JPH0230937Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Field of Application This invention relates to a tuner, such as a VHF tuner, that has a bandpass filter in which the filter constant is switched between a low channel band and a high channel band.

BACKGROUND TECHNOLOGY AND PROBLEMS For example, in a VHF tuner, the constant of a bandpass filter provided therein is switched between low channel band and high channel band, and the constant between the bandpass filter and mixer is changed. is provided with a capacitive coupling circuit.

FIG. 1 shows an example of the essential parts of a VHF tuner 1, in which 10 is a high frequency signal (television signal) amplification circuit, 20 is a bandpass filter having channel switching means and band switching means. , 30 is a mixer, and 40 is a capacitive coupling circuit.

The amplifier circuit 10 uses a quadrupole field effect transistor (FET) 2 in this example as an amplification element for the high frequency signal _SRF , and its first gate _G1 receives a high frequency signal from the terminal 3, in this example the VHF signal _SRF. is supplied to the second gate _G2 , an AGC signal (voltage) V _AGC is supplied from the terminal 4, and a gain-controlled VHF signal _SRF is output from the drain D side. This VHF signal _SRF is supplied to a bandpass filter ₂₀ via a DC cut capacitor C1.

The bandpass filter 20 has a plurality of coupled coils.
It has a filter 21 composed of _L1 to _L5 , and this filter ₂₁ includes switching diodes D1,
Band switching means 22 and 23 consisting of D ₂ are provided, and the switching pulse supplied to the terminal 24
The filter constant can be switched between low channel band and high channel band using P _SW . That is, by supplying the switching pulse P _SW that turns on the switching diodes D ₁ and D ₂ , the inductance of the filter 21 becomes smaller, so in this case, the high channel band becomes the pass band, and the other areas are the diodes. D ₁ and D ₂ are turned off, and the roach channel becomes a passband.

The filter 21 is provided with channel selection means 26 and 27 having variable capacitance diodes D ₃ and D ₄ at its input and output stages to constitute an electronic tuning tuner, and the channel selection voltage supplied to the terminal 29 is
The desired channel is selected by V _CH .

A capacitive coupling circuit 40 provided between the bandpass filter 20 and the mixer 30 includes a coupling capacitor _C2 and a variable capacitance diode _D5 .
Diode _D5 is provided to correct the disparity (deviation) in power gain between bands, and if this diode _D5 is not provided,
The gain in the roach channel band is the curve l ₁ in Figure 2.
Since the gain in the Haitian channel band is as shown by the curve _l2 in the figure, the gain difference _ΔG1 between the bands is very large.

This is because the amplification degree of the FET 2 for high frequency amplification is frequency dependent, and the gain tends to be high in the low channel band.

If a diode _D5 is connected in series with the coupling capacitor _C2 and its capacitance is varied by the channel selection voltage V _CH , the curve _L3 in Figure 2 will be obtained in the low channel band, and the same figure will be obtained in the high channel band. From the curve l ₄ , the coupling capacitor
Gain disparity between bands can be significantly reduced compared to using only _C2 .

However, even with this configuration, the gain difference between bands cannot be made sufficiently small.

Purpose of the invention Therefore, in this invention, the present invention proposes a tuner with excellent distortion characteristics and the like by reducing the gain difference between bands and the gain difference within the same band even more than in the past.

Summary of the invention Therefore, in this invention, the coupling capacitance of the capacitive coupling circuit is made larger in the low channel band and in the high channel band than in the former, thereby reducing the gain difference between bands and within the same band. This is a further reduction than in the case of curves l ₃ and l ₄ in Figure 2.

Specifically, this capacitive coupling circuit is composed of a variable capacitance diode for gain correction and a pair of reactance elements connected in series or parallel, and by selecting these pair of reactance elements according to band switching, Different coupling capacities can be obtained depending on the selected band.

Example Next, the tuner according to this invention is described above.
The application to a VHF tuner will be explained with reference to FIG. 3 and subsequent figures. However, the same reference numerals are given to the parts corresponding to those in the previous figure, and the explanation thereof will be omitted.

In the capacitive coupling circuit _{40 shown in} FIG.
A capacitive coupling means 41 is constructed, which is used as a capacitive coupling means when selecting a low channel band. In addition, this variable capacitance diode _D5 is used in common, and this and the second coupling capacitor
C _b and the switching diode D _b constitute a second capacitive coupling means 42, which is used as a capacitive coupling means when selecting a high channel band.
Therefore, the first coupling capacitor C _a and the diode
The series circuit of D _a and the series circuit of the second coupling capacitor C _b and diode D _b are connected in parallel.

Channel selection voltage V _CH is connected to terminal 2 on the cathode side of variable capacitance diode D ₅ as in the above case.
At the same time, a switching pulse P _SW (FIG. 4) for band switching is supplied from a terminal 44 to the connecting midpoints q ₁ and q ₂ in the above-mentioned series circuit. However, inverter 4 is connected to the connection midpoint q ₁ side.
Switching pulse _SW whose phase is inverted by 5
is supplied.

The first coupling capacitor C _a has a smaller capacitance than the coupling capacitor C ₂ used in Fig. 1, and conversely, the second coupling capacitor C _b has a smaller capacitance than the coupling capacitor C _b . The one with the highest value is used. Therefore, when a low-level switching pulse (the same pulse as the switching pulse supplied to the bandpass filter 20) P _SW as shown in FIG. 4 is supplied, only the diode D _a is turned on. The first capacitive coupling means 41 is selected.

Since C _a <C ₂ , the total coupling capacitance of the first capacitive coupling means 41 is smaller than in the case of FIG. 1, so its gain is lower than in the case of FIG. 1, and the rate of this reduction is Even within the low channel band, the gain is larger at the low end (the side where the channel frequency is low), and there is almost no change at the high end (the side where the channel frequency is high), so the gain within the low channel band is shown in curve ₅ in Figure 2. The characteristics are as follows.

When the high level switching pulse P _SW is supplied, the high channel band is selected and the second capacitive coupling stage 42 is also selected. At this time, since the relationship C _b > C ₂ exists, the total coupling capacitance of the second capacitive coupling main stage 42 becomes larger than in the case of FIG. The rate of increase is larger in the low end than in the high end even within the same Haiti channel band. Therefore, the gain within the channel band has a characteristic as shown in curve _l6 in FIG. 2.

For this reason, the level difference ΔG ₂ between bands is further reduced than before. At the same time, even within the same band, the level disparity between low-end and high-end is reduced, and gain disparity between channels can be eliminated. Therefore, a tuner with excellent distortion characteristics and S/N characteristics can be realized.

In other words, if the power gain of the tuner is too high, the distortion characteristics will deteriorate, and if it is too low, the output S/N will deteriorate.
Therefore, by lowering the gain of the lower channel band as a whole and reducing the gain difference between each channel in each band, both the distortion characteristics and S/N characteristics of the tuner can be improved. .

According to the above configuration, the gain of the lower channel band can be lowered as a whole, and the gain difference between bands is reduced, and the gain difference between channels even within the same band is reduced.
A tuner with excellent distortion characteristics and S/N characteristics can be realized.

FIG. 5 shows another example of the capacitive coupling circuit 40. In this example, a coupling capacitor _Cd is provided in series with the DC cut capacitor _Cc , and a switching diode _Dc is connected to the coupling capacitor _Cd . are connected in parallel. and,
The phase-inverted switching pulse _SW is supplied to the anode side of the switching diode D _c .

According to this configuration, the diode D _c is turned off when switched to the low channel band, so the variable capacitance diode D ₅ and the pair of capacitors C _c and C _d
acts as the first capacitive coupling means 41, and when switched to the high channel band, the diode D _c turns on and the variable capacitance diode ₅ and the capacitor C _c act as the second capacitive coupling means 42. The latter has a larger coupling capacity than the former, and almost the same effect as described above can be obtained. With this configuration, the circuit configuration can be simplified compared to the case of FIG.

Effects of the invention As explained above, in this invention, the coupling capacitance of the capacitive coupling means is made larger in the lower channel band and the higher channel band than in the former, so that the coupling capacity between bands and within the same band is It is possible to significantly reduce the disparity in power gain. Therefore, according to this invention, it is possible to easily construct a tuner that eliminates variation in gain within a band and has excellent distortion characteristics and S/N characteristics.

[Brief explanation of the drawing]

Figure 1 is a connection diagram showing an example of a conventional tuner.
Figure 2 is a gain characteristic diagram to explain its operation, Figures 3 and 5 are connection diagrams showing an example of the main parts of the tuner according to this invention, and Figure 4 is a waveform diagram of switching pulses for band switching. It is. 10 is a high frequency signal amplification circuit, 20 is a band pass filter, 30 is a mixer, 40 is a capacitive coupling circuit,
41 and 42 are first and second capacitive coupling means, C _a
~C _d is a reactance element.

Claims (1)

[Scope of utility model registration request] It has a bandpass filter equipped with a variable capacitance diode for channel selection and a band switching means between a low channel band and a high channel band, and a capacitive coupling circuit is provided between the bandpass filter and the mixer, This capacitive coupling circuit consists of a variable capacitance diode for gain correction and a pair of reactance elements connected in series or parallel.By selecting these pair of reactance elements according to band switching, A tuner that corrects the gain between each channel.