JPH0752809B2 - FM demodulation circuit - Google Patents

Description

The present invention relates to a configuration of an FM demodulation circuit for demodulating an FM modulated signal, and particularly to a phase locked loop (Phase Locked Loop) suitable for demodulation in a weak electric field such as satellite broadcasting. Loop; hereinafter abbreviated as PLL) The present invention relates to an FM demodulation circuit.

[Background of the Invention]

As a method of improving the threshold level of a conventional PLL / FM demodulation circuit, for example, as disclosed in Japanese Patent Laid-Open No. 57-171808, a wide band detection characteristic is secured by an optimum circuit constant in a medium and high electric field, and noise of PL in case of weak electric field
It is known that the demodulation band is narrowed by controlling the amplifier gain of L. The example shown here is FM
Although it relates to a radio broadcast receiver, the same applies to a satellite broadcast receiver. However, in order to further improve the threshold level, it is necessary to improve the threshold level of the demodulation band before narrowing the band and use a means for narrowing the band. In addition, satellite receivers
Since the S / N is almost determined not by the electric field level of the signal input to the FM demodulation circuit but by the C / N of the microwave signal received by the antenna, the narrowing band switching of the demodulation band shown in the example can be used. There was a problem of operating by the C / N of the signal instead of the method by the electric field level of the signal.

[Object of the Invention]

An object of the present invention is to solve the above-mentioned drawbacks of the PLL / FM demodulation circuit and to provide an FM demodulation circuit which can improve the threshold level in a weak electric field with a simple configuration.

[Summary of the Invention] In the present invention, the threshold level is improved by increasing the loop gain of the PLL in the demodulation band by feeding back the demodulation signal in the demodulation circuit, and input C / PLL by controlling the above feedback corresponding to N
The loop gain is controlled to improve the threshold level.

Example of Invention

An embodiment of the present invention will be described below with reference to FIG. The FM intermediate frequency signal is input from the input terminal 1 to form the PLL / FM demodulation circuit by the closed loop of the phase detector 2, the differential amplifier 3, the loop filter 4 and the voltage controlled oscillator 5, and the other output of the differential amplifier 3 It is fed back to the constant current source of the differential amplifier 3 via the amplifier 6, the output of the loop filter 4 is branched and connected to the loop gain control circuit 7, the output of the loop gain control circuit 7 is applied to the amplifier 6, and the loop The demodulated television signal is taken out from the output terminal 8 from the other branch of the output of the filter 4. The output of the phase detector 2 is band-shaped by the feedback amplifier including the differential amplifier 3 and the amplifier 6 with the frequency characteristic of the amplifier 6 and amplified. In addition, the loop gain control circuit 7 outputs from the output of the loop filter 4, that is, the demodulation output.
A control voltage corresponding to the C / N of the received signal obtained through is input to the amplifier 6 to control the demodulation bandwidth of the PLL / FM demodulation circuit.

FIG. 2 shows a circuit example of a feedback amplifier composed of the differential amplifier 3 and the amplifier 6 in the block diagram of FIG. 1. The differential amplifier 3 has transistors Q ₁ and Q having a constant current source of transistor Q _{3. It} consists of the differential amplifier circuit of ₂ and the emitter / follower of the transistor Q _6. The output of the phase detector 2 is connected to the base 9 of the transistor Q ₁ , the base of the transistor Q ₂ is grounded, and the collector of the transistor Q ₁ is connected. Transistor Q ₆
This output terminal outputs a signal through the emitter follower of
Connect 10 to loop filter 4 and connect the collector of transistor Q _{2 to} the input of amplifier 6. The amplifier 6 is composed of an amplifier circuit composed of transistors Q ₄ and Q ₅ , and has a differential amplifier 3
The output of is connected to the base of transistor Q ₄ through filter 11, and the collector of transistor Q ₄ is connected to transistor Q ₅
Is connected to the base of the transistor Q ₅ , and the collector output of the transistor Q ₅ , that is, the output of the amplifier 6 is connected to the transistor Q 3 of the differential amplifier _3.
Connected to the base of. Also, the collector of transistor Q ₄ is grounded by capacitor 12 and diode 13,
The output of the loop gain control circuit 7 is applied to the connection point between the diode 12 and the diode 13 from the terminal 14 via the short circuit control circuit 15.
In this circuit, since the collector voltage of the transistor Q ₂ is applied to the base of the transistor Q ₃ via the amplifier 6, the base voltage of the transistor Q ₃ fluctuates in proportion to the collector voltage of the transistor Q _{2. 3}
Similarly, the collector current of the transistor Q ₂ also fluctuates with respect to the collector voltage of the transistor Q ₂ , and the collector voltage of the transistor Q ₁ , that is, the output of the differential amplifier 3 increases. Performs an operation that increases the gain. Assuming that the gain of the amplifier 6 is sufficiently large, the rate of increase in gain is about twice as high as that of the amplifier 6 without feedback. By this feedback operation, a steep band characteristic can be obtained by combining the filter 11 and the loop filter 4 in the subsequent stage.

Figure 3 shows the open loop characteristics of the PLL, with the vertical axis representing the PLL.
The open loop gain is represented by the frequency on the abscissa, and is a characteristic 15 in which feedback by the amplifier 6 is not applied and a characteristic 16 in which feedback is not applied. Amplifier 6
When the feedback is applied by, the skirt characteristic of the band is not widened even if the closed loop gain is increased, and the noise bandwidth can be narrowed, and the demodulation characteristic of a wide band is obtained in a normal reception electric field and the threshold level is improved. In a weak electric field near the threshold where noise is a problem, the closed loop gain of the PLL becomes a low gain of characteristic 15 by stopping the feedback by the amplifier 6, and the demodulation range can be narrowed to further improve the threshold level. it can. In order to stop the feedback of the amplifier 6 in this embodiment, when the control voltage generated corresponding to the C / N of the signal from the loop gain control circuit 7 exceeds an appropriate threshold value, the short circuit control circuit 15 changes the DC voltage to the diode 13 Can be easily performed by turning on the diode 13 and grounding the signal with the capacitor 12, and the capacity of the capacitor 12 may be sufficient to ground the demodulated signal, for example, 1000 PF or more. The diode 13 and the capacitor 12 may be arranged at the base of the transistor Q ₄ , the collector of the transistor Q ₅ , or the like, or at a portion where a signal is detected by the amplifier 6 to obtain the same effect. Also,
Placement of the filter 11 effects be disposed to the collector of the base or the transistor Q ₅ of the transistor Q ₅ are the same.

FIG. 4 shows a specific example of the loop gain control circuit 7. The demodulated signal input to the terminal 17 is branched, one is input to the synchronization pulse generation circuit 18, and its output and the other demodulated signal are It is input to the sampling circuit 19, and a detection circuit 21 generates a control voltage through the amplification circuit 20 to control the amplifier 6.

FIG. 5 is an operation principle diagram of the loop gain circuit 7 of FIG.
From the demodulated signal a containing noise, a pulse signal b matching the synchronizing signal is generated by the synchronizing pulse generating circuit 18, and the pulse signal b
The sampling circuit 19 extracts the noise component 22 superimposed on the synchronization signal from the demodulation signal a, the amplification circuit 20 amplifies the noise component 22, and the detection circuit 21 detects the amplified noise component to detect the C / N of the received signal. DC voltage corresponding to is generated. Although the horizontal synchronizing signal is used in this embodiment, it is obvious that the vertical synchronizing signal can be used as well.

FIG. 6 shows another specific example of the feedback amplifier circuit including the differential amplifier 3 and the amplifier 6. The same reference numerals as those in FIG. 2 denote the same or corresponding portions, and the collector of the transistor Q ₄ is connected to the capacitor 12
And the variable capacitance diode 23 to ground, and a DC voltage corresponding to C / N of the received signal is applied from the loop gain control circuit 7 to the connection point between the capacitor 12 and the variable capacitance diode 23. The ground capacitance is changed by changing the capacitance of the variable capacitance diode 23, and the open loop gain of the PLL shown in FIG. 3 is continuously changed from the characteristic 16 to the characteristic 15 to prevent a sharp change in the image quality. You can The same effect can be obtained by arranging the capacitor 12 and the variable capacitance diode 23 not only at the base of the transistor Q ₄ or the collector of the transistor Q ₅ but also at a portion where a signal is detected by the amplifier 6.

〔The invention's effect〕

According to the present invention, by feeding back the demodulation signal in the demodulation circuit, the loop gain of the PLL in the demodulation band is increased to improve the threshold level, and the input C / N is dealt with in the weak electric field which causes a noise problem. By controlling the feedback described above, the loop gain of the PLL is controlled, the threshold level is improved, and good image quality can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an FM demodulation circuit according to the present invention, FIG. 2 is a circuit diagram showing an example of a feedback amplifier, and FIG.
Fig. 4 is a characteristic diagram showing the open loop gain of the PLL, Fig. 4 is a block diagram showing an example of the loop gain control circuit, Fig. 5 is an operation principle diagram of the loop gain control circuit, and Fig. 6 is another example of the feedback amplifier. It is a circuit diagram showing. 2 ... Phase detector 3 ... Differential amplification circuit 4 ... Loop filter 5 ... Voltage controlled oscillator 6 ... Amplification circuit 7 ... Loop gain control circuit

Claims (3)

[Claims] 1. A voltage-controlled oscillator, and a phase detector which inputs an FM signal to be demodulated and an oscillation signal from the voltage-controlled oscillator, compares the phases of both signals, and outputs a signal according to the difference. A differential amplifier that takes in and amplifies and outputs the output of the detector, and one of the differential outputs from the differential amplifier is taken in and filtered to the voltage controlled oscillator as an oscillation frequency control voltage of the voltage controlled oscillator. A loop filter for supplying, and a phase-locked loop type FM demodulation circuit for extracting a demodulated signal from the output of the loop filter, in which the other of the differential outputs from the differential amplifier is filtered and output. A circuit, an amplifier circuit that takes in and amplifies and outputs the output of the filter circuit, and an amplified output from the amplifier circuit is fed back to a constant current source that constitutes the differential amplifier. A feedback circuit that controls the band characteristic of demodulation, the output of the loop filter is branched and taken in, the C / N of the FM signal to be demodulated is detected, and a loop gain that outputs a control voltage corresponding to the C / N An FM demodulation circuit comprising: a control circuit; and a short circuit that detects when the control voltage from the loop gain control circuit exceeds a certain threshold value and short-circuits the feedback circuit in an alternating current manner. . 2. A voltage-controlled oscillator, and a phase detector which inputs an FM signal to be demodulated and an oscillation signal from the voltage-controlled oscillator, compares the phases of both signals, and outputs a signal according to the difference. A differential amplifier that takes in and amplifies and outputs the output of the detector, and one of the differential outputs from the differential amplifier is taken in and filtered to the voltage controlled oscillator as an oscillation frequency control voltage of the voltage controlled oscillator. A loop filter for supplying, and a phase-locked loop type FM demodulation circuit for extracting a demodulated signal from the output of the loop filter, in which the other of the differential outputs from the differential amplifier is filtered and output. A circuit, an amplifier circuit that takes in and amplifies and outputs the output of the filter circuit, and an amplified output from the amplifier circuit is fed back to a constant current source that constitutes the differential amplifier. A feedback circuit that controls the band characteristic of demodulation, the output of the loop filter is branched and taken in, the C / N of the FM signal to be demodulated is detected, and a loop gain that outputs a control voltage corresponding to the C / N A control circuit and a grounding short-circuit capacitance for AC short-circuiting the feedback circuit, to which a control voltage from the loop gain control circuit is applied, and the capacitance value is continuous depending on the applied voltage value. An FM demodulation circuit, comprising: 3. The method according to claim 1 or 2
In the FM demodulation circuit, the loop gain control circuit
It is composed of a circuit that extracts and detects the noise component superimposed on the horizontal sync signal or vertical sync signal in the video signal demodulated by the FM demodulation circuit and outputs the voltage corresponding to the noise level as a control voltage. FM demodulator circuit.