JPH01238321A - Input circuit for tuner - Google Patents

Abstract

PURPOSE:To block leakage of a high channel signal and a local oscillating frequency by acting a 1st varactor diode controlled in common by a tuning voltage for varying the cut-off frequency and acting the 2nd varactor diode for varying a trap frequency. CONSTITUTION:When a tuning voltage Vt is varied with the frequency of a desired reception channel, the capacitance of a varactor diode VD4 is varied to shift the cut-off frequency of the low pass characteristic. The varactor diode VD5 is varied to shift the trap frequency if the trap circuit Vp. The cut-off frequency of the low pass characteristic by the 1st varactor diode VD4 is shifted to block the high channel frequency being a back-talk disturbance surely. Moreover, the capability excluding input leakage of a local oscillating frequency is high.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to an input circuit for a tuner, and more specifically, when the input signal is a wide band, backtalk is caused by a high channel signal when receiving a low channel. The present invention relates to a tuner input circuit that prevents deterioration of reception characteristics due to occurrence of such phenomenon.

(Prior Art) A CATV receiver employs an up-down double heterodyne tuner.

This up-down tuner changes the oscillation frequency of a first local oscillator in accordance with the input RF signal, and fixes the oscillation frequency of a second local oscillator to receive a desired CATV broadcast signal.

FIG. 7 is a block diagram showing the configuration of a CATV up/down tuner. cable television signal (hereinafter referred to as
RF signal) passes through a bypass filter 71 and a low-pass filter 72 and is input to a first mixer 73. The first mixer 73 receives the first
It operates based on the local oscillation output from the local oscillator 74, and outputs a first intermediate frequency signal (hereinafter referred to as the 11th F signal). The first local oscillator 74 has an oscillation frequency t, lI controlled by a tuning voltage Vt, for example, 660 to 11
It is variable in the frequency range of 6Q [MH2]. input RF
The frequency is, for example, a signal in the range of 50 to 550 [MH2], and the first IF frequency loss is, for example, elo [MHz
i,=ffl is fixed.

The signal from the first mixer 73 is filtered through a bandpass filter 76;
The signal is guided to a second mixer 79 via a first IF amplifier 77 and a bandpass filter 78. The second i11 combiner 79 generates a 21st F signal using the local oscillation output output from the second station 811 oscillator 80, and the generated 21st F signal is received by a normal TV receiver via a bandpass filter 81. Output as possible RF signal.

The characteristics of the low-pass filter 72 can be changed by adjusting the tuning voltage (2) using a variable capacitance element such as a varactor diode.

The operation of such a tuner will be explained.

When the first local oscillator 14 generates a local oscillation output of frequency f1 based on 1 Yuning voltage Vt, this local oscillation output and the frequency fin that has passed through the low-pass filter 72
The input RF signal i) is mixed in the first mixer 73, and the 11th
An F signal is generated.

Here, when the first local oscillator 74 increases the tuning voltage vt, it generates a local oscillation output with a high frequency, and yt
If it is low, it will generate a low frequency local oscillation output. For example, when receiving a 50 [M) 12F low channel signal, lower the tuning voltage yt,
Set the local oscillation output to 660 [MHzl]. However, for example, when a high-frequency t/channel signal of 550 [MH7] passes through the bypass filter 71 and the low-pass filter 72 and is frequency-converted by the first local oscillation output, the frequency-converted component generated thereby is a low-channel signal. 610 is a component that is close to the signal frequency and leaks to the input terminal side.
[It creates an IF acid component near Ml-IZI.

This kind of interference is called backtalk. In addition, when receiving low channels, the local oscillation output moves to the lower frequency side, so the local oscillation output at that time may leak to the input terminal side (hereinafter referred to as input leakage), worsening cross-modulation characteristics. Conceivable.

For this reason, the low-pass filter 72 is designed so that its characteristics change depending on the tuning voltage Vt, so that the trap and cutoff characteristics appear particularly prominently during low channel reception, so as to avoid interference due to the backtalk characteristics and input leakage characteristics. It's Nishide.

FIG. 8 is a circuit diagram showing a specific configuration of the low-pass filter 72.

In FIG. 8, a bypass filter 71 is connected to the terminal Pin.
The signal from P is guided, and this terminal Pin performs a trap function with an inductance and a capacitor, and is connected in parallel with P.
Connected to 1. The parallel connection P1 is further followed by a trap parallel connection P2 of inductance and capacitor 1,
P3 are cascaded. Further, a capacitor C1a is connected between the terminal Pin and the reference potential point, and a capacitor C1a is connected between the connection point of Pl and P2 and the reference potential point, and between the connection point of P1 and P2 and the reference potential point.
A capacitor C1b, C is connected between the connection point of C and the reference potential point.
ICs are connected to each. Furthermore, the final stage parallel connection P3 has a capacitor C1d connected between its output end and the reference potential point. These PI, P
2.

P3 and capacitors C1a, C1b, C1c,
C1d constitutes a Dzievisiev type low-pass filter 12'.

The output terminal of the low-pass filter 72' is connected to an inductance L that constitutes a trap circuit for correcting the trap frequency.
A capacitor C2, an inductance 13, and a varactor diode VDI are connected in series between the connection point of the inductances L1 and L2 and the reference potential point. A tuning voltage Vt is applied to the varactor diode VD1 via a resistor R1. With this configuration,
When the tuning voltage Vt is changed, the capacitance value of the varactor diode VD1 is changed, and the characteristics of the low-pass filter 72 are changed as shown in FIG.

FIG. 9 is a characteristic diagram showing the frequency characteristics of the circuit shown in FIG. 8, where the horizontal axis is the frequency and the vertical axis is the response.

In FIG. 9, the solid line shows the characteristics when the tuning voltage Vt is high, and exhibits a cutoff frequency of approximately 550 [MH2]. Moreover, the broken line shows the characteristics when the tuning voltage Vt is lowered. In this way, the circuit shown in FIG. 8 attempts to improve the backtalk characteristics by creating a trap characteristic in the vicinity of the cutoff frequency.

The position of this characteristic A moves downward as the tuning voltage Vt decreases to eliminate high channel frequencies.

However, in the circuit shown in FIG. 8, as the position of the trap characteristic A moves downward, a characteristic B occurs in which the response becomes higher between the cutoff point and the upper trap characteristic C. , the backtalk characteristic deteriorates in high channel signals passing through the band of characteristic B. Furthermore, characteristic C improves input leakage in correspondence with the first local oscillation frequency range, but the level increases compared to the characteristic shown by the solid line, and the Rev. 93 K deteriorates accordingly. Therefore, as shown in Figure 10,
circuit is provided.

FIG. 10 is a circuit diagram showing another example of the low-pass filter 72. In FIG. 10, the same components as in FIG. 8 are given the same reference numerals.

The low-pass filter 72' shown within the dotted line is constructed by a combination of L and C as in FIG.

This circuit consists of the above-mentioned low-pass filter 72' and the terminal Po
Inductance L4, capacitor C3, C
A characteristic correction filter 728 whose cutoff frequency of the low-pass characteristic is determined by 4 is provided. This characteristic correction filter 72A is connected to the condensers C3 and C4! ! A varactor diode ■D2. VD3
is connected. These varactor diodes VD2
, VO2 have a resistor R2 connected between their respective cathodes, and a tuning voltage Vt is applied to the varactor diode VD3 via the resistor R3, and this voltage is further applied to the varactor diode VD2 via the resistor R2.

FIG. 11 is a characteristic diagram showing the characteristics of the circuit shown in FIG. 10, and the solid line shows the characteristics when the tuning voltage Vt is high.
The broken line shows the characteristics when the tuning voltage Vt is low. Also, △1.3+.

C' corresponds to the frequency range of characteristics A, B, and C in FIG. As can be understood from this characteristic, in the characteristic correction filter 72A, as shown in B of FIG. 9, the downward slope peculiar to a low-pass filter moves to the left without forming a characteristic that increases the response. There are some that perform such corrections. As a result, when the tuning voltage ■[ is low, the amount of attenuation from the cutoff frequency part (characteristic B' region) to the first local oscillation frequency (C' part) region is reduced by 19, and the input leakage characteristics are gently improved. has been done.

However, if the characteristic A' is compared with FIG. 10, sufficient attenuation H1 cannot be obtained, and the backtalk characteristic deteriorates in this portion.

(Problem to be Solved by the Invention) As described above, the conventional circuit that regulates the input frequency using a low-pass filter that follows the Tievisiev characteristic has a low-pass The characteristic at the cutoff point of the filter characteristic is corrected by using circuit means such as a trap filter or an O-pass filter configured in two stages. but,
It is difficult to sufficiently attenuate both of the above disturbance components, and a trap filter and a low-pass filter (if
The only way to solve the problem was to use For this reason,
The disadvantages were that the circuit configuration became complicated and the number of parts increased.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and provide a tuner input circuit that can reliably improve backtalk characteristics and input leakage characteristics with a simple circuit configuration.

[Structure of the Invention] (Means for Solving the Problems) The present invention uses an antenna as a signal source and achieves a predetermined low-pass characteristic by a combination circuit of a first capacitor, a coil, and a second capacitor connected in parallel. A first varactor diode that also serves as one of the first capacitors is used to vary the cut-off frequency of the low-pass filter, and a common input voltage is set by the first varactor diode and the tuning voltage. A trap characteristic section is created in the low-pass characteristic section by a trap circuit that connects a second varactor diode whose capacitance is varied to avoid changes in the trap characteristic section.

(Function) According to such a configuration, among the first and second varactor diodes that are commonly controlled by the tuning voltage, the first varactor diode is controlled by the second varactor diode because the frequency is variable. functions for varying the trap frequency. From this, it can be seen that the input horn selection characteristics when receiving the Rode I7 channel signal are characterized by large response characteristics that occur due to the movement of the trap frequency.
The characteristic is suppressed by the low-pass characteristic in which the 7th frequency is shifted downward. Therefore, it is possible to prevent leakage of the high channel signal and local oscillation frequency caused by the characteristic portion having a large response.

(Example of Actual Flow) The present invention will be described in detail below based on the drawings. FIG. 1 is a circuit diagram showing an embodiment of the input circuit of a chicona according to the present invention. In FIG. 1, the same components as in FIG. 8 are given the same reference numerals.

In FIG. 1, an RF multiplied signal is introduced into the terminal Pin. Parallel connection Pi, R2, R3 and capacitor C which perform trap function by inductance and capacitor
1a, C1b, and C1c have the same configuration as the conventional one.

The final stage parallel connection P3 is connected to a reference potential point via a capacitor C5 and a first varactor diode VD4, and a tuning voltage Vt is applied to the varactor diode VD4 via resistors R4 and R5.

The output end of the parallel connection P3 has an inductance L5,
A capacitor C6, an inductance L7, and a second varactor diode VD5 are connected in series between the connection point of the inductance L5.16 and the reference potential point. A tuning voltage Vt is applied to the varactor diode V[)5 via a resistor R5. These inductances L5, L6. L7, capacitor J-06, and varactor diode VD5 constitute a trap circuit Vp for preventing backtalk interference.

Note that the terminal pout is connected to a mixer (not shown),
It is combined with the RF multiplied signal local oscillation output output from the terminal pout and frequency converted.

Next, a second explanation will be given of the operation of the embodiment configured as described above.
This will be explained with reference to the figures.

FIG. 2 is a characteristic diagram showing the frequency characteristics of the above embodiment, where the horizontal axis represents the frequency and the vertical axis +M represents the response.

When the tuning voltage V[ is varied in accordance with the frequency of the desired reception channel, the first and second varactor diodes VD4 and VO2 commonly undergo a change in capacitance. When the capacitance of the varactor diode VD4 changes, the capacitance between the output end of the parallel connection P3 and the reference potential point changes, shifting the cutoff frequency of the low-pass characteristic. Further, by changing the varactor diode VD5, the trap frequency of the trap circuit 2p is moved. In this way, the filter characteristics change as the tuning voltage Vt changes, and the solid line in FIG.
The characteristics when t is high are shown, and the broken line shows the characteristics when t is low.

When the tuning voltage Vt is high, the capacitances of the varactor diodes VD4 and VO2 are small, and both the trap frequency and cutoff frequency move to the higher frequency side,
A slope peculiar to a 0-pass filter is formed and the response is lowered from the response point that gives the cutoff frequency. Conversely, when the tuning voltage Vt is lowered, the capacitance M of the varactor diodes VD4 and VO2 increases, and the trap frequency and cutoff frequency move to the lower frequency side. The trap characteristic portion indicated by the broken line indicates that the attenuation becomes large in the vicinity of the cutoff frequency due to a decrease in the trap frequency. Further, similar to the characteristics shown in FIG. 9 previously shown, a characteristic portion E where the response becomes large is generated above the trap characteristic portion D. However, in the case of this circuit, the first
As the cutoff frequency of the low-pass characteristic by the varactor diode VD4 moves, the above characteristic part E becomes as follows.
It is lower than the characteristics shown in FIG. Correspondingly, blocking of high channel reception, which causes backtalk interference, is reliably performed.

Next, the characteristic part F above the cutoff point prevents input leakage of the local frequency, but in the case of the characteristics of this circuit, the response does not increase much compared to Fig. 9. ,
The frequency shifts downward while keeping the trap level constant. Therefore, the ability to eliminate input leakage of the local oscillator frequency is higher than that of the circuit shown in FIG.

In this way, this embodiment improves the trap method shown in FIG.
It can be said that the low-pass characteristic variable function shown in FIG. 10 is added to the characteristic of p.

Due to these characteristics, when receiving a low channel by lowering the tuning voltage, it is possible to reliably cut the frequency on the vibration channel side and the local oscillation frequency, so the backtalk characteristic section and the local oscillator frequency can be cut off. can improve input leakage characteristics.

Now, in order to obtain such characteristics, Figs.
If the conventional circuit shown in the figure is simply combined, the number of varactor diode components is VDI.

Three points of VD2 and VD3, inductance is connected in parallel vc
The three inductance points forming P1 to P3 and Ll,
The 7 points of L2, L3, and L4, and the capacitors are connected to the 3 points of the capacitors forming the parallel connection P1 to P3 and C1a-C1.
d, 23 parts are required: 10 parts C2 to C4, and 3 parts R1 to R3 for resistance.

On the other hand, in the embodiment, the number of components of the varactor diode is V
Two points D4 and VD5, inductance connected in parallel P1
The three points of inductance that make up ~P3 are six points, 5 to L7, the capacitors are connected in parallel, the three points of capacitors that make up P1 to P3, and the eight points of C1a to C1c, C5, 06, and the resistors are R4 and R5. It can be constructed from a total of 18 parts, including two parts.

In this way, the circuit of this embodiment has a small number of parts and can reduce costs.

Incidentally, in FIG. 8, the cutoff frequency can be changed by changing the capacitance value of any of the capacitors C1a, C1b, C1c, and C1d installed on the reference potential point side.

Figure 3 shows the capacitor CIC in the circuit shown in Figure 1.
Capacitor C5' and varactor diode VD in place of
This is an example in which a 4' series circuit is provided, but in this case, the 4th
The characteristics are shown in the figure. In this figure as well, the solid line indicates the characteristic when receiving the high channel, and the broken line indicates the characteristic when receiving the low I-11 channel. According to the characteristics shown in FIG. 4, unlike the characteristics shown in FIG.
is not attenuated like E. Therefore, backtalk characteristics deteriorate in this band. Furthermore, when the capacitor C1a or the capacitor C1b is replaced with a varactor diode, there is a drawback that the local light source is leaked through the line that supplies the tuning voltage V. For this reason, the actual characteristics deviate from the designed characteristics, resulting in a decrease in performance. After examining the above experimental results, it was confirmed that the method of making the capacitor C1d variable as in this embodiment is optimal.

Note that the trap circuit Vp may be constructed as shown in FIGS. 5a and 5b. In FIG. 5a, T1 is the parallel connection P3
T2 corresponds to the terminal Pout.

Such a circuit is similar to the circuit shown in Fig. 1, where the coil L
The trap frequency can be varied by connecting the capacitor C8, the capacitor C8, and the varactor diode VD6 in parallel. Further, in FIG. 5, since two variable capacitance diodes, VD7 and VD8, are connected in anti-series, the tuning voltage Vt is applied to the intersection of these. Such a circuit improves trapping characteristics. Furthermore, L9゜Llo,
Lll is 15. in FIG. 1, respectively. L6. Compatible with L7.

FIG. 6 shows other circuit examples of the cutoff frequency variable section using the first varactor diode VD4.

T1 is a terminal connected to the output end of parallel connection P3, TI'
is the output terminal when the circuit is viewed as a 2-bot circuit.

FIG. 6a shows an inductance L12 connected in series with a varactor diode VD9. In this case, L1
2 is set small to improve the change ratio.

Figure 6 shows varactor diode VD10. VD11 is connected in series in the opposite direction. Using such a varactor diode can also improve the characteristics in the same way as shown in FIG. Figure 6C shows the varactor diode VD1 connected in parallel.
2. VD13 is provided, and a fixed capacitor C9 is further connected to these parallel connections. This ensures the necessary ground capacity tl value. When using one circuit shown in FIGS. 5 and 6, the number of parts increases by 1 or 2 points,
From the J embodiment shown in FIG. 1, it is clear that the change ratio of the trap frequency, cutoff frequency, etc. can be improved, and the backtalk characteristics and input leakage characteristics can be improved.

Note that although this embodiment is an example in which there are three stages of parallel connection having a trap effect, there is no particular need to limit the number of stages.

[Effects of the Invention] As described above, according to the present invention, backtalk characteristics and input leakage characteristics can be improved with a simple circuit configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the input circuit of a tuner according to the present invention, FIGS. 2 and 3 are characteristic diagrams and circuit diagrams for explaining the characteristics of the embodiment shown in FIG. Figure 4 is the third
5 is a circuit diagram showing other examples of the trap circuit used in the present invention, and FIG. 6 is a circuit diagram showing another example of the cutoff frequency control section. 7 is a block diagram showing an up-down tuner using the present invention, FIG. 8 is a circuit diagram showing an example of an input circuit of a conventional tuner, FIG. 9 is a characteristic diagram showing its selection characteristics, and FIG. The figure shows another example of a conventional input circuit;
FIG. 11 is a characteristic diagram showing the selection characteristics of A. Pin...input terminal, pout...output terminal, VD
4, VD5... Varactor diode, C1a-C1
c, C5, C6-Tl capacitor, p...Trap circuit Fig. 2 Fig. 3

Claims (1)

[Claims] A low-pass filter that has predetermined low-pass characteristics by connecting a combination circuit in multiple stages to a signal source, including a parallel-connected first capacitor, a serially-connected coil, and a parallel-connected second capacitor. and a first varactor diode that also serves as one of the first capacitors in this low-pass filter and that varies the cutoff frequency of the low-pass filter, and a capacitance that is variable in common with the first varactor diode by a tuning voltage. and a trap circuit that creates a trap characteristic section in the low-pass characteristic section and changes the trap characteristic section by changing the capacitance of the second varactor diode. circuit.