JPS6035302Y2 - automatic tuning circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency synthesizer tuner that receives a desired FM broadcast channel by automatically controlling the oscillation frequency of a local oscillator.

A conventional frequency synthesizer tuner uses an intermediate frequency signal extracted from a front end as a tuning signal having a predetermined bandwidth through a bandpass filter.

However, the interval between broadcast frequencies of FM broadcast stations differs from country to country, and if the set bandwidth of the tuning signal described above is too large, the interval between broadcast frequencies between FM broadcast stations on adjacent channels is narrow, Since the transmitted signals are mixed within the range of the tuning signals of the same channel, the tuning function of the frequency synthesizer tuner becomes incomplete.

Furthermore, if the set bandwidth of the tuning signal is narrowed, the tuning becomes delicate, and even if there is a slight frequency shift in the tuning signal, the received FM signal may not be fully extracted, and in this case, the demodulated signal may be incomplete. It becomes a thing.

The present invention has been proposed in view of this point, and is capable of arbitrarily setting the bandwidth of the above-mentioned tuning signal according to the above-mentioned conditions using the output of the FM detection circuit.

The present invention will be explained below according to the drawings.

Fig. 1 is a basic configuration diagram of a frequency synthesizer according to the present invention, Fig. 2 is a block diagram showing an embodiment of the present invention, and Fig. 3 is an embodiment of an automatic tuning signal generation circuit that constitutes the main structure of the present invention. 4 is a diagram showing the frequency characteristics of the frequency discriminator 16 shown in FIG. 3, and FIG. 5 is a diagram showing the input/output characteristics of the window comparator shown in FIG. 3.

The structure of the frequency synthesizer is shown in FIG.

In the figure, reference oscillator 1 takes out an oscillation output of a predetermined reference frequency, and this output is applied to phase comparator 2 at the next stage.

Further, an output having a certain frequency is applied to the phase comparator 2 from the N frequency divider 6, and a frequency difference between the output and the reference frequency described above is detected and a DC output representing this difference component is taken out.

This DC output is then passed through a low-pass filter 3 with a voltage holding function to a voltage controlled oscillator (hereinafter referred to as TVCO).
4 to control the oscillation frequency of the VCO.

Now, the oscillation output of the VCO 4 is returned to the N frequency divider 6 and frequency-divided by N.

A preset input designated in advance by the N variable circuit 5 is applied to the N frequency divider 6, and the value of N is set by this preset input.

This sets a certain frequency of the aforementioned output taken out from the N frequency divider.

The output of the VCO 4 is used as the oscillation output of the local oscillator constituting the frequency synthesizer tuner described above, and since the oscillation frequency is automatically controlled, the frequency synthesizer can automatically tune the channel by the preset input described above.

FIG. 2 shows a phase comparator 2 which is provided to VCO 4 by disposing a switch circuit 8 between the above-mentioned phase comparator 2 and low-pass filter 3 and controlling this switch circuit 8 by the output of an automatic tuning signal generator 9. The sensitivity of VCO4 can be improved by controlling the supply state of DC output from the VCO4.

A circuit diagram of an embodiment of the automatic tuning signal generator is shown in FIG.

In FIG. 3, a terminal 11 is an input terminal for an intermediate frequency signal, and is connected to a rectifying diode 12 and a frequency discriminator 16.

The diode 12 is grounded via a high frequency smoothing capacitor 13 and also via a low frequency Namekawa capacitor 14.

A connection point between the diode 12 and the capacitor 13 is connected to the base of the transistor 20 and the collector of the transistor 19 via the variable resistor 15.

The frequency discriminator 16 includes a window comparator 17 and a resistor 1.
It is connected to the base of the transistor 19 via a series circuit of 8.

The emitters of transistors 19 and 20 are each grounded.

The collector of the transistor 20 is connected to a DC power source 1B via a resistor 21 and serves as an output terminal 22.

Next, the operation will be explained.

When performing automatic tuning in a frequency synthesizer tuner, an intermediate frequency signal having a predetermined bandwidth is obtained by adding a variable oscillation frequency from a local oscillator (not shown) to the FM received signal.

An intermediate frequency signal of is applied to terminal 11.

Then, the intermediate frequency signal is rectified by the diode 12, smoothed by the capacitors 13 and 14, and taken out as a first DC signal.

The level of this first DC signal is set by a variable resistor 15 and applied to the base of a switching transistor 20.

Further, the above-mentioned intermediate frequency signal is applied to the frequency discriminator 16, where an FM demodulated output having frequency characteristics as shown in FIG. A second DC output corresponding to the frequency shift is extracted.

f shown in FIG. indicates the reference frequency of the intermediate frequency signal (10.7 MH2 in this case), and fl indicates the amount of frequency deviation of the intermediate frequency signal described above.

V again. is the reference frequency 10.7MH for the frequency discriminator 16.
Vl is the FM demodulation output formed in the frequency discriminator 16 when the intermediate frequency signal of
6 shows the fluctuation of the FM demodulation output when a frequency shift of f□ occurs in the intermediate frequency signal applied to the FM signal.

This FM demodulated output is further rectified and taken out as a second DC output.

Now, the second DC output taken out from the frequency discriminator 16 is applied to the window comparator 17 at the next stage.

The window comparator 17 is a comparator whose output is inverted when the input DC voltage falls within the range of two predetermined reference voltages, and as shown in FIG. When the DC output (for example, Vo) of the frequency discriminator 16 at is applied, O
Take out the output of v.

Then, the above-mentioned f is set to the center frequency of the intermediate frequency signal.

When there is a frequency deviation exceeding ±f1 from , the window comparator 17 takes out a predetermined control voltage 1B1 when the output of the frequency discriminator is applied.

Now, the output of the window comparator 17 is applied to the second switching transistor 19, and the transistor 19 is controlled to be turned off when the output is Ov, and turned on when the output is 10B.

As described above, since the DC voltage extracted from the intermediate frequency signal is applied to the base of the transistor 20, when the transistor 19 is turned on, the transistor 20
is turned off, and when transistor 19 is turned off, transistor 20 is turned on.

Also, when no intermediate frequency signal is applied, transistor 1
Even if transistor 9 is turned off, no DC voltage is applied to the base of transistor 20, so transistor 20 is turned off.

In this way, when the intermediate frequency signal whose bandwidth is set in advance is within this band, the transistor 20 is turned on and the switch circuit 8 is connected from the terminal 22. The output of the phase comparator 2 is controlled to be cut off, and the low-pass filter 3 causes the VCO 4 to oscillate at the voltage level previously held, thereby extracting a local oscillation output of a desired frequency.

Note that when the transistor 20 is off, the output of the phase comparator 2 is given to the low-pass filter 3 via the switch circuit 8, so the oscillation frequency of the VCO fluctuates.

As described above, according to the present invention, when performing FM demodulation of the intermediate frequency signal taken out from the front end, the bandwidth of the intermediate frequency signal can be substantially arbitrarily set in the frequency discriminator. This enhances the channel selection function of the TV.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of a frequency synthesizer according to the present invention, Fig. 2 is a block diagram showing one embodiment of the present invention, and Fig. 3 is an embodiment of an automatic tuning signal generation circuit that constitutes the main structure of the present invention. FIG. 4 is a diagram showing the frequency characteristics of the frequency discriminator 16 shown in FIG. 3 during FM demodulation, and FIG. 5 is a diagram showing the input/output characteristics of the window comparator shown in FIG. 3. 1, 2.5.6... Voltage control means, 3...
... Voltage holding means, 4... Voltage controlled oscillator, ... First switch circuit, 12, 13.14...
...DC conversion circuit, 16... Frequency discriminator, 17... Window comparator used as a detection circuit, 19... Third switch circuit, 20
...Second switch circuit.

Claims (1)

[Claims for Utility Model Registration] Voltage control means for extracting a control voltage for controlling the oscillation frequency of the voltage-controlled oscillator using the oscillation output of the voltage-controlled oscillator as a local oscillation output; A frequency synthesizer tuner in which the voltage controlled oscillator extracts a local oscillation output for receiving a desired FM broadcast channel by means of a voltage holding means that sets the oscillation frequency of the controlled oscillator at the voltage level of the control voltage; a DC conversion circuit that rectifies and smoothes a given intermediate frequency signal and takes it out as a first DC output; and a frequency converter that FM demodulates the applied intermediate frequency signal and further rectifies this demodulated output to take out a second DC output. a discriminator; a detection circuit that takes out a detection output indicating whether or not the second DC output is within a predetermined reference voltage range in response to the applied second DC output; and the voltage holding means; a first switch circuit that switches on and off the supply of control voltage from the voltage control means; and a first switch circuit that switches in response to the first DC output that is applied; a second switch circuit that takes out an output for controlling the first switch circuit to supply the control voltage to the voltage holding means;
and a third switch circuit that operates to cut off the first DC output applied to the second switch circuit when the DC output of the automatic tuning is outside the reference voltage range. circuit.