JPH0548550A - Fm demodulator - Google Patents

Abstract

PURPOSE:To improve the reception sensitivity by generating the synthesis signal of an FM demodulation signal, a carrier suppression signal and a pilot signal, converting the synthesis signal into an FM signal and implementing the FM negative feedback so as to reduce the reception band. CONSTITUTION:An input FM signal S1 is mixed with an output Sf of a VCO 32 at a mixer 12 and a differential component signal S1 is given to a BPF 13, an FM demodulator 14 and an LPF 15, from which a stereo L+R component S01 is obtained. Moreover, the signal S1 is given to a BPF 16, a demodulator 17 and an LPF 18, from which a pilot signal Sp' is obtained as a stereo L-R component signal S02. The signal Sp' is fed to a PLL 35, from which a subcarrier S7 of a stereo sub channel is generated and further a pilot signal Sp'' is generated. The signal Sp'', the stereo L+R component S01 and the sub channel signal S11 of the carrier suppression AM (DSB) obtained from the mixer 30 are synthesized by an adder circuit 31, from which a stereo composite signal S12 is obtained. The signal S12 is inputted to the VCO 32, from which a feedback signal Sf subjected to FM modulation is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM feedback type FM demodulator for performing high-sensitivity reception by demodulating an FM signal in a narrow band. Regarding a configuration for improvement.

[0002]

2. Description of the Related Art The reception bandwidth Bw of an FM receiver is determined by Bw = 2 (fd + fm) (1) fd: maximum frequency shift fm: maximum modulation frequency.

By the way, the FM demodulation noise when the C / N (carrier / noise power ratio) is small is almost occupied by the click noise. This click noise has a reception bandwidth B
As the reception width becomes wider in proportion to w, it becomes larger. Therefore, the reception sensitivity can be improved by reducing the reception width.

However, since the reception bandwidth Bw requires the value shown in (1), the reception sensitivity is naturally determined by the normal method, and it is impossible to increase the sensitivity. Therefore,
There is known an FM feedback system that improves the sensitivity by reducing the frequency shift, that is, by reducing the maximum frequency shift fd in the equation (1) to narrow the reception bandwidth Bw.

FIG. 2 shows the configuration of a conventional FM feedback type FM demodulator of this type. 1 is an input terminal for an input signal Si, 2 is a mixer, 3 is a BPF (bandpass filter), and 4 is a mixer. Is an FM demodulator, 5 is an LPF (low-pass filter), 6 is a delay correction circuit, 7 is a VCO (voltage controlled oscillator), and 8 is an output terminal of the output signal So.

In FIG. 2, the input signal Si from the input terminal 1 is input to the mixer 2 and mixed with the output Sf of the VCO 7, and the component SB of the difference frequency between them is extracted from the BPF (bandpass filter) 3. This signal SB is demodulated by the FM demodulator 4, the high frequency component is removed by the LPF (low pass filter) 5, and is output from the output terminal 8 as the output signal So, while being fed back to the VCO 7 via the delay correction circuit 6, FM modulation is performed by the VCO 7 so as to be in phase with the frequency shift of the input signal Si, that is, so that the frequency shift of the signal SB is minimum (= FM negative feedback). The input signal Si and the feedback signal Sf are

[0007]

Fo: carrier frequency of input signal Si fo ': carrier frequency of output Sf of VCO fd: maximum frequency deviation of input signal Si fd': maximum frequency deviation of output Sf of VCO

Then, a bandpass filter (BPF)
The output SB of 3 is

Therefore, if the frequency shift is fd≈fd ′, the equation (1) becomes Bw≈2fm (5).

In FM stereo broadcasting, fd = 75
Since kHz and fm = 53 kHz, the bandwidth Bw required by equation (1) is approximately 250 kHz, which is
When the M feedback method is used, from equation (5), about 1
It becomes 06 kHz. As a result, a narrow band can be achieved, click noise is reduced, and reception sensitivity is improved.

[0012]

However, in such a conventional FM feedback system, the bandwidth is 10 or less.
It is impossible to narrow the band below 6 kHz, and this state was regarded as the limit of sensitivity improvement. That is, (1) described above
As shown in the equation, even if fd = 0, Bw = 2
fm is the limit. This is because if it is smaller than this Bw, the bandwidth becomes insufficient, and the signal is distorted when the FM modulation spectrum is lost or demodulated.

An object of the present invention is to provide an FM demodulator which can further improve the receiving sensitivity by further narrowing the receiving bandwidth.

[0014]

In order to achieve such an object, the present invention provides a difference component signal extracting means for mixing an FM signal with a predetermined VCO output to obtain a difference component signal between the two signals, and Differential component signal is detected and stereo L + R and L
And a demodulation means for obtaining a demodulation signal of the R component, the demodulation signal being fed back to the VCO, and a control signal for performing modulation control substantially the same as the modulation signal of the FM signal is output to the VCO.
In the M demodulator, pilot component extraction means for extracting a pilot component signal containing a stereo L + R component based on the difference component signal, and stereo L having substantially the same frequency and phase as the pilot component signal based on the pilot component signal A frequency / phase control means for outputting a sub-carrier component signal of the -R component and a pilot signal, and a difference component signal is switched in response to a predetermined pulse signal corresponding to the sub-carrier component signal to demodulate a stereo LR component. A switching means for outputting, a demodulation signal of the stereo LR component, and a subcarrier component signal, and a carrier suppression means for outputting an amplitude-modulated carrier suppression signal, a pilot signal, and a carrier suppression signal, And a synthesizing means for synthesizing the demodulated signals of the stereo L + R components and outputting the synthesized signals to the VCO as the control signal. It is characterized in.

[0015]

In the present invention, the FM demodulated signal of the stereo L + R component is obtained from the FM signal, the FM signal is switched in synchronization with the subcarrier, and then the F of the stereo L-R component is generated.
The M demodulated signal is obtained, a composite signal of the FM demodulated signal, the carrier suppression signal, and the pilot signal is created, and the composite signal is converted into the FM signal to be the FM feedback signal for performing the FM negative feedback operation. Therefore, the reception band can be further reduced and the reception sensitivity can be improved.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows the configuration of an embodiment of an FM demodulator according to the present invention, in which 11 is an input terminal of an input signal Si,
12, 23, 30 are mixers, 13, 16, 20 are BPF
(Band pass filter), 14, 17 and 21 are FM demodulators, 15, 18, 22 and 24 are LPFs (low pass filters), 19 is a switch circuit, 25 and 32 are VCOs (voltage controlled oscillators), and 26 is a limiter circuit. , 27 is a 1/2 frequency divider, 28 is a π / 2 phase shifter, 29 is a delay circuit, 31 is a combiner, 33 and 34 are output terminals of stereo L + R component and LR component signals S01 and S02, respectively. 35
Indicates a PLL circuit.

The mixer 12 constitutes the difference component signal extracting means. The FM demodulators 14 and 21 constitute the demodulation means. The BPF 16 and the FM demodulator 17 constitute the pilot component extracting means. The mixer 30 constitutes carrier wave suppressing means. The synthesizer 31 constitutes the synthesizing means. Further, the PLL circuit 35 includes 23, 24, 25, 27-
29 circuits.

First, the outline of the present invention will be described. Figure 3
The spectrum of a stereo composite signal which is a modulation signal of the input signal Si is shown, and the frequency deviation of the signal S1 of the difference frequency component, which is the output of the mixer 12, is compressed. FIG. 4 shows the spectrum of the signal S1 at this time.

In FIG. 1, the BPF 13 extracts the upper and lower side wave components of the main channel signal, and the FM demodulator 1
The demodulation output SM of the main channel signal (stereo L + R signal) is obtained by 4, and the stereo L + R component signal S01 is further obtained by the LPF 15.

The BPF 16 extracts upper and lower side wave components (including the main channel signal component) of the pilot signal Sp, and the FM demodulator 17 and the LPF 18 extract the pilot signal Sp '(including the main channel signal SM). ) Signal is obtained as a stereo LR component signal S02.

The signal of the pilot signal Sp '(including the signal of the demodulation signal SM) is supplied to the PLL circuit 35 to form the subcarrier S7 of the stereo subchannel and further the pilot signal Sp ". The subcarrier S7 is a limiter circuit. 26, and converts it into a pulse signal S6. In the switch circuit 19, the signal S1 is switched by the pulse signal S6, and FIG.
A signal S4 having the spectrum of is obtained.

In FIG. 5, the sub-channel signal component,
The arrangement of the main channel signal components is in a state where the arrangement is replaced with that in FIG. Therefore, from the signal S4, the upper and lower side wave signals S5 of the sub-channel are taken out by the BPF 20, demodulated by the FM demodulator 21, passed through the LPF 22, and passed through the stereo LR.
The signal S03 is output.

Further, the signal S03 and the stereo subcarrier signal S7 are mixed by the mixer 30, and the carrier suppression AM (D
SB) sub-channel signal S11 is obtained. This signal S1
1, the pilot signal Sp ″, and the stereo L + R signal S
01 signals are combined to form a stereo composite signal S1.
Get 2. This signal S12 is input to the VCO 32, and FM
Obtain the modulated feedback signal Sf.

As a result, the feedback signal Sf can have the same modulation signal as the input signal Si, and the signal S1 in which the frequency deviation is suppressed can be obtained. Therefore,
The BPFs 13 and 20 can have a bandwidth of 30 kHz (twice the maximum frequency of the L and R channels of 15 kHz), and the BPF 16 can have a bandwidth of 38 kHz (twice the pilot signal frequency of 19 kHz). As a result, a receiving sensitivity higher than that of the conventional FM feedback method can be obtained. Hereinafter, details of the present invention will be described. Input signal Si and feedback signal S
Let f be the following equation.

[0025]

SM: main channel signal Sp: pilot signal Ss: sub-channel signal fd1: maximum frequency deviation of main channel signal of Si fd1 ': maximum frequency deviation of main channel signal of Sf fd2: maximum pilot signal of Si Frequency shift fd2 ': maximum frequency shift of Sf pilot signal fd3: maximum frequency shift of Si sub-channel signal fd3': maximum frequency shift of Sf sub-channel signal

Here, the main channel signal SM is
With the combination of the L (left) channel and the R (right) channel, SM = L + R (8) The pilot signal Sp is: Sp = cos2πfpt (9) and fp = 19 kHz.

The sub-channel signal Ss has a stereo component with a frequency twice as high as fp as a sub-carrier, and a difference signal L- of R
R is an amplitude-modulated carrier-suppressed AM (DSB) signal, which is expressed as Ss = (L−R) cos4πfpt (10). However, the L and R channels are signals with the maximum audio frequency fa (MAX) = 15 kHz.

Mixer 1 with signals Si and Sf as inputs
When mixed in 2, the output is converted to the sum of Si and Sf and the difference frequency component. Among these, if the signal of the difference frequency component is S1, then

[0030]

Here, fd1≈fd1 ′, fd2≈fd
If 2 ', fd3 ≈ fd3', then SM, S
Only the first-order upper and lower sidebands of the frequencies of the p and Ss signals are generated, and two or more sidebands are not generated. As a result, the spectrum of equation (11) becomes as shown in FIG. The signal S1 is input to the BPF 12 to obtain the signal S2. BPF1
The bandwidth Bw of 2 is set to the width (3
0 kHz)

[0032]

It becomes This signal S2 is FM-demodulated by the FM demodulator 14, and the LPF with the cut-off frequency fc = 15 kHz is further added.
A demodulated signal S01 is obtained through 15. This signal S01
Is expressed as S01 = k1SM.

K1 is the demodulation gain of the FM demodulator 13 and LP
The total gain of the loss of F15, where k1 = 1, the signal S01 is expressed as S01 = SM = L + R (13). Furthermore, the signal S1 is also given to the BPF 16,
The signal S3 is taken out. Bandwidth Bw2 of this BPF 16
Is the width of the upper and lower side waves of the pilot signal (38 kHz), the signal S3 is

[0035]

Is represented by This signal S3 is FM demodulated by the FM demodulator 17, and further demodulated output S02 is obtained through the LPF 18 having a cutoff frequency fc = 19 kHz. This signal S02 is expressed as S02 = SM + Sp ′ = L + R + cos2πfp (t−τ) (15) τ: delay time of the pilot signal from the BPF 16 to the LPF 18.

Here, the demodulation output S02 has the same L + R signal as in the equation (13), but the BPF13 to the BPF16.
Is wider than the signal shown in equation (13), S /
N is bad. Therefore, the signal expressed by the equation (15) is used for stereo subcarrier reproduction and is input to the PLL circuit 35. The output S7 of the VCO 25 of the PLL circuit 35 is expressed as S7 = cos4πfpt (16), and the output Sp ″ of the 1/2 frequency divider 27 is expressed as Sp ″ = cos2πfpt (17), and this output Sp ”Is delayed by π / 2 in the phase shift circuit 28, the output S8 is expressed as S8 = cos2πfpt (t−π / 2) (18).

Further, when the delay circuit 29 delays by time τ ', its output S9 is expressed as S9 = cos2πfpt (t-π / 2-τ') (19). Then, in the mixer 23, the signals S02 and S9 are
Are mixed and the DC component thereof is taken out by the LPF 24, the output S10 is S10 = cos (π / 2 + τ′−τ) (20). Here, the delay circuit 29 is set so that τ = τ ′.
When the characteristic of is set, S10 = cosπ / 2 (21)

The output signal S10 is a control signal of the VCO 25, and the pilot signals Sp 'and Sp of the formula (15) are π / 2 due to the feedback configuration of the PLL circuit 35.
The frequency of the VCO 25 is set so that there is a phase difference.
As a result, the pilot signal Sp ″ and the subcarrier signal S
Reference numeral 7 is a modulation signal of the signal Si. The signals have the same frequency and the same phase as the pilot signal Sp of the equation (9) and the subcarrier Ss of the equation (10). That is, a pilot signal and subcarrier without delay time τ from the BPF 16 to the LPF 18 can be obtained. The signal S7 is applied to the limiter circuit 26 and converted into the pulse signal S6. On the other hand, the difference frequency component S1 of the mixer 12 is applied to the switching circuit SW, and the signal S9 switched by the pulse signal S6 is output.

FIG. 6 (a) shows the frequency change of the sub-channel signal of the signal S1, which is an FM signal of the L-R component DSB-modulated by the stereo sub-carrier. FIG. 6B is a waveform of the pulse signal S6,
It is in phase with the stereo subcarrier of (a). When the switching circuit 19 operates so as to output the input signal when the signal S6 is 1 and cut off when the signal S6 is 0, the output S4 of the switching circuit 19 has a waveform as shown in FIG. 6C.
In FIG. 6C, a signal is output in the shaded portion, and the FM signal is not output in the time zone shown by the dotted line.

FIG. 5 shows the spectrum of the signal S4, and the sub-channel signal component is replaced with the position of the main-channel signal component of FIG. The bandwidth Bw3 is set so that the modulation component of the frequency band of 15 kHz of the stereo difference component signal L-R can pass through the signal S4 in such a state.
Input to the BPF 20 of = 30 kHz.

The BPF 20 removes components of ± 30 kHz or more from the FM modulation frequency fo-fo 'of the signal S4, and outputs the signal S5. As a result, the stereo LR which is the envelope of the sub-channel signal shown in FIG.
A signal S5 in which the component signal is FM-modulated is obtained. Signal S
5 is FM demodulated by the FM demodulator 21 and the cutoff frequency fc = 1.
A stereo LR component signal is obtained as a demodulation output S03 when passing through the LPF 22 of 5 kHz.

Next, the format of the feedback signal Sf will be described. When the stereo L-R component signal of the FM demodulation output S03 is mixed with the signal S7 by the mixer 30, the output S11 becomes S11 = (L-R) cos4πfpt (22) from the equation (16).

Further, when the signals S11, Sp ″ and S01 are combined, the output S12 is represented by S12 = L + R + cos2πfpt + (LR) cos4πfpt (23) (8), (9), (10). From the equation, the signal S12 is S12 = SM + Sp + Ss (24)

As described above, in the above-described embodiment of the present invention, the system as the FM feedback is established, and the stereo L + R component signal and the L-R component signal are L
It can be obtained with an FM signal having a bandwidth twice as high as 15 kHz which is the highest frequency of each audio of the channel and the R channel. The demodulation outputs S01 (L + R) and S03 (LR) are BPF13, LPF15 and BP.
Although the delay in F20 and LPF22 is actually added, in the embodiment of the present invention, since the delay time is smaller than the period of the L and R signal frequencies, there is no influence of the delay.

In the explanation of the equation (11), fd1
Although ≈fd1 ′, fd2≈fd2 ′, fd3≈fd3 ′ are set, this can be easily achieved by increasing the loop gain of FM feedback such as increasing the conversion gain of the VCO 32. Furthermore, the demodulation outputs S01 and S03 are
When applied to a matrix circuit (not shown) having an adder circuit and a subtractor circuit, it can be separated into an L channel signal and an R channel signal.

[0047]

In the conventional FM feedback system for receiving FM stereo broadcasting, the receiving bandwidth needs to be about 106 kHz, but in the system of the present invention, about 15 kHz is sufficient, and about 1/7 band. Can be reduced to width. Therefore, noise is removed by the narrow band, and the reception sensitivity can be improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of an FM demodulation device of the present invention.

FIG. 2 is a configuration diagram of a conventional FM demodulation device.

FIG. 3 is a diagram showing a spectrum of an FM stereo composite signal.

FIG. 4 is a diagram showing a spectrum of a signal S1.

FIG. 5 is a diagram showing a spectrum of a signal S4.

FIG. 6 is a diagram showing an example of waveforms of signals of respective parts.

[Explanation of symbols]

 12, 23, 30 Mixer 13, 16, 20 BPF 14, 17, 21 FM demodulator 15, 18, 22, 24 LPF 19 Limiter circuit 25, 32 VCO 27 1/2 divider 28 π / 2 Phase shifter 29 Delay Circuit 31 Adder Circuit 35 PLL Circuit

Claims (1)

[Claims] 1. An FM signal is mixed with a predetermined VCO output,
A difference component signal extracting means for obtaining a difference component signal of both signals and a demodulation means for detecting the difference component signal to obtain a demodulation signal of stereo L + R and LR components are provided, and the demodulation signal is fed back to the VCO. , An FM for outputting a control signal to the VCO for performing modulation control substantially the same as the modulation signal of the FM signal
In the demodulator, pilot component extraction means for extracting a pilot component signal containing a stereo L + R component based on the difference component signal, and stereo having substantially the same frequency and phase as the pilot component signal based on the pilot component signal The frequency / phase control means for outputting the subcarrier component signal of the L-R component and the pilot signal and the stereo component L-R for switching the difference component signal in response to a predetermined pulse signal corresponding to the subcarrier component signal. Switching means for outputting a demodulated signal of a component, carrier suppressing means for mixing the demodulated signal of the stereo L-R component and the subcarrier component signal, and outputting an amplitude-modulated carrier suppressing signal, and the pilot signal , The carrier suppression signal and the demodulated signal of the stereo L + R component are combined, and V is used as the control signal. FM demodulation apparatus characterized by having a synthesizing means for outputting a O.