JPH078031B2 - 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 TV broadcasting from geostationary satellites, a modulation method mainly used for FM modulation is used, and this FM modulation method is also often AC-coupled so that the average value of the modulated signal becomes a constant frequency. To be

However, there is a limitation in the transmission band, when the abrupt change of the average value voltage of the modulated signal, for example, when changing from a black screen to a white screen to prevent the FM signal spectrum from jumping out of the transmission band, or, The FM modulation method is used to fix the FM signal in the period that is the reference of the modulated signal to a fixed frequency when the reference DC voltage necessary for reproduction with a special video signal does not move depending on the content of the video signal. .

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 stable local oscillation frequency control is possible during steady reception and when the reference frequency constant period is long. However, when the power is turned on or the signal input is temporarily cut off, the control of the local oscillation frequency is disturbed and the recovery is delayed.
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 that eliminates the above-mentioned conventional drawbacks and enables stable local oscillation frequency control.

[Outline of Invention]

Until the pulse signal corresponding to the signal for a certain period of the reference frequency is generated, or when the pulse signal is stopped, the local oscillation frequency is controlled by the average value voltage of the demodulated video signal. The sample control circuit is configured to perform sample control by voltage, and the sample control circuit performs sample control of both the reference demodulation voltage and the comparison voltage, so that stable reception can be performed when the power is turned on or when the signal is temporarily cut off. Even if the frequency period is short, the local oscillation frequency can be 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, which is a configuration of a second heterodyne receiving circuit and a signal processing circuit of a satellite broadcast receiver. From the plurality of signals input from the terminal 1, the frequency mixing circuit 2 and the terminal 8
The local oscillator circuit 3 which determines the oscillation frequency with the tuning voltage from performs channel selection and frequency conversion of the desired signal, the band pass filter 4 removes interfering signals other than the desired signal, and the FM demodulator circuit 5 converts the video signal. It is mainly configured to demodulate, shape the waveform of the video signal in the video signal processing circuit 6, and output from the terminal 7.
In the FM receiving circuit, 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,
The output demodulation signal a of the FM demodulation circuit 5 is branched into two, one of which is converted into an average DC voltage signal by an integration circuit, and the switch 10 selects the demodulation signal and the output signal of the integration circuit. Controlled by c, the output signal of the integrating circuit is output when the pulse control signal b is not generated, and the demodulation signal a is output when the pulse control signal is generated. The output of the switch 10 is the switch 13 and the hold circuit 14. Connected to the first sample-hold circuit, the output of the first sample-hold circuit is input to the differential amplifier circuit 18, and the voltage of the comparison voltage generation circuit 15 is switched to the other input of the differential amplifier circuit 18. The output of the second sample hold circuit composed of 16 and the hold circuit 17 is connected, and the output of the differential amplifier circuit 18 is superimposed on the tuning voltage by the adder 19. Here, the switch 13 and the switch of the first and second sample and hold circuits
16 is controlled by the switch 11 controlled by the control signal c to select the pulse control signal b and the fixed voltage of the fixed voltage circuit 12, and the output signal d of this switch 11 is used. When b is not generated, the input / output is always conductive, and when the pulse signal b is generated, it 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 TV signal called MULTIPLEXED ANALOGUE COMPONENT signal, abbreviated as Mac signal (MAC) shown in the figure
The period of ₁ is a reference period indicating the center voltage of the signal, and T ₂ is the period of the signal and is composed of the color signal 20, the luminance signal 21, and the synchronization signal 22. Reference voltage period of T ₁ is required for signal processing, so that the voltage of the period T ₁ in the change of period T ₂ of the signal does not change, as the FM modulation method of the transmission side, the period T ₁ is T ₂
A constant frequency modulation is used regardless of the signal of the period.

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 the period of the period T ₂ of the FM signal T ₁ is at a constant frequency to receive the signal of varying frequency video signal, in FIG. 1, the period of the demodulated waveform a is T ₁ is the output of the FM demodulator circuit 5 The waveform of the video signal 24 is obtained with the DC voltage 23 for the period of T ₂ and the video signal processing circuit 6 generates the pulse control signal b corresponding to the period of T ₁ from the synchronization information of the video signal 24.
Further, from the video signal processing circuit 6, 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.
Is output. Now, as an example, the voltage of the pulse control signal b in the period T ₁ is a high voltage, the other period is a low voltage, the voltage of the control signal c in the period T ₀ is a high voltage, and the other period is a low voltage. Assuming that the voltage of the fixed voltage circuit 12 is substantially the same as the high voltage of the pulse signal b in the T ₁ period, the output of the switch 11 becomes the fixed voltage in the T ₀ period, and the pulse control signal b is generated by the pulse control signal b. Is output.

In the circuit operation in the period T ₀ , the output of the integrating circuit 9 is selected by the switch 10, the output waveform e of the switch 10 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 the output h of the hold circuit 14 become 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 generation circuit 15, and the differential amplifier circuit 18 acts to generate the demodulated signal. The frequency of the local oscillating circuit 3 is controlled so that the average value voltage of 1 and the output voltage of the comparison voltage generating circuit become equal. Therefore, even if the oscillation frequency of the local oscillation circuit 3 fluctuates due to temperature or the like, and is detuned, the demodulation signal a is quickly generated and the pulse control signal b is smoothly generated by the action of the average value AFC. However, during this T ₀ period, since the reference frequency changes with the average value of the video signal, the reference voltage 23 of the demodulation signal a changes, and the luminance signal of the video signal changes to white or black.

Next, when the pulse control signal b is generated, the output e of the switch 10 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 operated only for the period T ₁ of the pulse control signal b. It conducts and is cut off during the other T ₂ periods, and the output f of the switch 13 is the reference voltage 25 of the demodulated signal a only during the T ₁ period, and the output g of the switch 16 is the voltage of the comparison voltage generating circuit 15 T ₁ It is output only for a period and each hold circuit 14
When the voltage of the period T ₁ by 17, as the input h and i FIG differential amplifier circuit 18 for holding between the period T _2, is charged with period T _1, and the characteristic of little by little discharge period T ₂ But first
The sample-and-hold circuit and the second sample-and-hold circuit can reduce the change in the output voltage of the differential amplifier circuit during the T ₂ period by making the time constants of charging and discharging the same. That is, this circuit uses the reference voltage of the demodulated signal a during the T ₁ period.
The oscillation frequency of the local oscillation circuit 3 is controlled so that the output voltage of the reference voltage generation circuit 15 is equal to that of 25, the discharge circuit has a large discharge time constant during the T ₂ period, and at the time of discharging the reference voltage and the comparison voltage. By making the constants the same, a differential amplifier circuit
The output voltage fluctuation of 18 can be remarkably reduced, and the fluctuation of the oscillation frequency during the T ₂ period can be reduced. Here, by setting the output impedances of the FM demodulation circuit 5, the integration circuit 9, and the comparison voltage generation circuit 15 that are connected to the first and second sample hold circuits when the switch is conductive, to be small,
Since the charge time constant of the sample hold circuit can be made small, 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. CMOS gate switches are used for the switches 26, 27, 28 and 29, and the demodulated signal output transistor Q ₁ , the integration circuit output transistor Q ₂ and the comparison voltage generation circuit output transistor Q ₃ are emitter follower constant voltage circuits. As a result, the output impedance is made small, and the charging time constant formed by this impedance and the capacitances 30 and 31 of the hold circuit is made small to carry out charging for a short period of conduction, and at the time of holding, the transistors Q ₄ , Q ₅ , Q ₆ and Q _7, by increasing the impedance of the output side as viewed from the capacitor 30 and 31 in the multistage connection Q _8, Q _9, the discharge time constant is increased, with smaller voltage fluctuations during holding, two hold due to the discharge time constant of the circuit in the same, without the base voltage variation of transistor Q ₁₀ and Q ₁₁ of the differential amplifier circuit, good ho voltage of the differential amplifier circuit output in unchanged Field effect is obtained long period.

〔The invention's effect〕

As described above, according to the FM television receiver of the present invention, the local oscillation frequency is controlled by the average value voltage of the demodulated signal until the pulse signal corresponding to the signal in the constant frequency period serving as the reference is generated. After the pulse signal is generated, by using the present invention in which the local oscillation frequency is controlled by using the two sample-hold circuits for the demodulated signal and the comparison voltage, the average value AF can be obtained even when the power is turned on or the signal is temporarily cut off.
The function of C enables quick reception, and after the pulse signal is generated, the function of sampling AFC enables good reception with the reference frequency period at a fixed frequency, and the two sample-hold circuits provide a good hold effect. Is obtained, and stable frequency control is possible even in the case of a control signal for a short period.

[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 one specific circuit side of the AFC circuit constituent portion in FIG. 2 ... Frequency mixing circuit, 3 ... Local oscillation circuit, 4 ... Bandpass filter, 5 ... FM demodulation circuit, 6 ... Video signal processing circuit, 9
… Integrator circuit, 10,11,13,16,26,27,28,29… Switch circuit, 14,17… Hold circuit, 18… Differential amplifier circuit, 15… Comparison voltage generation circuit, 12… Fixed voltage generation circuit , 19 ... adder,
30,31… Capacity.

Claims (1)

[Claims] 1. A first television FM signal is received, frequency-converted into a second television FM signal by a frequency converter including a local oscillator and a mixer, and demodulated by the second television FM demodulator. In the FM television receiving device for obtaining a television signal by performing the following, first and second samples in which a fixed voltage and a pulse voltage signal synchronized with a specific period of the demodulated television signal can be selected as control signals. A hold circuit; and a control means for controlling the oscillation frequency of the local oscillator according to the output difference between the first and second sample-and-hold circuits. When the control signal is the fixed voltage, the first When the second sample and hold circuit holds the comparison voltage for the average value voltage of the television signal demodulated by the sample and hold circuit and the control signal is the pulse voltage signal, The sample hold circuit demodulates the signal voltage of the television signal during the specific period, and the second sample hold circuit samples and holds the comparison voltage by the pulse voltage signal, respectively. Television receiver.