JPS626754Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to a VHF tuning tuner for a television receiver. Generally, in the interstage circuit of the electronic tuning tuner of a television receiver, the equivalent circuit of the interstage circuit when receiving the low channel channel of the VHF band is the first one.
As shown in the figure. In this equivalent circuit, the inductance of the primary coil connected to the output side of the high frequency amplifier is _L1 , the inductance of the secondary coil connected to the local oscillation circuit and mixing circuit side is _L2 , and the mutual inductance is expressed by M. ing. In Figure 1, the loads on the primary side and secondary side are respectively
Assuming Q ₁ and Q ₂ , the loads Q ₁ and Q ₂ are expressed by the following equations. Q ₁ =ω ₀ Ct ₁ /gt ₁ …(1) Q ₂ =ω ₀ Ct ₂ /gt ₂ …(2) However, Ct ₁ and Ct ₂ are the total capacitance of the primary side and the secondary side, gt ₁ , gt ₂ is the total conductance of the primary side and secondary side, respectively. ω ₀ is 2π ₀ when the tuning frequency is set to ₀ . Also the coupling coefficient The coupling coefficient is expressed as K=k√ ₁・₂ (4). According to experiments, the above-mentioned coupling coefficient k≈0.115 has been obtained for domestic television broadcast channels. Regarding the adjusted waveform, since the waveform of the Haitian channel can be adjusted to near the critical coupling, it becomes a waveform between the maximum flatness characteristic and the delay flatness characteristic. Therefore, unless the low channel channel is adjusted in the same way as the high channel channel, there will be a difference in the waveform of the tuner as a whole, which is not desirable. Therefore, the value of the coupling index K is "1" to obtain the maximum flatness characteristic and "1" to obtain the delayed flatness characteristic.

Must be between [expression]. That is, substituting the conditions k=0.115 0.6≦K≦1 into equation (4), it is necessary that 5.2≦√ ₁・₂ ≦8.7 (5). However, in conventional electronically tuned tuners, when receiving the high channel, the gain is approximately 10 dB lower than that of the low channel due to the effects of the series resistance of the switching diode, etc., so a damping resistor Rd is inserted in parallel with the low channel coil to reduce the gain deviation.
I try to keep it within 6dB. Inserting this damping resistor Rd increases the overall conductance of the primary and secondary sides in equations (1) and (2) above, so the load on the primary and secondary sides is
Q ₁ and Q ₂ will decrease. If the above equation (5) is no longer satisfied due to the decrease in the loads Q ₁ and Q ₂ , adjustment becomes difficult. The value of the damping resistance Rd that satisfies the above formula (5) is 1KΩ≦Rd≦1.8KΩ (6). Equations (5) and (6) above are calculated assuming that the mutual inductance M is constant (approximately 25n ^H in actual measurements), but since the value of this mutual inductance M can be decreased, the above equation (6) There is no need to set an upper limit for this. That is, damping resistance Rd
should be 1KΩ≦Rd…(7). The above is a brief explanation of the conditions for the interstage circuit of an electronically tuned tuner associated with channel reception, but in reality, it is difficult to make the loads Q ₁ and Q ₂ on the primary and secondary sides the same size. . Now, if we set the coefficient b as b=Q ₁ /Q ₂ +Q ₂ /Q ₁ ...(8), the value of this coefficient b will not be "2",
The larger the difference between the primary side load Q ₁ and the secondary side load Q ₂ is, the larger the difference becomes. Also, at this time, the bonding index K is The value of this coupling index K also becomes large, and as a result, the left term of the above equation (7) becomes larger than 1KΩ. Furthermore, when the gain deviation between the low channel and high channel is increased to the allowable limit, the actual damping resistance Rd needs to have a value of around 1KΩ. The value of Rd is by no means a margin. As described above, conventional techniques have had the disadvantage that in order to improve the gain deviation between the high channel and the low channel, the waveform, selectivity, etc. in the low channel must be sacrificed. The present invention was made to improve the above-mentioned disadvantages, and by providing a circuit that switches the gain of the intermediate frequency amplifier circuit between the low channel band and the high channel band of the VHF band, the waveform and selection in the low channel band can be improved. An object of the present invention is to provide a VHF electronically tuned tuner capable of improving gain deviation between a low channel band and a high channel band without sacrificing performance. An embodiment of the present invention will be described below with reference to the drawings. Figure 2 shows the circuit of the VHF electronic tuning tuner of the present invention, where 11 is a high frequency amplification circuit;
2 is an interstage circuit (tuned circuit) connected to the output end of the high frequency amplification circuit 11; 13 is this interstage circuit 1;
A local oscillation circuit 14 is connected to the output of 2, and a mixing circuit 14 mixes the outputs from the interstage circuit 12 and the local oscillation circuit ₁₃ , which are supplied via the coupling capacitor C1.This mixing circuit has negative feedback. It is taking place. Further, 15 is an intermediate frequency tuning circuit, 16 is an intermediate frequency amplification circuit that amplifies the output from the intermediate frequency tuning circuit 15, which is supplied via a coupling capacitor C ₂ , and 17 is supplied via a coupling capacitor C ₃ An emitter follower type output circuit amplifies the output from the intermediate frequency amplification circuit 16, and 18 is an output terminal for supplying the output from the output circuit 17 to the next stage video intermediate frequency circuit via the coupling capacitor _C4 . be. Further, 19 is a switching circuit for switching the operation of the interstage circuit 12 and intermediate frequency amplification circuit 16 between high channel reception and low channel reception. One end of the primary side of the interstage circuit (tuned circuit) 12 is connected to the output end of the high frequency amplifier circuit 11,
Primary side coils _L1 , _L2 whose other ends are grounded via capacitor _C5 , capacitor _C6 and variable capacitance diode _VCD1 connected between one end of this primary side coil _L1 and the ground. , the primary coil L ₁
It has a switching diode D 1 whose cathode is connected to the connection point of the coil L ₂ and the switching diode D ₁ whose anode is grounded via a capacitor C ₇ . Furthermore, the secondary side of this circuit is the same as the primary side, as well as the secondary side coil.
L ₃ and L ₄ , capacitor C ₇ and variable capacitance diode
VCD ₂ , switching diode D ₂ , capacitor
The circuit is composed of _C8 . In addition, the variable capacitance diodes VCD ₁ and VCD ₂ on the primary side and secondary side
A channel selection DC voltage is supplied to the anode side of each of the channels through resistors R ₁ and R ₂ , respectively. On the other hand, the anode of the switching diode D ₁ on the primary side is grounded via the resistors R ₃ , R ₄ , R ₅ and the voltage E ₁ of the switching circuit 19, and the anode of the switching diode D ₂ on the secondary side is grounded. It is grounded via the resistor _R6 , the resistors _R4 and _R5 , and the power source _E1 . Also, the primary coil L ₂ and the secondary coil L ₄
The common connection point with the switch SW is connected via a resistor _R7 to the make contact side of the switch SW whose break contact side is grounded, and the negative electrode side is connected to the positive electrode side of the power source _E2 whose negative electrode side is grounded. This switch SW functions as a switch for switching between the roach channel and the high channel channel. The mixing circuit 14 has cascaded transistors Q ₁ and Q ₂ . The base of this transistor _Q1 is connected to one end of the coupling capacitor _C1 and is grounded via a resistor _R8 .
The emitter is grounded via a resistor _R9 and also via a capacitor _C9 , and the collector is connected to the emitter of the transistor _Q2 . The base of this transistor Q ₂ is a capacitor
It is grounded via _C10 and connected to the base of the transistor _Q1 via a resistor _R10 . Tuning coil in the intermediate frequency tuning circuit 15
One end of _L5 is the transistor _Q2 of the mixing circuit 14.
The other end of the coil _L5 is connected to the coupling capacitor _{C2 and} to the ground via the capacitor _C12 , and to one end of the choke coil _L6 . has been done. This chiyoke coil L ₆
The other end is grounded via a capacitor C ₁₃ and connected to the base of the transistor Q ₂ of the mixing circuit 14 via a resistor R ₁₁ . Further, the intermediate frequency amplification circuit 16 _has a transistor _{Q 3} whose base is connected to the other end of the coupling capacitor C _{2 .} The collectors are connected to one end of a coupling capacitor C ₃ and to the positive electrode of the power source E ₁ of the switching circuit 19 via a resistor R ₁₄ . Further, the base of the transistor Q ₃ is connected to the positive power source of the power source E ₁ via the resistors R ₁₅ and R ₅ of the switching circuit 19. The output circuit 17 has an emitter follower type transistor _Q4 whose base is connected to the other end of the coupling capacitor _C3 . The base of this transistor Q ₄ is grounded via a resistor R ₁₆ and also via a resistor R ₁₇ and the capacitor C ₁₃ , and the collector is grounded via a capacitor C ₁₄ as well as a resistor R ₁₈ and the capacitor C ₁₃ , and the emitter is grounded via a resistor _R19 and connected to the output terminal ₁₈ via a coupling capacitor C4. Next, the operation of the electronic tuning tuner constructed as described above will be explained. Now, when this electronic tuning tuner is receiving the Haiti channel, switch SW
is in a short-circuited state, so the switching diodes D ₁ and D ₂ are conductive. If the total current flowing through these switching diodes D ₁ and D ₂ is Id, the voltage V _BH applied to the base of the transistor Q ₃ of the intermediate frequency amplifier circuit 16 is V _BH = (E ₁ - R ₅ × Id) R ₁₂ /R ₁₅ +R ₁₂ . Note that since a value of approximately 1KΩ is used for the resistor _R5 , the voltage drop due to the base current of the transistor _Q3 can be ignored. On the other hand, when receiving the low channel channel, the switch SW is open, so the switching diode D ₁ ,
D ₂ is in the cut-off state. At this time, the voltage V _BL applied to the base of the transistor Q ₃ of the intermediate frequency amplification circuit 16 is given by V _BH = E ₁ × R ₁₂ /R ₅ + R ₁₅ + R ₁₂ , and the voltage V when receiving this low channel is
_BL becomes higher than the voltage V _BH when receiving the Haitian channel. Therefore, the emitter current of the transistor _Q3 of the intermediate frequency amplifier circuit 16 becomes larger during low channel reception. Therefore, the resistance R ₃ of the switching circuit 19 is adjusted so that the maximum gain is achieved when receiving the Haiti channel.
~R ₇ , R ₁₅ and resistance of intermediate frequency amplification circuit 16
By setting the values of R ₁₂ and R ₁₃ , the gain decreases during low channel reception. The amount by which this gain decreases can be freely changed by resetting the values of the resistors R ₃ to R ₇ and the resistors R ₁₂ and R ₁₃ . Therefore, according to this circuit, it is possible to improve the gain deviation at the time of low channel. Furthermore, as is well known, when the operating current of the intermediate frequency amplification circuit 16 is changed, the input impedance of the circuit changes, which may lead to a change in waveform. This is prevented by providing an intermediate frequency harmonic circuit 15 at the center. That is, the output impedance of the mixing circuit 14 is very high, but the input impedance of the intermediate frequency amplifier circuit 16 is very high.
can be very low depending on This can be achieved by making the value of capacitor C ₁₂ of intermediate frequency tuning circuit 15 relatively large (usually 3 to 6 times larger) than the value of capacitor C ₁₁ . Furthermore, since the absolute value of the input capacitance value of the intermediate frequency amplifier circuit 16 is considerably smaller than the capacitance value of the capacitor _C12 , the influence on the intermediate frequency waveform is extremely small. Furthermore, since the output terminal of the intermediate frequency amplifier circuit 16 is connected to the input terminal of the emitter follower transistor _Q4 in terms of high frequency, the overall waveform seen from the output terminal 18 becomes stable. Further, the present invention has the advantage that the dynamic range of the tuner as a whole does not change due to the operating current of the intermediate frequency amplification circuit 16. As explained above, according to the present invention, by providing a circuit that switches the gain of the intermediate frequency amplifier circuit between the low channel band and the high channel band of VHF, waveform and selectivity can be sacrificed with a simple circuit. It is possible to provide a VHF electronically tuned tuner that can improve the gain deviation between the low channel band and the high channel band without having to do so.

[Brief explanation of the drawing]

Figure 1 is an equivalent circuit diagram of the interstage circuit of a general electronically tuned tuner when receiving a low channel.
The figure is a circuit diagram of a VHF electronic tuning tuner which is an embodiment of the present invention. 12...Interstage circuit (tuned circuit), 14...Mixing circuit, 15...Intermediate frequency tuning circuit, 16...Intermediate frequency amplification circuit, 19...Switching circuit, _D1 , _D2 ...
switching diode.

Claims (1)

[Scope of utility model registration request] an intermediate frequency tuning circuit having capacitors on the input and output sides, interposed between the mixing circuit and an intermediate frequency amplification circuit having an amplification transistor having a bias resistor between the base and ground, and placed in the input path of the mixing circuit; a switching diode for switching the tuning inductance according to the high and low channels; at least two series resistors connected in series between one end of the diode and the power supply; and a connecting point of the resistors and the intermediate DC coupling means that connects the bases of the amplification transistors constituting the frequency amplification circuit and supplies a bias voltage to the bases of the amplification transistors; and a DC coupling means that is DC coupled to the other end of the switching diode for switching the diode. 1. A VHF electronic tuning tuner, comprising: a switching circuit, wherein the output side capacitance of the intermediate frequency tuning circuit is set larger than the input side capacitance.