JPS63146508A - Intermediate frequency circuit - Google Patents

Abstract

PURPOSE:To reduce the distortion caused in a transistor (TR) by connecting a collector output and an emitter output of the TR of a preamplifier to a couple of input terminals of a surface acoustic wave filter (SAW) filter. CONSTITUTION:Distances A', B' between an output terminal (g) and input terminals (a), (e) are equal to each other and distances C', D' between an output terminal (h) and the input terminals (a), (e) are nearly equal to each other in the printed pattern of a printed board with a SAW filter 2 mounted thereupon. Since an undesired signal coming from the input terminals (a), (e) to the output terminal (g) has an nearly equal absolute value and is in opposite phase to each other, the signals are cancelled at a point of time when they come across the output terminal (g). This is applied similarly as to output terminals (c), (d), (h). Moreover, the dual input is adopted for the SAW filter 2, then the gain is improved, then the amplification factor of the TR is selected to be comparatively small, and the distortion caused in the TR is minimized without gain reduction.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an intermediate frequency circuit in a television receiver.

(B) Conventional technology Surface acoustic wave filters (hereinafter referred to as 8 and A) used in intermediate frequency circuits C and 12 circuits of television receivers
As shown in Patent No. 11'B59-25524 (HO3H9/25), the wiring pattern of the printed circuit board on which the SAW filter is placed is approximately equidistant from the input terminal of the SAW filter and the pair of output terminals. Since the output terminals are arranged so that the output terminals are arranged in such a manner that unnecessary signals such as feedthrough that appear at the pair of output terminals are canceled out by the late-acting amplifier in the subsequent stage.

On the other hand, in recent years, sound quality has become more important in audio multiplex broadcasting, AV systems, etc., and IP circuits dedicated to video and audio, called parallel IP circuits, are being provided. At this time, the n SAW filter used is shown in Figure 5 (a) (
As shown in (b), the T! 5B2201 is used.

The terminal arrangement of this SAW filter is as shown in the bottom view of Fig. 4: input terminal (a) ground terminal (b) audio output terminal (C) (d) and input terminal (61), ground terminal φ, video terminal The output terminals (g) and Ch) are arranged in a dual in-line manner, and the wiring pattern of the printed circuit board on which f'L is mounted is as shown in FIG. That is, for a single input input to the input terminal (El), the output is:
They are dual outputs from the output terminals (A) and (C) and (D), the input terminal (A) and the ground terminal (S), respectively.
1)) φ are each connected to a ground pattern. Therefore, the distance between input terminal (e) and each output terminal @ A - D
Then, since it is A to B, n appears at the output terminal (sudan).
Ninthly, the unnecessary signals having different magnitudes can be canceled out by an intermediate frequency circuit having a differential amplifier at the rear stage. Since it is D to C, the same can be said about the output terminal (CXd).

(c) Problems to be Solved by the Invention The present invention has been devised in view of the above points, and is an S having two different band characteristics used in a parallel IF circuit.
The present invention provides an intermediate frequency circuit that can remove unnecessary signals between input and output even in AT filters.

B.) Means for Solving the Problems The present invention provides a surface acoustic wave filter (1) comprising one pair of input terminals and two pairs of output terminals, and having two different band characteristics.
It consists of a preamplifier (1) which applies a pair of opposite phase input signals from a tuner to a pair of input terminals, and first and second intermediate frequency circuits connected to two pairs of output terminals, respectively.

(e) Operation By means of the above-described means, the unnecessary signals of opposite phases coming from the pair of input terminals of the SAW filter are substantially canceled out at each output terminal.

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

FIG. 1 is a circuit diagram of the IF circuit in this embodiment, in which the output of tuner +11 is applied to the base of a transistor (Tr) constituting a preamplifier via a coupling capacitor (C). (R1) (R2) are base bias resistances,
(R13) is a collector bias resistance, and (R4) is an emitter bias resistance. The collector of the transistor (Tr) is a 3AW filter (such as the TSB2201).
2), and its emitter is connected to the other input terminal (6). Therefore, both input terminals (
The signals input to a) and (e) have opposite phases.

(31 (41 is a matching coil connected to each pair of output terminals (0), (d) and (g), respectively) of the SAW filter (2), (51d is connected to a matching coil (3)) The audio intermediate frequency circuit (6) is an fL visual intermediate frequency circuit connected to the matching coil (4).The wiring pattern of the printed circuit board on which the above SAW filter is mounted is as shown in Figure 2. , the distances A' and B' between the output terminal (g) and each input terminal (a) and (e) are approximately equal to each other, and the distance C between the output terminal Ql) and each input terminal (a) and (e) is approximately equal to each other.
' and D' are also substantially equal to each other. Therefore, the input terminal (aXe
), the unnecessary signals that enter the output terminal (g) have substantially equal absolute values and opposite phases, so they are canceled out when they enter the output terminal (g). Also output terminals (C,) (a) (h)
The same applies to

In addition, by making the input of the SAW filter (2) dual, it is possible to improve the gain by up to 6 dB compared to the conventional single input. It is possible to minimize the noise generated in the transistor (Tr) without reducing the gain.

Further, although the distances A and B' between the input and output terminals were assumed to be approximately equal, in reality, they have a slight difference. Therefore, since the output terminals (g) and (h) to which the video intermediate frequency circuit (6) is connected are slightly closer to the input terminal (61) than the input terminal (a), the influence from the input terminal (e) is reduced. more receptive.

However, the resistor (Rs) (
Since R4) is set, the unnecessary signals coming from the input terminals (a) and (8) to the output terminals (g) and (h) have substantially equal absolute values and cancel each other out.

On the other hand, the output terminal (
0) Contrary to the above, the unnecessary signals coming in from both input terminals in (d) have different absolute values and cannot be canceled out completely, but compared to video, audio is less affected by unnecessary signals, so it is practically There is no problem.

(G) Effects of the Invention As described above, according to the present invention, since the collector output and emitter output of the preamplifier transistor are connected to the pair of input terminals of the SAY filter, the transistor The degree of amplification can be reduced, and the distortion generated in the transistor can be reduced.

In addition, since signals of opposite phase are applied to a pair of inputs of the SAW filter, unnecessary signals such as feed-through can be removed even if the output terminal is placed in-line with the input terminal, and the reproduced image No adverse effects.

Just in case, in order to connect each of the pair of input grid electrodes of the SAW filter to the collector and emitter of the transistor,
The potential difference between the electrodes is smaller than the conventional one, which connects to the collector and ground, making it possible to connect the preamplifier and SAT filter directly.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an intermediate frequency circuit in an embodiment of the present invention, Fig. 2 is a wiring pattern diagram of a SAW filter in this embodiment, and Fig. 6 (a) middle) is an image of a conventional SAW filter. and voice band characteristics, Figure 4 shows the conventional SAW
The bottom view of the filter, FIG. 5, is a wiring pattern diagram of a conventional SAW filter.

Claims (3)

[Claims] (1) A surface acoustic wave filter comprising one pair of input terminals and two pairs of output terminals and having two different band characteristics;
An intermediate frequency circuit comprising a preamplifier that applies a pair of opposite phase input signals from a tuner to a pair of input terminals, and first and second intermediate frequency circuits connected to each of the two pairs of output terminals. (2) The intermediate frequency circuit according to claim 1, wherein the preamplifier is constituted by a transistor whose collector and emitter are connected to each of the pair of input terminals. (3) The intermediate frequency circuit according to claim 1, wherein the first and second intermediate frequency circuits are audio and video intermediate frequency circuits, respectively.