JPH04585Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention relates to an automatic frequency control device used to stabilize the oscillation frequency of a local oscillation section, and particularly relates to an improvement of an integrating circuit within the automatic frequency control device.

<Prior Art> Conventionally, the above-mentioned automatic frequency control device includes one shown in FIG. 3, which is used, for example, in a satellite broadcast receiving device. In the figure, reference numeral 2 denotes a frequency mixing section which mixes an input signal, which is an FM modulated wave, with a local oscillation signal from a local oscillation section 4, and converts it into an intermediate frequency signal. Reference numeral 6 denotes an intermediate frequency amplifying section that amplifies the intermediate frequency signal from the frequency mixing section 2 and supplies it to the frequency discriminating section 8 . The frequency discriminator 8 performs FM demodulation on the intermediate frequency signal. This FM demodulated signal is supplied to the AFC circuit 10. The AFC circuit 10 generates an error voltage representing how much the intermediate frequency signal deviates from the normal frequency of the intermediate frequency signal based on the FM demodulated signal. This error voltage is supplied to an integrating circuit 12, and the output voltage of this integrating circuit 12 is supplied to the local oscillation section 4. The local oscillation section 4 is configured to be able to change the oscillation frequency according to the output voltage of the integrating circuit 12. Therefore, even if the frequency of the local oscillation signal of the local oscillator 4 fluctuates due to changes in ambient temperature, etc., the oscillation frequency can be automatically maintained at the normal frequency.

<Problems to be Solved by the Invention> As the integrating circuit 12, for example, as shown in FIG. 4, one consisting of a resistor 14 and a capacitor 16 is used. The time constant of the integrating circuit 12 is set so that the rise and fall of the error voltage of the AFC circuit 10 can be promptly transmitted to the local oscillation section 4, and the frequency fluctuation of the local oscillation signal can be corrected promptly.
Smaller is preferable. Particularly in satellite broadcast receivers, fluctuations in the frequency of local oscillation signals cause blurring on the screen, so it is necessary to promptly correct the fluctuations. On the other hand, since the error voltage of the AFC circuit 10 includes unnecessary components such as video signals,
In order to prevent the frequency of the local oscillation signal from fluctuating due to these factors, it is desirable that the time constant of the integrating circuit 12 be large. That is, there is a problem in that the integrating circuit 12 needs to satisfy two contradictory requirements.

<Means for Solving the Problems> In order to solve the above problems, this invention uses a local oscillation section configured so that the frequency of the local oscillation signal changes depending on the magnitude of the control voltage, and an input signal. a mixer that mixes the input signal with the local oscillation signal to convert the input signal into an intermediate frequency signal; a circuit that generates an error voltage representing the frequency deviation of the intermediate frequency signal with respect to the normal intermediate frequency; It includes an integrating circuit that supplies the output voltage as a control voltage to the local oscillator. Then, the integrating circuit connects two resistors in series between the input terminal and the output terminal, connects a capacitor between the output terminal and the reference potential point, and connects one of the two resistors to the other. Diodes are connected in antiparallel.

<Function/Effect> When the error voltage supplied to the input terminal of the integrating circuit rises, the capacitor is not charged, so the potential difference between the two terminals of the two diodes increases, and one diode becomes conductive. This diode and the resistor connected in parallel are shorted. Therefore, the time constant of this integrating circuit is a small value obtained by multiplying the value of the unshorted resistor by the value of the capacitor. Eventually, as the capacitor is charged, the potential difference between the ends of the conducting diode becomes smaller, and the conducting diode becomes non-conducting. As a result, the time constant of the integrator circuit is 2
A large value is obtained by multiplying the sum of the resistance values of the two resistors and the value of the capacitor. When the error voltage falls, the potential difference across the two diodes increases, the other diode becomes conductive, and the resistor connected in parallel with this diode is short-circuited, causing the integrator circuit to open in the same way as described above. The time constant becomes smaller. Therefore, the time constant of this integrating circuit quickly responds to the rise and fall of the error voltage, and quickly corrects fluctuations in the oscillation frequency of the local oscillator. Furthermore, since the time constant from the rise to the fall is large, it is possible to prevent the local oscillation frequency from varying in response to unnecessary components such as a video signal. Moreover, the circuit configuration for changing the time constant in this way consists of two anti-parallel connected diodes connected in parallel to one of the two series resistors of the integrating circuit. There is no need to provide a switch or the like that is controlled on/off by a comparator, and the circuit configuration is simplified.

<Embodiment> FIG. 1 is a circuit diagram of an integrating circuit of an embodiment of the automatic frequency control device according to this invention, and this integrating circuit is also used in the same way as the conventional integrating circuit 12 shown in FIG. 3. . That is, the input terminal 20 has an AFC
An error voltage is supplied from the circuit 10, and the output terminal 22
The output voltage is supplied to the local oscillator 4 as a control voltage.

Two resistors 24 and 26 are connected in series between the input terminal 20 and the output terminal 22. A capacitor 28 is connected between the output terminal 22 and the reference potential point. Diodes 30 and 32 are connected in antiparallel between both ends of the resistor 24. That is, the diode 30 has its cathode connected to the input terminal 20 and its anode connected to the resistor 2.
The diode 32 has its anode connected to the input terminal 20 and its cathode connected to the connection point of the resistors 24 and 26, respectively.

In this integrating circuit, when the capacitor 28 is in a steady state, the time is determined as shown in FIG. 2a.
V ₁ (>V _A ) is maintained from t ₁ to time t ₂ , and thereafter V ₂
Input the step voltage V _i that maintains (<V _A ) to the input terminal 2.
0 as an error voltage, a large forward voltage drop occurs between both ends of the diode 32 at time _t1 , and the diode 32 becomes conductive. As a result, the time constant of the integrating circuit becomes a small value obtained by multiplying the value of the resistor 26 by the value of the capacitor 28, and the output voltage _V0 at the output terminal 22 rises rapidly as shown by the symbol A in FIG. 2b. . Eventually, capacitor 2
8 becomes large, for example V ₁ ', the potential difference across the diode 32 becomes small, the diode 32 becomes non-conducting, and the time constant of this integrating circuit becomes equal to the voltage of the resistors 24 and 26. It is a value obtained by multiplying the sum of the resistance values by the value of the capacitor 28, and is larger than the time constant when the diode 32 is conductive. As a result, it changes gradually as shown by the symbol B in FIG. 2b.

When the input voltage V _i drops to V ₂ at time t ₂ ,
A large potential difference is generated in the diode 30 in the opposite direction to that when the diode 32 was conductive, and the diode 30 becomes conductive. As a result, the time constant of this integrator circuit is again the value of resistor 26 and capacitor 2.
8, and the output voltage V ₀ falls rapidly, as shown by the symbol C in FIG. 2b. Eventually, when the voltage across the capacitor 28 reaches V ₂ ', the voltage across the diode 30 decreases and the diode 30 becomes non-conductive. As a result, the time constant of this integrator circuit is again
The value obtained by multiplying the resistance value of 26 by the value of capacitor 28 is obtained, and the output voltage V ₀ changes gradually as shown by the symbol D in b in FIG.

As described above, in this embodiment, frequency fluctuations can be corrected by rapidly responding to the rise or fall of the error voltage. Thereafter, even if unnecessary components such as video signals are included in the error voltage, the frequency will not fluctuate due to this.

In the above embodiment, this invention was implemented in an automatic frequency control device for stabilizing the local oscillation signal of a satellite broadcast receiver, but it was also implemented in an automatic frequency control device for stabilizing the local oscillation signal of a television receiver. You may. In that case, the video carrier wave is extracted from the video intermediate frequency amplification circuit, the amplitude modulation part is removed by a limiter, and the change in the video carrier frequency is extracted as a change in DC voltage by FM detection, which is then supplied to the integrating circuit. Bye.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of an integrating circuit of an embodiment of the automatic frequency control device according to this invention, Figures 2a and b are diagrams showing the waveforms of the input voltage and output voltage of the circuit of Figure 1, and Figure 3 is FIG. 4 is a block diagram of a conventional automatic frequency control device. FIG. 4 is a circuit diagram of an integrating circuit used in the device of FIG. 2... Frequency mixing section, 4... Local oscillation section, 10
...AFC circuit, 12 ... Integrating circuit, 20 ... Input terminal, 22 ... Output terminal, 24, 26 ... Resistor, 28 ... Capacitor, 30, 32 ... Diode.

Claims (1)

[Scope of utility model registration request] a local oscillation unit configured to change the frequency of the local oscillation signal according to the magnitude of the control voltage; and a configuration configured to mix the input signal and the local oscillation signal to convert the input signal into an intermediate frequency signal. a mixing section that generates an error voltage representing a frequency deviation of the intermediate frequency signal with respect to a normal intermediate frequency, and an integrator that receives the error voltage and supplies the output voltage to the local oscillator section as the control voltage. The integrating circuit includes two resistors connected in series between an input terminal and an output terminal, a capacitor connected between the output terminal and a reference potential point, and the two resistors. An automatic frequency control device comprising two diodes connected in antiparallel across one end of the device.