dash line in 3b in Fig. 3. The resultant data becomes as shown in a waveform 2d. This signal is filtered by a low pass filter 25 to eliminate the influence by the noise, so that a signal of a waveform 2e is derived. This signal is waveform shaped by a waveform reshaper 26 to thereby form a pulse. A data signal clock 2g is extracted by a timing detection circuit 28. A data signal such as a waveform 2f is obtained by a regenerator 27. The demodulating accuracy of the foregoing demodulator is determined by the ratio of the carrier frequency of the FSK signal and the frequency of the counter clock (waveform 2c in Fig. 3) from the oscillator 23. As the counter clock frequency is higher than the FSK carrier frequency, an allowance for the discriminating count number is large as shown in 3b in Fig. 3. An allowance for the noise is large and the demodulation can be performed at a high demodulating accuracy. However, when considering the semiconductor device and circuit technique which are used, the counter clock frequency cannot be set to a high value without limitation. When the counter clock frequency is set to about 10 MHz, the carrier frequency is at most about 100 kHz and the transmission data signal speed is about a few kb/sec. It is unpractical to transmit a data signal of a speed higher than that value by the transmission system using the foregoing demodulator. Further, the conventional apparatus needs averaging with the low pass filter 25 and has a drawback such that the whole demodulator cannot be constituted by only logic circuits. US-A-4 535 297 discloses a circuit to demodulate a data signal from a chirped-frequency-modulation or FSK signal including a means for generating a clock signal of a frequency which is equal to the frequency which is integer times as high as the frequency of the input signal. IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, Vol. VT-34, no. 4, November 1985, pages 157 to 158, IEEE, New York, US; H. SUZUKI et al.: "A single-chip MSK coherent demodulator for mobile radio transmission" discloses a circuit to demodulate a data signal from an FSK signal as mentioned in the preambles of claims 1 and 5. SUMMARY OF THE INVENTION It is the object of the invention to ensure a correct demodulation of a data signal by a demodulator which is simply constituted, whereby the cost of the demodulator can be remarkably reduced. This object is solved by the invention as given by claims 1 and 5. A mark and a space are discriminated by using the presence or non-presence of a phase change at a section (time slot) corresponding to a period of the data signal. The level of the sampled data is inverted in dependence on the phase difference between the oscillation signal, i.e. the clock signal, from the oscillator and the FSK signal. But irrespective of the phase difference, the level inverted signal occurs merely in the mark signal interval. On the basis of whether the level has been inverted in the time slot of the data signal or not, it is possible to discriminate whether the time slot corresponds to a mark or a space. According to the invention the data signal itself is used for discriminating a mark and a space for a correct demodulation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing an example of a constitution of the present invention; Fig. 2 is a block diagram showing a constitution of a conventional FSK demodulator; Fig. 3 is a waveform diagram in each section in Fig. 2; Figs. 4A and 4B are a constitutional diagram and a waveform diagram of a sampling circuit in Fig. 1; Fig. 5 is an explanatory diagram of a level inverting position; Fig. 6 is a block diagram showing an embodiment of the invention; Fig. 7 is a waveform diagram in each section in Fig. 6; Fig. 8 is a block diagram showing an example of a circuit of a phase detecting circuit section; and Fig. 9 is a waveform diagram in each section in Fig. 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows an embodiment of the present invention. In the diagram, the FSK signal 3a is converted into a pulse by the band pass filter 1 and slicer 2 and becomes the signal 2a. The oscillator 3 generates a clock signal 4a having a frequency which is nearly equal to the frequency which is integer times (one time in this example) as high the frequency of the signal 2a. A sampling circuit 4 samples the signal 2a by the clock signal 4a. Thus, the FSK signal is converted into a signal 4b having a frequency which is almost the same as that of the data signal. Since the signal 4b is the low-speed signal, the demodulation can be performed by the signal processing at a speed which is nearly equal to the carrier frequency. This frequency converted FSK signal 4b is binarized by a frequency decision circuit 6 on the basis of the digital counting system, thereby demodulating the data signal. A data signal clock timing which is needed at this time is extracted from the FSK signal by a data clock detection circuit 5. Fig. 4A shows an example of the sampling circuit 4 using a D flip-flop. Fig. 4B shows a waveform in each section. The FSK signal 2a whose waveform was shaped by the band pass filter 1 and slicer 2 is connected to a D input terminal of a flip-flop 41. The sampling clock 4a having a frequency which is nearly equal to the frequency assigned to a space signal of the FSK signal is connected to a Ck input terminal of the flip-flop 41. Thus, the signal 4b is output from the flip-flop 41. Namely, the signal 4b is divided into the portion (DC component) whose level does not change and the portion (AC component) whose level changes in one time slot in correspondence to the signal 3a. Since the level of the AC component is inverted twice for one time slot, its frequency is equal to the data signal speed. In this manner, the FSK signal is frequency converted into the DC component and AC component. The position at which the level is inverted as shown in the signal 4b in Fig. 4 differs in dependence on the phase difference between the oscillation signal from the oscillator 3 and the FSK signal. This point will be explained with reference to Fig. 5. In the diagram,  denotes a phase difference between the FSK space signal frequency and the sampling clock. As will be obvious from the diagram, the level inverted signal occurs in the mark signal interval irrespective of the phase of . Namely, on the basis of whether the level has been inverted in the time slot of the data signal or not, it is possible to discriminate whether the time slot corresponds to a mark M or a space S. In this discrimination, it is sufficient to discriminate between the DC component and the AC component. Therefore, a discriminating clock of a speed higher than that of the data signal is counted by such a counter as to perform the counting operation only when the output of the flip-flop 41 is at the "H" level. If the count number is 0 or n (n is the number of discriminating clock pulses which are generated in the time slot of the data signal), it is decided that the time slot indicates the space. If the count number is n/2, it is determined that the time slot represents the mark. Namely, a discrimination allowance of n/2 is obtained. In this case, even when n is set to a large value above 10, the discriminating clock has a frequency which is almost equal to the carrier frequency. A high-speed clock as in the conventional apparatus is unnecessary. Further, a clock of the oscillator 3 for frequency conversion can be also used. The time slot corresponding to the data signal is detected and the timing to reset the discriminating counter is extracted in the following manner. As will be understood from Fig. 5, the level inverted signal does not overflow from this time slot. Namely, by adjusting the position of the time slot such that the level inverted signal does not overflow, the accurate counter reset timing (data signal clock) can be extracted. In the foregoing demodulating method, when  = 0, π, and 2π, there is a possibility such that a level inverted signal is output for the space interval. This state corresponds to the case where a jitter due to the noises occurred in the FSK input to the D flip-flop. In this case, there is a possibility such that an error exceeding the discrimination allowance of the discriminating counter occurs. To prevent this error, it is sufficient to perform the discrimination by two phases by use of clocks whose phases are deviated by π/2. When the phase  of one clock is equal to 0, π, and 2π, the phase  of the other clock is π / 2 and 3 / 2π, so that the accurate discrimination can be performed. Moreover, since the foregoing error occurs in only the space interval, the accurate demodulation can be performed by merely calculating the AND of the results of the discrimination of two phases. In the case of the data signal clock extracted by the foregoing method, if a jitter is large, this jitter can be reduced in the following manner. In the case of the phase continuous FSK signal, the phases of two frequencies which are key-input coincide at the timing of the data signal clock, i.e., the frequency switching timing. Therefore, after this coincident point was detected by the foregoing method, by locking the data signal timing clock at the coincident point in a PLL manner, the jitter can be reduced to a value which is determined by the characteristics of the PLL even if the phase of the level inverted signal in the mark interval fluctuated due to a frequency deviation. Fig. 6 shows a practical embodiment of the invention. Fig. 7 shows a waveform in each section in Fig. 6. An explanation will now be made with respect to the example in which the data clock frequency is set to f₀, the "mark" of the data signal is set to 17f₀ (= f c + Δf) , and the "space" of the data signal is set to 16f₀ (= f c - Δf) . In Fig. 7, a waveform 7a indicates a data clock and a waveform 7b represents a transmission data signal. The waveform of the received FSK signal is shaped as shown in a waveform 7c by the band pass filter 1 and slicer 2. On the other hand, the oscillator 3 on the reception side outputs a clock of the frequency 2f b [2(f c - Δf)] which is almost twice as high as the frequency corresponding to the "space" signal. One of the clock signals is converted into a sampling clock 7d of a frequency f b by a 1/2 frequency divider 62. The other signal is converted into the signal having the opposite phase by an inverter 61 and thereafter, it is frequency divided into 1/2 by a frequency divider 62′. Thus, the signal is converted into the sampling clock whose phase is deviated by π/2 from the phase of the sampling clock 7d. Further another signal is input to a DPLL (digital phase locked loop) consisting of a phase comparator 68, a synchronizing protection circuit 69, and a variable frequency divider 60. The former two signals are input to clock terminals of flip-flops 63 and 63′. The FSK signal is input to the D input terminals of the flip-flops 63 and 63′. Thus, the signals having waveforms 7e and 7f derived by frequency converting the FSK signal 7c are output from the flip-flops 63 and 63′, respectively. When it is assumed that hexadecimal counters 64 and 64′ count the foregoing sampling clocks of two phases only when the outputs of the flip-flops 63 and 63′ are at the "H" level, the count numbers of the counters 64 and 64′ become as diagrammatically shown in 7g and 7h. In this case, the count values 7g and 7h are discriminated as shown in 7i and 7j by use of such decision circuits 65 and 65′ as to decide the "mark" when the count value lies within a range of 4 to 12 and the "space" when the count value falls within a range of 0 to 4 or 12 to 16, respectively. However, since the phase difference between the clock and the FSK signal is about 0 or π in the case of the waveform 7f, the erroneous frequency conversion is performed in the "space" interval. Therefore, the result 7j of the discrimination differs from the transmission data 7b. However, a data signal 7k without an error is obtained by an AND circuit 66. On the other hand, the data clock is reproduced in the following manner. As shown in Fig. 5, if the frequency f b of the sampling clock is slightly deviated from one frequency f c - Δf of the FSK signal, the outputs of the flip-flops 64 and 64′ change in accordance with this deviation. For example, when f b is slightly higher than f c - Δf, the waveforms change upwardly from the lower portion in Fig. 5. When the phase  is near π, the signal corresponding to the "space" interval changes from the "L" level to the "H" level. At this time, the trailing edge of the signal corresponding to the "mark" coincides with the frequency switching timing of the FSK signal. A phase detecting circuit 67 operates as mentioned above and controls the synchronizing protection circuit 69 in the DPLL. Fig. 8 shows an example of the phase detection circuit. Fig. 9 shows a waveform in each section of the circuit of Fig. 8. An n=0 detection circuit 81 detects that the output of the counter 64 in Fig. 6 is 0 and outputs a pulse 9d. The n=0 detection circuit 81 detects that the output of the flip-flop 63 is continuously at the "L" level at least sixteen times at the period of f b . When this state is detected, a flip-flop 84 is set. After the flip-flop 84 was set, it is detected by an n=16 detection circuit 82 that the output of the flip-flop 63 is continuously at the "H" level at least sixteen times at the period of f b . When this state is detected, a flip-flop 85 is set. Reference numeral 83 denotes a pull-up resistor. Thus, a gate circuit 87 is made operative and the output of the flip-flop 63 is input to a flip-flop 89. The flip-flop 89 is set in response to the trailing edge of the output of the flip-flop 63. Reference numeral 88 denotes a pull-up resistor. A variable frequency divider 60 is reset by the output of the flip-flop 89 through the synchronizing protection circuit 69 in Fig. 6. Thus, although a phase deviation as shown in a waveform 9a occurred at first, this phase deviation is corrected and the data clock of the correct phase is output as an output pulse from the variable frequency divider 60. As described above, the phase which has once been detected is locked by controlling the variable frequency divider 60 on the basis of the result of the phase comparison by the phase comparator 68 between the FSK signal and the output pulse 9a of the variable frequency divider 60. The case where the frequencies which are sixteen and seventeen times as high as that of the data signal are switched has been described in the foregoing embodiment. However, the method of the invention can be also applied to the case where other frequencies are used. The case where the sampling clock frequency f b was deviated to a high frequency has been described in the foregoing embodiment. However, the invention can be also applied to the opposite case where the sampling clock frequency was deviated to a low frequency. In this case, the leading edge after the change from n = 0 to n = 16 may be used. On the other hand, the change from n = 16 to n = 0 can be also used. Further, by checking a change in output of the flip-flop 63, it is possible to discriminate whether the sampling clock frequency has been deviated to a high frequency or a low frequency. Therefore, by switching the phase detection by use of this method, an arbitrary sampling clock can be used. In the foregoing embodiment, the sampling clocks of two phases having the phase difference of only π/2 have been formed by use of the oscillator of the oscillating frequency of 2f b . However, these sampling clocks can be also formed by use of an oscillator of the frequency of f b and a fixed delay circuit. This is because there is no need to accurately set the phase difference between two sampling clocks to π/2. In the foregoing embodiment, the discriminating ranges of the "mark" and "space" have been set to the range of 4 to 12 and the range of 0 to 4 or 12 to 16, respectively. Even if these ranges were slightly changed, the influence on the resetting operation is small. These ranges can be also changed to simplify the circuit constitution.

Claims: A circuit to demodulate a data signal from an FSK signal, comprising: means (3) for generating a clock signal of a frequency which has an integer relationship to the frequency of an FSK pulse signal, means (4) for sampling said FSK pulse signal by said clock signal, means (5) for extracting a data signal clock timing from said FSK pulse signal, and means (6) for obtaining a data signal by binarizing said sampled signal on the basis of said data signal clock timing, characterized in that said circuit further comprises means (1 and 2) for converting an input FSK signal into said FSK pulse signal, that said frequency with said integer relationship to the frequency of said FSK pulse signal is integer times as high as the frequency of said FSK pulse signal, wherein a phase difference exists between said clock signal and said FSK pulse signal, and that a mark (M) or a space (S) are discriminated by using the presence or non-presence of a phase change at a section corresponding to a period of data signal. A demodulating circuit according to claim 1, wherein said sampling means (4) includes: a first frequency divider (62) to frequency divide said clock signal into 1/2; a second frequency divider (62') to frequency divide a signal obtained by converting said clock signal into the signal having the opposite phase into 1/2; a first flip-flop (63) which receives said FSK pulse signal and an output of said first frequency divider (62) as a clock input and operates; and a second flip-flop (63') which receives the FSK pulse signal and an output of said second frequency divider (62') as a clock input and operates. A demodulating circuit according to claim 1 or 2, wherein said means (5) for extracting the data signal clock timing includes: a digital phase locked loop (60, 68, 69) which receives said FSK pulse signal and outputs the data signal clock timing on the basis of said clock signal; and a phase detection circuit (67) which receives said data signal clock timing and controls said digital phase locked loop. A demodulating circuit according to claim 2 or 3, wherein said means (6) for obtaining the data signal includes: a first counter (64) to count the output of said first flip-flop (63) on the basis of said data signal clock timing; a second counter (64') to count the output of said second flip-flop (63') on the basis of the data signal clock timing; a first decision circuit (65) to discriminate said mark or said space of the signal on the basis of a count number of said first counter (64); a second decision circuit (65') to discriminate said mark or said space of the signal on the basis of a count number of said second counter (64'); and an AND circuit (66) to calculate the AND of the outputs of said first and second decision circuits (65 and 65') and to set an output indicative of the AND to the data signal. A method of demodulating a data signal from an FSK signal, comprising the steps of: sampling an FSK pulse signal by a clock signal of a frequency which has an integer relationship to the frequency of said FSK pulse signal; and obtaining the data signal by binarizing said sampled signal on the basis of the data signal clock timing extracted from said FSK pulse signal, characterized in that said FSK pulse signal is generated by converting the input FSK signal into a pulse, that said frequency with said integer relationship to the frequency of said FSK pulse signal is integer times as high as the frequency of said FSK pulse signal, wherein a phase difference exists between said clock signal and said FSK pulse signal, and that a mark (M) or a space (S) are discriminated by using the presence or non-presence of a phase change at a section corresponding to a period of said data signal.

Inventors: TAKASE AKIHITO, TAKASE, AKIHITO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1987
Main classification: H04L27
IPC codes: H04L27