JPH04207802A - Digital fm signal demodulator - Google Patents

Abstract

PURPOSE:To improve the threshold characteristic and at the same time to omit the highly complicated circuit adjustment for a digital signal demodulator by operating the difference between the in-phase and orthogonal components for each phase angle in order to obtain the corrected value, i.e., the differential value kept in an allowable range and securing the differential arithmetic result in an in-allowable range mode and the corrected value in an out-allowable range mode respectively. CONSTITUTION:A level deciding circuit 37 transmits a control signal to a selection circuit 37 so that the output of a subtractor circuit 35 is selected at a level lower than the reference value and then the output of a delay circuit 38 is selected at a level higher than the reference value respectively. In other words, the output value of the circuit 35 is sent as it is to a D/A converter 36 in a normal state and then the value preceding by one stage and delayed by the circuit 38 is sent to the converter 36 as the corrected value. Thus it is possible to improve the S/N of a demodulated signal with no adjustment applied to the circuit even though an FM modulated wave superposed with an impulsive noise is inputted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention (Industrial Application Field) The present invention relates to a digital FM signal demodulation device that demodulates FM modulated waves in a digital format.

(Prior Art) A typical example of a conventional FM signal demodulator is the configuration shown in FIG. FM modulated wave S input to this device
IN is divided into two systems by the IF distributor 11, and tuned circuits 12 . . . with center frequencies f, f2. ．． I am led to 122. Each tuning circuit 121.12□ is shown in Figure 2 (a).
) has the frequency characteristics shown in fo is the carrier frequency of the FM modulated wave S. Each tuned circuit 12+,'
The frequency signal obtained at l'22 is passed through the detection diode 13.
.

The signals are detected with opposite polarities at 132, and added at adder 14. That is, this adder 14 has a character-8 demodulation characteristic as shown in FIG. The output of the adder 14 is filtered by a low-pass filter 15 to remove unnecessary high frequency components, and is then converted into an FM demodulated signal S. It is output as U7. However, although this method has a simple circuit, it has the disadvantage that it is difficult to obtain good demodulation characteristics because it is affected by the balance of the tuning circuit and the nonlinearity of the detector.

On the other hand, as shown in FIG. 6, the -FM modulated wave SIN is converted into a rectangular wave by the rectangular wave converter 21, and the monostable multivibrator 22 is operated at the edge of the rectangular wave, so that a constant width wave as shown in FIG. A method has been put into practical use in which a coarse pulse train is generated and a DC component is extracted by a low-pass filter 23. In addition, FIG. 7(a) shows the waveform of the modulation signal, and FIG. 7(b) shows the carrier wave signal (
a) FM when FM modulated by the modulation signal shown in the figure
The waveform of the modulated wave S, (c) is the output waveform of the rectangular wave converter 21, (d) is the 8 waveform of the monostable multivibrator 22, and (e) is the output of the low-pass filter 23. Waveform(
SQLIT). Although this method has a simple circuit configuration, it requires a digital collector that operates at a considerably higher frequency than the frequency of the input signal SIN, and the rectangular wave converter 21 and monostable multivibrator 22 operate as analog circuits. It has drawbacks such as requiring adjustment and being susceptible to temperature changes.

However, in recent years, as the speed of A/D converters has increased,
A digital FM signal demodulator as shown in the figure has been proposed. In this device, the input FM modulated wave SIN is
The in-phase component (0
°) and orthogonal component (90'), respectively.
The signal is converted into a digital signal by /D converters 321 and 322 and input to an arctangent circuit (phase angle calculator) 33. This arctangent circuit 33 takes the in-phase component at the time of sampling as sin θ and the orthogonal component as COS θ from the data after A/D conversion, and calculates the phase angle θ by calculating tan-'θ.
(1) is sought.

nth sample value S. and the previous sample value S n−
Each phase angle θ of 1. , θ. -1 is shown in FIG.

f between So and S I+-1. represents the frequency of the n-th sample, f, -dθ/dt-Δθ/Δt-(θ.-θ.-1)/T.

The output of the arctangent circuit 33 is transmitted to the 1-sample delay circuit 34.
, and to the differentiation circuit 34 by the subtraction circuit 342.

This differentiating circuit 34 delays input phase angle data θ(T,) by one sample through a delay circuit 341.
, ) ), the subtraction circuit 342 subtracts the values before and after the delay,
The change molecule (T,)(-θ(T,)-θ('r,-,
)). The 8 forces of this differential circuit 34 are:
After a frequency component unique to the carrier wave is removed by a subtraction circuit 35, the signal is converted into an analog signal by a D/A converter 36, and the FM demodulated signal S is converted into an analog signal. Output as LIT. This method is superior to the above two methods in terms of operational stability and demodulation characteristics because all signal processing is digital.

To explain more specifically, the input signal S is defined as follows.

S IN-sin ((JJO+Δω) t
-(1) However, ω. : Carrier frequency Δω: Frequency change due to modulation signal In this case, the output of the arctangent circuit 33 is given as (ω0+Δω)t (2). Here, the sampling times of the A/D converters 321 and 322 are To, T, .

Tll, and the interval is ΔT, the outputs of the arctangent circuit 33 are θ(T,), θ(T+), "', θ(T,,
)...(3) is given by. However, θ(T) represents the phase angle of the input signal at time T. Therefore, the output of the differentiating circuit 34, Sa (T), is given by the following equation.

5B(T,)-θ (T,)-θ +', T,-,
)−(ω.+Δω)IT, −+ 1ΔT)−(ω0 〒
ΔωIT,.

-ω0 ・ΔT±Δω ・ΔT −・
(4) The first term in equation (4) indicates the amount of phase angle change due to the carrier frequency component. ω. , ΔT are constant values and become constants. Therefore, the subtraction circuit 35 has ω. - By reducing ΔT, a component Δω·ΔT (8T is a constant) proportional to the modulation signal can be extracted.

However, the above FM signal demodulator has a threshold level (C/N of 106B: C is carrier,
There is a problem in that the demodulation S/N rapidly decreases below the threshold (N is noise). This occurs when a noise exceeding the carrier level is added to the input FM modulated wave, causing a sudden change in the phase angle.
It becomes an impulsive noise. When the C/N is large (S/N is small) in Fig. 10(a), the Fig. 10(b) shows C, , /
The case where N is small (S/N is large) is shown.

That is, the noise in the communication line is mainly thermal noise, and its amplitude distribution follows a Gaussian distribution. Therefore, C/N10d
The above phenomenon occurs below B? 1 is remarkable. This is a phenomenon specific to the FM communication system, and there is no difference between the demodulators shown in FIGS.

Here, as a method to improve the above problems, FM feedback (FMFB) method and phase synchronization detection (PL
D: Phase Lock Detect, tor
) method etc. have been proposed, but in both cases, by making the local oscillation frequency follow the modulation component in the FM signal,
It is characterized by lowering the apparent IF multiplied signal frequency deviation position and narrowing the bandpass filter to improve the C/N. However, even with this type of system, demodulation linearity or voltage controlled oscillator (V C
Since it depends on the modulation linearity of O), the modulation frequency characteristic, or the frequency characteristic of the loop, a great deal of effort is required to adjust the characteristic.

(Problems to be Solved by the Invention) As mentioned above, the conventional digital FN11 code demodulator has problems unique to the FM communication system, such as the demodulation S/N rapidly decreasing below the threshold level, and advanced and It has a problem that requires complicated circuit adjustment.

This invention was made to solve the above problems, and an object of the present invention is to provide a digital FM signal demodulation device that does not require sophisticated and complicated circuit adjustment while improving the threshold characteristics specific to the FM communication system. With the goal.

[Structure of the invention] (Means for solving the problem) In order to achieve the above object, a digital type F according to this invention is provided.
The M signal demodulator includes a hybrid divider that inputs an FM modulated wave and divides it into an in-phase component and a quadrature component, and sets a sampling period to an integral multiple of the period when the input F Pv1 modulated wave is unmodulated; Analog/digital conversion means for converting the in-phase component and quadrature component into digital signals, respectively, at the sampling period, and phase angle calculation means for calculating the phase angle of the input signal at the time of sampling from the data after analog/digital conversion by this means. and a difference calculating means for calculating the difference between the phase angles of the in-phase component and the orthogonal component obtained by this means, and determining whether or not the difference value obtained by this means exceeds a predetermined tolerance range. a correcting means for determining a difference value that falls within the allowable range; a correcting means for deriving the calculation result of the difference calculating means when it is determined by the determining means that the difference is within the allowable range; and switching means for introducing the difference value determined by the correction means.

(Operation) The digital FM signal demodulator having the above configuration calculates the difference between the phase angles of the in-phase component and the quadrature component, and determines whether or not this difference value exceeds a predetermined allowable range. Then, a correction value, which is a difference value that falls within the permissible range, is determined, and when it is determined that it is within the permissible range, the difference calculation result is directly derived, and when it is determined that it is outside the permissible range, the correction value is derived.

(Embodiment) Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1 to 3. However, in FIG. 1, the same parts as in FIG. 8 are designated by the same reference numerals, and the two parts mainly focus on different parts. , an embodiment of the present invention is shown in FIG.
A correction circuit is added between 6 and 6. This correction circuit includes a level determination circuit 37 that determines whether the output level of the subtraction circuit 35 exceeds the reference level, a delay circuit 38 that delays the output value of the subtraction circuit 35 by one sample, and a subtraction circuit 35 and a selection circuit 39 selectively outputting the output of the delay circuit 38 to the D/A converter 36 according to the determination result of the level determination circuit 37.

The operation of the above configuration will be described below with reference to FIG. 2.

First, when there is no noise in the input FM modulated wave SIN, the equations (1) to (4) are as shown. but,
Since there is noise in the actual received signal, the arctangent circuit 3
The output of 3 is θs (t) + θN (t) ... (5
). Note that θ5 is a modulation signal component, and θ is a phase noise component. Therefore, the output of the subtraction circuit 35 is given as follows.

(θs(T,)+θN(T-)1-(θ5(T-1)+θN(T-+)!=(ω
0+Δω)ΔT+ fθN(T,)−θN (T−+
) 1 (6) The second term in the above equation is the FM noise component.

As is clear from equation (6), if C/N is sufficiently large, 1θN(T,)−θN(T−+) l <
<Δω·ΔT, but when the C/N becomes 10 dB or less, impulse noise exceeding the modulation component Δω·ΔT is generated due to noise, as shown in FIG. 7(b). Therefore, the level determination circuit 37 determines the output S+ of the subtraction circuit 35.
It is determined whether or not N (S is a demodulated signal, N is noise) exceeds a reference level.

In other words, the frequency difference for each sample value due to the signal component is
Assuming that the frequency shift amount is ΔF1 and the highest modulation frequency is fffi, it does not exceed ΔF·2πf,·ΔT. Is it the output of the subtraction circuit 37 ((ΔF・2πf0・ΔT)?
+Ml (Pvf is a margin for minute noise), it can be determined that impulsive noise is occurring. The level determination circuit 37 sends a control signal to the selection circuit 39 to select the output of the subtraction circuit 35 if the reference level is not exceeded, and to select the output of the delay circuit 38 if the reference level is exceeded. .

That is, normally, the output value of the subtraction circuit 35 is derived as is, and only when it exceeds the reference level, the previous value delayed by the delay circuit 38 is derived as a correction value to the D/A converter 36.

Therefore, even if the digital FM signal demodulator having the above configuration receives the FM modulated wave on which impulsive noise is superimposed as shown in FIG. 2(a), the digital FM signal demodulator shown in FIG. The S/N of the demodulated signal can be improved as shown in FIG.

FIG. 3 shows another embodiment of the invention.

However, in FIG. 3, the same parts as in FIG. 1 are given the same reference numerals, and their explanations will be omitted.

In this embodiment, in addition to the level determination circuit 37, first to third delay circuits 40°41.42,
It is composed of an adder 43 and a 1/2 arithmetic unit 44. That is, in this device, the level determination circuit 37
When it is determined that the output level 5 (T9) of 5 does not exceed the reference level, the first delay circuit 40 derives the value S (T, -1) delayed by one sample, and determines that the output level 5 (T9) has exceeded the reference level. 2 at the second and third delay circuits 41 and 42.
The value S(Tfi-2> delayed by the sample and the current value S(T,) are added by the adder 43, and the 1/2 arithmetic unit 44
The average value (S (T, -2)
10S (T, ))/2 is determined and derived as a correction value. In this case, compared to the embodiment shown in FIG. 8, errors caused by data replacement can be further reduced.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a digital FM signal demodulation device that does not require advanced and complicated circuit adjustment while improving the threshold characteristics specific to the FM communication system. I can do it.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing an embodiment of a digital FM signal demodulator according to the present invention, FIG. 2 is a waveform diagram for explaining the operation of the embodiment, and FIG. A block circuit diagram showing another embodiment, FIG. 4 is a conventional F
FIG. 5 is a characteristic diagram for explaining the operation of the device in FIG. 4; FIG. 6 is a block circuit diagram showing another configuration of the conventional device; FIG. 6 is a waveform diagram illustrating the operation of the device shown in FIG. 6, FIG. 8 is a block circuit diagram showing the configuration of a conventional digital FM signal demodulation device, and FIG. 9 is a diagram illustrating the operation of the device shown in FIG. Vector Diagram for Explanation FIG. 10 is a vector diagram for explaining impulsive noise, which is a problem with the apparatus shown in FIG. SIN/input FM modulated wave, 31...Hybrid distributor, 321.32°...A/D converter, 33...
Arctangent circuit (phase angle calculator), 34. ...1 sample delay circuit, 342...Subtraction circuit, 34...Differentiation circuit, 3
5 ・= subtraction circuit, 36- D/A converter, so,
- Demodulated signal, 37... Level judgment circuit, 38 Delay circuit, 3... Selection circuit, 40, 41.42 Delay circuit, 43... Adder, 44. U/2 arithmetic unit. Applicant's agent Patent attorney Takehiko Suzue 4 Figure ゛(b) Figure 6 Figure 9

Claims (3)

[Claims] (1) A hybrid distributor that inputs an FM modulated wave and divides it into an in-phase component and a quadrature component, and a sampling period that is set to an integral multiple of the period when the input FM modulated wave is unmodulated; Analog/digital conversion means for converting the in-phase component and quadrature component into digital signals, phase angle calculation means for calculating the phase angle of the input signal at the time of sampling from data after analog/digital conversion by this means, and this means a difference calculation means for calculating the difference between the phase angles of the in-phase component and the orthogonal component obtained by the step; and a determination means for determining whether the difference value obtained by this means exceeds a predetermined tolerance range. , a correction means for determining a difference value that falls within the tolerance range; a calculation result of the difference calculation means is derived when the discrimination means determines that the difference value is within the tolerance range; and a calculation result is derived by the difference calculation means when it is determined that the difference value is outside the tolerance range; 1. A digital FM signal demodulator, comprising: switching means for deriving a difference value. (2) The digital FM signal demodulation device according to claim 1, wherein the correction means outputs the previous difference value obtained by the difference calculation means. (3) The digital FM signal demodulation device according to claim 1, wherein the correction means outputs an average value of the difference values before and after the difference values obtained by the difference calculation means.