JP2019033317A - FSK demodulator - Google Patents

Abstract

To increase FSK demodulation accuracy.SOLUTION: A frequency conversion unit 10 generates a low-band FSK modulation signal produced by converting an FSK modulation signal to a low band in frequency, and output to an amplitude regulating unit 12. The amplitude regulating unit 12 determines an absolute value of the low-band FSK modulation signal, divides the low-band FSK modulation signal by the absolute value to normalize the low-band FSK modulation signal, and outputs to a multiplication unit 14. The multiplication unit 14 multiplies, by 2, a frequency of the normalized low-band FSK modulation signal to produce a 2-multiplied FSK modulation signal and outputs to a decoding unit 16. The decoding unit 16 decodes digital data based on the 2-multiplied FSK modulation signal having two frequencies consisting of a high frequency and a low frequency.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an FSK demodulator, and more particularly to improvement of demodulation performance.

  FSK modulation is widely used as a method for modulating a signal transmitted in wireless communication or wired communication. In FSK modulation, the frequency of a carrier wave is changed according to the sign of digital data to be transmitted. The FSK modulation has an advantage that noise and amplitude distortion appearing in the amplitude component of the signal have a small influence on the communication quality.

  From the FSK modulation signal, the digital data is demodulated by detecting its frequency. Patent Document 1 below describes an FSK demodulator. The FSK demodulator described in this document generates a pulse width modulation signal whose pulse width changes according to the frequency of the FSK modulation signal, and decodes digital data based on the pulse width of the pulse width modulation signal.

JP 2001-251372 A

  In a communication system using FSK modulation, the frequency of the FSK modulated signal varies due to noise or the like depending on the condition of the signal transmission path from the transmitter to the receiver. At this time, if the difference between the higher frequency (high frequency) and the lower frequency (low frequency) of the FSK modulated signal is not sufficient, it becomes difficult to distinguish between the high frequency and the low frequency, and the accuracy of FSK demodulation is reduced. May decrease.

  An object of the present invention is to improve the accuracy of FSK demodulation.

  The present invention provides a multiplier that multiplies the frequency of the FSK modulation signal, a measurement unit that measures the frequency of the signal multiplied by the multiplier, and a demodulated signal based on the frequency measured by the measurement unit. And a demodulated signal generating section for outputting.

  Preferably, a frequency conversion unit that converts the frequency of the FSK modulated signal before frequency multiplication to a low frequency is provided.

  Preferably, the frequency conversion unit performs frequency conversion of the FSK modulation signal so that the lower limit of the occupied frequency band does not return at 0 frequency and is close to 0 frequency.

  Preferably, the FSK modulation signal before frequency multiplication is detected, and a signal adjustment unit that adjusts the magnitude of the FSK modulation signal before frequency multiplication based on the magnitude of the detection signal. .

  According to the present invention, the accuracy of FSK demodulation can be improved.

It is a figure which shows the structure of a FSK demodulator. It is a figure which shows the structural example of a frequency conversion part. It is a figure which shows each frequency component of a FSK modulation signal, a low region and a FSK modulation signal, and a 2 ^N multiplication and a FSK modulation signal. It is a figure which shows the structural example of an amplitude adjustment part. It is a figure which shows the structural example of a multiplication part. It is a figure which shows the structural example of a decoding part.

  FIG. 1 shows an FSK demodulator according to an embodiment of the present invention. The FSK demodulator is mounted on, for example, a wireless receiver. The mounted radio receiver receives a radio signal subjected to FSK modulation with digital data, and converts the radio signal into an FSK modulated signal which is an electric signal. Here, it is assumed that the FSK modulation signal is used such that the frequency of the FSK modulation signal when the digital data value is 1 is higher than the frequency of the FSK modulation signal when the digital data value is 0. .

  The FSK demodulator includes a frequency conversion unit 10, an amplitude adjustment unit 12, a multiplication unit 14, and a decoding unit 16. The frequency converter 10 receives an FSK modulated signal as a signal to be processed. The frequency conversion unit 10 multiplies the FSK modulation signal by the local signal to generate a low-frequency / FSK modulation signal obtained by frequency-converting the FSK modulation signal to a low frequency, and outputs the low-frequency / FSK modulation signal to the amplitude adjustment unit 12. The amplitude adjustment unit 12 obtains an absolute value (magnitude) of the low-frequency / FSK modulation signal and divides the low-frequency / FSK modulation signal by the absolute value, thereby normalizing the low-frequency / FSK modulation signal, and a multiplication unit. 14 for output.

The multiplier 14 multiplies the frequency of the standardized low frequency / FSK modulated signal by ^2N to generate a ^2N multiplied / FSK modulated signal, and outputs it to the decoder 16. Here, N is an integer of 1 or more.

The decoding unit 16 decodes the digital data based on the ^2N multiplication / FSK modulation signal having two frequencies, a high frequency and a low frequency. That is, the decoding unit 16 outputs 1 as a decoded value when the ^2N multiplied / FSK modulated signal has a high frequency, and outputs 0 as a decoded value when the ^2N multiplied / FSK modulated signal has a low frequency. .

  FIG. 2 shows a configuration example of the frequency conversion unit 10. In this configuration example, the frequency conversion unit 10 includes a local oscillator 20, a mixer 18, and a low-pass filter 22. The mixer 18 receives an FSK modulation signal whose frequency changes between the low frequency f0 and the high frequency f1. The local oscillator 20 generates a local signal having a frequency fa and outputs the local signal to the mixer 18. The mixer 18 generates a frequency mixed signal obtained by multiplying the FSK modulated signal and the local signal, and outputs the frequency mixed signal to the low-pass filter 22. The frequency mixed signal includes the frequency component of the sum (f + fa) of the frequency f (f0 or f1) of the FSK modulated signal and the frequency fa of the local signal, and the difference between the frequency f of the FSK modulated signal and the frequency fa of the local signal. The frequency component of the absolute value Δ is included. The low-pass filter 22 suppresses or removes the sum frequency component and passes the low-frequency / FSK modulated signal that is the difference frequency component. Note that the local frequency fa is set, for example, so that the lower limit fu of the occupied frequency band of the FSK modulated signal does not return at 0 frequency and is as close to 0 as possible (near 0 frequency). Preferably, the local frequency fa is set so that fu−Δ = 0.

  FIG. 3A shows frequency components of the FSK modulation signal. FIG. 3B shows the frequency components of the low-frequency / FSK modulated signal. The horizontal axis represents frequency and the vertical axis represents amplitude. FIG. 3 conceptually shows the frequency component of each signal, and does not strictly show the frequency spectrum.

  As shown in FIG. 3A, the frequency of the FSK modulation signal is the low frequency f0 or the high frequency f1, and the frequency difference between the low frequency f0 and the high frequency f1 is d = f1-f0. The frequency of the low frequency / FSK modulation signal output from the frequency converter 10 is lower than the frequency of the FSK modulation signal by the conversion frequency Δ. Here, the conversion frequency Δ is an absolute value of a difference between the frequency f of the FSK modulated signal and the frequency fa of the local signal.

  The frequency of the low-frequency / FSK modulated signal is F0 = f0−Δ or F1 = f1−Δ. The frequency F0 corresponds to the digital data value “0” included in the low frequency / FSK modulation signal, and the frequency F1 corresponds to the digital data value “1”. The frequency difference between the low frequency F0 and the high frequency F1 is d, which is the same as that of the FSK modulated signal. That is, the frequency difference d does not change before and after the frequency conversion by the frequency conversion unit 10 is performed in the low frequency range.

  FIG. 4 shows a configuration example of the amplitude adjustment unit 12. In this configuration example, the amplitude adjustment unit 12 includes an orthogonal detection unit 24, an absolute value calculation unit 26, and a normalization unit 28. The quadrature detection unit 24 performs quadrature detection on the low-frequency / FSK modulation signal B and outputs an in-phase component signal I and a quadrature component signal Q. The absolute value calculation unit 26 obtains the absolute value A of the low-frequency / FSK modulation signal B by obtaining the square root of the sum of the square of the in-phase component signal I and the square of the quadrature component signal Q, and outputs the absolute value A to the normalization unit 28. . The normalization unit 28 divides the low-frequency / FSK modulated signal B by the absolute value A to obtain and output a standardized low-frequency / FSK modulated signal. By standardizing the low-frequency / FSK modulation signal, the accuracy of the decoding process in the decoding unit 16 is improved.

  FIG. 5 shows a configuration example of the multiplication unit 14. In this configuration example, the multiplier 14 includes N multipliers 30-1 to 30-N. Each multiplier includes a squarer 32 and a band pass filter 34. The squarer 32 outputs a square signal obtained by squaring the input signal to the band pass filter 34. In general, the square signal includes a DC component and a harmonic component in addition to a doubled signal obtained by doubling the frequency of the input signal. Therefore, the band pass filter 34 extracts the doubled signal by suppressing or removing the direct current component and the harmonic component. In this way, each multiplier doubles the frequency of the input signal and outputs it.

When N is 2 or more, the preceding multiplier of the adjacent multipliers doubles the frequency of the input signal and outputs the doubled signal to the subsequent multiplier. The rear stage multiplier doubles the frequency of the input signal from the previous stage and outputs it. The final stage multiplier doubles the frequency of the input signal from the previous stage, and outputs a 2 ^N multiplied / FSK modulated signal from the multiplier 14. When N is 1, the multiplier 30-1 doubles the frequency of the FSK modulated signal input to the multiplier 14 and outputs a 2 ^N multiplied / FSK modulated signal from the multiplier 14.

FIG. 3C shows the frequency of the ^2N multiplication / FSK modulation signal. The low frequency of the 2 ^N multiplication / FSK modulation signal is 2 ^N F0, and the high frequency is 2 ^N F1. The low frequency 2 ^N F0 corresponds to the digital data value “0” included in the 2 ^N multiplication / FSK modulation signal, and the high frequency 2 ^N F1 corresponds to the digital data value “1”. The frequency difference between the frequency 2 ^N F0 and the frequency 2 ^N F1 is 2 ^N d, which is 2 ^N times the frequency difference d in the FSK modulated signal. That is, the frequency difference d is ^2N times as the frequency is multiplied by ^2N by the multiplier 14.

FIG. 6 shows a configuration example of the decoding unit 16. The decoding unit 16 includes a measurement unit 36 and a demodulated signal generation unit 38. The measurement unit 36 includes a zero cross detection unit 40 and a counter 42. The zero cross detector 40 outputs a pulse signal to the demodulated signal generator 38 and the counter 42 at a timing when the ^2N multiplication / FSK modulation signal as an input signal becomes 0. The counter 42 generates a count value that increases by 1 each time the predetermined period T elapses, and outputs the count value to the demodulated signal generation unit 38. The counter 42 resets the count value to 0 at the timing when the pulse signal is output from the zero cross detector 40.

With such a configuration and processing, the measurement unit 36 generates a demodulated signal together with a pulse signal that represents a count value representing a half period of the ^2N multiplied / FSK modulated signal according to the timing when the ^2N multiplied / FSK modulated signal becomes 0. To the unit 38. The inverse of the value obtained by multiplying the count value by 2T corresponds to the frequency of the ^2N multiplication / FSK modulation signal.

The demodulated signal generator 38 determines whether or not the count value is equal to or greater than a predetermined threshold every time a pulse signal is output from the measuring unit 36. When the count value is greater than or equal to the threshold value, 0 is output as the decoded value, and when the count value is less than the threshold value, 1 is output as the decoded value. When the count value is equal to or greater than a predetermined threshold, the frequency of the 2 ^N multiplied / FSK modulated signal is the low frequency 2 ^N F0, and when the count value is less than the predetermined threshold, the 2 ^N multiplied / FSK modulated signal The frequency is a high frequency 2 ^N F1. Therefore, the demodulated signal generation unit 38 outputs a decoded value obtained from the 2 ^N multiplied / FSK modulated signal in accordance with the timing when the 2 ^N multiplied / FSK modulated signal becomes 0, and the digital data is decoded.

  The process of determining the decoded value based on whether the count value is equal to or greater than a predetermined threshold is performed based on whether the evaluation value obtained based on the count value satisfies a predetermined condition. May be. For example, every time a pulse signal is output from the measurement unit 36, the demodulated signal generation unit 38 determines whether the frequency obtained from the reciprocal of the value obtained by multiplying the count value by 2T is equal to or higher than a predetermined frequency threshold value. Also good. In this case, the demodulated signal generation unit 38 outputs 0 as a decoded value when the frequency is less than the threshold, and outputs 1 as the decoded value when the frequency is equal to or higher than the frequency threshold.

According to the FSK demodulator according to the present embodiment, a low-frequency / FSK-modulated signal obtained by frequency-converting an FSK-modulated signal as a processing target signal to a low frequency is generated. Then, a 2 ^N multiplied / FSK modulated signal obtained by multiplying the frequency of the low frequency / FSK modulated signal by 2 ^N is generated, and the digital data is decoded based on the frequency of the 2 ^N multiplied / FSK modulated signal. The frequency difference of the ^2N multiplication / FSK modulation signal is ^2N times the frequency difference of the original FSK modulation signal. Therefore, even when the frequency difference becomes sufficiently large and the frequency of the FSK modulated signal fluctuates due to noise or the like, the possibility of erroneously discriminating between the high frequency and the low frequency is reduced, and the accuracy of FSK demodulation is reduced. Will improve.

In general, when the modulation frequency at which the high frequency and the low frequency are switched is higher than the high frequency, it is difficult to determine the low frequency and the high frequency, and FSK demodulation is difficult. In the FSK demodulator according to the present embodiment, the frequency of the ^2N multiplication / FSK modulation signal is sufficiently larger than the modulation frequency. This avoids difficulty in FSK demodulation due to the high modulation frequency.

Further, in the FSK demodulator according to the present embodiment, in the process of generating the ^2N multiplication / FSK modulation signal, the FSK modulation signal is frequency-converted to a low frequency to generate the low frequency / FSK modulation signal. Thus, as compared with the case where it is ^{2 N} multiplying the frequency of the FSK-modulated signal, a frequency of ^{2 N} multiplying · FSK modulation signal is low. Therefore, while the frequency difference between the high frequency and the low frequency is increased by ^2N multiplication, the frequency of the signal to be processed by the demodulated signal generation unit 38 is maintained at a low frequency, and the circuit design of the demodulated signal generation unit 38 is improved. It becomes easy.

Further, in the process of multiplying the frequency of the low-frequency / FSK modulated signal by ^2N , the size of the low-frequency / FSK modulated signal is normalized. As a result, the rate of change (gradient with respect to time) of the low-frequency / FSK modulation signal at the timing when the low-frequency / FSK modulation signal becomes 0 becomes uniform, and the zero-cross detection unit 40 sets the ^2N multiplication / FSK modulation signal to 0. The accuracy of the timing detection process is improved, and consequently the accuracy of FSK demodulation is improved.

In the above embodiment, the case where the frequency of the FSK modulation signal when the digital data value is 1 is higher than the frequency of the FSK modulation signal when the digital data value is 0 is described. When the value of the digital data is in the opposite relationship, the decoded values 1 and 0 may be reversed. That is, the demodulated signal generation unit 38 in the decoding unit 16 outputs 0 as a decoded value when the frequency of the ^2N multiplication / FSK modulation signal is high, and the frequency of the ^2N multiplication / FSK modulation signal is low. What is necessary is just to comprise so that 1 may be output as a decoded value at a certain time.

  In the above description, the embodiment in which the FSK demodulator according to the present invention is mounted on a wireless receiver has been described. The FSK modulator according to the present invention may be mounted on a receiver in wired communication. In this case, the FSK demodulator decodes digital data from the FSK modulated signal received from an optical fiber cable, a wire cable or the like and converted into an appropriate electrical signal.

  DESCRIPTION OF SYMBOLS 10 Frequency conversion part, 12 Amplitude adjustment part, 14 Multiplication part, 16 Decoding part, 18 Mixer, 20 Local oscillator, 22 Low-pass filter, 24 Quadrature detection part, 26 Absolute value calculation part, 28 Normalization part, 30- 1 to 30-N multiplier, 32 squarer, 34 band pass filter, 36 measuring unit, 38 demodulated signal generating unit, 40 zero cross detecting unit, 42 counter.

Claims (4)

A multiplier for multiplying the frequency of the FSK modulated signal;
A measurement unit for measuring the frequency of the signal whose frequency is multiplied by the multiplication unit;
A demodulated signal generator for outputting a demodulated signal based on the frequency measured by the measuring unit;
An FSK demodulator characterized by comprising: The FSK demodulator according to claim 1, wherein
An FSK demodulator comprising: a frequency conversion unit that converts the frequency of the FSK modulated signal before frequency multiplication into a low frequency range. The FSK demodulator according to claim 2,
The FSK demodulator, wherein the frequency conversion unit frequency-converts the FSK modulation signal so that a lower limit of an occupied frequency band is not folded back to 0 frequency and is close to 0 frequency. The FSK demodulator according to any one of claims 1 to 3,
A signal adjusting unit that detects the FSK modulated signal before frequency multiplication and adjusts the magnitude of the FSK modulated signal before frequency multiplication based on the magnitude of the detected signal; FSK demodulator.

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