JPH0759060B2 - FM television receiver - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an FM television receiver suitable as a second heterodyne receiver of a satellite broadcast receiver, and more particularly to a received television FM signal. The present invention relates to an AFC that controls the oscillation frequency of a local oscillator in a frequency converter.

[Background of the Invention]

In satellite broadcasting that directly receives television broadcasting from geostationary satellites, a modulation method mainly used for FM modulation is used, and this FM modulation method often uses a method in which the average value of the modulated signal is a constant frequency.

However, there is a limit to the transmission band, and when the FM signal spectrum is prevented from jumping out of the transmission band when the average value voltage of the modulated signal changes rapidly, for example, when the black screen changes to a white screen, or In order to prevent the reference DC voltage necessary for reproduction with a different video signal from moving according to the content of the video signal, an FM modulation method is used in which the FM signal of the period to be the reference of the modulated signal is fixed to a constant frequency.

An example of this modulation type receiving circuit is shown in FIGS. 3 and 5 of JP-A-57-135582. This figure shows a configuration for stabilizing the local oscillation frequency of the second heterodyne receiver of the satellite broadcast receiver, and the stable local oscillation frequency can be controlled during steady reception and when the reference frequency constant period is long. Although it can be done, the local frequency control is disturbed and the recovery is delayed when the power to the receiver is turned on or when the signal input is temporarily cut off. Further, there is a drawback that the control becomes incomplete when the reference frequency fixed period is short.

[Object of the Invention]

It is an object of the present invention to provide an FM television receiving device which solves the above-mentioned drawbacks of the prior art and enables stable local oscillation frequency control at all times.

[Outline of Invention]

In the present invention, the local oscillation frequency is controlled by the average value voltage of the demodulated video signal until the pulse signal corresponding to the signal of the reference frequency constant period is generated, or when the pulse signal is stopped, The sample control circuit is configured to perform sample control by the demodulated video voltage, and the sample control circuit performs sample control of both the reference demodulation voltage and the comparison voltage to ensure stable reception when the power is turned on and when the signal is temporarily cut off. Even if the fixed frequency period is short, the local oscillation frequency is controlled stably.

Example of Invention

Hereinafter, the present invention will be described in detail according to the embodiments shown in the drawings.

FIG. 1 is a configuration diagram showing an embodiment of an FM television receiving apparatus according to the present invention, and shows the configurations of a second heterodyne receiving circuit and a signal processing circuit of a satellite broadcast receiver.
Frequency mixing circuit 2 from a plurality of signals input from terminal 1
And the local oscillator circuit 3 which determines the oscillation frequency by the tuning voltage from the terminal 8 selects the desired signal and converts the frequency, and the band pass filter 4 removes interference signals other than the desired signal, and the FM demodulator circuit 5 The video signal is demodulated, and the video signal processing circuit 6 shapes the waveform of the video signal and outputs it from the terminal 7. The video signal processing circuit 6 outputs a pulse control signal b corresponding to a constant frequency period of the FM signal and a control signal c indicating whether or not this pulse control signal is generated, and the output demodulation signal a of the FM demodulation circuit 5 is integrated. When the pulse control signal b is not generated, a signal obtained by integrating the demodulation signal a is output, and when the pulse control signal b is generated, the demodulation signal a is input to the circuit 9. The output of the integrating circuit 9 is connected to a first sample-hold circuit composed of a switch 13 and a hold circuit 14, and the output of the first sample-hold circuit is input to a differential amplifier circuit 18, The voltage of the comparison voltage generator 15 is applied to the other input of switch 18
The output of the second sample hold circuit composed of the hold circuit 17 and the output of the differential amplifier circuit 18 is superimposed on the tuning voltage by the adder 19. Here, the control of the switches 13 and 16 of the first and second sample hold circuits is as follows.
The pulse control signal b and the fixed voltage of the fixed voltage circuit 12 are selected by the switch 11 controlled by the control signal c, and the output signal d of this switch 11 is used to perform the switch 13 and the switch 16
When the pulse signal b is not generated, the input / output is always conductive, and when the pulse signal b is generated, the input / output is conductive only for the period of the pulse signal b and cut off for the other periods.

FIG. 2 shows an example of a signal waveform of a television signal which is the reception processing target of this embodiment. This is an Ivy Axial and Development Report (IBA).
EXPERIMENTAL & DEVELOPEMENT Report) 5th of 116/81
The television signal called MULTIPLEXED ANALOGUE COMPONENT signal, abbreviated as Mac signal (MAC)
The period T ₁ is a reference period indicating the center voltage of the signal, and the period T ₂ is a signal period, which includes a color signal 20, a luminance signal 21, and a synchronization signal 22. The reference voltage in this period T ₁ is necessary for signal processing, and this period T ₁ is set as the FM modulation method on the transmission side so that the voltage in this period T ₁ does not change due to the signal change in period T _2. Modulation with a constant frequency is used regardless of the signal in the period T ₂ .

As the signal having such a reference voltage period, that is, a reference frequency period, there is a MUSE signal planned by the Japan Broadcasting Corporation (NHK) in addition to the above-mentioned Mac signal. When receiving an FM signal having these reference frequency periods, the receiver must also perform frequency control so that the reference frequency period becomes a constant frequency.

FIG. 3 shows a waveform example for explaining the operation of the embodiment shown in FIG. If FM signal received by the signal which changes the frequency of the video signal in the period T ₂ of the T ₂ at a constant frequency in the period T _1, in FIG. 1, the demodulated waveform a, which is the output of the FM demodulator circuit 5 during the period
The DC voltage 23 is obtained at T ₁ , the waveform of the video signal 24 is obtained during the period T ₂ , and the video signal processing circuit 6 generates the pulse control signal b corresponding to the period T ₁ from the synchronization information of the video signal 24. Further, the video signal processing circuit 6 outputs a control signal c corresponding to the time T ₀ until the pulse control signal b is generated when the power is turned on or when the signal is temporarily cut off due to channel selection or the like. Now, as an example, the pulse control signal b in the period T ₁
Of the control signal c is a high voltage and the other period is a low voltage.
When the voltage in the period T ₀ is high and the other periods are low, and the voltage of the fixed voltage circuit 12 is substantially the same as the high voltage of the pulse signal b in the period T ₁ , the output of the switch 11 is
At T ₀ , the voltage becomes a fixed voltage, and the pulse control signal b is output when the pulse control signal b is generated.

In the circuit operation in the period T ₀ , since the integral output is selected by the integrating circuit 9, the output waveform e of the integrating circuit 9 becomes the average value voltage of the demodulated signal a, and the switch 13 and the switch 16 are always conductive, so that the switch 13 Output f and output h of hold circuit 14
Is the above-mentioned average value voltage, and the output g of the switch 16 and the output i of the hold circuit 17 become the voltage of the comparison voltage generating circuit 15. The differential amplifier circuit 18 acts to output the average value voltage of the demodulated signal and the output of the comparison voltage generating circuit. The frequency of the local oscillator circuit 3 is controlled so that the voltages become equal. Therefore, even if the oscillation frequency of the local oscillation circuit 3 fluctuates due to temperature or the like, and even if the tuning is detuned, the average value AFC serves to quickly generate the demodulation signal a and smoothly generate the pulse control signal b. However, since the reference frequency changes with the average value of the video signal during this T ₀ period, the reference voltage 23 of the demodulation signal a changes, and the luminance signal of the video signal changes to the white or black side.

Next, when the pulse control signal b is generated, the output e of the integrating circuit 9 becomes the demodulation signal a and the output d of the switch 11 becomes the pulse control signal b, so that the switches 13 and 16 are in the T ₁ period of the pulse control signal b. only conductive, the reference voltage 25 only the demodulated signal a T ₁ period in the output f of the switch 13 becomes the other period T ₂ is cut off, the output g of the switch 16 the comparison voltage generating circuit
The voltage of 15 is output only for the period of T ₁ , and the hold circuit 14 and
The voltage of period T ₁ as the input h and i FIG differential amplifier circuit 18 for holding between the period T ₂ by 17, although the characteristics of discharging portions at period T ₂ is charged with period T ₁ The first sample-hold circuit and the second sample-hold circuit have the same charge-discharge time constant, so that the change in the output voltage of the differential amplifier circuit during the T ₂ period can be reduced.
That is, this circuit controls the oscillation frequency of the local oscillation circuit 3 so that the reference voltage 25 of the demodulated signal a during the T ₁ period and the output voltage of the comparison voltage generation circuit 15 become equal, and during the T ₂ period, when the hold circuit is discharged. By making the constant large and making the discharge time constants of the reference voltage and the comparison voltage the same, it is possible to significantly reduce the output voltage fluctuation of the differential amplifier circuit 18 and reduce the fluctuation of the oscillation frequency during the T ₂ period. it can. Here, the FM demodulation circuit 5, the integration circuit 9, and the comparison voltage generation circuit 15 which are connected to the first and second sample and hold circuits when the switch is conductive.
Since the charging time constant of the sample hold circuit can be made small by setting the output impedance of the sample output to a small value, a stable sampling AFC can be constructed even with a pulse signal of a short period due to the hold effect.

FIG. 4 shows a concrete circuit example of the AFC circuit constituent portion in FIG. The switch 26, the resistor 32, and the capacitor 33 form an integrating circuit. When the capacitor 33 is grounded by the switch 26, the integrated signal of the demodulation signal a is output to the base of the transistor Q ₂ , and the switch 26 disconnects the grounding of the capacitor 33. Then, the demodulated signal a is output to the base of the transistor Q ₂ . Here, the switch 26 can be easily realized by using an electronic switch having a low on resistance, for example, the forward characteristic of a diode. Further, according to this configuration, the DC level applied to the base of the transistor Q ₂ when the demodulated signal a is not modulated is the same for both the integrated output and the non-integrated output, so that level adjustment between both outputs is unnecessary. The transistor Q ₂ that outputs the demodulated signal or the integrated signal of the demodulated signal and the output transistor Q ₃ of the comparison voltage generation circuit function as a constant voltage circuit of the emitter follower to reduce the output impedance and are formed by this impedance and the capacitances 30 and 31 of the hold circuit. The charging time constant is reduced and charging is performed for a short period of conduction, and at the time of holding, the transistors Q ₄ , Q ₅ , Q ₆ and Q ₇ , Q ₈ , Q ₉ are connected in multiple stages to see from the capacitors 30 and 31. Since the impedance on the output side is increased, the discharge time constant is increased to reduce the voltage fluctuation during hold, and the discharge time constants of the two hold circuits are made the same, the transistors of the differential amplifier circuit are
The difference between the base voltages of Q ₁₀ and Q ₁₁ does not change, the voltage of the differential amplifier circuit output does not change, and a good hold effect can be obtained for a long period.

〔The invention's effect〕

As described above, according to the FM television receiver of the present invention, the AFC unit of the local oscillator specifies the demodulated signal in the period in which the pulse signal corresponding to the signal in the constant frequency period serving as the reference is generated. Since the frequency of the local oscillator is controlled by using the signal voltage of the period as the sampling hold,
Even when the specific period is short, the frequency control of the local oscillator is stabilized, and the sample hold circuit integrates the demodulated signal by an integrating circuit even when the power is turned on or the signal is temporarily cut off. Since the output is held and the frequency of the local oscillator is limited, stable reception is possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an FM television receiving apparatus according to the present invention, and FIG. 2 is a waveform diagram showing an example of a television signal which is a reception processing target of the embodiment shown in FIG.
FIG. 3 is a waveform diagram showing signals at various parts in FIG. 1, and FIG.
FIG. 1 is a circuit diagram showing a specific circuit example of the AFC circuit constituent portion in FIG. 2 ... Frequency mixing circuit, 3 ... Local oscillation circuit, 4 ... Band pass filter, 5 ... FM demodulation circuit, 6 ... Video signal processing circuit, 9 ... Integration circuit, 11,13,16,26, 27,28,29 …… Switch circuit, 14,17 …… Hold circuit, 18 …… Differential amplifier circuit, 15 …… Comparison voltage generation circuit, 12 …… Fixed voltage generation circuit, 19 …… Adder, 30, 31,33 …… Capacity, 32 …… Resistance.

Claims (1)

[Claims] 1. A frequency converter comprising a local oscillator and a mixer for receiving a first television FM signal and converting the frequency into a second television FM signal, and the second television.
An FM demodulator that demodulates an FM signal to demodulate a television signal, an AFC unit that has a sample hold circuit and controls the oscillation frequency of the local oscillator according to the hold output of the sample hold circuit, and the AFC unit. A first mode in which the signal voltage of the demodulated television signal during a specific period is sample-held to operate and a second mode in which the average voltage of the demodulated television signal is held to operate are selectively set. In the FM television receiver having a control means, the time constant is made small to pass the demodulated television signal in the first mode, and the time constant is made large in the second mode. Integrating means for selectively outputting the integrated voltage value of the demodulated television signal is provided, and the output of the integrating means is supplied to the sample hold circuit. FM television receiver that.