JPH0124982Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an input circuit for a tuner, and particularly to an input circuit for an electronically tuned tuner that selectively switches between a high channel band and a low channel band.

As the reception band of the tuner becomes wider, mismatching occurs due to frequency characteristics in the input circuit between the input signal source and the amplification element for the high frequency amplification stage. Such a mismatch occurs, for example, when a VHF tuner selects a low channel band and a high channel band. Matching with the amplification element side is achieved by changing the step-up ratio and changing the equivalent signal source admittance. Such a change in the metamorphic ratio between the high channel band and the low channel band may not be satisfied depending on the amplification element used. For example, in a FET amplifier element, the conductance increases as the frequency increases, and the real part of the equivalent input admittance strongly depends on the frequency. Therefore, securing the necessary band in the high channel band by changing the impedance change ratio reduces the conductance in the low channel band, making the band under matching conditions extremely narrow. On the other hand, if the required band is secured in the low channel band, the high channel band becomes too wide, making it impossible to receive the desired band. That is, matching by changing the admittance on the power supply side has a degree of freedom for the high channel band, but since the resistance on the FET side cannot be lowered, matching for the low channel band is restricted. Further, in order to obtain a predetermined band for each band, a damping element may be inserted into the input circuit, but this is not satisfactory because it causes a loss of the input signal and deteriorates the NF.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to overcome the above-mentioned deficiencies and to provide an improved input circuit for a tuner.

According to the present invention, the ON-OFF switching of the band switching diode in the tuner input circuit
Using this operation, the equivalent input admittance on the amplifier element side is changed between the high channel band and the low channel band. In other words, an inductance circuit that has two coils connected in series and supplies an input signal to a common connection point of each coil, a capacitance circuit that includes a variable capacitance element connected in parallel to this inductance circuit, and a high-channel coil and band switch. A band switching circuit that connects switching elements in series and connects this coil in parallel to an inductance circuit when selecting a Haitian channel band, and connects two series-connected capacitors in parallel to this Haitian channel coil, and connects these capacitors in a common connection point. An input circuit for a two-band tuner is provided which includes a matching circuit coupled to the input side of an amplifying element. When receiving the Haiti channel band with the switching element in a conductive state, this input circuit connects the Haiti channel coil and two series-connected capacitors to the main tuning circuit, which includes an inductance and a capacitance circuit, in a high-frequency parallel relationship. The equivalent input admittance on the amplification element side seen from the input main tuning circuit is set to a predetermined value suitable for the reception frequency of the Haitian channel, and the switching element is cut off.When receiving the low channel band, the main tuning circuit and Two capacitors substantially in parallel are inserted between the input sides of the amplification element, and the equivalent input admittance on the amplification side viewed from the input main tuning circuit is set to a predetermined value suitable for the receiving frequency of the low channel. Therefore, as the channel band is switched, the input impedance of the amplifying element having frequency characteristics is matched, and the equivalent input admittance on the amplifying element side as seen from the input circuit is changed, thereby always enabling good reception.

Embodiments of the present invention will be described in detail below with reference to the drawings. Figure 1 shows the input circuit of a VHF tuner according to the present invention, in which the input circuit as a preselector of the present invention is coupled between a signal input terminal 1 connected to an antenna and a high frequency amplification element 2 which is an FET. . This preselector circuit includes a tuning inductance circuit 3, a tuning capacitance circuit 4, and a band switching circuit 5.
and a matching circuit 6. The tuned inductance circuit 3 consists of a series circuit of a first coil 7 and a second coil 8.
An input signal from a signal input terminal 1 is supplied to a connection point 9 of 8. The tuning capacitance circuit 4 consists of a variable capacitance diode element 12 to which a tuning voltage is applied from a tuning voltage supply terminal 10 via a resistor 11 and a DC blocking capacitor 13, and works together with the tuning inductance circuit 3 to form a main tuning circuit. do.

On the other hand, in order to configure the VHF tuner with two bands, a band switching circuit is constructed by connecting a third coil 14 for high channel and a switching diode 15 for band switching in series, and further connecting a capacitor 16 for DC blocking and high pass. 5 is connected in parallel with the main tuning circuit in an alternating current manner. The switching diode 15 operates in an ON-OFF state of conduction or cutoff by the switching voltage applied from the band switching voltage supply terminal 17 through the resistor 18, and when it is conductive, the third coil 14 for the high channel is connected to the tuning inductance. The circuit 3 or the capacitive circuit 4 is connected in high frequency in parallel to enable high channel band reception, and when it is cut off, the third coil 14 having a relatively small inductance value is disconnected to enable low channel band reception. do. Here, two capacitors 19 forming a matching circuit 6,
The capacitance value of 20 is determined according to the characteristics of the amplifying element 2 with respect to the receiving frequency of each band depending on the operating state of the band switching circuit 5, and a matching state is obtained under a predetermined band in any receiving band. I can do it.

2 and 3 are high frequency equivalent circuits when receiving high channel band and low channel band in the input circuit of the VHF tuner shown in FIG. 1. Here, the FET amplifying element 2 is represented by an equivalent capacitance C ₃ and a resistance r _p , and the input signal power supply side is a resistance r _i
(=75Ω). When receiving the Haitian channel band, the switching diode 15 becomes conductive, forming the equivalent circuit shown in FIG. 2. In this case, what should be noted is the capacitors 19 and 20 of the matching circuit.
is coupled to the input on the amplifier element side in a state where the input signal is voltage-divided. Here, the inductance values of the respective coils 7 and 8 of the tuning inductance circuit 3 are specifically indicated by L ₁ (=200nH) and L ₂ (=40nH), and the respective capacitance values of the respective capacitors 19 and 20 of the matching circuit 6 are Specifically, expressed as C ₁ (=10pF) and C ₂ (=4pF), the admittances Y _i and Y _p and conductances 1/R _i and 1/R _p on the power supply side and amplification side as seen from the tuning circuit are as follows. It is calculated using the following formula. First, on the power supply side, Z _i =1/1/r ₁ +1/jωL ₂ +jωL ₁ = r _i (jωL ₂ )/r
_i +jωL ₂ +jωL ₁ =jωr _i (L ₁ +L ₂ )−ω ² L ₁ L ₂ /r _i +jωL ₂ ∴Y _i =r _i +jωL ₂ /jωr _i (L ₁ +L ₂ )−ω ² L ₁ L ₂ = (r _i +jωL ₂ ) {−ω ² L ₁ L ₂ −jωr _i (L ₁ +L ₂ )}
/(ω ² L ₁ L ₂ ) ² +ω ² r _i ² (L ₁ +L ₂ ) ² = {ω ² r _i L ₂ (L ₁ +L ₂ )−ω ² r _i L ₁ L ₂ }−jω{r _i ² (
L ₁ +L ₂ )−ω ² L ₁ ² L ₂ }/(ω ² L ₁ L ₂ ) ² +ω ² r _i ² (L ₁ +L ₂
) ² = 1/R _i +A, then ∴R _i = (ω ² L ₁ L ₂ ) ² + ω ² r _i ² (L ₁ + L ₂ ) ² /ω ² r _i L ₂ ² = (L ₁ + L ₂ /L ₂ ) ² r _i +ω ² L ₁ ² /r _i When calculating R _i in the specific example, the first term is approximately 2.7 kΩ, and the second term of frequency fluctuation is in the low band (f =
Approximately 50Ω at 50MHz), high band (f = 200MHz)
It becomes about 800Ω. Therefore, in the low band, the first
can be ignored for terms. In addition, although it affects the high band, the inductance _L2 of the coil 8 is adjusted to secure the band in the high channel, and the matching condition depends on the amplification side (here, r _p is the high band resistance).

Next, on the amplification side, Z _p =1/jω(C ₂ +C ₃ )+1/r ₀ +1/jωC ₁ =r _p /1+jω(C ₂ +C ₃ )r _p +1/jωC ₁ =1+jω(C ₂ +C ₃ ) r _p +jωC ₁ r _p /jωC ₁ −ω ² C ₁ (
C ₂ +C ₃ ) r _p = 1 + jωr _p (C ₁ +C ₂ +C ₃ )/jωC ₁ −ω ² C ₁ (C ₂ +C
₃ ) r _p ∴Y _p = jωC ₁ −ω ² C ₁ (C ₂ + C ₃ ) r _p /1 + jωr _p (C ₁ + C
₂ +C ₃ ) = {(jωC ₁ −ω ² C ₁ (C ₂ +C ₃ ) r _p }・{1−jωr _p
(C ₁ +C ₂ +C ₃ )}/1+ω ² r _p ² (C ₁ +C ₂ +C ₃ ) ² =ω ² r _p C ₁ (C ₁ +C ₂ +C ₃ )−ω ² r _p C ₁ (C ₂ +C ₃ )/1
+ω ² r _p ² (C ₁ +C ₂ +C ₃ ) ² +jωC ₁ +jω ³ r _p ² C ₁ (C ₂ +C ₃ )・(C ₁ +C ₂ +C ₃ )
/1+ω ² r _p ² (C ₁ +C ₂ +C ₃ ) ² =1/R _p +B ∴R _p =1+ω ² r _p ² (C ₁ +C ₂ +C ₃ ) ² /ω ² r _p C ₁ ( C ₁ +C ₂
+C ₃ )−ω ² r _p C ₁ (C ₂ +C ₃ ) =1+ω ² r _p ² (C ₁ +C ₂ +C ₃ ) ² /ω ² r _p C ₁ ² =(C ₁ +C ₂ +C ₃ /C ₁ ) ² r _p +1/ω ² C ₁ ² r _pThe second term of the resistance component R _p in the above equation is (ωC ₁ ) ² r _p = constant and can be ignored in calculations.

On the other hand, when receiving the low channel band, the switching diode 15 is cut off to form the equivalent circuit shown in FIG. 3, and the matching circuit capacitor 19,
20 is to couple the main tuning circuit and the amplification element in parallel relationship. In the figure, the inductance value L ₃ of the high-channel coil 14 is selected to be around 40 nH, and the apparent capacitive impedance is compared to the capacitance C ₂ (=4 to 8 pF) of the capacitor 20 connected in series with it. It is only 10-30% larger, but can be ignored since it has a beneficial effect on alignment. Therefore, the equivalent input admittance on the amplification element 2 side seen from the dashed-dotted line A-A where the main tuning circuit is located in Fig. 1 is changed by band switching, and a good reception condition is obtained over the entire channel. .

In other words, the amplification side is a Haitian channel (f=
200MHz) and roach channel (f=50MHz)
For each selection in the case of , the ratio of the resistance R when each band is selected is as follows.

(For Haitian channel) R _p = (C ₁ +C ₂ +C ₃ /C ₁ ) ² × r _p , where r _p ≒1000Ω (For Low channel channel) R _pL = (C ₁ +C ₂ +C ₃ /C ₁ +C ₂ ) ² × r _pL , where r _pL ≒40
00Ω ∴R _pL /R _p = (C ₁ /C ₁ +C ₂ ) ² × r _pL / r _p = (10/14) ² × 4≒2.0 On the other hand, if each channel is used in the equivalent circuit shown in Figure 2, Then, this ratio becomes:

R _pL ′/R _p ′=(C ₁ +C ₂ +C ₃ /C ₁ ) ² /(C ₁ +C ₂ +C ₃ /C ₁
) ² ×r _pL /r _p =4.0 Therefore, in the present invention, when switching channel bands, the variation between bands is halved and the matching state is improved.

As described above, unlike the conventional case of changing the equivalent power supply admittance on the signal source side, the present invention changes the equivalent admittance on the amplification element side, so the use of a damping element for band adjustment for the frequency characteristics of the amplification element can be avoided. Incidentally, among the main constituent elements in the embodiment, the specific values used for the coil and the capacitor can be set within the following ranges. In addition, the coil inductance is Japan channel, US channel,
It can also be varied by different frequency coverage ranges such as European channels.

1st coil 7...around 200nH, 2nd coil 8...
...about 60 nH, third coil 14...about 40 nH, first capacitor 19...5 to 10 pF, and second capacitor 20...4 to 8 pF.

[Brief explanation of the drawing]

FIG. 1 is an input circuit diagram of a VHF tuner according to the present invention, and FIGS. 2 and 3 are equivalent circuit diagrams of the tuner of FIG. 1 when receiving a high channel band and a low channel band, respectively. 1...Antenna side input terminal, 2...Amplification element,
3... Tuning inductance circuit, 4... Tuning capacitance circuit, 5... Band switching circuit, 6... Matching circuit,
7...First coil, 8...Second coil, 12...
Variable capacitance diode, 14...Third coil, 15
...Switching diode, 19...First capacitor, 20...Second capacitor.

Claims (1)

[Scope of utility model registration request] a tuned inductance circuit in which an input terminal is coupled to a connection point between a first coil and a second coil connected in series;
A tuning capacitor circuit in which a variable capacitance diode to which a tuning voltage is applied is connected in parallel with the inductance circuit at high frequency, and a switching diode to which a band switching voltage is applied are connected in series to the third coil. A band switching circuit connects a third coil in parallel with the inductance circuit at high frequency, and a series circuit of a first capacitor and a second capacitor is connected in parallel to the third coil, and the connection point of these capacitors is connected to the input of the FET amplifier element. The input circuit of the tuner with a matching circuit coupled to the side.