JPH0349481Y2 - - Google Patents

Description

[Detailed explanation of the invention] Industrial application field This invention converts the intermediate frequency signal of a UHF tuner into a VHF tuner.
The present invention relates to a high frequency signal coupling circuit in a combination tuner coupled to a tuner.

2. Description of the Related Art A combination tuner including a UHF section and a VHF section has a block configuration as shown in FIG.

In the figure, 1 is a VHF tuner, and 2 is a UHF tuner, each of which includes high frequency amplifiers 3 and 7, bandpass filters 4 and 8, mixing circuits 5 and 9, and local oscillation circuits 6 and 10. The mixing circuit 5 of the VHF tuner 1 and the mixing circuit 9 of the UHF tuner 2 are connected by a tuning circuit 11.
During reception, the mixing circuit 5 of the VHF tuner is used as an amplifier to amplify the IF output of the UHF tuner. Reference numeral 12 in the figure is a low-pass filter, and the IF outputs of each tuner 1 and 2 are outputted to an IF processing circuit (not shown) through this filter 2.

By the way, the above-mentioned tuning circuit 11 has two coils LA and LB which are electromagnetically coupled and arranged as shown in FIG. 4A. However, such electromagnetic coupling requires a large area to be arranged, and to make it more compact, three coil elements L ₁ , L ₂ , and L ₃ are arranged in a T-shape as shown in Figure 2. There is. Here, the self-inductance of the two electromagnetically coupled coils is
If LA, LB and mutual inductance are M, then
There are relationships such as L1=LA−M, L2=LB−M, and L3=M.

In actual use, as shown in FIG. 5, capacitors C1 and C2 are inserted to tune around the intermediate frequency. In the figure, SD is a switching diode that turns on when receiving UHF and turns off when receiving VHF. C3 is VHF tuner mixing circuit 5
The coupling capacitor C4 between is a capacitor connected in series to the coupling coil element L3. For example, as a coupling circuit of this type,
number is known.

Problems that the invention aims to solve By the way, even if the intermediate frequency (IF) is relatively high like in our country, depending on the destination country,
There are places where it is as low as about 30 MHz, and in such cases, it is necessary that the coil elements L ₁ and L ₂ in FIG. 4 have a large inductance. For this reason, cores made of ferrite or the like are currently inserted into the coil elements L ₁ and L ₂ to increase their inductance values.

However, when the cores are inserted into the coil elements L ₁ and L ₂ in this way, the Q of those coil elements L ₁ and L ₂ themselves becomes the coupling coil element consisting of an air-core coil.
This results in a very low value for the Q of _L3 .

Therefore, the high frequency signal coupling circuit consisting of the tuning circuit, switching diode SD, and coupling capacitor C3 shown in Fig. 5 becomes the equivalent circuit shown in Fig. 6 when receiving UHF.
This will lead to Here, since the Q of the coil element L ₂ is low, L ₂ ' and R ₂ are expressed by dividing them into an inductance component and an equivalent resistance component. The inductance value of the coil element _L3 is not so large that it is necessary to downsize it as a core-containing coil, which would increase the cost.

Therefore, since an air-core coil is used, Q is high and there is no need to consider equivalent resistance.

Therefore, the coupling capacitor C ₃ → the capacitance of the switching diode SD → the inductance of the coil element L ₂ L ₂ ′ → the main resonance created by the coil of the coil element L ₃ , and the coupling capacitor C ₃ → the capacitance of the switching diode SD → Equivalent resistance R ₂ of coil element L ₂ → Minor resonance created by the root of coil element L ₃ occurs. Furthermore, since the Q of the coil element _L3 is high, a considerably sharp resonance characteristic is produced.

This secondary resonance frequency may fall into the VHF band because the inductance value of the coil element _L3 is small.

Note that since the inductance value (L ₂ '+L ₃ ) is large, the frequency of the main resonance is sufficiently lower than the VHF reception band and does not cause any particular problem.

Specifically, L3 = 0.7μH, C3 = 6pF, and SD capacitance = 1.3pF, so the resonant frequency is becomes. In other words, the high frequency signal coupling circuit shown in FIG. 5 functions as a circuit that traps the received signal around 190 MHz when receiving VHF, and the reception condition in that frequency band becomes extremely poor.

Means for Solving the Problems The present invention aims to provide a useful high frequency signal coupling circuit that solves the above problems.

To achieve this object, the present invention couples the IF output of the UHF tuner to the VHF tuner through a tuning circuit, and the _IF output frequency is below 40 MHz, and the tuning circuit connects three coil elements to T. The coil element having a letter-shaped configuration is characterized in that each of the three coil elements is a chiyoke assembled coil element.

Embodiment FIG. 1 shows a high frequency signal coupling circuit as an embodiment of the present invention. This circuit is almost the same as the conventional circuit shown in FIG. uses one with a core, and connects capacitors C1 and C2 on both sides of both coil elements L1 and L2 to tune the tuning circuit to a predetermined intermediate frequency (IF). The input side of the tuning circuit is connected to the mixing circuit 9 of the UHF tuner, and the output side is connected to the switching diode SD,
It is connected to the VHF tuner mixing circuit 5 via a coupling capacitor C3. Therefore, the coil element L3 for coupling the chiyoke assembly consists of two coil elements.
Capacitor C4 is connected between the midpoint of L1 and L2 and ground.
It has been inserted through. Since the chiyo assembly is a coil with a core of ferrite etc. inserted, iron loss occurs and the coil element L ₃ is compared to an air core coil.
The Q of the coil element becomes low, and there is no big difference from that of the other coil elements L1 and L2. Therefore, the high frequency signal coupling circuit connected to the mixing circuit 5 of the VHF tuner during VHF reception becomes an equivalent circuit shown in FIG. Here
L ₃ ' and R ₃ represent the inductance and equivalent resistance of the coil element L ₃ , respectively.

Here, in addition to the main resonance due to the route of coupling capacitor C ₃ → capacitance of switching diode SD → inductance of coil element L ₂ L ₂ ′ → inductance of coil element L ₃ L ₃ ′, coupling capacitor C ₃ → Capacitance of switching diode SD → Equivalent resistance of coil element L ₂ R ₂ → Coil element
The first secondary resonance due to the route of the inductance of L ₃ L ₃ ′, and the coupling capacitor C ₃ → the capacitance of the switching diode SD → the equivalent resistance of the coil element L ₂ R ₂ ′ → the inductance of the coil element L ₃ L ₂ ′→Second by the route of equivalent resistance R ₃ of coil element L ₃
A secondary resonance occurs.

However, the first secondary resonance is caused by the coil element L ₃
Since its Q is low (there is an equivalent resistance R ₃ ), it does not have sharp resonance characteristics and does not act as a trap.

Note that the frequency of the main resonance and the frequency of the second sub-resonance are both low and out of the reception band, so there is no problem.

As a result, the trap frequency can be removed from the VHF reception channel, and VHF reception conditions can be maintained in good condition.

Effects of the invention As explained above, according to the invention, the trap frequency of the trap circuit constituted by the high-frequency signal coupling circuit that couples the UHF tuner and the VHF tuner can be made lower than that of the VHF receiving channel. , the VHF reception condition can be maintained in good condition without being affected by the trap circuit.

In addition, since the trap frequency of the trap circuit is lowered by constructing the coupling coil with a choke assembly, it is extremely easy to implement.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a high frequency signal coupling circuit as an embodiment of the present invention, Fig. 2 is a diagram showing an equivalent circuit (trap circuit) when the above circuit is viewed from a mixing circuit of a VHF tuner, and Fig. 3 is a diagram showing a high frequency signal coupling circuit as an embodiment of the present invention. A block diagram showing a UHF/VHF combination tuner using a high frequency signal coupling circuit, FIG. 4 is a diagram showing a tuning coil and an equivalent T-shaped coil, FIG. 5 is a diagram showing a conventional high frequency signal coupling circuit, FIG. 6 is a diagram showing an equivalent circuit (trap circuit) when the circuit of FIG. 5 is viewed from a VHF tuner mixing circuit. FIG. 7 is an equivalent circuit showing the equivalent circuit of FIG. 6 when it resonates. 1...VHF tuner, 2...UHF tuner, L1,
L2, L3...Coil element, L3...Coupling coil element.

Claims (1)

[Scope of utility model registration request] A circuit for coupling an _IF output signal of a UHF tuner to a VHF tuner through an _IF tuning circuit, the circuit comprising:
The I _F output signal is 40MHz or less, and the I _F tuning circuit has three coil elements arranged in a T-shape, and each of the coil elements is a chiyoke assembled coil element. Signal coupling circuit.