JPS6327886B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to provide a circuit configuration suitable for applying a surface wave filter to a frequency selection circuit of a video intermediate frequency signal processing device (hereinafter referred to as a VIF device in this invention). a first piezoelectric transducer, in particular arranged on the surface of the elastic substrate;
Regarding the practical application of a surface wave filter having a so-called three-transducer structure, which has second and third piezoelectric transducers arranged at a predetermined interval on both sides of the first piezoelectric transducer, the present invention is small and compact. This realizes a high-performance video intermediate frequency signal processing device. First, a conventional example related to this invention will be explained.

FIG. 1 shows an example of a well-known surface wave filter having a two-transducer structure, in which a surface wave transmitting transducer 2 receiving transducer 3 arranged on the surface of an elastic substrate 1 is separated by a predetermined interval. It is set aside. The transmitting converter 2 and the receiving converter 3 are connected to a signal source 5 having an internal impedance 4 and a load 6 under coupling conditions such that electrical mismatch loss occurs, respectively.
Such mismatch loss increases the actual loss, which is the sum of the surface wave filter's own inherent power loss, and reduces unnecessary reflected signal components, especially TTE (triple transit echo) components, from the main signal. This is necessary to achieve a relative attenuation of 40dB or more. For example, as described in "Application of Surface Acoustic Wave Filter to Television Receiver" in Telbi Magazine Vol. 31, No. 11, the inherent power loss of the surface acoustic wave filter itself, that is, the transmitting converter, receiving converter, and external circuit, respectively, is electrically By generating a mismatch loss, the actual loss can be reduced to 18 to 13 dB compared to the loss of 10 to 13 dB when matched to
This increases the TTE component to 20 dB, thereby suppressing the TTE component. It is appropriately selected whether the mismatch loss for suppressing the TTE component is generated equally in each of the transmitting converter 2 and the receiving converter 3, or only in one of them. Also send,
The receiving converters 2 and 3 have their respective converter impedances set in order to obtain desired frequency characteristics and phase characteristics, or to facilitate application to a VIF device in which the output impedance of the tuner is low and the input impedance of the VIF amplifier is high. They are different. Therefore, in a surface wave filter, either of the two converters can be used as the transmitting or receiving converter, but in a VIF device, the relatively low impedance converter of the two converters is connected to the tuner side, and the high impedance converter is connected to the tuner side. It is advantageous to couple the impedance side transducer to the VIF amplifier side and configure it to produce the required mismatch losses within such coupling conditions. In this way, when applying a surface wave filter to a VIF circuit such as a television tuner, its insertion loss is greater than that of a normal LC filter, which not only degrades the signal-to-noise ratio of the image but also reduces the reception of the receiver. This will also make the sensitivity worse. Therefore, a preamplifier for preventing such performance deterioration has practical drawbacks, such as requiring an amplification element with good noise characteristics and no distortion.

In contrast to such a surface wave filter having a two-converter structure, a three-converter structure as shown in FIG. 2 is described in US Pat. No. 3,582,840. This is intended to reduce the loss by 3 dB by arranging the receiving converters 3 and 3' on both sides of the transmitting converter 2.

In this surface wave filter with a three-transducer structure, the distance between the transmitting converter placed in the center of the elastic substrate and the receiving converters placed on both sides of the transmitting converter is appropriately selected, and the transmitting converter and the signal source are connected electrically. It has become clear that by matching, it is possible not only to reduce loss but also to suppress unnecessary reflected signal components, as reported in the academic journal IEEE Transaction on
Microwave theory and techniques, VOL.
It can be derived from p856-p864 of MTT-17, No.11, NOV.1969.

In other words, when the surface acoustic wave signals sent out from the transmitting transducer 2 are reflected from the receiving transducers 3 and 3' formed at the same distance from these two and arrive at the transmitting transducer 2 at the same time, the transmitting transducer 2 is impedance matched by an external circuit and a matching circuit, the reflected wave 3-b reflected by the receiving converter 3 and further reflected by the transmitting converter 2 is reflected by the receiving converter 3' and transmitted to the transmitting converter 2. Since they have the same amplitude and a 180° phase difference from the passing wave 3'-a which has passed through the receiving converter 3, they cancel each other out, and the reflected wave at the receiving converter 3 no longer affects the receiving converter 3 again. The same thing applies to the reflected wave 3'-b that was reflected by the receiving converter 3' and further reflected by the transmitting converter 2, and the passing wave 3-a that was reflected by the receiving converter 3 and passed through the transmitting converter 2. I can say it. However, this state is achieved when the external matching circuit matches the impedance of the transmitting converter 2 and the external circuit at the maximum response frequency of the transmitting converter 2, and the above conditions must be fully satisfied. The amplitude and phase of the reflected wave and the passing wave at the transmitting converter 2 are shifted, and the reflected waves at the receiving converters 3 and 3' are reflected again at the receiving converter 3.
and 3'. The above operation can be similarly explained when converter 2 is used as a receiving converter and converters 3 and 3' are used as transmitting converters. In a surface wave filter with such a three-transducer structure, the receiving converters 3 and 3' have relatively low impedance compared to the transmitting converter 2, so the output impedance is low when actually applied to a VIF device. In particular, converters 3 and 3' are connected to the tuner side to form a transmitting converter, and converter 2 is connected to the VIF amplifier side, which has a relatively high input impedance, to form a receiving converter. This is suitable for the purpose of suppressing the TTE component by matching the i.e. the transmitting and receiving converters of the surface wave filter and the external circuits tuner and VIF.
If it is necessary to cause mismatch loss between the amplifier and the amplifier, either of the two types of converters may be used as the transmitting converter or the receiving converter, but in the case of the three converter structure described above, converter 2 and the external It is necessary to match the circuits, and a method of arranging matching circuits under coupling conditions with relatively low mismatch loss is usually used.

However, since the VIF amplifier is a variable gain type, its input impedance is likely to fluctuate due to gain control operations, and since it is a high gain amplifier, a feedback loop is likely to be formed due to undesired coupling between the output and the input. The input impedance will change significantly. Therefore, as mentioned above, converter 2 of the three-transducer structure surface wave filter is
In the usual method of coupling via a matching circuit on the amplifier side, the amount of suppression of unnecessary reflected signals becomes insufficient due to slight changes in the electrical coupling conditions with converter 2 due to fluctuations in the input impedance of the VIF amplifier. As a result, this kind of surface wave filter not only has drawbacks such as deterioration of the quality of the reproduced image, but also the influence of the variable gain type amplification stage has hindered the practical use of this type of surface wave filter.

In the VIF device of the present invention, when a fixed gain amplifier having a low output impedance and a variable gain amplifier having a high input impedance are coupled through a surface wave filter having a three-converter structure, the surface wave filter is brought into an optimal operating state. The aim is to provide a VIF device configuration that can be maintained. In the present invention, at least the transducer disposed centrally among the transducers is
They are suitably arranged to be coupled directly or via a desired impedance transformer to the input terminal of an amplifier with a fixed gain. Meanwhile, the other two converters are connected in parallel inside or outside the enclosure for surface wave filter implementation and to other variable gain amplifiers either directly or via the desired impedance converter. be combined. According to such arrangement conditions of the present invention,
Unwanted changes in its input and output impedance due to changes in the operating point of any amplifier of the VIF circuit can be reliably prevented from affecting the operation of the transducer located centrally among the piezoelectric transducers of the surface wave filter. An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 3 shows the radio frequency amplification stage and VIF stage of a television receiver in which the present invention is implemented. The radio frequency amplifier 7 is of the type of a variable gain amplifier and is provided at the input stage of the receiver, and the fixed gain amplifier 8
to which both the output of the radio frequency amplifier 7 and the output of the local oscillator 9 are applied constitutes a frequency conversion stage. The surface wave filter 10 has a transmitting transducer 2 consisting of a first transducer in the center coupled to the output terminal of this fixed gain amplifier 8, and other transducers, namely the first transducer 2, arranged on both sides. a receiving converter 3 consisting of second and third converters electrically connected in parallel;
3' is coupled to variable gain amplifier 11. The radio frequency amplifier 7 and the variable gain amplifier 11 of the VIF adjust the level of the signal supplied to the detector placed after the variable gain amplifier 11, depending on the level of the received signal, as in a normal receiver. Its gain is controlled by an automatic gain control (AGC) signal to keep it constant. The radio frequency amplifier 7 normally receives large signals, and its gain is controlled so as to maintain the level of the signal supplied to the fixed gain amplifier 8 having a frequency conversion function at a predetermined value. Radio frequency amplifier 7 generated by the above gain control operation
The change in output impedance of the fixed gain amplifier 8
This does not affect the transmitting converter 2 of the surface wave filter 10, and therefore, the transmitting converter 2 and the fixed gain amplifier 8 always maintain an impedance matching state, so unnecessary reflections such as the TTE component mentioned above are avoided. The amount of signal component suppression can be maintained at a desired value. On the other hand, it is shown in FIG. 3 that the other transducers of the surface wave filter, namely the receiving transducers by the second and third transducers 3, 3', can be coupled to the input side of the variable gain amplifier 11. The amplifier 11 includes, for example, buffer and common emitter amplification transistors 110, 111 and 112, 113 as shown in FIG.
Currently, many devices are used that are constructed with variable impedance elements 114 and 115 whose operating impedance is varied by AGC.

In this type of amplifier, transistors 112,1
The pace input impedance of 13 changes in principle by the gain control operation, and this influence is easily transmitted to the input side of the buffer transistors 110 and 111 even if the buffer transistors 110 and 111 are arranged.
The coupling conditions between the receiving converter and the amplifier 11 are not constant. Therefore, the coupling conditions between the second and third converters 3, 3' constituting the receiving converter of the surface wave filter 10 and the amplifier 11 change depending on the gain control operation of the amplifier 11, but the three converters as described above change. In the structure of the surface wave filter, these second and third transducers 3,
A change in the coupling conditions between 3' and the external circuit does not significantly change the amount of suppression of the TTE component. That is, a change in the coupling conditions between the converters 3, 3' and the external circuit increases or decreases the actual loss of the surface acoustic wave filter, and accordingly only a very small change in the amount of suppression of the TTE component occurs. The converter 2 forming the transmission converter of the surface wave filter 10 is therefore reliably separated by the amplifier 8 from the radio frequency amplifier 7 whose output impedance is variable, and the variable gain amplifier 1 whose input impedance is variable.
1 can be reliably separated in terms of impedance by the surface wave propagation path of the surface wave filter 10, and this brings about great practical advantages such as being able to always operate the surface wave filter under optimal conditions.

In the above explanation, the amplifier 11 uses a configuration example in which the input impedance changes in principle by the gain control operation, but a common base amplifier or a cascode amplifier is used as a buffer to change the input impedance of the amplifier 11. Even with a circuit configuration that prevents this, if these amplifiers are formed in a known integrated circuit together with other signal processing circuits, the amplifier 11 needs to have a gain of about 50 dB.
Therefore, it is difficult to eliminate changes in input impedance due to interference between the input and output of such a high gain amplifier. Even with such a configuration, the present invention has the feature that the surface acoustic wave filter can be maintained at optimal operating conditions.

As is clear from the above embodiments, in the video intermediate frequency signal processing device according to the present invention, when a surface wave filter with a three-transducer structure is put into practical use, the piezoelectric transducer located at the center of the surface wave filter is replaced with a fixed gain amplifier. It is characterized in that the other two converters of the surface wave filter are electrically connected in parallel and coupled to the output terminal of the surface acoustic wave filter and a variable gain amplifier.

In particular, when coupling a tuner with a relatively low output impedance and a variable gain VIF amplifier with a high input impedance through a surface wave filter with a three-converter structure, unlike the usual method, the surface wave filter The converter 2, which has a relatively high impedance, is coupled to the tuner, which has a low output impedance, and the other converter 3, 3', which has a low impedance, is coupled to the VIF amplifier 11, which has a high input impedance. be.

According to the configuration of the present invention, the three-converter type surface acoustic wave filter can always be operated under optimal conditions, and it is possible to suppress the signal component of the reflected wave and prevent deterioration of image quality. become. This eliminates the need for a preamplifier, which is required in a conventional two-converter structure surface wave filter, and the VIF
This has extremely great industrial effects, not only by making the stage smaller but also by achieving higher performance.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram using a surface wave filter with a two-converter structure, FIG. 2 is a circuit configuration diagram showing a surface wave filter with a three-converter structure and its connections according to the present invention, and FIG. FIG. 4 is a block diagram showing an embodiment of the video intermediate frequency signal processing apparatus, and FIG. 4 is a specific circuit diagram of an amplifier of the embodiment. 1... Elastic substrate, 2... First piezoelectric transducer, 3,
3'... second piezoelectric transducer, 7... radio frequency amplifier,
8... Fixed gain amplifier, 9... Local oscillator, 10... Surface wave filter, 11... Variable gain amplifier, 12... Fixed gain amplifier.

Claims (1)

[Claims] 1 A first piezoelectric transducer written on the surface of an elastic substrate and a plurality of piezoelectric transducers arranged at predetermined intervals on both sides of the first piezoelectric transducer are electrically connected in parallel. and a second piezoelectric transducer whose impedance is relatively lower than the first piezoelectric transducer, and at least the connection between the first piezoelectric transducer and an external circuit is electrically matched. The first piezoelectric transducer of the surface wave filter is connected directly or through a predetermined impedance conversion means to a frequency having a fixed gain. The second piezoelectric transducer of the surface wave filter is coupled to the input terminal of a variable gain video intermediate frequency amplification stage either directly or via a predetermined impedance conversion means. Video intermediate frequency signal processing device.