MXPA00002692A - Television tuner with suppressed attenuation of level of video intermediate frequency with respect to rise in ambient temperature - Google Patents

Abstract

A television tuner comprising:a frequency converter for frequency converting a received television signal into an intermediate frequency signal and outputting the intermediate frequencysignal;and an intermediate frequency tuning circuit for passing the intermediate frequency signal, wherein the tuning frequency of the intermediate frequency tuning circuit is changed to a higher value in association with a rise in the ambient temperature.

Description

TELEVISION TUNER WITH RESTRICTED ATTENUATION INTERMEDIATE VIDEO FREQUENCY LEVEL WITH REGARD TO THE INCREASE IN AMBIENT TEMPERATURE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a television tuner.

DESCRIPTION OF THE RELATED TECHNIQUE A conventional television tuner will be described in relation to FIGS. 5 and 6. A television tuner comprises: a frequency converter 31 for converting, as to the frequency, a received television signal into an intermediate frequency signal (below) in this document, it is abbreviated as an IF signal) and produce the IF signal; an intermediate frequency tuning circuit (hereinafter, abbreviated as an IF tuning circuit) 32 for passing the IF signal produced from the frequency converter 31; and an intermediate frequency amplifier 33 for amplifying the IF signal and producing the amplified IF signal.

First, a television signal that was received by an antenna (not shown) is amplified by an amplifier (not shown) and the amplified signal is supplied to the amplifier. frequency converter 31. The frequency converter 31 has first and second output terminals 31a and 31 b and, although not shown, a voltage regulation oscillator and a mixer. In the mixer, an oscillation signal produced from the voltage regulating oscillator is mixed with the television signal, thereby performing the frequency conversion and obtaining an IF signal. The IF signals are produced from the first and second output terminals 31a and 31b in an equilibrium state. For example, in a television tuner that complies with a national specification, an intermediate frequency of SIF sound is 54.25 MHz, an intermediate frequency of color CIF is 55.17 MHz and an intermediate frequency of PIF image is 58.75 MHz. The tuning circuit IF 32 has a capacitor 32a and a tuning coil 32b. The capacitor 32a and the tuning coil 32b are connected in parallel. One end of the tuning coil 32b is connected to the first output terminal 31a of the frequency converter 31 and the other end of the tuning coil 32b is connected to the second output terminal 31b of the frequency converter 31. It is placed a tuning frequency fO of the tuning circuit IF 32 so as to be an almost central frequency (57 MHz in the national specification) between the intermediate image frequency PIF and the intermediate color frequency CIF.

The intermediate frequency amplifier 33 is connected to both ends of the tuning coil 32b by means of coupling capacitors 34 and 35. The IF signals produced from the frequency converter 31 pass through the tuning circuit IF 32 and are supplied to the intermediate frequency amplifier 33 in a state of equilibrium. The IF signals are amplified and produced. As shown by a full line in Figure 6, the level of the IF signal produced from the tuning circuit IF 32 becomes the maximum value (point A) at the tuning frequency fO. A characteristic output is obtained which draws a curve of the level of the IF signal, which is gradually reduced as the frequency becomes greater or less than the tuning frequency fO. The level of the intermediate image frequency PIF is lower than that of the tuning frequency fO (point B). Similarly, the level of each intermediate color frequency CIF and the intermediate frequency of sound SIF is lower than that of the frequency of tuning fO (points C and B). For example, transistors (not shown) are used for the mixer of the frequency converter 31 and an amplifier (not shown) is provided in the front stage of the frequency converter 31. Generally, the gain of the transistor decreases when the ambient temperature increases. In the characteristic output of the IF signal of the frequency converter 31, as shown by a broken line in Figure 6, the gain decreases as a whole. Therefore, as shown by the dashed lines in Figure 6, the levels of the intermediate frequency signal of SIF sound and the intermediate frequency signal of picture PIF decrease from points D and B to the points D 'and B', respectively. Especially, the decrease in the level of the PIF image intermediate frequency signal causes a problem so that the S / N ratio of an image displayed on a television screen deteriorates.

BRIEF DESCRIPTION OF THE INVENTION The invention attempts to solve the problem and its object is to suppress the attenuation of the level of an intermediate image frequency PIF when the ambient temperature is increased. To solve the problem, according to the invention, a television tuner is provided comprising: a frequency converter for converting, as regards frequency, a received television signal into an intermediate frequency signal and producing the frequency signal intermediate; and an intermediate frequency tuning circuit for passing the intermediate frequency signal, wherein a tuning frequency of the intermediate frequency tuning circuit is changed to a higher value with an increase in ambient temperature. According to the television tuner of the invention, the intermediate frequency tuning circuit has at least one capacitor and one coil, and a capacitor having a characteristic negative temperature, such as the capacitor, is used. According to the television tuner of the invention, when the tuning frequency of the intermediate frequency tuning circuit is set to an almost central frequency between an intermediate image frequency and an intermediate color frequency of the television signal at a temperature specific on an ordinary temperature scale, the tuning frequency when the ambient temperature is increased to 25 ° C is set to be higher than the frequency which is almost the center between the intermediate image frequency and the intermediate color frequency of the signal of television and less than the intermediate frequency of image.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram of a television tuner of the invention. Figure 2 is a graph showing characteristic output of an intermediate frequency tuning circuit in the television tuner of the invention. Figure 3 is a circuit diagram showing another embodiment of the television tuner of the invention.

Figure 4 is a circuit diagram showing another embodiment of the television tuner of the invention. Figure 5 is a circuit diagram of a conventional television tuner. Figure 6 is a graph showing a characteristic output of an intermediate frequency tuning circuit in the conventional television tuner.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES A television tuner of the invention will be described with reference to figures 1 to 4. Figure 1 is a circuit diagram of a television tuner of the invention. Figure 2 is a graph of a characteristic output of an intermediate frequency tuning circuit of the television tuner of the invention. Figures 3 and 4 are circuit diagrams of another embodiment of the television tuner of the invention. The television tuner of the invention comprises a frequency converter 1 for converting, as a frequency, a television signal at an intermediate frequency (further down in this document, it is abbreviated as an IF signal) and producing the IF signal, a intermediate frequency tuning circuit (hereinafter abbreviated as IF tuning circuit) 2 to pass the IF signal produced from the frequency converter 1, and an intermediate frequency amplifier 3 to amplify the IF signal and produce the IF signal amplified. First, a television signal received by means of an antenna (not shown) is amplified by means of an amplifier (not shown) and the amplified signal is supplied to the frequency converter 1. The frequency converter 1 has first and second output terminals 1a and 1 b and, although not shown, a voltage regulation oscillator and a mixer. In the mixer, an oscillation signal produced from the voltage regulating oscillator is mixed with the television signal, thus performing the frequency conversion to obtain an IF signal. The IF signals are produced from the first and second output terminals 1a and 1b in a state of equilibrium. For example, in a television tuner that complies with the specifications for Japan, the intermediate frequency of sound SIF is 54.25 MHz, the intermediate frequency of color CIF is 55.17 MHz and the intermediate frequency of image PIF is 58.75 MHz. The tuning circuit IF 2 comprises a capacitor 4 having a characteristic negative temperature and a tuning coil 5. The capacitor 4 and the tuning coil 5 are connected in parallel. One end of the tuning coil 5 is connected to the first output terminal 1a of the frequency converter 1 and the other end of the tuning coil 5 is connected to the second output terminal 1b of the frequency converter 1.

The characteristic temperature of the capacitor 4 is as shown by the full line in FIG. 2. The level of the IF signal reaches the maximum value (point A) when a tuning frequency fO of the tuning circuit IF 2 at a temperature specific (eg, 25 ° C) on an ordinary temperature scale (eg, 20 ° C to 25 ° C) equals an almost central frequency (57 MHz in the national specification) between the intermediate CIF color frequency ( 55.17 MHz) and the intermediate image frequency PIF (58.75 MHz). For example, when the temperature is increased from the ordinary temperature to 25 ° C, as shown by the broken line in Figure 2, a characteristic output is obtained so that the level of the IF signal reaches the value maximum (point E) at a tuning frequency fO 'of the tuning circuit IF 2 which is greater than the tuning frequency fO at the time of the ordinary temperature and lower than the intermediate image frequency (PIF). As a consequence, the tuning frequency fO 'after the increase in ambient temperature is closer to the intermediate image frequency PIF than the tuning frequency fO. As shown in Figure 2, the level of the IF signal becomes greater as the frequency gets closer to the tuning frequency fO. The tuning frequency fO becomes fO '(point E) as the ambient temperature increases and the tuning frequency fO' is closer to the intermediate image frequency PIF. Therefore, the difference between the level of the tuning frequency fO 'and the level of the intermediate image frequency PIF is reduced when compared with that of the conventional technique, so that attenuation of the frequency level can be suppressed. intermediate PIF image when the ambient temperature of the television tuner rises. Since the tuning frequency fO is set to the specific temperature to be a center frequency between the intermediate color frequency CIF and the intermediate image frequency PIF, when the level of the IF signal is attenuated due to a change in temperature, the level of the intermediate frequency of SIF sound is prevented from decreasing more than necessary and a sound output deteriorates. Both ends of the tuning coil 5 are connected to the intermediate frequency amplifier 3 by means of coupling capacitors 6 and 7. The IF signals produced from the frequency converter 1 pass through the tuning circuit IF 2 and are supplied to the amplifier of intermediate frequency 3. The IF signals are amplified by means of the intermediate frequency amplifier 3 and the amplified signals are produced. Although it has been described, the case in which the IF signals are produced in parallel from the frequency converter 1, the IF signals do not necessarily occur in a state of equilibrium from the frequency converter 1. It is also possible to connect the capacitor 4 and the tuning coil 5 in series in the intermediate frequency circuit 2 as shown in figure 3 or connecting the capacitor 4 and the tuning coil 5 in parallel, as shown in figure 4. Also in the other configurations, Similar effects can occur. As described above, the television tuner of the invention comprises: the frequency converter for converting, as a frequency, a received television signal into an intermediate frequency signal and producing the intermediate frequency signal; the intermediate frequency tuning circuit for passing the intermediate frequency signal; and the amplifier for amplifying the intermediate frequency signal and producing the amplified signal, and the tuning frequency of the intermediate frequency tuning circuit is changed to a higher value in relation to an increase in ambient temperature. Thus, the difference between the tuning frequency fO 'and the intermediate image frequency PIF is reduced, thereby enabling the attenuation of the intermediate image frequency PIF level to be suppressed. According to the television tuner of the invention, the intermediate frequency tuning circuit has at least the capacitor having a characteristic negative temperature and the tuning coil. The intermediate frequency tuning circuit is constructed by connecting the capacitor and the tuning coil in parallel, thereby allowing the television tuner to be provided with a suppressed attenuation of the intermediate image frequency signal PIF even when the room temperature.

According to the television tuner of the invention, when the tuning frequency of the intermediate frequency tuning circuit is set at an almost central frequency between the intermediate image frequency and the intermediate color frequency of the television signal at a temperature specific on an ordinary temperature scale, the tuning frequency when the ambient temperature is increased to 25 ° C is set to be higher than the frequency which is almost the center between the intermediate image frequency and the intermediate color frequency of the signal of television and less than the intermediate frequency of image. Therefore, it can be avoided that the level of the intermediate frequency of SIF sound decreases more than necessary at the time when the room temperature increases and the sound output deteriorates.

Claims (3)

NOVELTY OF THE INVENTION CLAIMS

1. - A television tuner comprising: a frequency converter for converting, in terms of frequency, a received television signal into an intermediate frequency signal and producing the intermediate frequency signal; and an intermediate frequency tuning circuit for passing the intermediate frequency signal, wherein a tuning frequency of the intermediate frequency tuning circuit is changed to a higher value in relation to an increase in ambient temperature.

2. The television tuner according to claim 1, further characterized in that the intermediate frequency tuning circuit has at least one capacitor and a coil and a capacitor having a characteristic negative temperature, such as the capacitor, is used.

3. The television tuner according to claim 2, further characterized in that when the tuning frequency of the intermediate frequency tuning circuit is established at an almost central frequency between an intermediate image frequency and an intermediate color frequency of the television signal at a specific temperature on an ordinary temperature scale, the tuning frequency, when the ambient temperature is increased to 25 ° C, is set to be higher than the frequency which is the quasi-central frequency between the intermediate image frequency and the intermediate color frequency of the television signal and less than the intermediate image frequency. •• - -