JPS62216515A - Band pass filter - Google Patents

Abstract

PURPOSE:To obtain a filter excellent in characteristic with less number of components by arranging inductors so as to apply the electrode induction coupling to the inductors of different stages. CONSTITUTION:A channel signal from the 2nd mixer 19 is fed to an input terminal 40. A capacitor C1 and an inductor L1 constitute the 1st stage resonance circuit. The capacitor C4 acts like a coupling capacitor and an inductor L2 and a capacitor C2 act like a resonance circuit of the 2nd stage. The air-core inductors L1, L2 are arranged in the relation of the electromagnetic induction coupling as shown in figure and a characteristic being the addition of a new trap characteristic to the substantial band characteristic of a dual tuning circuit is formed. Thus, the increase in number of components is not required and a steep trap is formed by an inexpensive circuit.

Description

[Detailed Description of the Invention] [Purpose of the Invention (Industrial Application Field) This invention is applicable to cable television (hereinafter referred to as CAT
This invention relates to a bandpass filter used in an up/down tuner (referred to as V).

(Prior Art) In a tuner for a CATV system or the like, an up-down tuner as shown in FIG. 6 is used. This tuner selects a desired channel signal from among the high-frequency television signals transmitted from the main line, converts it to a higher frequency, and then converts it to a channel that can be received by the tuner of the television receiver (for example, channel 2). converted into frequency. Once converted to a higher frequency in this way,
Since there are many transmission channels in a CATV system, this is to keep the image channel after frequency conversion away from the original channel and to make it easier to select a channel. In FIG. 6, the signal supplied to the input terminal 11 is
It is supplied to a first mixer 13 via an input circuit 12. The first mixer 13 mixes the oscillation output from the first oscillator 14 and the input signal to create an intermediate frequency signal. First oscillator 1
4 is a local oscillator for tuning, the oscillation frequency of which is varied by a control signal from the tuning circuit 15; The intermediate frequency signal from the first mixer 13 is passed through a bandpass filter 16
°, is supplied to a second mixer 19 via an intermediate frequency amplifier 17 and a bandpass filter 18. The second mixer 19 is
The intermediate frequency signal and the oscillation output from the second oscillator 20 are mixed and frequency-converted to, for example, the frequency of the second channel.

The second channel signal obtained in this way is supplied to the output circuit 21. Note that 22 is a switching circuit used to switch the oscillation frequency of the second oscillator 20 when changing the channel frequency supplied to the television receiver.

The above-mentioned up-down tuner is housed in a shield box 30 as shown in FIG. 7, for example, to ensure its shielding. In each part of the shield box 30,
The locations of the above circuits are indicated by the same numbers.

Incidentally, in CATV systems, in order to improve services, it is being considered to establish a pay program system in which a special contract is made between subscribers and program providers. This system allows only those who have made a viewing contract for a specific program to receive the program through the tuner described above. To achieve this, it is necessary to perform scrambling when transmitting a specific program signal, while descrambling on the receiving side. Although various scrambling methods are being considered, there is a desire for one that can be realized at the lowest possible cost.

(Problems to be Solved by the Invention) As the above-mentioned scrambling method, there is a method in which the video carrier and the audio carrier are exchanged and transmitted.

Then, the program signals scrambled in this manner are arranged and transmitted every other channel in the transmission band.

However, the issue here is whether or not it is possible to sufficiently prevent unauthorized viewing of the scrambled program signal. For example, when looking at the frequency characteristics of the output circuit 21 of the above-mentioned up-down tuner, it becomes as shown in FIG. fP is a video carrier, and fs is an audio carrier. Even in systems that do not adopt a pay program system, it is necessary to trap unnecessary video carriers such as adjacent channels, but even in systems that transmit scrambled program signals, it is necessary to trap unnecessary video carriers such as those from adjacent channels. When is a scrambled program signal, it is necessary to trap unnecessary video carriers, but
A video carrier appears at fP 12MHz,
By slightly shifting the tuning frequency of the output circuit 21 or the television receiver side, it becomes possible to secretly watch programs for which the user has not subscribed (programs on the next channel). To prevent this, a surface acoustic wave filter with good frequency cut characteristics may be used as the output circuit 21, but this becomes expensive and the circuit configuration becomes complicated. Furthermore, in the case of a surface acoustic wave filter, since the amount of attenuation is large, an amplifier must be used in conjunction with the filter, and as a result, there is a problem in that the cause of generation of noise components increases.

Also, Q
It is possible to increase the output side attenuation by increasing the Q, but increasing the Q increases the rate at which the tracking error increases. Further, even if the number of stages on the output circuit side is increased unnecessarily, a sufficient effect cannot be expected at the cost of an increase in price.

Therefore, the aim of this invention is to provide a bandpass filter that can form a steep trap using an inexpensive double-tuned circuit in which a plurality of parallel resonant circuits using capacitors and inductors are connected via a capacitor. It is something.

[Structure of the Invention] (Means for Solving the Problems) In this invention, in a double-tuned circuit in which a plurality of parallel resonant circuits each consisting of a capacitor and an inductor are coupled by a coupling capacitor, the inductors between different stages are connected to each other by electromagnetic induction. The inductors are arranged so as to be coupled with each other to add a trap characteristic.

(Function) By arranging the inductors between different stages so that they perform electromagnetic induction coupling as described above,
A new trap characteristic can be added, and a filter with good characteristics can be obtained with a small number of parts.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows one embodiment of the present invention, and will be described as a circuit used in the output circuit of the up-down tuner explained in FIG. The channel signal from the second mixer 19 is supplied to the input terminal 40 . This terminal 40 is grounded via a capacitor C1 and also via an inductor L1. Terminal 40 is also connected to one electrode of coupling capacitor C4. Capacitor C1 and inductor L
1 constitutes the first stage resonant circuit.

The other electrode of capacitor C4 is grounded via inductor L2 and grounded via capacitor C2, and is also connected to one electrode of capacitor C5. Capacitor C4 acts as a coupling capacitor, and inductor L2 and capacitor C2 act as a second stage resonant circuit. The other electrode of capacitor C5 is grounded via capacitor C3 and also via inductor L3. The other electrode of capacitor C5 is output terminal 4.
1 has been derived.

Here, in this invention, when the above circuit is formed on a printed wiring board, the air-core inductors L1 and L2 are arranged in a relationship of facing electromagnetic induction coupling as shown in FIG. This is a characteristic in which a new trap characteristic is added to the original band characteristic of the circuit.

Now, to make the explanation easier to understand, consider a simple filter as shown in FIG. 3(a). That is, capacitor C1l,
Considering a filter consisting of C12, C13 and inductors Lll, L12, the inductors Lll, L1 of this filter
2 is assumed to be coupled by electromagnetic induction. In this circuit, if the winding ratio of the inductors Lll and L12 is 1:1 and the mutual conductance is M, it can be expressed as an equivalent circuit shown in FIG. 3(b). Furthermore, this circuit is Y−△
When converted, it can be expressed as shown in FIG. 3(C).

Here, La, Lb, and Lc in FIG. 3(c) are Lll, L
12. There is the following relationship with M.

In other words, the inductors L11 and L12 are coupled by electromagnetic induction, thereby making it possible to set the trap frequency.

If the circuit in Figure 1 is rewritten as shown in Figure 3(c), the fourth
The result will be as shown in the figure. In this embodiment, the trap formed in this way is set at a position 12 MHz lower than the video carrier frequency fP, as shown in Fig. 5, to suppress signals from the adjacent channel, and suppress signals of 20 dB or more. We were able to obtain an improvement in the amount of attenuation.

Although the above embodiment has been described as a filter for an output circuit in an up-down tuner, the present invention is not limited to this and can be applied to filters in general. In this case, the trap position is set to an arbitrary frequency.

[Effects of the Invention] As described above, the present invention can provide a bandpass filter that can form a steep trap using an inexpensive circuit without the need to increase the number of components.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an example of the arrangement of parts of the circuit of FIG. 1, and FIG. 3 is a circuit diagram for explaining the function of the filter of the present invention. 4 is a diagram showing an example of an equivalent circuit of the circuit in FIG. 1, FIG. 5 is a characteristic diagram obtained by an embodiment of the present invention, and FIG. 6 is an example of an up-down tuner for a cable television system. FIG. 7 is a block diagram showing the configuration, FIG. 7 is a perspective view for explaining the arrangement of the blocks in FIG. 6, and FIG. 8 is a frequency characteristic diagram of the output circuit in FIG. 6. 01-C5...Capacitor, L1-L4...Inductor.

Claims (1)

[Claims] In a double-tuned circuit in which multiple parallel resonant circuits each consisting of a capacitor and an inductor are coupled by a coupling capacitor, the inductors between different stages are arranged so that the inductors are electromagnetically coupled to each other, and a trap characteristic is added. bandpass filter.