JP3288251B2 - CATV receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CATV receiving apparatus capable of receiving a CATV broadcast, and more particularly to improving the performance of a high-frequency unit tuner by preventing local oscillation leakage and return loss, and realizing low cost. The present invention relates to a CATV receiving apparatus that is suitable for

[0002]

2. Description of the Related Art With the recent digitalization and multimedia, in the field of broadcasting, not only the current radio broadcasting but also cable television broadcasting (hereinafter referred to as CATV) in which broadcasting and communication are integrated. ) Is drawing attention.

[0003] CATV has been widely used as a cable-based broadcasting form, and recently, with the realization of the CATV interactive service business in the United States, which is a CATV advanced nation, in Japan, CATV interactive services have been developed. Commercialization of the service is underway.

[0004] In CATV, the retransmission of existing terrestrial television and BS / CS satellite television and the like, and the spread of urban-type cable television that provides independent programs in cities are remarkable. Broadcasting service systems, such as cable Internet and cable karaoke, that use TV are also very popular.

A CATV receiver capable of receiving an analog or digital CATV has a tuner for receiving a signal in a desired transmission band from a transmission signal transmitted through a transmission medium such as a cable or a radio wave. Such a tuner is usually called a double super system and includes two frequency converters and two local oscillators. Such a double super type CAT
FIG. 4 shows the V tuner.

FIG. 4 is a block diagram showing a conventional CATV receiving apparatus and showing an example of a double super system CATV tuner.

As shown in FIG. 4, the CATV tuner is provided with an input terminal 1a for receiving an input signal. The input terminal 1a receives a signal from a center facility called a head end (not shown) and a cable or the like. Signal (RF signal) transmitted through the transmission medium
Is supplied. The RF signal input via the input terminal 1a is supplied to the AGC circuit 2 after passing only a predetermined frequency band by the LPF and HPF1.

The AGC circuit 2 receives an input RF signal based on an RFAGC signal from an AGC circuit control unit (not shown).
The signal gain is limited to an optimum level and output. That is, when the input radio wave is strong, the gain of the high-frequency amplifier circuit is controlled while keeping the gain of the video intermediate amplifier circuit (not shown) at a constant small gain, thereby suppressing the occurrence of cross-modulation interference and the like.

The output signal of the AGC circuit is amplified by an amplifier 3 and then applied to a first mixer 4. The first mixer 4 includes an oscillation frequency supplied from a separately provided first local oscillator (hereinafter, also referred to as an LO) 13 whose oscillation frequency is variable, and a frequency of the supplied RF signal. Are mixed to generate a beat signal having a beat frequency equal to the difference between the two frequencies and output. That is, the mixer 4 is a first frequency converter, and the first local oscillator 1
By using the oscillation frequency from the amplifier 3, the input signal from the amplifier 3 is up-converted, converted into a 1st IF signal (intermediate frequency signal), and provided to the HPF 5.

The HPF 5 passes the high frequency component of the input 1st IF signal and supplies it to the BPF 6, and the 1st IF signal of the high frequency component passed through is restricted in a predetermined band by the BPF 6, so that a band corresponding to one CATV channel is obtained. Signal. After that, the output signal of the BPF 6 is amplified by the amplifier 7 and then supplied to the second mixer 8.

The second mixer 8 is a second frequency converter, which down-converts the input signal from the amplifier 7 using the oscillation frequency from the second local oscillator 15 to obtain a second frequency.
The signal is converted into an ndIF signal (intermediate frequency signal) and supplied to the LPF 9.

The LPF 9 passes the low-frequency component of the input 2nd IF signal and supplies it to the amplifier 10. Amplifier 10
Amplifies the input signal, and the amplified signal is
The band is limited to a predetermined band, and after being amplified by the amplifier 12, the band is output to a signal processing unit (not shown).

With such a configuration, for example, by converting the input frequency to a higher frequency by the first frequency converter of the first stage, the oscillation frequency of the local oscillator can be set outside the band of the input RF. It is possible to reduce the frequency change ratio of the oscillator. Further, by providing and using a fixed frequency filter (also referred to as a band-pass filter) such as BPF6 between the first frequency converter and the second frequency converter, the output waveform of any input signal can be stabilized. It becomes possible.

By the way, CA in recent digital broadcasting
The use of the single conversion type has been reconsidered because TV tuners tend to have low cost and advantageous advantages because they are each provided with one frequency converter and local oscillator. Generally, as a single conversion type tuner currently used, a tuner employed in a television receiver (hereinafter abbreviated as TV) is well known. FIG. 5 shows an example of such a single conversion type TV tuner.

FIG. 5 shows an example of the configuration of a conventional single-conversion type TV tuner. FIG. 5A is a block diagram showing the configuration of the tuner, and FIG. 5B is a specific example of a PLL circuit used in the tuner. FIG. 3 is a block diagram illustrating a configuration. Note that the description of the configuration includes only one band out of three bands for simplification of the description.

As shown in FIG. 5A, the TV tuner is provided with an input terminal 11a for receiving an input signal, and the input terminal 11a is provided with a reception signal (not shown) received by a reception antenna (not shown). A television signal,
Hereinafter, referred to as an RF signal). The RF signal input via the input terminal 11a is L
After the PF and HPF 11 pass only a predetermined frequency band, the signal is supplied to the switch 12.

The switch 12 switches the input RF signal to three in accordance with the respective frequencies, and outputs the three to the corresponding variable tracking filters (13, 19, 25). For example, if the band is switched to the band based on the input terminal b by the switch 12, the input RF signal is transmitted to the variable tracking filter 19 synchronized with the local oscillation.
And a predetermined band is limited by the variable tracking filter 19 so that the amplifier (FET AMP) 20
Given to.

The amplifier 20 receives an input signal from an AG (not shown).
The level is adjusted so that the signal is amplified at a level based on the gain control signal from the C circuit, and is provided to the variable tracking filter 21 at the subsequent stage. The tracking filter 21 further limits the band of the input signal and outputs the signal. This output signal is then amplified by an amplifier 22 and then applied to a mixer (frequency conversion unit) 23.

The mixer 23 converts the input signal from the amplifier 22 into an IF signal (intermediate frequency signal) using the oscillation frequency from the local oscillator 24 and outputs the IF signal. This IF signal is thereafter given to a BPF (single tuned filter) 31 so that a predetermined band is limited, further amplified by an amplifier 32, and then output to a signal processing unit (not shown) via an output terminal 32a. .

A PLL circuit 34 shown in the figure generates a control signal based on the oscillating frequencies of the local oscillators 18, 24 and 30, and transmits the control signal to the local oscillators 18, 24.
And 30 to control the oscillation frequency of each oscillator.

More specifically, as shown in FIG.
The LL circuit 34 detects the oscillation frequency Lo from any one of the local oscillators 18, 24 and 30, divides the frequency by 1 / N by the fixed frequency dividing prescaler 38, and further 1 / M by the variable frequency divider 37. The phase of the divided signal is compared with a signal from a fixed frequency divider 35 for dividing the frequency of a signal REF from a high-precision fixed oscillator (not shown) by a phase comparator 36. The control voltage VT is generated based on the phase comparison result, and the oscillation frequency of any one of the local oscillators is varied. That is, the use of the PLL circuit 34 makes it possible to control the oscillation frequencies of the local oscillators 18, 24, and 30.

Therefore, in the TV tuner which is a single-conversion type tuner having the above configuration, P
Since the LL circuit 34 is provided with the prescaler 38 having a fixed frequency division, the local oscillation tuning step width and the PLL
Have different values from the comparison frequency.

When the above two types of tuners are compared, there are the following differences. For example, the current double conversion type of analog and digital CATV tuners has two local oscillators for performing the frequency conversion process twice. It is disadvantageous in price as compared with the conversion type tuner. That is, there is a disadvantage that the cost is high.

In the double-conversion type, the frequency of the two local oscillations is as high as 1-2 GHz and the variable width is as wide as 1 GHz, so that the frequency sensitivity to the voltage is increased (about 35 MHz / V). Phase noise tends to deteriorate. Further, as for the phase noise of the output signal, since the phase noise of the two local oscillators is added and output, there is only one local oscillator and the frequency is as low as 100 to 900 MHz (ex. 15 MHz /
It is disadvantageous when compared with the single conversion type.

In digital broadcasting CATV, which has been put into practical use in recent years, especially in multilevel QAM transmission (256 QAM transmission) for transmitting a large amount of data at a high rate, the interval between symbols in a constellation (signal arrangement diagram) is narrow. That is, if the eye pattern is small and the phase noise is bad, each symbol has a spread in the phase direction, and consequently, the constellation is blurred (the eye pattern is broken), which causes an increase in fixed deterioration.

Therefore, in order to avoid such inconvenience, when considering the improvement of the phase noise and the like by using a single conversion type tuner having only one local oscillator, the single conversion type tuner as described above is considered. It is conceivable to refer to a TV tuner as a tuner.

However, in the configuration of the conventional TV tuner, the isolation between the input oscillation frequency of the mixer and the input signal is not sufficient, and the two tracking filters at the preceding stage eliminate the influence of the oscillation frequency. Processing is not enough. Furthermore, because the isolation between the input and output of the amplifier is not sufficient, the leakage performance of the oscillation frequency to the input terminal (hereinafter referred to as LO leakage) is insufficient, or the return loss outside the band due to a variable filter is reduced. Since the state is close to the total reflection, the return loss performance in all the bands may be insufficient or the performance level required for CATV may not be reached. For this reason,
It may interfere with the subscribers connected via the transmission cable, resulting in the inconvenience of image degradation, and it is not possible to use it as it is .

On the other hand, in the single conversion type tuner, since the fixed dividing prescaler for LO is provided in the PLL circuit as described above, the tuning step width (step frequency) of the local oscillation and the comparison frequency of the PLL are compared. Have different values. For example, currently 4MH
The reference signal of z is divided by 512 to 7.8125K
Although the PLL is operated at the comparison frequency of Hz, if the fixed frequency division for LO is reduced to 1/8, the LO tuning step width becomes 62.5 KHz. In other words, in the current tuner, the comparison frequency becomes 1 / (fixed frequency division) of the tuning step width, becomes as small as several KHz, and the PLL fluctuation is multiplied by the fixed frequency division for the LO fixed frequency divider. Therefore, the carrier is in a state where FM is applied. Therefore, low phase noise at a low-frequency offset of about 1 KHz is difficult to realize and is one of the factors for increasing fixed deterioration.

[0029]

As described above, the conventional C
If an ATV receiver uses a double-conversion type tuner, it becomes more expensive than a single-conversion type tuner, and the phase noise due to the local oscillation frequency also deteriorates. In order to avoid such inconvenience, it is optimal to use a single conversion type. However, if a CATV reception signal is tuned as it is, the LO leakage or LO leakage or the like is insufficient due to insufficient isolation. Inconvenience occurs in return loss and the like, and as a result, there is a problem that ghosts and flickers occur to other subscribers connected to the transmission cable, resulting in image degradation.

Therefore, the present invention has been made in view of the above-mentioned problems, and realizes high performance by improving the phase noise of low-frequency offset, LO leak, return loss, etc., and realizes low cost. It is an object of the present invention to provide a CATV receiving apparatus capable of performing such operations.

[0031]

A CATV receiver according to the present invention has an input terminal to which a high-frequency signal is supplied, and a gain control circuit capable of controlling the signal supplied to the input terminal to a predetermined level and outputting the signal. And connected to the gain control circuit,
An amplifier circuit for amplifying and outputting a signal whose gain is controlled,
A circuit that mixes an output signal from the amplifier circuit and a local oscillation signal from the local oscillator to generate an intermediate frequency signal by a single-stage frequency conversion unit including a voltage-controlled local oscillator and a mixing circuit; A variable filter disposed between an amplifier circuit and the frequency conversion means and having a pass band limited by a control voltage for controlling the local oscillator, wherein the local oscillator has a P filter capable of controlling a local oscillation frequency.
An PLL circuit is provided, wherein the PLL circuit has a comparison frequency of the same frequency as a tuning step frequency of a local oscillator.

In the present invention, by providing the gain control circuit, the amplifying circuit and the variable filter,
While the isolation of the input of the variable filter can be improved and the leak and return loss of the local oscillation signal can be suppressed, the comparison frequency of the PLL is set to the same frequency as the tuning step frequency of the local oscillator. Phase noise at the band offset can be improved (low phase noise).

Further, the CATV receiver according to the present invention comprises:
An input terminal to which a high-frequency signal is supplied, and an input terminal for inputting the high-frequency signal supplied through the input terminal, which is input to the input terminal by a single-stage frequency conversion unit including a local oscillator and a mixing circuit. A tuner that mixes the high-frequency signal and the local oscillation signal from the local oscillator in the mixer, converts the mixed signal into an intermediate frequency signal, and outputs the intermediate frequency signal, and a gain control connected between the input terminal and the input terminal of the tuner. A circuit comprising a series circuit of a circuit and an amplifier, and a low-pass filter connected in series at a stage subsequent to the series circuit, and comprising an isolation circuit for reducing signal leakage from the local oscillator to the input terminal. It is.

Further, the CATV receiver according to the present invention has an input terminal to which a high-frequency signal is supplied, and an input terminal to which the high-frequency signal supplied through the input terminal is input, and includes a local oscillator and a mixing circuit. A tuner that mixes the high-frequency signal input to the input terminal and the local oscillation signal from the local oscillator with the mixer, converts the signal into an intermediate frequency signal and outputs the signal, and the input terminal; An isolation circuit that includes a series circuit of a gain control circuit and an amplifier connected between the input terminal of the tuner and an amplifier, and that is provided at a stage preceding the isolation circuit to reduce signal leakage from the local oscillator to the input terminal; And a filter circuit composed of a high-pass filter and a low-pass filter.

[0035]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a CATV receiving apparatus according to the present invention.

As shown in FIG. 1, the CATV according to the present invention
The receiving apparatus (which may be described as a CATV tuner) 101 is a single conversion type that can improve the phase noise by having only one local oscillator as described in the related art in order to improve the phase noise. It is configured by adopting the ones. That is, the circuit configuration is similar to that of a TV tuner. However, if the TV tuner is used as it is as a CATV tuner,
There may be a problem caused by LO leak, return loss, and the like, and a problem such as phase noise at a low-frequency offset. Therefore, a CATV receiver according to the present invention is provided with means for solving the above-mentioned problems. A specific circuit configuration employing these means will be described below.

As shown in the figure, the CATV tuner 10
1 is provided with an input terminal 41a for receiving an input signal. The input terminal 41a receives a reception signal received by a center facility called a headend (not shown) and transmitted via a transmission medium such as a cable. (RF
Signal). Input terminal 41a
Is supplied to the HPF and the LPF 41.

The HPF and LPF 41 perform a wide-band filtering process on the input RF signal and output it. This HPF, L
The output of the PF 41 is thereafter supplied to an AGC circuit 70 as an additional circuit newly provided in the present embodiment.

The additional circuit is arranged, for example, between the input terminal 41a and a tuner main part described later, and has a configuration in which an AGC circuit 70, an amplifier 80, and an LPF 90 are sequentially connected to the input terminal 41a. I have.

The AGC circuit 70 receives an input R signal based on an RFAGC signal from an AGC circuit control unit (not shown).
The F signal gain is limited to an optimum level and output. That is, when the input radio wave is strong, the gain of the high-frequency amplifier circuit is controlled while keeping the gain of the video intermediate amplifier circuit (not shown) at a constant small gain, thereby suppressing the occurrence of cross-modulation interference and the like.

The output signal of the AGC circuit 70 is
After being amplified by the LPF 90. LP
F90 passes a low frequency component of the input signal and supplies it to the input terminal of the switch 42 that operates in the same manner as in the prior art. That is, the input signal applied to the main part of the tuner after the switch 42 is used as the output RF signal of the additional circuit, so that the input isolation in the mixers 47, 53, and 59 at the subsequent stage is improved. It is possible to do.

The switch 42 switches the input RF signal to three in accordance with the respective frequencies and outputs the signals to the corresponding variable trunking filters (43, 49, 55). For example, assuming that the band is switched to the band based on the input terminal b by the switch 42, the input RF signal is a control voltage (tuning voltage V) for controlling the local oscillator 54.
t) is supplied to a variable tracking filter 49 whose frequency pass band is adjusted, and a predetermined band is limited by the tracking filter 49 so that an amplifier (FET)
AMP) 50.

The amplifier 50 receives an input signal from an AG (not shown).
The level is adjusted so that the signal is amplified at a level based on the gain control signal from the C circuit, and is provided to the variable tracking filter 51 at the subsequent stage. The variable tracking filter 51 includes a local oscillator 54 similarly to the variable tracking filter 49.
The frequency pass band is adjusted by a control voltage (tuning voltage Vt) for controlling the input signal, and the output signal is further restricted in the input signal band. This output signal is then amplified by an amplifier 52 and then applied to a mixer (frequency conversion unit) 53.

The mixer 53 converts the input signal from the amplifier 52 into an IF signal (intermediate frequency signal) using the oscillation frequency from the local oscillator 54, and outputs the IF signal. The IF signal is thereafter given to a BPF (single tuned filter) 61 to restrict a predetermined band, and is further amplified by an amplifier 62 before being output to a signal processing unit (not shown) via an output terminal 62a. .

The other bands, ie, the switches 42
When the band is switched to the band based on the input terminal a and the input terminal c, the operation is substantially the same as that of the band described above, and the output IF signal of the mixer 47 or the mixer 59 is
The signal is output from the output terminal 62a via the PF 61 and the amplifier 62.

According to such a configuration, it is possible to effectively reduce the phase noise at a higher frequency offset than that of the double conversion type. In digital transmission, particularly in multi-valued QAM such as 256 QAM, fixed deterioration is not possible. It is possible to suppress the increase.

As described above, by providing the additional circuit blocks of the AGC circuit 70, the amplifier 80, and the LPF 90 before the variable tracking filters 43, 49, and 55, the input isolation in each mixer can be improved. Therefore, as a result, it is possible to suppress the occurrence of the LO leak. Also, as for the return loss, only the reception channel is good and the others are not close to total reflection as in the conventional TV tuner. By providing an additional circuit block, the influence of the variable tracking filter at the input terminal is reduced. As a result, good return loss performance can be obtained in all bands.

On the other hand, the PLL circuit 64 shown in the figure generates a control signal based on the oscillating frequency of the local oscillators 48, 54 and 60, and transmits the control signal to the local oscillators 48 and 54.
And 60 to control the oscillation frequency of each oscillator.

Further, the PLL circuit 64 in the present embodiment is devised to reduce the phase noise of the low frequency offset of the local oscillation frequency.

For example, as shown in FIG. 2, the PLL circuit 64 includes a fixed frequency divider 65 for dividing the reference signal (REF), a variable frequency divider 67 for dividing the local oscillation frequency (LO), There is a phase comparator 66 for comparing the output of the fixed frequency divider 65 with the output of the variable frequency divider 67, and the local oscillation frequency is controlled by the phase comparison output VT. In other words, the fixed frequency dividing prescaler 3 conventionally used
8 (see FIG. 5B). By removing the fixed frequency dividing prescaler, the comparison frequency of the PLL is made equal to the step frequency of the LO (local oscillator).

Therefore, the PLL in this embodiment is
In the circuit 64, as shown in FIG.
And 60 are divided by the variable divider 67 by (1 / N) / (1 / N + 1), and the divided signal is combined with a high-precision fixed oscillator (not shown). The phase comparator 66 compares the phase of the signal (REF) from the signal (REF) with the signal divided by the fixed frequency divider 65, and varies the oscillation frequency of the local oscillator according to the control voltage VT resulting from the phase comparison. Thus, the PLL circuit 64 controls the oscillation frequency of the local oscillators 48, 54 and 60.

Thus, in a single-conversion type tuner for digital transmission, a variable frequency divider 67 is used instead of a fixed frequency prescaler of a PLL circuit due to the occurrence of phase noise at a low frequency offset. By circulating, the comparison frequency of the PLL and the tuning step width of the local oscillation can be matched, and as a result, the phase noise at the low frequency offset can be reduced.

Note that the PLL circuit 64 may employ the ordinary PLL circuit configuration including the fixed frequency dividing prescaler shown in the prior art (FIG. 5) to constitute the CATV receiver shown in FIG. In the case where the PLL circuit 64 shown in FIG. 2 is adopted, the fixed deterioration can be more effectively improved.

FIG. 3 is a block diagram showing another embodiment of the CATV receiving apparatus according to the present invention, showing an application example of the receiving apparatus. FIG. 3 (a) shows an IF signal demodulation type. 3 (b) shows an I and Q signal demodulation type. It is assumed that the tuner 101 shown in FIG. 3 has the same configuration requirements as the tuner shown in FIG.

In the present embodiment, an additional circuit group is added at the subsequent stage of the CATV receiver shown in FIG. 1 so that two types of CATV tuners capable of different demodulation processing, that is, an IF output type are provided. , I, and Q output types.

More specifically, as shown in FIG.
The F output type tuner includes a BPF 102 for inputting an output IF signal from an output terminal 62a of the CATV tuner 101 shown in FIG. The BPF 102 has an input IF
A predetermined band of the signal is limited and supplied to the amplifier 103.
The amplifier 103 amplifies the output signal of the BPF 102,
Further provided mixer (described as MIX in the figure) 10
4

The mixer 104 further down-converts and outputs the input IF signal by mixing the local oscillation frequency of the local oscillator 107 and the output signal from the amplifier 103. At this time, an IF in which an A / D converter (not shown) (A / D converter connected to the receiving device) directly operates.
Downconvert to the frequency of the signal.

The IF signal further frequency-converted by the mixer 104 passes only the low-frequency component of the signal by the LPF 105, is thereafter amplified by the amplifier 106, and is output to a signal processing unit (not shown). In this manner, by further down-converting and demodulating, it is possible to obtain an IF signal to be provided to a signal processing unit (not shown).

On the other hand, as shown in FIG. 3B, another I / Q output type tuner has a BPF 102 for inputting an output IF signal from the output terminal 62a of the CATV tuner 101 shown in FIG. Is provided. BPF10
2 restricts a predetermined band of the input IF signal, and
Supply 3 The amplifier 103 amplifies the output signal of the BPF 102 and supplies the amplified signal to a distributor 108 that distributes an input IF signal according to the frequency.

The distributor 108 distributes the input IF signal according to the frequency, and outputs one to the mixer 109 and the other to the other mixer 112. These mixers 109, 11
2 is supplied with a local oscillation frequency signal whose phase is shifted by 90 degrees by the phase shifter 115 provided with the phase of the local oscillation frequency from the local oscillator 117.

The mixers 109 and 112 mix the IF signals input thereto and the local oscillation frequency signals having different phases to perform frequency conversion.
The signal is supplied to the connected LPFs 110 and 113, respectively. Then, after the low-frequency components of the input signal are passed by the LPFs 110 and 113, respectively,
1 and 114 are amplified by the respective output terminals 11
From 1a and 114a, it is supplied to a signal processing circuit (not shown). By performing the demodulation processing in this manner, it is possible to obtain an I signal and a Q signal that are 90 degrees out of phase.

Therefore, according to this embodiment, FIG.
In addition to the same effects as those of the above-described embodiment, two types of tuners that perform different demodulation processes can be configured, and a high-performance CATV tuner can be realized with a simple circuit configuration. Can be planned.

In the embodiment according to the present invention,
In the tuner configuration shown in FIG. 1, a description has been given of a three-band system capable of switching to three bands. However, the present invention is not limited to this. Alternatively, it may be configured as a two-band system. Even in such a case, it is possible to obtain the same effect as the embodiment.

[0065]

As described above, according to the present invention, A
By providing an additional circuit block composed of the GC circuit 70, the amplifier 80, and the LPF 90 at the preceding stage of the variable tracking filter, the input isolation of the mixer can be improved. In addition to reducing the influence of the noise, phase noise can be suppressed, which contributes to higher performance and lower cost. As a result, it is possible to prevent image degradation for other subscribers connected via the transmission cable. In addition, by causing the PLL circuit to perform phase comparison so that the value of the PLL comparison frequency matches the value of the tuning step width of the local oscillation frequency, phase noise at a low frequency offset of the local oscillation frequency can be reduced. Thus, it is possible to improve fixing deterioration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a CATV receiver according to the present invention.

FIG. 2 is a block diagram illustrating a configuration of a PLL circuit in FIG. 1;

FIG. 3 is a block diagram showing another embodiment.

FIG. 4 is a block diagram showing an example of a conventional double conversion type tuner.

FIG. 5 is a block diagram showing an example of a conventional single conversion type tuner.

[Explanation of symbols]

41a: input terminal, 42: switch, 43, 45, 4
9, 51, 55, 57: variable tracking filter, 4
4, 50, 56 ... amplifier (FET AMP), 47, 5
3, 59: mixer (MIX), 48, 54, 59: local oscillator (LO), 61: BPF, 64: PLL circuit, 6
2a ... output terminal.

Continuation of front page (56) References JP-A-5-176317 (JP, A) JP-A-5-268583 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 7 / 10 H04N 7/14-7/173 H04N 7/20-7/22 H04N 5/00

Claims (3)

(57) [Claims] 1. An input terminal to which a high-frequency signal is supplied, a gain control circuit capable of controlling a signal supplied to the input terminal to a predetermined level and outputting the signal, and a gain control circuit connected to the gain control circuit for controlling a gain. Amplifying circuit for amplifying and outputting the obtained signal, and a single-stage frequency conversion means including a voltage-controlled local oscillator and a mixing circuit, the output signal from the amplifying circuit and the local oscillation signal from the local oscillator are converted A circuit that mixes and generates an intermediate frequency signal; and a variable filter that is disposed between the amplifier circuit and the frequency conversion unit and whose pass band is limited by a control voltage that controls the local oscillator. The oscillator is a PLL that can control the local oscillation frequency
A CATV receiver comprising a circuit, wherein the PLL circuit sets the comparison frequency of the PLL to be the same as the tuning step frequency of the local oscillator. 2. An input terminal to which a high-frequency signal is supplied, and an input terminal to which the high-frequency signal supplied through the input terminal is input, and a single-stage frequency conversion means including a local oscillator and a mixing circuit. A tuner that mixes the high-frequency signal input to the input terminal and the local oscillation signal from the local oscillator with the mixer, converts the mixed signal into an intermediate frequency signal, and outputs the intermediate frequency signal, between the input terminal and the input terminal of the tuner; A series circuit of a connected gain control circuit and an amplifier, and a low-pass filter connected in series at a stage subsequent to the series circuit, and an isolation circuit for reducing signal leakage from the local oscillator to the input terminal. A CATV receiver provided. 3. An input terminal to which a high-frequency signal is supplied, and an input terminal to which the high-frequency signal supplied through the input terminal is input, and a single-stage frequency conversion means including a local oscillator and a mixing circuit. A tuner that mixes the high-frequency signal input to the input terminal and the local oscillation signal from the local oscillator with the mixer, converts the mixed signal into an intermediate frequency signal, and outputs the intermediate frequency signal, between the input terminal and the input terminal of the tuner; An isolation circuit including a series circuit of a connected gain control circuit and an amplifier, configured to reduce a signal leak from the local oscillator to the input terminal; and a high-pass filter and a low-pass filter provided in a stage preceding the isolation circuit. CATV receiving apparatus comprising: a filter circuit;