JPH08172371A - Fm receiver - Google Patents

Abstract

PURPOSE: To obtain a demodulated output reduced at its distortion even when overmodulation is requested and to prevent the generation of a noise even when an adjacent sound subcarrier is jumped into a received stereophonic broadcasting wave at the time of receiving the stereophonic broadcasting by selecting a narrow band by band width switching. CONSTITUTION: An FM demodulating means is constituted of a MIX circuit 12, a BPF circuit 13, an FM demodulation circuit 14, and a voltage controlled oscillator 20, a channel selecting PLL circuit is constituted of the oscillator 20, a variable frequency dividing circuit 22, a phase comparator 23, a local oscillation circuit 24, and an LPF circuit 25 and an FM negative feedback loop is constituted of a loop filter circuit 17, a DC interruption circuit 18, a switch 13, and an adder 19. A switch 32 can supply the output of the FM demodulating means to either one of an HPF 33 with a large time constant and an HPF 34 with a small time constant, and in a wide band selecting state, a switch 31 is OFF and the switch 32 is in an HPF 34 selecting state. In a narrow band selecting state, the switch 31 is ON and the switch 32 is in an HPF 33 selecting state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM receiver using an FM negative feedback system, and more particularly to a demodulation system effective when overmodulation is required.

[0002]

2. Description of the Related Art As is well known, in a transmission system of television image signals and audio signals using a geostationary satellite, an FM modulation system which has a wider band and lower noise than an AM modulation system is widely adopted. There is. However, in this FM modulation system, a so-called threshold phenomenon occurs in which impulse noise rapidly increases when a weak electric field is generated, and the quality of images and sound is significantly deteriorated.

Therefore, in the FM receiver, a threshold improvement type demodulation circuit (TED; Threshold Ex) is generally used.
tension demodulator) is used. The threshold improvement type demodulation circuit includes various systems such as a tracking filter system, an FM negative feedback system and a negative feedback phase detection system.

FIG. 3 shows a conventional FM receiver which employs an FM negative feedback method suitable for a voice signal transmission system. That is, the audio subcarrier supplied to the input terminal 11 is converted into an intermediate frequency signal by the mixer (MIX) circuit 12, and then a bandpass filter (BPF).
The audio signal is demodulated by the FM demodulation circuit 14 via the circuit 13, and is extracted from the output terminal 16 via the amplifier circuit 15.

In this case, the voice signal output from the FM demodulation circuit 14 is the loop filter (LF) circuit 17, DC
A feedback loop 21 including a (DC) cutoff circuit 18, an adder circuit 19 and a VCO (voltage controlled oscillator circuit) 20.
By being fed back to the MIX circuit 12 via the, the frequency of the input audio subcarrier and the VCO
The intermediate frequency signal having a frequency that is different from the frequency of the output signal of 20 is generated.

Further, the output signal of the VCO 20 is supplied to the phase comparison circuit 23 via the variable frequency division circuit 22, and the phase comparison circuit 23 outputs the output signal of the variable frequency division circuit 22 and the local oscillation circuit 24. A signal corresponding to the frequency difference component from the output constant frequency signal is generated. Then, this signal is the low-pass filter (LPF) circuit 2
The voltage is converted into the voltage V _T in step 5, added to the output voltage of the DC cutoff circuit 18 by the adder circuit 19 and supplied to the VCO 20, so that the oscillation frequency of the VCO 20 is controlled so as to eliminate the frequency difference component. .

Therefore, the VCO 20 and the variable frequency divider 2
2, the phase comparison circuit 23, the variable frequency dividing circuit 22, the local oscillator circuit 24, and the LPF circuit 25 select the audio signal by changing the frequency division ratio of the variable frequency dividing circuit 22, the audio tuning PLL (phase A synchronous loop) circuit 26 is configured.

Here, the above-described FM receiver is a normal super-heterodyne FM receiver with a feedback loop 21 added. So, for example, F
If the frequency deviation ΔF of the M signal is ± 75 kHz and the maximum modulation frequency f _max is 15 kHz, the bandwidth B required for the super-heterodyne FM receiver is twice the sum of them according to the Carson law, that is, B = 2 (ΔF + f _max ) = 2 (75 + 15) = 180 kHz (1)

Therefore, the threshold level P
Letting _TH be the Boltzmann constant and T be the absolute temperature, _TH becomes P _TH = 8 kTB = 8 kT × 180 × 10 ⁻³ (2)

Next, considering the existence of the feedback loop 21, the frequency of the intermediate frequency signal output from the MIX circuit 12 is always the input signal (voice subcarrier).
And the frequency of the output signal of the VCO 20. Therefore, the polarity of the feedback loop 21
If the oscillation frequency of the VCO 20 is changed so as to follow the frequency change of the input signal, that is, if negative feedback is selected, the frequency change of the intermediate frequency signal is compressed more than the frequency change of the input signal. Becomes smaller.

Such a feedback system is a kind of negative feedback similar to the feedback of a normal low frequency amplifier circuit, and an FM signal is transmitted to a part of the loop. Therefore, it is called FM negative feedback. Has been done.

As described above, the frequency deviation ΔF ₃ of the intermediate frequency signal is equal to the frequency deviation ΔF ₁ of the input signal and VCO 2
It is equal to the difference from the frequency deviation ΔF ₂ of the output signal of 0, and ΔF ₃ = ΔF ₁ −ΔF ₂ (3)

The output amplitude V _o of the audio signal obtained from the output terminal 16 has a frequency deviation ΔF _{3 of the} intermediate frequency signal.
And the demodulation sensitivity K ₁ of the FM demodulation circuit 14 and the amplification circuit 15
Since it is the product of the gain G ₁ and G ₀ , V _o = ΔF ₃ · K ₁ · G ₁ (4)

Further, the frequency deviation ΔF ₂ of the output signal of the VCO ₂₀ is the modulation sensitivity K ₂ , the output amplitude (V _o / G ₁ ) of the audio signal output from the FM demodulation circuit 14, and L
Since it is a product of the gain of the F circuit 17 and G ₂ , ΔF ₂ = K ₂ (V _o / G ₁ ) G ₂ (5)

From the above equations (3), (4) and (5), the frequency deviation ΔF of the output signal of the VCO 20 is calculated.
Eliminating ₂ and the frequency deviation ΔF ₃ of the intermediate frequency signal,
When the relationship between the frequency deviation ΔF ₁ of the input signal and the output amplitude V _o of the audio signal obtained from the output terminal 16 is obtained, V _o = K ₁ · G ₁ · ΔF ₁ / (1 + K ₁ · K ₂ · G ₂ ) ...... (6), and this equation (6) is an FM that employs the FM negative feedback method.
It is a basic equation showing the operation of the receiving device.

Next, from equation (4) and (6), when determining the relationship between the frequency shift [Delta] F ₁ and the frequency deviation [Delta] F ₃ of the intermediate frequency signal of the input signal, ΔF ₃ = ΔF ₁ / ( 1 + K ₁・ K ₂・ G ₂ ) ... (7) In this expression (7), the frequency deviation ΔF ₃ of the intermediate frequency signal is 1 / (1 of the frequency deviation ΔF ₁ of the input FM signal.
+ K ₁ · K ₂ · G ₂ ) times more compressed.

By the way, considering only the above description,
As the gain of the negative feedback loop is increased, the threshold level seems to improve forever, but in reality, there is noise that circulates through the FM negative feedback loop,
This noise increases as the loop gain increases,
The disturbance deteriorates the threshold. Therefore, the modulation degree and frequency of the input FM signal, and the VCO
There is an optimum loop gain determined by the linearity of 20 and the like, and the best threshold level will be obtained at that time.

Next, considering the application of the FM negative feedback system as described above to satellite broadcasting, for example, in the 4 GHz band SH.
In F broadcasting (communication), a 4 GHz carrier is FM-modulated and transmitted by a composite signal of an image signal and an FM-modulated audio subcarrier. Of these, a plurality of voice subcarriers exist in the range of 5.0 to 8.5 MHz, and the user selects and receives the desired voice subcarrier from among them.

Therefore, in the FM receiving apparatus described above, as described above, the audio tuning PLL circuit 26 causes the LPF to operate.
By controlling the oscillation frequency of the VCO 20 with a voltage obtained by adding the tuning voltage V _T output from the circuit 25 to the output voltage of the DC cutoff circuit 18, FM negative feedback and voice tuning are performed simultaneously. There is.

However, in the conventional FM receiver using such an FM negative feedback system, the degree of modulation exceeds a certain value (which is determined by the dynamic range of the BPF circuit 13 and the FM demodulation circuit 14), that is, , When the overmodulation is requested, the LF circuit 1 which applies the FM negative feedback
Since the amount of feedback of 7 is small, there is a problem that the audio output is distorted.

However, even if this is the case, if an amplifier circuit or the like is inserted and amplification is performed in order to increase the feedback amount of FM negative feedback, negative feedback is applied to the low frequency range, and the audio tuning PL is used.
The low-frequency DC component of the L circuit 26 collides with the band-passed AC (alternating current) component of the FM negative feedback loop circuit, resulting in the inconvenience of not operating properly. Also, since analog tuning is adopted, when FM negative feedback is applied, loop negative feedback is repeated in the state where the AC component of the negative feedback is mismatched at the time of sound voice tuning, resulting in large voice distortion and tuning. It becomes difficult to remove.

[0022]

As described above, in the conventional FM receiver using the FM negative feedback system, when the overmodulation is required to have a certain value or more (determined by the BPF circuit and the dynamic range), the FM Since the amount of feedback is small, there is a problem that the demodulation output waveform is distorted. Also, although bandwidth switching is performed in the receiver, if the negative feedback loop is operated regardless of this bandwidth switching, overmodulation can be tolerated when wideband (wideband) is selected. However, when narrow band (narrow band) is selected, when negative feedback is applied, there is a problem that adjacent audio subcarriers jump in when receiving stereo broadcasting. Furthermore, regardless of bandwidth switching. If FM negative feedback is applied during audio tuning, the audio subcarrier frequency will be several hundred KH above the tuning frequency.
The value may be shifted by z, and the distortion of the sound becomes large, and a loud sound such as “blur” is generated.

Therefore, the present invention has been made in consideration of the above circumstances, and obtains a demodulated output waveform with less distortion even if overmodulation is requested, and further switches the bandwidth to select a narrow band to receive a stereo broadcast. An FM receiver that can prevent noise from occurring even when adjacent audio subcarriers jump in at the same time, and can eliminate analog distortion and facilitate analog tuning when a user selects a desired audio subcarrier The purpose is to provide.

[0024]

According to the present invention, the output of a voltage controlled oscillator is multiplied by an input FM signal to be converted into an intermediate frequency signal, the intermediate frequency signal is band limited, and the band limited output is FM demodulated. The FM demodulation means and the output of the voltage controlled oscillator are frequency-divided by a frequency division ratio according to a channel selection request, the frequency division output and the oscillation output from the reference oscillator are phase-compared, and an error signal thereof is obtained. The unnecessary high frequency component of the error signal is limited to obtain the low frequency component, and the phase locked loop for feeding back to the control terminal of the voltage controlled oscillator and the output of the FM demodulating means are subjected to band limitation and DC cut. In an FM receiver including a negative feedback loop that feeds back to a voltage controlled oscillator, the first switch means provided in the negative feedback loop and the output of the FM demodulation means are output to a high pass filter having a large time constant. Fixed A second switch means which can be supplied to one of a small high-pass filters,
When the means for amplifying the output of each high-pass filter through the low-pass filter and the FM demodulation means are switched to the wide band selection state, the first switch means is turned off and the second switch means is turned off. When the switch means is switched to the high-pass filter selection state of the smaller time constant and is switched to the narrow band selection state, the first switch means is turned on and the second switch means is switched to the time constant. And a means for switching to a high-pass filter selection state of the larger one. Furthermore, in the present invention, in addition to the above configuration, a mute means for muting at the final output stage is provided at the time of a sound subcarrier channel selection operation, and a means for forcibly turning off the first switch is provided. There is.

[0025]

According to the above-mentioned structure, when the mode is switched to the wide band selection state, the HPF circuit having the smaller time constant is selected, and the FM negative feedback loop is turned on to increase the low frequency gain. The minute part is suppressed by the HPF circuit having the smaller time constant, and the frequency characteristic is flattened. As a result, even in the case of overmodulation, it is possible to receive the signal without a problem in wideband selection. When the narrow band selection state is selected, the HPF circuit with the larger time constant is selected, and the FM
Since negative feedback is not applied, if the time constant in the subsequent stage is small, the gain in the low frequency range is lowered. Therefore, the whole frequency characteristic is made flat by the HPF circuit with the larger time constant. As a result, when stereo is received, even if the narrow band is selected, adjacent audio subcarriers do not jump in to generate noise. Further, by providing a means for forcibly turning off the FM negative feedback loop at the time of selecting the channel of the voice subcarrier, a large voice noise does not occur at the time of channel selection, and analog tuning becomes easy.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention. The same parts as those of the circuit of FIG. 3 are designated by the same reference numerals. Therefore, the description will focus on the different parts. That is, by switching between the DC cutoff circuit 18 and the adder circuit 19 between the ON state and the OFF state, D
A switch 31 that allows and blocks the output of the C cutoff circuit 18 to be supplied to the adder circuit 19 is inserted and connected.

Further, the following circuit is provided between the output terminal of the FM demodulation circuit 14 and the amplification circuit 15. That is, the output end of the FM demodulation circuit 14 is connected to the input end of the switch 32, one output end of this switch 32 is connected to the high pass filter (HPF) circuit 33 having a large time constant, and the other output. The end is connected to a high pass filter (HPF) circuit 34 having a small time constant. Switch 3
An output is obtained from either one of the HPF circuits 33 and 34 by the selection operation of No. 2, and this output is input to the low pass filter (LPF) circuit 35, and this LPF circuit 35.
Is output to the amplifier circuit 15.

In the above system, the switch 31,
32 is controlled as follows. That is, when the broadband selection state is switched, the switch 31 is turned on and the switch 3 is turned on.
2 is controlled to select the HPF circuit 34 having the smaller time constant. In this state, the FM negative feedback loop is turned on, so that the low-frequency gain is increased.
Therefore, the increase in the low-frequency gain of the FM demodulation output is suppressed by the HPF circuit 34 having the smaller time constant, and the frequency (f)
The characteristics are flat. Therefore, in the case of overmodulation, the signal can be received without any problem even if the narrow band (for example, 300 KHz) is selected.

On the contrary, when the narrow band selection state is selected, the switch 31 is turned off and the switch 32 is controlled to select the HPF circuit 33 having the larger time constant. In this state, FM negative feedback is not applied. Then, when the time constant in the subsequent stage is small, the gain in the low frequency band is lowered. Therefore, the HPF circuit 33 having the larger time constant is selected to flatten the entire frequency characteristic. Therefore, when receiving stereo, a narrow band (eg 150
Even in the KHz) selection state, adjacent audio subcarriers do not jump into each other to generate a noise sound.

As described above, according to this embodiment, distortion of the audio output level does not occur due to overmodulation,
In addition, even when a stereo broadcast is received, there is no occurrence of an interfering sound due to the adjacent audio subcarriers.

FIG. 2 shows another embodiment of the present invention. FIG.
The same parts as those of the circuit of FIG. Therefore,
The different parts of the configuration will be mainly described. That is, a mute circuit 36 that can mute sound is added between the amplifier circuit 15 and the output terminal 16.
Further, in this embodiment, means for controlling the switches 31, 32 and the mute circuit 36 is also clarified. That is, 4
Reference numeral 0 denotes a microcomputer as a system control unit, which has a band switching control unit 41 and a tuning control unit 42. The band switching control unit 41 can control the switches 31 and 32 in response to the band switching operation, and the tuning control unit 42 can control the switch 31 and the mute circuit 36 in response to the tuning operation. it can.

The system described above operates as follows.
The switching of the band width is performed by the output of the band switching control unit 41, and the operation at this time is controlled by the switches 31 and 32 as in the previous embodiment. That is, when switched to the wide band selection state, the switch 31 turns on and the switch 32 selects the HPF circuit 34 having the smaller time constant. The increase in the low frequency gain due to the turning on of the FM negative feedback loop is suppressed by the HPF circuit 34 having the smaller time constant, and the frequency characteristic is flattened. As a result, even in the case of overmodulation, it is possible to receive the signal without a problem in wideband selection.

When switched to the narrow band selection state, the switch 31 is turned off and the switch 32 selects the HPF circuit 33 having the larger time constant. In this state,
Since FM negative feedback is not applied, the low-frequency gain decreases when the time constant in the subsequent stage is small. Therefore, the HPF circuit 33 having the larger time constant is selected to flatten the overall frequency characteristic. As a result, when stereo is received, even in the narrow band selection state, adjacent audio subcarriers do not jump into and generate a noise sound.

Next, in this embodiment, a function of controlling the switch 31 is added even when the voice subcarrier channel selection operation is performed regardless of band switching. That is, when a voice subcarrier selection operation is performed in the wideband selection state, the switch 31 in the on state is turned off, then the channel selection operation is performed, and when the channel selection is completed, the switch 31 is turned on. .
In this way, the mute circuit 36 is controlled and the audio mute is applied until the switch 31 is turned off and then turned on. Next, when the voice subcarrier selection operation is performed in the narrow band selection state, the switch 31 in the off state
The channel selection operation is performed when the mute circuit 36 is controlled and the audio mute is performed while maintaining the above condition.
When the tuning is finished, the audio mute is released and the switch 31 is kept in the off state.

As described above, in addition to the above embodiment, this embodiment has means for forcibly turning off the FM negative feedback loop at the time of tuning operation, so that voice noise is generated at the time of tuning. Instead, analog tuning becomes easier.

[0036]

As described above, according to the present invention,
Even if overmodulation is required, a demodulated output waveform with little distortion is obtained,
Furthermore, it is possible to prevent noise even when adjacent audio subcarriers jump in when a narrow band is selected by narrow band selection and stereo broadcasting is received, and when the user selects a desired audio subcarrier. It can eliminate analog distortion and make analog tuning easier.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an FM receiver according to the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the present invention.

FIG. 3 is a circuit diagram showing a conventional FM receiver.

[Explanation of symbols]

11 ... Input terminal, 12 ... MIX circuit, 13 ... BPF circuit, 14 ... FM demodulation circuit, 15 ... Amplification circuit, 16 ... Output terminal, 17 ... Loop filter (LF) circuit, 18 ... DC
Cutoff circuit, 19 ... Addition circuit, 20 ... Voltage controlled oscillator (V
CO), 21 ... Feedback loop, 22 ... Variable frequency dividing circuit, 23 ... Phase comparison circuit, 24 ... Local oscillation circuit,
25 ... Low-pass filter (LPF) circuit, 26 ... Audio tuning PLL circuit, 27 ... Switch, 31, 32 ... Switch, 33, 34 ... High-pass filter (HPF) circuit, 3
5 ... Low-pass filter (LPF) circuit, 36 ... Mute circuit, 41 ... Band switching control unit, 42 ... Tuning control unit.

Claims (2)

[Claims] 1. An FM for multiplying an output of a voltage controlled oscillator and an input FM signal to convert the signal into an intermediate frequency signal, band limiting the intermediate frequency signal, and FM demodulating the band limited output.
The demodulation means and the output of the voltage controlled oscillator are divided by a division ratio according to a tuning request, and the phase comparison between the divided output and the oscillation output from the reference oscillator is performed to obtain the error signal. Of the undesired high frequency component to obtain the low frequency component and feed back to the control terminal of the voltage controlled oscillator, and the output of the FM demodulating means is band-limited and the direct current cut is performed to perform the same voltage control. In an FM receiver including a negative feedback loop for feeding back to an oscillator, a first switch means provided in the negative feedback loop and an output of the FM demodulation means are used as a high-pass filter having a large time constant and a time constant. Second switch means for supplying to one of the small high-pass filters, means for amplifying the output of each high-pass filter by passing it through a low-pass filter, and the FM demodulating means for selecting a wide band. When the switch is switched to, the first switch means is turned off and the second switch means is switched to the high pass filter selection state of the smaller time constant, and when switched to the narrow band selection state. An FM receiver, comprising means for turning on the first switch means and switching the second switch means to a high-pass filter selection state having a larger time constant. 2. An FM for multiplying an output of a voltage controlled oscillator and an input FM signal to convert into an intermediate frequency signal, band limiting the intermediate frequency signal, and FM demodulating the band limited output.
The demodulation means and the output of the voltage controlled oscillator are divided by a division ratio according to a tuning request, and the phase comparison between the divided output and the oscillation output from the reference oscillator is performed to obtain the error signal. Of the undesired high frequency component to obtain the low frequency component and feed back to the control terminal of the voltage controlled oscillator, and the output of the FM demodulating means is band-limited and the direct current cut is performed to perform the same voltage control. In an FM receiver including a negative feedback loop for feeding back to an oscillator, a first switch means provided in the negative feedback loop and an output of the FM demodulation means are used as a high-pass filter having a large time constant and a time constant. Second switch means for supplying to one of the small high-pass filters, means for amplifying the output of each high-pass filter by passing it through a low-pass filter, and the FM demodulating means for selecting a wide band. When the switch is switched to, the first switch means is turned off and the second switch means is switched to the high pass filter selection state of the smaller time constant, and when switched to the narrow band selection state. , Means for switching on the first switch means to switch the second switch means to a high-pass filter selection state with a larger time constant, and audio mute during a channel selecting operation period of the audio subcarrier. And an FM receiver including means for forcibly turning off the first switch means.