JPS6127218Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bandpass filter suitable for use in an antenna input circuit of an electronically tuned receiver using a variable capacitance diode.

The bandpass filter of this invention is an electronically tuned reception type in which the distance from the antenna to the high frequency amplification circuit is long, as in car radios, etc., and the distributed capacity of the antenna input circuit is increased by the coaxial cable connecting therebetween. It is extremely effective when used on machines.

It is well known that when the antenna input circuit of a receiver is made of a non-tunable type, various radio waves outside the band of the broadcast radio waves desired to be received are inputted to the high frequency amplifier circuit, which tends to cause cross-modulation phenomena.

Generally, in order to prevent this cross-modulation phenomenon, measures are used such as making the antenna input circuit a tuned type or providing a bandpass filter in a non-tuned type antenna input circuit. By the way, in the case of an electronically tuned receiver, using a variable capacitance diode to make the antenna input circuit a tuned type is difficult because the capacitance value of the variable capacitance diode cannot be as large as that of a variable capacitor, and the variable range is narrow, so the distribution of the antenna input circuit is Capacity sensitive. Receivers such as regular radio cassettes and portable stereos have short antenna input circuits, so it is possible to ignore the effects of distributed capacitance and use a tunable type receiver, but unlike car radios, the distance from the antenna to the high-frequency amplification circuit is short. When is long, the distributed capacitance cannot be ignored compared to the capacitance value of the variable capacitance diode, and the variable range of the tuning frequency becomes narrower, making it impossible to use a tunable type.

In other words, the maximum and minimum capacitance values of existing AM variable capacitance diodes are 500pF and 30pF, respectively.
For example, if the distributed capacitance value of 70 pF of 1 m of coaxial cable is added in parallel by using a tuned type, the maximum and minimum capacitance values of the antenna input circuit will be approximately 570 pF and 10 pF, respectively. In this state, the frequency variable ratio is √ maximum capacity value minimum capacity value = √ 570100 = 2.4 times, and in the medium wave broadcasting band starting from 520KHz, the limit that can be received is 520 × 2.4 = 1248KHz, and the medium wave from 520KHz to 1620KHz It is not possible to cover the entire broadcast band. It has also been impossible to construct a bandpass filter due to the presence of distributed capacitance. Therefore, in the past, the antenna input circuit of an electronically tuned receiver was a non-tunable type as shown in the circuit diagram of the high frequency amplification circuit part in Figure 1, and furthermore, because a bandpass filter could not be used, cross modulation phenomena were likely to occur. It was.

The present invention is located in the antenna input circuit 12 between the antenna 1 and the high frequency amplification circuit, as shown in the circuit diagram of the high frequency amplification circuit portion of an electronically tuned receiver in Figure 2, and is a non-tunable antenna input circuit. It is an object of the present invention to provide a bandpass filter 2 configured such that the adverse effects of the distributed capacitance of No. 12 can be almost ignored. In Figures 1 and 2, 3 is a junction FET transistor for high frequency amplification.
4 is a variable capacitance diode for tuning the high frequency amplifier circuit. The bandpass filter of the present invention has a winding separated by a tap into a first part and a second part that are magnetically coupled to each other, and the end of the first part of the winding is grounded, and the tap side of the first part, the end side of the second part, and the ground side of the first part are arranged close to each other and the first part. A distributed capacitance is formed between the second portion, the tap is used as an input terminal connected to the antenna via a non-tunable antenna input circuit, and the end of the second portion is connected to a high frequency amplification circuit. It is characterized by having an output terminal.

The following description will be made with reference to FIG. 3a, which is a wiring diagram showing an embodiment of the bandpass filter of the present invention, and FIG. 3b, which is an equivalent circuit diagram of FIG. 3a.

The winding located between terminals 5 and 7 in FIG. 3a is divided by a tap 6 into a first part _N1 and a second part _N2 . The first portion N ₁ and the second portion N ₂ are magnetically coupled to each other.
Also, the winding direction is the same.

The end of the first portion _N1 is grounded by grounding the terminal 5, and the tap 6 is positioned close to the grounded end.
The second portion _N2 is located close to the grounded end of the end portion _N1 connected to the terminal 7. A distributed capacitance is formed inside the first portion N ₁ and between the first portion N ₁ and the second portion N ₂ . Tap 6 is connected to antenna 1 via a non-tunable antenna input circuit 12, and terminal 7 is connected to a high frequency amplification circuit. Therefore, tap 6 and terminal 7 serve as the input terminal and output terminal of the bandpass filter, respectively.

The equivalent circuit diagram of the bandpass filter configured as shown in FIG. 3a is shown in FIG. 3b.

L ₁ , L ₂ are the self-inductances of the first part N ₁ and the second part N ₂ of the winding, respectively, and M is the mutual inductance between the first part N ₁ and the second part N ₂ . By configuring the first part N ₁ and the second part N ₂ as described above, C ₁ , C ₂ , and C ₃ are arranged inside the first part N ₁ and between the first part N 1 and the second part N ₁ . is the distributed capacitance created between portion _N2 , and CS is the distributed capacitance present in antenna input circuit 12 between antenna 1 and tap 6. In addition, in FIG. 3b, self-inductance L ₁ , L ₂ , mutual inductance M, distributed capacitance C ₁ , C ₂ , C ₃ , and distributed capacitance CS also display inductance and capacitance values, respectively. Tap 6 and terminal 7
An inductance (-M) and an inductance (L ₂ +M) are connected in series between the two, and the connection point thereof is grounded via the inductance (L ₁ +M). Distributed capacitance C ₁ and distributed capacitance C ₂ are connected in parallel to inductance (L ₁ +M) and inductance (L ₂ +M), respectively, and tap 6 and terminal 7 have distributed capacitances, respectively.
CS, grounded via distributed capacitance _C3 .

FIG. 4 is a diagram showing filter characteristics of the bandpass filter of the present invention constructed as shown in FIGS. 3a and 3b.

In the filter characteristics shown in Figure 4, P ₁ is the parallel resonance point determined by the inductance (L ₁ +M) and the distributed capacitance C ₁ , and the part that includes the frequency range where the output gain is almost flat against changes in frequency, that is, the frequency The portion higher than f ₁ has distributed capacitances C ₁ , C ₂ , C ₃ , inductance (L ₂ +M), and inductance (−
This is a so-called low-pass characteristic determined by M).
Also, P ₂ is the distributed capacitance C ₂ and inductance (L ₂ + M)
This is the pole of the low-pass characteristic determined by .

As described above, the bandpass filter of the present invention is composed of a frequency characteristic element consisting of parallel resonance and a frequency characteristic element consisting of a low pass filter. The distributed capacitance CS existing at the input terminal together with the inductance (-M) constitutes a low-pass filter, so the presence of the distributed capacitance CS causes the filter characteristics to show an effect of increasing the degree of attenuation at frequencies ₂ and above. Rather preferable.

Thus, the present invention uses the uncertain distributed capacitance CS existing in the antenna input circuit 12 in order to improve the filter characteristics, and since it does not cause any disadvantage, it can be used as a low-pass filter between frequencies ₁ and ₂ . Can be used.
Moreover, since the distributed capacitance obtained between the windings is utilized, there is no need to connect a separate capacitor, and the number of windings is extremely economical since only one winding having the tap 6 is required. Note that the values of distributed capacitance and inductance can be adjusted to the capacitance by changing the winding method of the winding.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of the high frequency amplification circuit of a conventional electronically tuned receiver with a large distributed capacitance of the antenna input circuit, and Figure 2 is a circuit diagram of the electronic tuning circuit to explain the position of the bandpass filter of the present invention. It is a circuit diagram of the high frequency amplification circuit part of the type receiver, and the third
Figure a is a wiring diagram showing an embodiment of the band pass filter of the present invention, Figure 3 b is an equivalent circuit diagram of the band pass filter of the present invention, and Figure 4 shows the filter characteristics of the band pass filter of the present invention. FIG. 1...Antenna, 2...Band pass filter,
4...Variable capacitance diode, 12...Antenna input circuit, _N1 ...First part of the winding, _N2 ...Second part of the winding, _C1 , _C2 , _C3 ...Distributed capacitance , CS…
...Distributed capacitance of the antenna input circuit 12, L ₁ ... Self-inductance of the first part N ₁ , L ₂ ... Second
The self-inductance of the part N ₂ .

Claims (1)

[Scope of utility model registration request] A first part and a second part magnetically coupled to each other have a winding separated by a tap, the end of the first part of the winding being grounded, and the first part The tap side of the second part, the end side of the second part, and the ground side of the first part are arranged close to each other,
A distributed capacitance is formed between the first parts and between the first part and the second part, the tap is used as an input terminal connected to the antenna via a non-tunable antenna input circuit, A bandpass filter characterized in that an end of the section is used as an output terminal connected to a high frequency amplification circuit.