JPH0548482A - Feedback type fm demodulator - Google Patents

Abstract

PURPOSE:To control a band width of an intermediate frequency signal and a level of an FM demodulation signal to be fed back in response to an S meter voltage so as to maximize the S/N of an FM demodulation signal. CONSTITUTION:A control signal generator 10 controls a gain (FM demodulation signal level) of a variable gain low frequency amplifier 6 and a band width BW of a variable band filter 2 based on an S meter voltage VSM from an IF amplifier 3. An AC component of an FM demodulation signal is fed back to an adder 8 and added to a bias voltage Vb from a bias voltage generator 9 and its combined output is fed to a VCO 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a feedback-type FM demodulation device in which the FM demodulation output S / N is always maximized with respect to the received input power.

[0002]

2. Description of the Related Art FIG. 8 shows an example of a conventional negative feedback demodulator. In the figure, 1 is a mixer, 2 is a narrow band filter, 3 is an IF amplifier, 4 is a limiter, 5 is an FM demodulator,
Reference numeral 6 is a low frequency amplifier, and 7 is a voltage controlled oscillator (VCO).

In the above device, the input FM signal f + Δf
(F is the carrier frequency, Δf is the frequency deviation) and is mixed with the VCO output signal fv + Δfv (fv is the VCO carrier frequency and Δfv is the frequency deviation) in the mixer 1 to obtain a narrow band. IF signal fi + Δ with filter 2
fi (fi represents the center frequency of the IF signal, and Δfi represents the frequency shift of the IF signal). The following is a mathematical expression.

Fi + Δfi (f + Δf) − (fv + Δfv) (1) The IF signal is amplified by the IF amplifier 3 and subjected to amplitude limitation by the limiter 4, and then FM demodulated by the FM demodulator 5 to be the low frequency amplifier 6. Is input to. The output of the low-frequency amplifier 6 is an FM demodulation output signal, and at the same time, is fed back to the VCO 7 and input to modulate the output signal of the VCO 7. The following is a mathematical expression.

VDC + VAC = KD · KA (fi + Δfi) (2) fv + Δfv = KV (VDC + VAC) (3) Substituting the equation (2) into the equation (3), fv + Δfv = KV · KD · KA ( fi + Δfi) (3) ′ Substituting the expression (3) into the expression (1), fi + Δfi = (f + Δf) −KV · KD · KA (fi + Δfi) (4)

Formula (4) is rearranged and IF signal fi + Δfi
Ask for. fi + Δfi = 1/1 + KV · KD · KA (f + Δf) (4) ′ However, VDC: DC voltage generated in the FM demodulator by the FM-FB loop during non-modulation, that is, the FM input signal is Δf = 0. In the case of f, the FM demodulator output KD · KA · fi VAC: AC voltage generated in the FM demodulator by the FM-FB loop at the time of modulation, that is, when the FM input signal is Δf ≠ 0, f + Δ
Of the FM demodulator output when f, KD, KA, Δfi KD: demodulation sensitivity of FM demodulator 5 KA: gain of low frequency amplifier 6 KV: modulation sensitivity of VCO 7

If KVKDKA> 0 in the equation (4) ', the IF signal fi + Δfi is obtained by compressing the frequency deviation of the FM input signal f + Δf. Therefore, the band of the bandpass filter 2 in the IF signal can be narrowed by the amount of the compressed frequency shift. Since the threshold level of the FM detection circuit is determined by the C / N of the IF signal,
Noise decreases as the signal band decreases, and the threshold level can be improved in the FM negative feedback system (the noise N and the IF signal bandwidth have a proportional relationship).

FIG. 2 shows received input power (corresponding to C / N) vs. FM in the case of an ordinary FM demodulator and an FM negative feedback demodulator.
The relationship of demodulation output S / N is shown. Generally, the threshold level Vth is defined as the point at which the S / N is abruptly changed and deteriorated by 1 dB. Each threshold level Vth of the normal FM demodulator and the FM negative feedback type demodulator of FIG.
Vth 'also follows the above. As described above, in the FM negative feedback system, the threshold level can be improved and the sensitivity can be increased.

[0009]

However, as shown in FIG. 1, in the range where the reception input is sufficient, the FM negative feedback demodulator is ΔS / N as much as the FM demodulation output S compared with the ordinary FM demodulator.
There is a drawback that / N deteriorates. The reason for this deterioration is that the signal level S of the FM demodulation output is reduced by that amount because the frequency shift is suppressed in the FM negative feedback system (the noise level N is determined by the band of the FM demodulation output, so normal FM demodulation is performed). Will be the same as the vessel).

The object of the present invention is to make the FM demodulation output S / N when the received input power is high in the negative feedback type FM demodulator equal to the normal FM demodulation output S / N, and to always perform FM demodulation with respect to the received input power. FM to maximize the output S / N
It is to control the feedback amount of the negative feedback and the bandwidth of the IF signal BPF.

[0011]

In order to achieve the above object, the first invention of the present application is a mixing and converting means for mixing an FM signal with a predetermined VCO output, converting it into a predetermined intermediate frequency signal, and outputting the signal. In the feedback FM demodulation device, which includes demodulation means for detecting the intermediate frequency signal and outputting an FM demodulation signal, and VCO output means for feeding back the FM demodulation signal to modulate the VCO output, the intermediate frequency A control means for extracting an S-meter component from the signal, controlling the level of the FM demodulation signal based on the S-meter component, and controlling the frequency bandwidth of the intermediate frequency signal, and the fed-back FM demodulation signal. Of the AC component and a predetermined reference DC component are combined, and the combined output is supplied to the VCO output means to generate a VCO having a frequency corresponding to the reference DC component. And a synthesizing means for generating a force, the feedback level and the frequency bandwidth according to the S meter components, the FM demodulation signal S /
It is characterized by controlling so that N becomes maximum.

The second invention of the present application is a mixing and converting means for mixing an FM signal with a predetermined VCO output, converting it to a predetermined intermediate frequency signal and outputting the mixed signal, and an FM demodulated signal by detecting the intermediate frequency signal. And a V for modulating the VCO output by feeding back the FM demodulated signal.
A feedback type FM demodulator including a CO output unit, extracts an S meter component from the intermediate frequency signal, controls the level of the FM demodulated signal based on the S meter component, and outputs the intermediate frequency signal. Controlling means for controlling the frequency bandwidth of the variable gain amplifier and the gain of the variable gain amplifier, and the AC component of the FM demodulated signal fed back through the variable gain amplifier and a predetermined reference DC component are combined. The synthesized output is supplied to the VCO output means to generate a VCO output having a frequency corresponding to the reference DC component, and the feedback level and the frequency bandwidth are set according to the S meter component. , So that the S / N of the FM demodulated signal is maximized.

[0013]

In the feedback type FM demodulator of the first invention of the present application, the control means controls the level of the FM demodulated signal based on the S meter component extracted from the intermediate frequency signal from the mixing and converting means, and Controls the frequency bandwidth of the intermediate frequency signal. The AC component of the FM demodulated signal fed back to the VCO output means side and the reference DC component are combined and supplied to the VCO output means, and a VCO output having a frequency corresponding to the reference DC component is generated.

In the feedback type FM demodulator of the second invention of the present application, the control means controls the gain of the variable gain amplifier in addition to the above control. Further, the AC component of the FM demodulated signal is fed back through the variable gain amplifier and is combined with the reference DC component.

[0015]

Embodiments of the present invention shown in the drawings will be described below.
1 is an embodiment of a feedback type FM demodulator according to the first invention of the present application, the same reference numerals as those in FIG. 8 denote the same or similar circuits, and the configuration different from FIG. A control signal generator 10, a bias voltage generator 9, for controlling the bandwidth BW of the variable band filter 2 and the gain of the variable gain low frequency amplifier 6 (the level of the FM demodulation signal) based on the S meter voltage VSM extracted from And the AC voltage VAC of the FM demodulated signal fed back and the bias voltage (reference DC component) Vb from the bias voltage generator 9.
The point is that an adder 8 that combines and outputs is added.

In this embodiment, the IF signal fi + is obtained by using the signal meter voltage representing the received input power from the IF amplifier 3.
The variable band filter 2 and the variable gain low frequency amplifier 6 for extracting Δfi are controlled by the control signal generator 10.

First, when the input FM signal is not modulated, that is, Δ
Consider the case of f = 0. Since Δf = 0, the output VAC of the variable gain low-frequency amplifier 6 becomes VAC = 0 (VDC ≠ 0). The adder 8 adds the constant DC voltage Vb generated by the bias voltage generator 9 and the AC signal VAC via the DC voltage blocking capacitor C.

Therefore, in this case, the adder 8 changes Vb to VCO.
Input to 7. In VCO 7, the output frequency f according to Vb
Output v. In the mixer 1, the input FM signal f and the VCO output signal fv are mixed and passed through the variable band filter 2 to obtain the IF signal fi.

The IF signal fi is amplified by the IF amplifier 3, limited in amplitude by the limiter 4, and then FM demodulated by the FM demodulator 5 and input to the variable gain low frequency amplifier 6, and the output of the variable gain low frequency amplifier 6 is output. The FM demodulation output VDC is obtained. The above can be expressed by the following formulas.

Fi = f−fv = f−KV · Vb (5) VDC = KD · fi = KD (f−KV · Vb) = constant (6)

That is, the center frequency of the IF signal becomes independent of the FM demodulation low frequency output VDC, and is determined by the output signal Vb of the bias voltage generator 9 and the modulation sensitivity KV of VCD7.

Next, when the input FM signal is modulated, that is, when f + Δf, the IF signal is fi + Δfi = f + Δf- (fv + Δfv) (7) VDC + VAC = KD · KA '( fi + Δfi) (8) fv + Δfv = KV (Vb + VAC) = KV · Vb + KV · KD · KA ′ · Δfi (9) Equation (9) is substituted into Equation (7), and fi + Δfi = f + Δf − (Fv + Δfv) = f−KV · Vb + Δf−KV · KD · KA ′ · Δfi (10)

The right side f-KV.Vb of the equation (10) represents the center frequency fi of the IF signal as can be seen from the equation (5). Therefore, if equation (10) is rewritten only for the frequency deviation Δfi of the IF signal, Δfi = Δf−KV · KD · KA ′ · Δfi ∴Δfi = 1 / (1 + KV · KD · KA ′) Δf (10) '.

That is, in the FM negative feedback system of the present embodiment, the center frequency fi of the IF signal is constant even if the gain KA 'of the variable gain low frequency amplifier 6 is changed, and only the frequency deviation Δfi of the IF signal is generated. 2 will change (if KA is changed in the conventional method of FIG. 2, since not only Δfi but also fi changes, an FM demodulated output signal cannot be obtained normally). The gain KA 'of the variable gain low frequency amplifier 6 and the bandwidth BW of the variable band filter 2 are controlled by the control signal generator 10 as shown in FIG. 4 based on the signal meter voltage VSM representing the received input power of the IF amplifier 3. Receive.

That is, when the received input power is sufficient to some extent (area where VSM is constant), KA '= 1, BW = BWmax (F
In the case of M stereo broadcasting, applying Carson's rule, BWmax
= 2 × (53 + 75) = 256 kHz). When the received input power decreases (VSM decreases, FM demodulation output S
/ N approaches near the threshold level) K
A ′ gradually increases, and BW decreases in accordance with the decrease in frequency deviation Δfi of the IF signal. Since the BW is reduced, the noise N in the IF signal is also reduced, the C / N at the input of the FM demodulator 5 is improved, and the threshold level is improved.

These states are shown in FIG. Since the feedback amount of the FM-FB and the bandwidth of the bandpass filter that passes the IF signal are controlled according to the received input power, the FM demodulation output S / N can always be maximized with respect to the received input power.

Next, FIG. 5 shows an embodiment of the negative feedback type FM demodulator according to the second invention of the present application. In FIG.
1 indicates the same or similar circuit. In the present embodiment, a variable gain amplifier 11 is further provided in addition to the configuration of FIG. 1, and the AC component of the FM demodulated signal is passed through this variable gain amplifier 11. While being fed back, the control signal generator 10 controls the gain of the amplifier 11 based on the S meter voltage VSM.

In the above apparatus, IF signal fi + Δfi
Is amplified by the IF amplifier 3, limited in amplitude by the limiter 4, and then FM demodulated by the FM demodulator 5. FM
The demodulated signal is input to the variable gain low frequency amplifier A ₁ (6) via the DC voltage blocking capacitor C and becomes the FM demodulated signal Va, and the other is input to the variable gain low frequency amplifier A ₂ (11). Is entered. The output of the variable gain low frequency amplifier A ₂ (11) is added to the DC bias voltage Vb generated by the bias voltage generator 9 by the adder 8 and input to the VCO 7, which becomes the VCO output signal fv + Δfv.

The following is a mathematical expression. Va = KD · KA ₁ · Δfi (11) fv + Δfv = KV {Vb + (KA ₂ / KA ₁ ) Va} (12) Substituting equation (11) into equation (12), fv + Δfv = KV {(Vb + KA ₂ / KA ₁ ) KD · KA ₁ · Δfi} = KV (Vb + KA ₂ · KD) Δfi (13) Substituting equation (13) into equation (1), fi + Δfi = (f- KV · Vb) + (Δf−KV · KA ₂ · KD · Δfi) (14) where KA ₁ : variable gain low frequency amplifier A ₁ (6) gain KA ₂ : variable gain low frequency amplifier A ₂ (11) ) Gain KD: demodulation sensitivity of FM demodulator 5 KV: modulation sensitivity of VCO 7

The term on the right side of f-KV.Vb in the equation (14) is I
It is none other than the center frequency fi of the F signal. Therefore, if the formula (14) is rewritten only for the frequency deviation Δfi of the IF signal, Δfi = Δf−KV · KA ₂ · KD · Δfi ∴Δfi = 1 / (1 + KV · KA ₂ · KD) · Δ f (15)

If KV · KA ₂ · KD> 0 in the equation (15), the IF signal fi + Δfi is the one in which the frequency deviation of the FM input signal f + Δf is compressed. The gains KA ₁ and KA _{2 of the} variable gain low frequency amplifiers A ₁ (6) and A ₂ (11) and the bandwidth BW of the variable band filter 2 are control signals based on the signal meter voltage VSM representing the reception input of the IF amplifier 3. The generator 7 receives control as shown in FIG.

That is, if the received input power is sufficiently high (VSM1 is constant), KA ₂ = 0, KA ₁ = 1 and BW =
BWmax (BWmax = 2 × (53 + 75) = 256 kHz when Carson's rule is applied in FM stereo broadcasting). When the received input power decreases (VSM decreases, F
When the M demodulation output S / N approaches the threshold level, KA ₁ and KA ₂ gradually increase, and the bandwidth BW of the variable band filter decreases as the frequency deviation Δfi of the IF signal decreases. Therefore, the noise N in the IF signal is also reduced to F
The C / N at the input of the M demodulator 5 is improved, the threshold level is also improved, and the sensitivity is higher than that of a normal FM demodulation method.

FM demodulation output S / N with respect to received input power
7 shows the relationship. FM-FB depending on the received input power
Of the FM demodulation output S with respect to the received input power in order to control the amount of feedback of B and the bandwidth of the bandpass filter that passes the IF signal.
It is possible to maximize / N. The variable gain low frequency amplifier A ₁ is intended to suppress the fluctuation of the FM demodulation output signal caused by the magnitude of the feedback amount of the FM negative feedback.

From the equations (11) and (15), the FM demodulated signal Va is Va = KD · KA ₁ · Δfi = KD · KA ₁ · {1 / (1 + KV · KA ₂ · KD)} Δf (16) Becomes Here, the condition that Va is not changed by the feedback amount of FM-FB is KA ₁ · {1 / (1 + KV · KA ₂ · KD)} = C (C is a constant) (17). Therefore, the relationship between KA ₁ and KA ₂ is KA ₁ = (1 + KV · KA ₂ · KD) · C (18).

In FIG. 6, C = 1 and KA ₂ =
When KA ₁ at 0 and KA ₂ max is KA ₁ min and KA ₁ max, the following results. KA ₁ min = 1 (KA ₂ = 0) (19) KA ₁ max = 1 + KV / KD / KA ₂ max (KA ₂ = KA ₂ max) (20)

As described above, in the FM negative feedback system in which the feedback amount is variable, the FM demodulation output S / N is always maximized with respect to the received input power, and high sensitivity is achieved.

[0037]

As described above, according to the present invention, when the high received input power is used, compared with the conventional FM demodulation method, the conventional F demodulation method is used.
The FM demodulation output S / N, which was disadvantageous in the M negative feedback system, can be made equal to the normal FM demodulation system, and the FM demodulation output S / N can always be maximized with respect to the received input power, and the sensitivity is high. Is achieved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a first invention of the present application.

FIG. 2 is a characteristic diagram showing a relationship between threshold improvement by an FM negative feedback method and demodulation S / N.

FIG. 3 is a characteristic diagram showing a relationship between threshold improvement and demodulation S / N by the FM negative feedback system of FIG.

FIG. 4 is a characteristic diagram for explaining the operation of the control signal generator of FIG.

FIG. 5 is a block diagram showing an embodiment of the second invention of the present application.

6 is a characteristic diagram for explaining the operation of the control signal generator of FIG.

7 is a characteristic diagram showing a relationship between threshold improvement and demodulation S / N by the variable feedback FM demodulation method of FIG.

FIG. 8 is a block diagram showing a conventional feedback type FM demodulation device.

[Explanation of symbols]

 1 mixer 2 variable band filter 3 IF amplifier 5 FM demodulator 6 variable gain low frequency amplifier 7 VCO 8 adder 9 bias voltage generator 10 control signal generator 11 variable gain amplifier

Claims (2)

[Claims] 1. A mixing and converting means for mixing an FM signal with a predetermined VCO output, converting it to a predetermined intermediate frequency signal for output, and a demodulating means for detecting the intermediate frequency signal and outputting an FM demodulated signal, A feedback type FM demodulation device comprising: a VCO output means for feeding back an FM demodulated signal to modulate the VCO output; and extracting an S meter component from the intermediate frequency signal, and based on the S meter component, the FM meter component Control means for controlling the level of the demodulated signal and for controlling the frequency bandwidth of the intermediate frequency signal, synthesizes the AC component of the fed-back FM demodulated signal with a predetermined reference DC component, and outputs the synthesized output. Is supplied to the VCO output means to generate a VCO output having a frequency corresponding to the reference DC component. The feedback type FM demodulator, wherein the feedback level and the frequency bandwidth are controlled so that the S / N of the FM demodulated signal is maximized. 2. A mixing and converting means for mixing an FM signal with a predetermined VCO output and converting it to a predetermined intermediate frequency signal for output, and a demodulating means for detecting the intermediate frequency signal and outputting an FM demodulated signal, A feedback type FM demodulation device comprising: a VCO output means for feeding back an FM demodulated signal to modulate the VCO output; and extracting an S meter component from the intermediate frequency signal, and based on the S meter component, the FM meter component Control means for controlling the level of the demodulated signal, controlling the frequency bandwidth of the intermediate frequency signal, and controlling the gain of the variable gain amplifier; and FM fed back through the variable gain amplifier.
And a synthesizing means for synthesizing the AC component of the demodulated signal and a predetermined reference DC component, supplying the synthesized output to the VCO output means, and generating a VCO output having a frequency corresponding to the reference DC component. A feedback type FM demodulation device, wherein the feedback level and the frequency bandwidth are controlled according to the S meter component so that the S / N of the FM demodulation signal becomes maximum.