JPH0969790A - Multipath distortion reduction circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely reduce the multipath distortion without manual adjustment of the delay time. SOLUTION: The intermediate frequency signal of an FM modulated wave is demodulated by an FM demodulator 1 and is added to a prescribed signal by an adder 3 and is delayed in a delay device 4A by a variable delay time and is inputted to a computing element 11 together with the output of the adder 3. A delay time detection circuit 21 detects the delay time of a reflected wave component from the intermediate frequency signal of the FM modulated wave, and a controller 22 controls the delay device 4A based on the detected delay time to adjust the delay time, meanwhile, the intermediate frequency signal has the envelope detected by an envelope detection circuit 6 and has the DC component eliminated by a DC component elimination circuit 7 and has the DC component detected by a DC component detection circuit 8, and outputs are outputted to the computing element. The computing element 11 performs prescribed operation and outputs the result to the adder 3. The adder 3 adds the FM demodulation output and the output of the computing element 11 to output the modulation signal where the multipath distortion is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multipath distortion reduction circuit, and more particularly to a multipath distortion reduction circuit capable of removing and reducing a multipath distortion component generated when an FM modulated signal is received.

[0002]

2. Description of the Related Art When an FM modulated signal is received and demodulated, multiple transmission (multipath transmission) occurs in which an unnecessary reflected wave component due to a building, a mountain, etc. is superimposed on a desired direct wave component,
It is known that reception quality is deteriorated, such as an increase in distortion of a demodulated signal. This is a phenomenon similar to ghost when receiving television, and it can be dealt with by fixing the directivity of the antenna and matching it with the direct wave in the receiver where the installation location is fixed, but in the case of reception by mobile such as in-vehicle As described above, it is a phenomenon that becomes a particular problem when a special antenna cannot be installed and the state of multipath changes with movement.

Therefore, in mobile receivers, various techniques have been proposed to reduce multipath distortion. Of these, one shown in FIG. 5 is one that can be realized in a relatively small circuit scale. This multipath distortion reduction circuit is configured to calculate and remove the multipath distortion component in the FM demodulated signal from the AM component of the intermediate frequency signal. Multipath component intermediate frequency signal when receiving the FM signal containing Ccos a direct wave component in (angular frequency = omega _c at the time of _{non-modulation) (ω c t + f (} t)), the reflection coefficient of the reflected wave (direct wave The amplitude ratio of the reflected wave to the amplitude of K), the delay time of the reflected wave is τ, and the local oscillation angular frequency for converting the reception frequency to the intermediate frequency is ω _L0 , the intermediate frequency signal c ₀ is c ₀ = C ( cos (ω _c t + f (t)) + K cos (ω _c (t−
τ) + f (t−τ) + ψ)) where ψ = −ω _L0 τ and X = Kcos (f (t) −f (t−τ) + ω _c τ +
ω _L0 τ), if K << 1, the intermediate frequency signal c
_The output e _{0 obtained} by demodulating ₀ by the FM demodulator is e ₀ = f ′ (t) − (X / (1 + 2X)) · (f ′ (t) −f ′ (t−τ)) (1 However, f ′ (t) is a modulated signal. When the formula (1) is modified, f ′ (t) = e ₀ + (X / (1 + 2X)) · (f ′ (t) −f ′ (t−τ)) (2)

CX is the amplitude variation of the intermediate frequency signal c ₀ , C
Is the amplitude of the direct wave component. CX is obtained by envelope-detecting the intermediate frequency signal c ₀ and then removing the DC component, and C is obtained by envelope-detecting c ₀ and then detecting the DC component. FIG. 5 is an example of a circuit configuration that realizes the calculation of Expression (2), and the analog intermediate frequency signal c ₀ is input. e ₀ obtained by demodulating c ₀ by the FM demodulator 1 is A / D converted by the A / D converter 2 and input to the digital adder 3, and the output of the digital adder 3 is delayed by the digital delay unit 4 by the delay time τ. Delay only. The output of the digital adder 3 is D / A converted by the D / A converter 5 and output to the subsequent stage. On the other hand, after the analog intermediate frequency signal c ₀ is envelope-detected by the envelope detection circuit 6, the DC component is removed by the DC component removal circuit 7 to obtain CX, and the DC component is detected by the DC component detection circuit 8 to obtain C Ask for. Then, the signals are A / D converted by the A / D converters 9 and 10, respectively, and then input to the digital arithmetic unit 11. The digital calculator 11 subtracts the output s of the digital delay unit 4 from the output w of the digital adder 3 to obtain m, and the output of the DC component removing circuit 7 p = CX
From the output q = X of the DC component detection circuit 8 ((p / q) /
Calculate (1 + 2 (p / q)) (X / (1 + 2X))
Is calculated, multiplied by m, and output to the digital adder 3. At this time, the modulated signal f ′ (t) in which the multipath distortion is removed and reduced is obtained at the output of the digital adder 3.

According to the multipath distortion reduction circuit of FIG.
Since the arithmetic processing for removing and reducing the multipath distortion is performed on the demodulated signal in the relatively low frequency range output from the M demodulator 1, the operation speed required for the circuit element can be slow, and the circuit scale is large. It can be small.

[0006]

However, the delay time τ of the reflected wave is usually unknown. Therefore, when the receiver is used, the delay time of the delay device 4 must be manually adjusted so that the distortion of the modulation output f ′ (t) is minimized. Since it changes with the passage of time, it is practically difficult to adjust and distortion cannot be reduced. The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to provide a multipath distortion reduction circuit that can reliably reduce the multipath distortion without manually adjusting the delay time.

[0007]

SUMMARY OF THE INVENTION In one aspect of the present invention, an FM demodulation means for inputting an FM-modulated intermediate frequency signal having a multipath component to perform FM demodulation and an output side of the FM demodulation means are provided. The addition means, the delay means for delaying the output of the addition means by a variable delay time, the delay time detecting means for detecting the delay time from the direct wave component of the reflected wave component included in the intermediate frequency signal, and the delay Delay time control means for controlling the means to vary the delay time of the delay means to a value detected by the delay time detection means, envelope detection means for inputting the intermediate frequency signal and performing envelope detection, A direct current component removing means for removing a direct current component from the output of the envelope detecting means, a direct current component detecting means for detecting a direct current component from the output of the envelope detecting means, and an output of the direct current component removing means p, the direct current component Of detection means An operation for performing an operation of v = ((p / q) / (1 + 2 (p / q))) · (w−s), where force is q, output of the adding means is w, and output of the delay means is s. Means, and the addition means is configured to add the output of the FM demodulation means and the output v of the calculation means and output the result.

In another aspect of the present invention, the delay time detecting means inputs a digital signal having a variable filter coefficient for inputting the intermediate frequency signal and performing a predetermined filtering operation, and a constant output amplitude of the digital filter. Filter coefficient updating means for calculating the update value of the filter coefficient for converting the digital filter to the digital filter, and estimation for estimating the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means. And means are provided.

In still another aspect of the present invention, the delay time detecting means is provided in the preceding stage of the FM demodulating means, and the output of the digital filter in the delay time detecting means is input to the FM demodulating means. Is characterized by. Also,
The estimating means is characterized in that the delay time of the reflected wave having the second largest reflection coefficient is estimated.

In another aspect of the present invention, an FM-modulated intermediate frequency signal having a multipath component is input to the FM
An FM demodulation means for performing demodulation, an addition means provided on the output side of the FM demodulation means, a delay means for delaying the output of the addition means by a variable delay time, and a front stage of the FM demodulation means, and an intermediate frequency A digital filter for performing a predetermined filtering process on the signal, and a filter coefficient updating means for calculating an updated value of the filter coefficient for making the output amplitude of the digital filter constant at a fixed value Q and updating the filter coefficient of the digital filter. , Estimating means for estimating the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means, and controlling the delay means to vary the delay time of the delay means to a value obtained by the estimating means Delay time control means, envelope detecting means for performing envelope detection by inputting the intermediate frequency signal output from the digital filter, and the envelope detecting means. A DC component removing means for removing the DC component from the output of the detection means, the output of the DC component removing means p,
The output of the addition means is w and the output of the delay means is s, and an operation means for performing an operation of v = ((p / Q) / (1 + 2 (p / Q))) (w-s) is provided. , The adding means is the F
It is characterized in that the output of the M demodulating means and the output v of the calculating means are added and output. Further, the estimating means is characterized in that the delay time of the reflected wave having the second largest reflection coefficient is estimated.

[0011]

According to one aspect of the present invention, the FM-modulated intermediate frequency signal having multipath components is FM-demodulated by the FM demodulating means, and then the predetermined signals are added by the adding means and output.
The output of the adding means is delayed by the delay means with a variable delay time, and is input to the calculating means together with the output of the adding means. Also, the intermediate frequency signal is envelope-detected by the envelope detection means, the direct-current component is removed by the direct-current component removal means, and the direct-current component is detected by the direct-current component detection means. In the computing means, the output of the DC component removing means is p, the output of the DC component detecting means is q, the output of the adding means is w, and the output of the delay means is s.
, V = ((p / q) / (1 + 2 (p / q))) · (ws) is calculated and output to the adding means. The addition means is F
By adding M demodulation output and v, the modulated signal is output to the subsequent stage. On the other hand, the delay time detection means detects the delay time from the intermediate frequency signal to the direct wave component of the reflected wave component included in the intermediate frequency signal, and the delay time control means controls the delay means to detect it by the delay time detection means. Change to the specified value.
Thus, the delay means can be automatically adjusted to an appropriate delay time without the user having to manually adjust the delay time of the delay means, and multipath distortion can be effectively removed or reduced. In particular, even in the case of mobile reception where it was difficult to adjust the delay time in the past, it is possible to automatically adjust to an appropriate delay time by following changes in the multipath state, and it is appropriate to remove or reduce multipath distortion. It becomes feasible.

According to another aspect of the present invention, a digital filter having a variable filter coefficient for inputting an intermediate frequency signal and performing a predetermined filtering operation is provided, and the output amplitude of the digital filter is made constant by the filter coefficient updating means. The update value of the filter coefficient for is calculated, and the filter coefficient of the digital filter is updated. Then, the estimating means estimates the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means. As a result, the unknown reflected wave delay time can be stably detected by digital processing, and the removal and reduction of multipath distortion can be stably performed. According to yet another aspect of the present invention, the output of the digital filter in the delay time detecting means is input to the FM demodulating means. When the filter coefficient is updated so that the output amplitude of the digital filter is constant,
The intermediate frequency signal whose reflected wave component is suppressed by the digital filter can be input to the FM demodulation means. Therefore, multipath distortion can be further removed and reduced.

According to another aspect of the present invention, a digital filter having a variable filter coefficient for inputting an FM-modulated intermediate frequency signal having a multipath component and performing a predetermined filtering operation is provided, and the filter coefficient updating means is provided. The update value of the filter coefficient for making the output amplitude of the digital filter constant at the fixed value Q is calculated, and the filter coefficient of the digital filter is updated. On the other hand, after the output of the digital filter is FM demodulated by the FM demodulating means, a predetermined signal is added by the adding means and output. The output of the adding means is delayed by the delay means with a variable delay time, and is input to the calculating means together with the output of the adding means. A value obtained by estimating the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means by the estimating means, and controlling the delay means by the delay time controlling means to estimate the delay time of the delay means by the estimating means. Change to. Further, the intermediate frequency signal output from the digital filter is subjected to envelope detection by the envelope detection means, the direct current component is removed by the direct current component removing means, and then output to the computing means. In the arithmetic means, the output of the DC component removing means is p, the output of the adding means is w, and the output of the delay means is s. V = ((p / Q) / (1 + 2 (p / Q))) * (ws ) Is calculated and output to the adding means. The addition means is F
By adding M demodulation output and v, the modulated signal is output to the subsequent stage. This also allows the delay means to be automatically adjusted to an appropriate delay time without the user having to manually adjust the delay time of the delay means, which can effectively remove and reduce multipath distortion, and the envelope. Since the DC component detecting means for detecting the DC component from the output of the line detecting means can be omitted, the circuit configuration becomes simple.

According to another aspect of the present invention, when the delay time detecting means is provided before the FM demodulating means, the estimating means estimates the delay time of the reflected wave having the second largest reflection coefficient. To do. When a plurality of reflected waves are present, the digital filter of the delay time detecting means suppresses the reflected wave having the largest reflection coefficient, so that the delay time of the delay means is set to the value of the reflected wave having the second largest reflection coefficient. If adjusted, the multipath distortion due to the reflected wave having the largest reflection coefficient and the reflected wave having the second largest reflection coefficient as a whole can be removed and reduced.

[0015]

1 is a circuit diagram of a multipath distortion reducing circuit according to a first embodiment of the present invention, in which the same components as those in FIG. 5 are designated by the same reference numerals. Reference numeral 20 denotes an A / D that performs A / D conversion of the intermediate frequency signal c ₀ having the FM-modulated multipath component.
D converter, 21 is a digitized intermediate frequency signal c ₀
Is a delay time detector for detecting the delay time τ from the direct wave component of the reflected wave component in the intermediate frequency signal c ₀ ,
Here, the intermediate frequency signal c ₀ is passed through a digital filter having a variable filter characteristic, and the intermediate amplitude signal of the FM modulated wave has a constant amplitude (constant envelope) so that the output amplitude of the digital filter becomes constant. By varying the filter characteristic, the delay time τ is estimated from the filter characteristic that is adaptively varied so as to suppress the reflected wave component. A delay time 22 is input with the delay time τ detected by the delay time detecting section 21, and the delay time is variable.
A controller for controlling A to adjust the value of the delay time of the digital delay unit 4A to τ.

In the delay time detecting section 21, 30 is a digital filter composed of an FIR filter having a variable filter coefficient, and performs a predetermined filtering operation on the input digital signal x from the A / D converter 20. Reference numeral 31 is a filter coefficient updating unit that obtains an updated value of the filter coefficient from the output of the digital filter 30 by the steepest gradient method and sets the updated value in the digital filter 30, and 32 is input with the filter coefficient updated by the filter coefficient updating unit 30. It is an estimation unit that estimates the delay time τ of the reflected wave based on the filter coefficient.

The operation of the delay time detector 21 will now be described. The value of the time n of the input digital signal of the digital filter 30 is b _n , the order of the digital filter 30 is N, and the filter coefficient of the digital filter 30 is a _k (k
= 0 to N), and the value of the time n of the output digital signal of the digital filter 30 is y _n , Given in. In matrix notation, y _n = A ^T B. However, A ^T = [a ₀ , a ₁ , a ₂ , ...
a _N ], B ^T = [b _n , b _n-1 , b _n-2 , ...
b _nN ], and the superscript “ ^T ” represents a transposed matrix.
The error ε _n when the reference amplitude value is an arbitrary fixed value Q is ε _n = | y _n | ² −Q ² ··· (3).

The evaluation function F in the adaptive algorithm is F = E [ε _n ² ]. Here, E [•] indicates the expected value calculation. Suppressing the reflected wave component is synonymous with minimizing F, and the filter coefficient a _k for minimizing F is determined by the steepest slope of F, so a _k ← a _k −α ( The filter coefficient a _k is updated by ∂F / ∂a _k ), and the next time (n + 1)
You can use it with. However, α is a fixed convergence parameter. In this embodiment, for simplicity, Q = 1 in the equation (3) is set. When the expected value calculation is replaced with the instantaneous value calculation and the steepest gradient method (LMS algorithm) is applied, the update formula of the filter coefficient matrix A is as follows. A ← A−α (∂F / ∂A) = A−2αε _n B ^* y _n ··· (4) However, the subscript “ ^* ” indicates the conjugate prime number.

In the example of FIG. 1, the filter coefficient updating unit 31 obtains the square of the absolute value of y _n , and subtracts the reference amplitude value 1 from this to obtain ε _n . Then, the expected value calculation and the filter coefficient update calculation are performed, and the filter coefficients a _{0 to} a _N of the digital filter 30 are updated and set. The digital filter 30 and the filter coefficient updating unit 31 constitute an adaptive filter that suppresses the reflected wave component from the input digital signal.

The controller 22 controls the delay time detector 21 to cause the delay time detector 21 to start detecting the delay time τ of the reflected wave so that A =
[A ₀ = Q = 1, a ₁ = 0 _, a ₂ = 0, ..., a _N =
0] is initialized and the same value is initialized in the filter coefficient updating unit 31, and then the adaptive operation is started. During the adaptive operation, the filter coefficient updating unit 21 sequentially updates each filter coefficient a _k of the digital filter 30 according to the equation (4).
Since the digital filter 30 is an FIR filter, it is possible to suppress the multipath component from the input digital signal in a stable operation after the adaptive operation is started, and the error ε _n becomes small to a certain degree or less after a certain period of time. At this time,
The absolute value of each filter coefficient a _k has various magnitudes. The one having the maximum absolute value corresponds to the direct wave component, and the one having the second largest absolute value corresponds to the reflected wave component. In other words, the filter coefficient a _k having the second largest absolute value is most effective in suppressing the reflected wave component. The order i of the maximum absolute value of the filter coefficients a _k k (1), the order of the large filter coefficients a _k absolute value to the second and k (2), the unit delay time of the delay elements D of the digital filter 30 u , Τ = | k (2) −k (1) | · u ·· (5), where τ is the delay time of the reflected wave with respect to the direct wave. The initial setting value of the filter coefficient is A = [a ₀ = Q =
1, a ₁ = 0 _, a ₂ = 0, ..., A _N = 0], k (1) = 0.

The estimator 32 sets the filter coefficient A, which is updated and set in the digital filter 30, after the adaptive operation by the digital filter 30 and the filter coefficient updater 31 has been started and a steady state has elapsed for a certain time. Input from the update unit 31. Then, the delay time τ of the reflected wave is estimated based on the input filter coefficient A according to the equation (5), and is output to the controller 22. In this embodiment, the estimation unit 32 estimates a new delay time τ and outputs delay time data to the controller 22 every time the filter coefficient A is updated. On the other hand, the controller 22 uses the delay time detector 21
When is initialized, the delay time of the digital delay unit 4A is initially adjusted to τ = 0, and thereafter, each time delay time data is input from the estimation unit 32, the delay time data is compared with the value previously adjusted for the digital delay unit 4A. If different, the delay time of the digital delay unit 4A is readjusted to the delay time τ newly input from the estimation unit 32. Since the digital delay unit 4A handles the modulation signal, the operating frequency may be lower than that of the digital filter 30 handling the intermediate frequency signal, and the adjustment interval of the delay time τ may be longer than the update interval of the filter coefficient. For example, the maximum frequency of the modulated signal f ′ (t) is smaller than f _M kHz, and the sampling frequency fs of the A / D converter 2 is 2
f _M kHz, and the intermediate frequency fc is 455 kH
z, the sampling frequency fs' of the A / D converter 20 is 912
When the unit delay time u of the delay element D of the digital filter 30 is u = 1 / fs ′ in kHz, the estimation unit 32 estimates the delay time τ each time the filter coefficient is updated (each time the unit delay time u elapses). Instead, t = u · (fs ′ / f
s) Based on the filter coefficient set in the digital filter 30 at every time interval longer than u, such as every time t = 1 / f _M kHz, every t = 1 ms, every t = 10 ms, etc. The delay time τ is estimated and the controller 2
You may make it output to 2.

The larger the order N of the digital filter 30, the more accurately the reflected wave delay time can be estimated. However, since the suppression of the reflected wave component is not the purpose, for example, the intermediate frequency fc
Is 455kHz, sampling frequency fs of A / D converter 20
If ′ is 912 kHz and the unit delay time u of the delay element D of the digital filter 30 is u = 1 / fs ′, the delay time can be estimated even when N is about 32 to 64. The estimation unit 3
In the second method, the estimation of the reflected wave delay time is started after a fixed time has elapsed after the start of the adaptive operation. However, the error ε _n calculated by the filter coefficient updating unit 31 is monitored and the magnitude of ε _n is The delay time estimation may be started after the value becomes smaller than a certain value. Further, the filter coefficient updating unit 31 updates the filter coefficient according to the formula of A ← A−α (∂ (| y | ² −Q ² ) / ∂A) ··· (6) instead of the formula (3). It may be done. In addition, Q is an arbitrary fixed value in the expressions (3) and (6).

Returning to FIG. 1, a portion indicated by reference numeral 40 is a multipath distortion reducing section, which has the same structure as that of FIG.
1 is F for FM demodulating the intermediate frequency signal c ₀ in the analog domain
M demodulator, 2 A / D for A / D converting the FM demodulated signal e ₀
D converter, 3 is a digital adder for adding the output of the digital calculator described later to the demodulated signal e ₀ after A / D conversion,
A digital delay unit 4A delays the output of the digital adder 3 by a variable delay time τ, and the delay time τ is adjusted by the control of the controller 22. Reference numeral 5 is a D / A converter for D / A converting the output of the digital adder 3 to output an analog modulation signal, and 6 is an envelope detection for performing envelope detection on the intermediate frequency signal c ₀ to detect the amplitude. Reference numeral 7 is a circuit, 7 is a direct current component removing circuit for removing a direct current component from the output of the envelope detection circuit, 8 is a direct current component detecting circuit for detecting the direct current component of the output of the envelope detection circuit, and 9 and 10 are direct current component removing circuits, respectively. A / D converter for A / D converting the output of the
11 is a digital adder 3 and digital delay devices 4A, A
This is a digital arithmetic unit that removes multipath distortion from the output of the digital adder 3 by inputting each output of the / D converters 9 and 10 and performing a predetermined arithmetic operation and outputting to the digital adder 3.

Here, the operation principle of the multipath distortion reduction section 40 will be described. If the carrier central angular frequency ω of the FM radio wave, the reception amplitude R of the direct wave at the antenna, the delay time τ of the reflected wave with respect to the direct wave, and the reflection coefficient K, the reception signal r at the antenna is r = R (cos (ωt + f (T)) + Kcos (ω (t-
τ) + f (t−τ))). When r is converted to an intermediate frequency signal c ₀ having a central angular frequency ω _c by a local oscillator having an angular frequency ω _L0 , c ₀
The amplitude as C _{is, c 0 = C (cos (} ω c t + f (t)) + Kcos (ω c (t-
τ) + f (t−τ) + ψ)) where ψ = −ω _L0 τ.

[0025] By modifying _{c 0, c 0 = Ecos (} ω c t + θ) where, E = C (1 + K 2 + 2Kcos (f (t) -f (t-τ)
+ Ω _c τ−ψ)) ^1/2 Becomes Differentiating both sides of the equation (7) by t, (sec ² θ) dθ / dt = (1 / D ² ) · (f ′ (t) + K ² f ′ (t−τ) + K (f ′ (t) + F ′ (t−τ)) · cos (f (t) −f (t−τ) + ω _c τ−ψ)) ·· (8) where D = cosf (t) + Kcos (f (t−τ)) −ω _c
τ + ψ).

In the relational expression of sec ² θ = 1 + tan ² θ, (7)
By substituting the formulas and rearranging, sec ² θ = (1 / D ² ) · (E ² / C ² ) ·· (9). X = Kcos (f (t) -f (t-τ) + ω c τ-
ψ), the output e _{0 obtained} by demodulating the intermediate frequency signal c ₀ by the FM demodulator is obtained by referring to the equations (8) and (9), Becomes Usually K << 1, so However, f ′ (t) is a modulated signal. By transforming equation (10), Becomes

CX is the amplitude variation of the intermediate frequency signal c ₀ , C
Is the amplitude of the direct wave component. CX is obtained by envelope-detecting the intermediate frequency signal c ₀ and then removing the DC component, and C is obtained by envelope-detecting c ₀ and then detecting the DC component. Therefore, assuming that the output of the DC component removing circuit 7 is p and the output of the DC component detecting circuit 8 is q, X = CX / C = p / q. From the equation (11), the output w of the digital adder 3 is f '(t) and the output s of the digital delay unit 4A is f' (t.
−τ), the digital computing unit 11 has p,
It suffices to calculate v based on the following equation using q, w, and s and output it to the digital adder 3. v = ((p / q) / (1 + 2 (p / q))) · (ws−) ·· (13) The digital adder 3 adds v to the FM demodulated signal e ₀ to obtain multipath distortion. Of the modulated signal f ′
(T) can be output.

Next, the operation of the above embodiment will be briefly described. Here, the formula (4) is used as the formula for updating the filter coefficient (Q = 1). The center frequency of the intermediate frequency signal is 455 kHz, the sampling frequency f of the A / D converter 20
s ′ = 912 kHz, unit delay time u of delay element D of digital filter 30 = 1/912 kHz, digital filter 30
And the delay time τ for the direct wave is 30.
Intermediate frequency signal c of FM modulated wave with one reflected wave of u added
Consider the case where ₀ is input to the multipath reduction circuit of FIG. The intermediate frequency signal c ₀ is A / D converted by the A / D converter 20, and then input to the digital filter 30 of the delay time detection unit 21 and subjected to predetermined filtering processing. The controller 22 first initially adjusts the delay time τ of the digital delay unit 4A to 0, and the filter coefficient A of the digital filter 30 of the delay time detection unit 21 is A = [a ₀ , a ₁ ,
a ₂ , ..., A _N ] is initialized to [1, 0, 0, ..., 0], and the same value is initialized in the filter coefficient updating unit 31. Then, the digital filter 30 and the filter coefficient updating unit 31 are caused to start the adaptive operation. When the adaptive operation is started, the filter coefficient updating unit 31 sequentially calculates the update value of the filter coefficient A that can make the amplitude of the output y of the digital filter 30 constant according to the equation (4), and the digital filter 3
Set to 0. By this adaptive operation, the output y of the digital filter 30 becomes such that the reflected wave component is suppressed.

After a certain time has passed since the adaptive operation was started,
The error ε _n between the square of the absolute value of y _n and the reference amplitude value 1 becomes smaller than a certain value. At this time, the estimating unit 32 inputs the filter coefficient A currently set in the digital filter 30 from the filter coefficient updating unit 31, and based on the filter coefficient, delays the reflected wave from the direct wave according to the equation (5). The time τ is estimated and the result is output to the controller 22. For example, when the filter coefficient having the maximum absolute value is a ₀ and the filter coefficient having the second largest absolute value is a ₃₀ , | 30−
The delay time is 0 | .u = 30u. Controller 22
Since the delay time τ estimated this time by the estimation unit 22 is different from the previously adjusted delay time τ = 0, the delay time of the digital delay circuit 4A is readjusted to τ = 30u. After that, the estimation unit 32 re-estimates a new delay time and outputs the new delay time to the controller 22 each time a predetermined time longer than u or u elapses, and the controller 22 has already input the new delay time. If it is different from the value adjusted in, readjust. Delay time of reflected wave is 30u
If it does not change, the delay time in the digital delay circuit 4A also remains 30u.

When the delay time of the digital delay circuit 4A is properly adjusted to the delay time of the reflected wave, the digital calculator 11 of the multipath distortion reducing section 40 causes the output p of the DC component removing circuit 7 and the DC component detecting circuit 8 to operate. Output q, digital adder 3
W and the output s of the digital delay unit 4 (1
By calculating v based on the equation (3) and outputting it to the digital adder 3, multipath distortion due to a reflected wave is removed from the output of the digital adder 3, and the modulated signal f ′ is reduced.
(T) can be obtained. If the delay time of the reflected wave fluctuates, the digital filter 30 follows the fluctuation.
The filter coefficient A that is updated and set to is changed. Therefore,
The delay time output from the estimation unit 32 varies, and the digital delay unit 4A is always adjusted to the current delay time of the reflected wave.
Note that, here, Q = 1 has been described, but a fixed value other than 1 may be used (it may be updated by the formula (3) or the formula (5)).

According to this embodiment, the delay time detector 21 is added to the multipath reduction circuit, and the delay time detector 2 is added.
1, the intermediate frequency signal c ₀ is passed through the digital filter 30 whose filter coefficient is variable,
Of the filter coefficient updating unit 31 so that the output amplitude of
Obtains the updated value of the filter coefficient and digital filter 3
The adaptive operation for suppressing the reflected wave component is performed by updating and setting to 0, and during the adaptive operation, the order difference between the filter coefficient having the maximum absolute value and the filter coefficient having the second largest absolute value is obtained. Since the delay time of the reflected wave is represented, the delay time of the reflected wave can be detected in real time by the estimation unit 32 estimating the delay time of the reflected wave based on the updated value of the filter coefficient.
Then, since the controller 22 adjusts the delay time of the digital delay unit 4A based on the delay time data estimated by the estimation unit 32, the controller automatically adjusts the delay time of the digital delay unit 4A to an appropriate delay time without manual adjustment. The multipath distortion can be adjusted, and the multipath distortion reducing unit 40 can effectively remove and reduce the multipath distortion. Especially,
Even in mobile reception, where the delay time adjustment is difficult, the digital delay unit 4A can be automatically adjusted to an appropriate delay time in real time by following changes in the multipath state, and removing the multipath distortion. , Mitigation can be performed appropriately. Further, the adaptive digital processing by the digital filter 30 and the filter coefficient updating unit 31 makes it possible to stably detect an unknown reflected wave delay time, and it is possible to stably remove and reduce multipath distortion.

FIG. 2 is a circuit diagram of a multipath reducing circuit according to the second embodiment of the present invention. The same components as those in FIG. 1 are designated by the same reference numerals. In the first embodiment, the analog intermediate frequency signal c ₀ is input to the FM demodulator as it is, but in the second embodiment of FIG.
The output of the digital filter 30 of No. 1 is input. However, since the output of the digital filter 30 is digital, it is input to the FM demodulator 1 of the multipath distortion reduction unit 40A after being D / A converted by the D / A converter 41. The other components are constructed exactly as in FIG.

As described above, the delay time detecting section 21 has a function of suppressing the reflected wave component in the intermediate frequency signal c ₀ by the adaptive operation. Therefore, as shown in FIG. 2, by using the input of the FM demodulator 1 as the output of the delay time detector 21 in which the reflected wave component is suppressed, the digital adder 3 outputs a modulated signal in which multipath distortion is further reduced. It is possible to output f ′ (t).

It should be noted that there are usually not only one reflected wave but also a plurality of reflected waves. Therefore, when it is considered that the delay time detection unit 21 sufficiently suppresses the reflected wave having the largest reflection coefficient, the multipath distortion reduction unit 40.
In A, the multipath distortion due to the reflected wave having the second largest reflection coefficient may be reduced. For example, of the filter coefficients during adaptive operation, the filter coefficient a with the maximum absolute value is
The order i of the _{k k} (1), the order of the large filter coefficients a _k absolute value to the second k (2), when the order of the filter coefficient having a large absolute value to the third a _k and k (3), ｜ k (2) −k
(1) If | <N / 2 (where N is the filter order), then k
It is considered that (2) and the filter coefficients of the orders before and after are effective and the suppression operation is sufficiently performed for the reflected wave having the largest reflection coefficient inside the delay time detection unit 21.

Therefore, the estimating unit 32 determines whether or not | k (2) -k (1) | <N / 2 ... (14) based on the filter coefficient. If NO, the following equation, τ = | K (2) −k (1) | · u ·· (15) The delay time data estimated according to (15) is output, and the multipath distortion reduction unit 40A also suppresses distortion due to the reflected wave with the largest reflection coefficient. Let On the contrary, if the judgment of the equation (14) is YES, the delay time data estimated according to the following equation, τ = | k (3) -k (1) | The path distortion reduction unit 40A suppresses distortion due to the reflected wave having the second largest reflection coefficient. As a result, it is possible to reduce the multipath distortion due to both the reflected wave having the maximum reflection coefficient and the reflected wave having the second largest reflection coefficient.

Even if the equation (14) is not satisfied, the delay time detecting section 21 suppresses the reflected wave having the maximum reflection coefficient to some extent. Therefore, the delay time is estimated according to the equation (16). The multipath distortion reducing unit 40A may reduce the multipath distortion due to the reflected wave having the second largest reflection coefficient. Further, as in the modification shown in FIG. 3, the envelope detection circuit 6 of the multipath distortion reduction unit 40B.
Alternatively, the output of the D / A converter 41 may be input. At this time, the output amplitude of the digital filter 30 is Q
Since it is fixed to (an arbitrary fixed value), if the digital calculator 11 replaces q in the equation (13) with a fixed value Q to perform digital calculation, the DC component detection circuit 8 of FIG. And the A / D converter 10 can be omitted, and the circuit configuration becomes simple.

Further, as shown in FIG. 4, if the FM demodulator of the multipath distortion reducing section 40C is replaced with a digital FM demodulator 1C that performs FM demodulation in the digital domain, then D in FIG.
The output of the digital filter 30 can be directly input to the digital FM demodulator 1C without passing through the A / A converter 41 to perform FM demodulation operation in the digital domain (in this case, the A / D converter 2 performs sampling). The frequency is f _S ′
It should be replaced with D / D converter 42 for converting the f _S from). Further, the envelope detection circuit 6, the DC component removal circuit 7, and the DC component detection circuit 8 are also replaced with a digital envelope detection circuit 6C, a digital DC component removal circuit 7C, and a digital DC component detection circuit 8C that operate in the digital area, By directly inputting the output of the digital filter 30 to the digital envelope detection circuit 6C, the D / A converter 4 of FIG.
1 can be eliminated (in this case, the A / D converters 9 and 10 are also replaced with D / D converters 43 and 44 that convert the sampling frequency from f _S ′ to f _{S. In} addition, the digital calculator 11 is used. If q is replaced with a fixed value Q to perform digital calculation, the digital DC component detection circuit 8C and the D / D converter 43 in FIG. 4 can be omitted).

[0038]

According to one aspect of the present invention, an FM-modulated intermediate frequency signal having a multipath component is F
After M demodulation, a predetermined signal is added and output by the adding means, the output of the adding means is delayed by the delay means with a variable delay time, and is input to the calculating means together with the output of the adding means. Also, the intermediate frequency signal is envelope-detected by the envelope detection means, the direct-current component is removed by the direct-current component removal means, and the direct-current component is detected by the direct-current component detection means. In the computing means, the output of the direct current component removing means is p, the output of the direct current component detecting means is q, the output of the adding means is w, and the output of the delay means is s. V = ((p / q) / (1 + 2 (p / q))). (w-s) is calculated and output to the adding means. The addition means is F
By adding M demodulation output and v, the modulated signal is output to the subsequent stage. On the other hand, the delay time detection means detects the delay time from the intermediate frequency signal to the direct wave component of the reflected wave component included in the intermediate frequency signal, and the delay time control means controls the delay means to detect it by the delay time detection means. Change to the specified value.
Thus, the delay means can be automatically adjusted to an appropriate delay time without the user having to manually adjust the delay time of the delay means, and multipath distortion can be effectively removed or reduced. In particular, even in the case of mobile reception where it was difficult to adjust the delay time in the past, it is possible to automatically adjust to an appropriate delay time by following changes in the multipath state, and it is appropriate to remove or reduce multipath distortion. It becomes feasible.

According to another aspect of the present invention, a digital filter having a variable filter coefficient for inputting an intermediate frequency signal and performing a predetermined filtering operation is provided, and the output amplitude of the digital filter is made constant by the filter coefficient updating means. The update value of the filter coefficient for is calculated, and the filter coefficient of the digital filter is updated. Then, the estimating means estimates the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means. As a result, the unknown reflected wave delay time can be stably detected by digital processing, and the removal and reduction of multipath distortion can be stably performed. According to yet another aspect of the present invention, the output of the digital filter in the delay time detecting means is input to the FM demodulating means. When the filter coefficient is updated so that the output amplitude of the digital filter is constant,
The intermediate frequency signal whose reflected wave component is suppressed by the digital filter can be input to the FM demodulation means. Therefore, multipath distortion can be further removed and reduced.

According to another aspect of the present invention, a digital filter having a variable filter coefficient for inputting an FM-modulated intermediate frequency signal having a multipath component and performing a predetermined filtering operation is provided, and the filter coefficient updating means is provided. The update value of the filter coefficient for making the output amplitude of the digital filter constant at the fixed value Q is calculated, and the filter coefficient of the digital filter is updated. On the other hand, after the output of the digital filter is FM demodulated by the FM demodulating means, a predetermined signal is added by the adding means and output. The output of the adding means is delayed by the delay means with a variable delay time, and is input to the calculating means together with the output of the adding means. A value obtained by estimating the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means by the estimating means, and controlling the delay means by the delay time controlling means to estimate the delay time of the delay means by the estimating means. Change to. Further, the intermediate frequency signal output from the digital filter is subjected to envelope detection by the envelope detection means, the direct current component is removed by the direct current component removing means, and then output to the computing means.
In the arithmetic means, the output of the DC component removing means is p, the output of the adding means is w, and the output of the delay means is s. V = ((p / Q) / (1 + 2 (p / Q))) * (ws ) Is calculated and output to the adding means. The addition means is F
By adding M demodulation output and v, the modulated signal is output to the subsequent stage. This also allows the delay means to be automatically adjusted to an appropriate delay time without the user having to manually adjust the delay time of the delay means, which can effectively remove and reduce multipath distortion, and the envelope. Since the DC component detecting means for detecting the DC component from the output of the line detecting means can be omitted, the circuit configuration becomes simple.

According to another aspect of the present invention, when the delay time detecting means is provided before the FM demodulating means, the estimating means estimates the delay time of the reflected wave having the second largest reflection coefficient. To do. When a plurality of reflected waves are present, the digital filter of the delay time detecting means suppresses the reflected wave having the largest reflection coefficient, so that the delay time of the delay means is set to the value of the reflected wave having the second largest reflection coefficient. If adjusted, the multipath distortion due to the reflected wave having the largest reflection coefficient and the reflected wave having the second largest reflection coefficient as a whole can be removed and reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a multipath distortion reduction circuit according to a first exemplary embodiment of the present invention.

FIG. 2 is a circuit diagram of a multipath distortion reduction circuit according to a second embodiment of the present invention.

FIG. 3 is a partially omitted circuit diagram showing a modified example of the second embodiment.

FIG. 4 is a partially omitted circuit diagram showing another modification of the second embodiment.

FIG. 5 is a circuit diagram of a conventional multipath distortion reduction circuit.

[Explanation of symbols]

1 FM demodulator 1C Digital demodulator 3 Digital adder 4A Digital delay device 6 Envelope detection circuit 6C Digital envelope detection circuit 7 DC component removal circuit 7C Digital DC component removal circuit 8 DC component detection circuit 8C Digital DC component detection circuit 11 Digital arithmetic unit 21 Delay time detecting unit 22 Controller 30 Digital filter 31 Filter coefficient updating unit 32 Estimating unit 40, 40A, 40B, 40C Multipath distortion reducing unit

Claims (6)

[Claims] 1. An FM demodulation unit for inputting an FM-modulated intermediate frequency signal having a multipath component to perform FM demodulation, an addition unit provided on the output side of the FM demodulation unit, and an output of the addition unit variable. Delaying means for delaying only the delay time, delay time detecting means for detecting the delay time from the direct wave component of the reflected wave component contained in the intermediate frequency signal, and controlling the delay means to adjust the delay time of the delay means. The delay time control means for varying the value detected by the delay time detection means, the envelope detection means for performing the envelope detection by inputting the intermediate frequency signal, and the direct current component removed from the output of the envelope detection means. DC component removing means, DC component detecting means for detecting a DC component from the output of the envelope detecting means, p output of the DC component removing means, q output of the DC component detecting means, and output of the adding means W, The output means of the delay means is s, and an operation means for performing an operation of v = ((p / q) / (1 + 2 (p / q))) · (ws) is provided, and the addition means is the FM. A multipath distortion reduction circuit characterized in that the output of the demodulating means and the output v of the calculating means are added and output. 2. The delay time detecting means receives the intermediate frequency signal and performs a predetermined filtering operation on the filter coefficient variable digital filter, and updates the filter coefficient for stabilizing the output amplitude of the digital filter. Filter coefficient updating means for calculating a value and updating the filter coefficient of the digital filter; and estimating means for estimating the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means. The multipath distortion reduction circuit according to claim 1, which is characterized in that. 3. The delay time detecting means is provided before the FM demodulating means, and the output of the digital filter in the delay time detecting means is input to the FM demodulating means. Multipath distortion reduction circuit. 4. The multipath distortion reduction circuit according to claim 3, wherein the estimating means estimates the delay time of the reflected wave having the second largest reflection coefficient. 5. An FM demodulation means for inputting an FM-modulated intermediate frequency signal having a multipath component to perform FM demodulation, an addition means provided on the output side of the FM demodulation means, and an output of the addition means being variable. Delaying means for delaying by the delay time, a digital filter provided in the preceding stage of the FM demodulating means, for performing a predetermined filtering process on the intermediate frequency signal, and for making the output amplitude of the digital filter constant at a fixed value Q. Filter coefficient updating means for calculating an updated value of the filter coefficient and updating the filter coefficient of the digital filter; estimating means for estimating the delay time of the reflected wave based on the filter coefficient calculated by the filter coefficient updating means; A delay time control means for controlling the delay means to vary the delay time of the delay means to a value estimated by the estimation means; An envelope detection means for performing an envelope detection by inputting an intermediate frequency signal output from the output signal, a direct current removal means for removing a direct current component from the output of the envelope detection means, and an output of the direct current removal means. p, an output of the adding means is w, and an output of the delay means is s. v = ((p / Q) / (1 + 2 (p / Q))) · (w−s) The multi-path distortion reducing circuit is characterized in that the adding means adds the output of the FM demodulating means and the output v of the calculating means and outputs the result. 6. The multipath distortion reduction circuit according to claim 5, wherein the estimating means obtains a delay time of a reflected wave having the second largest reflection coefficient.