JPH0278315A - Inter-stage dual tuning circuit - Google Patents

Abstract

PURPOSE:To reduce the deviation of the band width at a full tuning frequency band and the loss characteristic by splitting a secondary tuning coil of inter- stage dual tuning circuit of a tuner receiving a television signal, and connecting a tap of the splited point to an input terminal of a mixer circuit. CONSTITUTION:The primary tuning circuit consists of a coil 8 and a varactor diode 11 and the secondary tuning circuit consist of a series inductance comprising the coils 9, 10 and a varactor diode 12 and each tuning circuit forms a resonance circuit in an inter-stage dual tuning circuit 4. Moreover, the coils 8, 9 are coupled inductively to form a dual tuning circuit. Furthermore, a tap of the intermediate point of the coils 9, 10 in the inter-stage dual tuning circuit 4 is connected to an input terminal of a mixer circuit and the dual tuning circuit with less loss over the range of the change in the tuning frequency and less band width deviation is constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the configuration of an interstage double-tuned circuit for a tuner that receives television signals.

[Conventional technology]

In conventional interstage double-tuned circuits such as television tuners, the mixer and the secondary side tuning circuit of the interstage double-tuned circuit are coupled using a fixed capacitor, as shown in Figure 3 of Japanese Patent Publication No. 59-12205. .

[Problem to be solved by the invention]

The above-mentioned conventional technology has no problem when a mixer circuit with high input impedance and an interstage double-tuned circuit are coupled with a small fixed capacitance. However, some mixers have a FET with the gate grounded and input the RF multiplied signal into the source terminal, while others have a transistor with the base grounded and input the RF multiplied signal into the emitter terminal.
The one that inputs the doubled signal has excellent distortion performance such as third-order distortion.
The input impedance of these mixers ranges from a few + ohms to several hundred ohms.
and low. Therefore, the Q of the secondary side resonant circuit of the interstage double-tuned circuit is
In order to increase
Since the Q of the secondary-side resonant circuit becomes higher than necessary, the bandwidth becomes narrower, the loss increases, and there is a problem that a deviation in the bandwidth or loss characteristics occurs in the entire tuning frequency band.

The object of the present invention is to provide an interstage double-tuned circuit, an input impedance,
When connecting a mixer circuit with low resistance, increase the equivalent load resistance of the mixer circuit to a nearly constant value regardless of one frequency, and reduce the bandwidth deviation and loss characteristic deviation in the entire tuning frequency band. There is a particular thing.

[Means to solve the problem]

The above object is achieved by dividing the secondary side tuned coil of the interstage double-tuned circuit, taking a tap from the dividing point, and connecting it to the input end of the mixer circuit.

[Effect]

This was obtained by dividing the secondary tuning coil of the interstage double-tuned circuit. The two coils operate like a step-up circuit. As a result, the equivalent load resistance of the low input impedance mixer circuit connected to the interstage double-tuned circuit is increased to a nearly constant value regardless of the frequency, and the bandwidth deviation in the entire tuning frequency band is increased. and deviations in loss characteristics are reduced.

〔Example〕

A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of one embodiment of the present invention.

1 is an input terminal, 2 is a high-frequency amplification circuit, and the amplified signal is inputted via a coupling capacitor 3 to an interstage double-tuned circuit 4 shown in a portion surrounded by a dotted line. The output signal from the interstage double tuning circuit 4 is as follows.

is input to the mixer circuit 6 via the coupling capacitor 5,
Derived from terminal 7. In the interstage double-tuned circuit 4, a primary side tuning circuit is composed of a coil 8 and a variable capacitance diode 11, a secondary side tuning circuit is composed of a series inductance of a coil 9 and a coil 10, and a variable capacitance diode 12, Each forms a resonant circuit. Further, the coil 8 and the coil 9 are inductively coupled to form a double tuned circuit. Further, 13 and 14 are bias resistors, 15 is a bypass capacitor, and 16 is a tuning voltage application terminal. Interstage double tuning circuit 4
As shown in FIG. 1, a tap is taken from the midpoint between the coils 9 and 10, and the tap is connected to the input terminal of the mixer circuit. Currently, in mixer circuits, the gate of the FET is grounded, taking into consideration the third-order distortion characteristics of the mixer, and the RF
Since the circuit configuration is such that the multiplied signal is input, or the base of the transistor is grounded and the RF multiplied signal is inputted from the emitter, the input impedance is low, ranging from several +Ω to several hundred Ω. Also, L due to coil 9 and coil 10
by step-up circuit.

If the equal input load resistance of the mixer is RL, then the second order LL,
L2 is the inductance of the coil 10 and the coil 9, respectively. The circuit configuration in which the L step-up circuit is connected to the low-impedance mixer circuit equivalently increases the degree of coupling at low frequencies compared to the conventional circuit configuration in which the L step-up circuit is connected to the cathode of the variable capacitance diode 12 with a small capacitance. , it has the advantage of securing a bandwidth at a low tuning frequency and having little loss. This makes it possible to construct a double-tuned circuit with less loss and less bandwidth deviation over the entire tuning frequency change range.

FIG. 2 shows a second embodiment of the invention, in which a variable capacitance diode 17.1 is connected in series with the tuning inductances 8, 9 of FIG.
It has a configuration in which 8 are inserted respectively. This circuit is
At the same time as changing the capacitance of the parallel variable capacitance diodes 11 and 12, the capacitance of the series variable capacitance diodes 17 and 18 is also changed, equivalently changing the parallel inductances 8 and 9 as well. It constitutes a tuned circuit. The step-up operation of this circuit is similar to that of the circuit of FIG. In the parallel tuned circuit configuration shown in Fig. 1, there are cases where the value of the step-up coil 10 must be selected to be small. There were cases where the change occurred. However, by adopting a circuit configuration as shown in FIG. 2, it is possible to select a large value for the step-up coil 10, and it is possible to further reduce variations in the degree of coupling.

FIG. 3 shows a third embodiment of the present invention, and is a circuit diagram in which a band switching circuit is added to the circuit of FIG. 2.

In FIG. 3, in the interstage double-tuned circuit 4, the primary side tuning circuit is composed of coils 8, 19 and variable capacitance diodes 11, 17, and the secondary side tuning circuit is composed of coils 9, 10, 20, .
21 and variable capacitance diodes 12 and 18, each forming a resonant circuit. Further, the coil 8 and the coil 9, and the coil 19 and the coil 20 are inductively coupled to form a double tuned circuit. 22 and 23 are high frequency switching diodes that switch the inductance value of the resonant circuit between when receiving VHF low band and when receiving VHF high band. Also, 24, 25, 26° 27.28.29
30, 31°, 32.33 are bypass capacitors, and 34.35 are band switching voltage application terminals. The operation when the interstage double tuning circuit 4 shown in FIG. 3 is used in a VHF tuner will be explained. When receiving VHF low band,
A positive voltage is applied to the band switching voltage application terminal 35.

A reverse voltage is applied to diode 22.23 via bias resistor 28, placing diode 22.23 in a blocking state. As a result, the main tuning coils are coil 8 and coil 19.
, the coil 9, the coil 10, the coil 20, and the coil 21 become series inductances, respectively, and VHF low band reception becomes possible. Also, when receiving VHF high band,
A positive voltage is applied to the band switching voltage application terminal 34, current flows through the diode 22.23 through the bias resistor 26.27, and the diode 22.23 becomes conductive. As a result, the connection point between the coil 8 and the coil 19 is grounded at high frequency, and the connection point between the coil 9 and the capacitor 31 and the connection point between the coil 2o and the coil 21 are short-circuited at high frequency. The main tuning coil at this time is coil 8. Coil 9 and Coil 2
Each has a series inductance of 1, making it possible to receive VHF high band. The circuit shown in Fig. 3 is a circuit in which a switching diode 22°23 is added to the circuit shown in Fig. 2.
F changes the inductance of the resonant circuit, forming a double-tuned circuit with a wider variation range. Third
The L step-up during VHF low band reception of the circuit shown in the figure is coil 9 and coil 10, coil 20 and coil 2.
1, respectively, and has the same effect as the circuit shown in Fig. 1, but more than the circuit shown in Fig. 2.
It is possible to select the value of the step-up coil 10.20 over a wider range, and it is possible to further reduce variations in the degree of coupling. FIG. 4 shows a high frequency equivalent circuit diagram during VHF high band reception. In FIG. 4, the L step-up operation is determined by coil 9 and coil 21, and has the same effect as the circuit shown in FIG. 1. At this time, the coil 36 is equivalently connected to the load of the mixer circuit 6 as a parallel inductance of the coil 1o and the coil 20. This has the effect of reducing the impact of capacity.

〔Effect of the invention〕

According to the present invention, coupling between a low input impedance mixer such as a source input of a common-gate FET or an emitter input of a common-base transistor and an interstage double-tuned circuit is achieved by dividing the secondary inductor of the tuned circuit. , L step-up configuration has the effect of reducing bandwidth deviation and loss characteristic deviation in the tuning frequency change range.

[Brief explanation of the drawing]

1 is a circuit diagram of a first embodiment of the present invention, FIG. 2 is a circuit diagram of a second embodiment of the present invention, FIG. 3 is a circuit diagram of a third embodiment of the present invention, and FIG. The figure is a high frequency equivalent circuit diagram at the time of VHF high band reception in FIG. 3. 2...High frequency amplifier circuit, 6...Mixer circuit, 22°23...High frequency switching diode. 1st collection 2 V name 3 Z collection 4- times

Claims (1)

[Claims] 1. a primary side tuning circuit coupled to the output of the high frequency amplifier circuit and comprising a first variable capacitance diode and a first inductive impedance; a second variable capacitance diode, a second inductive impedance and a third inductive impedance; The first inductive impedance and the second inductive impedance are inductively coupled to a secondary side tuned circuit consisting of a series connection of impedance circuits, and the connection point of the second and third inductive impedances is connected to the mixer circuit. An interstage double-tuned circuit characterized by being coupled.