MXPA97008126A - Ant tuning circuit - Google Patents

Abstract

The present invention relates to an antenna tuning circuit with an input terminal and an output terminal, comprising: a first low band coil connected between said input terminal and a ground, a first high band coil whose end it is connected to earth at the reception of a high band, a second band coil placed between said output terminal and the other end of said first high band coil, a first capacitor connected between said input terminal and said output terminal; and a second capacitor connected between said other end of said first high band coil and said output terminal, wherein a coil is provided to be connected between said input terminal and said other end of said first high band coil upon receipt of a high band, and a first trap circuit including said first capacitor and said coil is established between said input terminal and said output terminal and further u n second trap circuit including said second capacitor and said second highband coil is made between the same

Description

ANTENNA TUNING CIRCUIT BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to an antenna tuning circuit of a tuner with a band switching device. 2. DESCRIPTION OF THE RELATED TECHNIQUE An antenna tuning circuit will be described later with reference to FIGS. 4 to 8. FIG. 4 shows an antenna tuning circuit of a VHF tuner, FIG. 5 illustrates a circuit equivalent to high band reception. , and Figure 6 illustrates a circuit equivalent to receiving a low band. In addition, FIGS. 7 and 8 are illustrations of transmission characteristics of the antenna tuning circuit upon reception of a high band and reception of a low band. First, in Figure 4, an input terminal 2 of an antenna tuning circuit 1 is connected through a series adaptation coil 3 to a preceding circuit (not shown), for example, an antenna filter (attenuating the intermediate frequency band). This adaptation coil 3 adapts the antenna tuning circuit 1 with the antenna filter or the like, the preceding circuit being the reception of a high band. An output terminal 4 of the antenna tuning circuit 1 is connected through a coupling capacitor 5 to a high frequency amplifier (not shown). A first low band coil 6 is connected between the input terminal 2 and the ground, although both end portions of a first high band coil 9 for receiving a high band are connected through first and second switching diodes 7. , 8 at both ends of the first low band coil 6. That is, one end of the first high band coil 6 is connected to ground by a second switching diode 8 while the other end thereof is connected to the end of input 2 by the pnrner commutator diode 7. Also, between the input terminal 2 and the other end portion of the first high-band coil 9, in addition to the first commutator diode 7, a second low-band coil 10 for reception of a low band is connected in parallel. The other end of the first high-band coil 9 is connected through a second high-band coil 11 to a varactor diode 12 which in turn is connected to ground. In addition, another varactor diode 13 is connected to the node between the second high-band coil 11 and the varactor diode 12, and the other end of this varactor diode 13 serves as an output terminal 4 of the antenna tuning circuit 1. Accordingly , the second high-band coil 11 adopts a condition placed between the other end of the first high-band coil 9 and the output terminal 4. In addition,? n first capacitor 14 is coupled between the input terminal 2 and the outlet 4, while a second capacitor 15 is coupled between the other end of the first high-band coil 9 and the output terminal 4. Incidentale, a capacitor 16 connected in series to the second low-band coil 10, a capacitor 17 placed between the first commutating diode 7 and the second high-band coil 11 and a capacitor 18 making the first low-band coil 6 connected to ground are for blocking direct currents. In the antenna tuning circuit 1 so arranged, in accordance with the band switching voltages to be applied to the band switching terminals Hb, LB, the first and second switching diodes 7, 8 respectively adopt the condition of conduction or non-conduction for high band reception or low band reception, and a tuning voltage of terminal tuning voltage terminal TU is applied to varactor diodes 12, 13, thus being tuned to give a frequency in each band. Figure 5 is an illustration of an equivalent circuit of the antenna tuning circuit 1 when the first and second switching diodes 7, 8 in Figure 4 arrive at driving conditions to allow reception of high band. At the high band reception, the first and second switching diodes 7, 8 assume the driving conditions, and thus the first low band coil 6 and the first high band coil 9 are coupled in parallel with each other, and are shown as a coil 19 connected between the input terminal 2 and the ground in Figure 5. Furthermore, in a similar manner, the first switching diode 7 reaches the driving state, and both ends of the second low band coil 10 make short circuit, so that the second high-band coil 11 is connected directly to the input terminal 2 as shown in Figure 5. Furthermore, since the first switching diode 7 adopts the driving state, the first and second capacitors 14, 15 in FIG. 4 are coupled in parallel to each other, and thus are shown as a capacitor 20 in FIG. 5. The tuning frequency of the antenna tuning circuit 1 in FIG. 5 depends mainly on the first coil of ba High side 19 and second high-band coil 11 coupled in series with each other and the varactor diode 12 connected in parallel to these coils 19, 11. A capacitor 20 between the input terminal 2 and the output terminal 4 in the figure 5 is connected in parallel to the second high-band coil 11 and varactor diode 13 connected in series to each, thus organizing a parallel resonance circuit whose resonance frequency is set at a substantially intermediate frequency (eg 650 MHz) of UHF band, with the result that a signal is attenuated at this frequency. That is, the capacitor 20, the second high-band coil 11 and the varactor diode 13 constitute a trap circuit 21 that attenuates the UHF band at high band reception, thus preventing the penetration of a UHF band signal to the high band reception. In Figure 7, a transmission characteristic of the antenna tuning circuit 1 occurring at high band reception is indicated by a curve A. This curve A goes up when the tuning circuit 1 is tuned to 343.25 MHz of a high band , and a reference line Al in curve A represents the tuning frequency of 343.25 MHz and a reference line A2 denotes the trap frequency of 650 MHz. On the other hand, at the reception of low band, the first and second diodes switches 7, 8 in figure 4 reach the non-conduction conditions, and therefore the second low-band coil 10 and the second high-band coil 11 are coupled in series with each other. In this case, the second high-band coil 11 is sufficiently smaller than the second low-band coil 10 and its reactance is ignored in the case of the low band, and thus the second capacitor 15 in Figure 4 can be observed to be coupled in parallel to the varactor diode 13, with the result that the equivalent circuit of Figure 4 becomes what is shown in Figure 6. In Figure 6, a variable capacitor 22 is due to the parallel connection of the second capacitor 15 and the varactor diode 13. Furthermore, the first capacitor 14 is connected in parallel to the second low-band coil 10 and capacitor-variable 22 connected in series with each other, and its parallel resonance frequency is set to a closeness of a frequency of image at the low band frequency. In FIG. 8,? A curve B shows a transmission characteristic adopted when the antenna tuning circuit l in FIG. 4 is tuned to 126.25 MHz of the low band, where the reference line Bl represents the frequency of tuning of 126.25 MHz while B2 represents the trap frequency of 240 MHz established at a closeness of the image frequency (European Specification). At high band reception, for example, when 343.25 MHz is received, the image frequency is converted to 421.05 MHz, although as is obvious from Figure 7 the level of transmission on (A3 in curve A) to eeta frequency in the antenna tuning circuit 1 described above only attenuates by approximately 17 dB (XI in Figure 7) with respect to the Al tuning frequency. For this reason, it is disturbed from the image frequency to the reception of high band. A possible solution to this problem is to reduce the previous trap frequency (A2 in curve A in figure 7) adjusted to the UHF band. However, in this case, the attenuation of the UHF band does not occur and, thus, the disturbance of the UHF band frequency occurs, which makes the resolution of the problems difficult.

In addition, in Figure 6, a circuit equivalent to the low band reception, the trap circuit 23 shows capacitance at both ends, ie, between the input terminal 2 and the output terminal 4 in the tuning circuit, at frequencies higher than the trap frequency, and thus, it has a resonance frequency in series with the adaptation coil 3. This frequency appears in the lowest band of the UHF band, that is, 400 MHz, so that a maximum point (B3 in curve B in figure 8) as the so-called reflected radiation. As indicated by Yl in FIG. 8, 'this maximum point B3 only attenuates by 3 dB with respect to the low-band tuning frequency Bl, so that the low band is disturbed by this frequency.

BRIEF DESCRIPTION OF THE INVENTION Accordingly, an object of the present invention is to provide an antenna tuning circuit that is capable of eliminating disturbances due to the image frequency and the UHF band frequency. Another object of this invention is to eliminate the disturbance due to the reflected radiation. To eliminate the aforementioned problems, in accordance with the present invention, an antenna tuning circuit comprises an input terminal and an output terminal, and is further composed of a first band coil ba a connected between the input terminal and the earth, and a first high-band coil whose end is connected to ground at the reception of a high band, a second high-band coil placed between the output terminal and the other end of the first high-band coil, a first capacitor connected between the input terminal and the output terminal, and a second capacitor connected between the other end of the first high-band coil and the output terminal, where upon receipt of a high band? a coil is provided for connect between the input terminal and the other end of the first high-band coil and? n first trap circuit including the first capacitor and the coil is established between the input terminal and the output terminal and further a second trap circuit including a second capacitor and the second high band coil is made between them. Further, in the antenna tuning circuit according to this invention, the first trap circuit catches a closeness of an image frequency at a high band reception frequency, while the second trap circuit catches a frequency of a band of UHF. Furthermore, in the antenna tuning circuit according to this invention, the coil is placed between the input terminal and the other end of the first high band coil to be in series with a first switching diode adopting a driving condition at the reception of a high band, and the end of the first high band coil is connected to ground through a second switching diode which assumes a driving condition upon reception of a high band. In addition, in the antenna tuning circuit according to this invention, a preceding circuit is connected to the input terminal, and upon receipt of a high band the coil is used as an adaptation coil to achieve an impedance matching. with the preceding circuit, and a second low band coil to be connected to the reception of a low band is placed between the input terminal and the other end of the first high band coil.

BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Figure 1 shows an antenna tuning circuit according to the present invention; Figure 2 shows an equivalent circuit of the antenna tuning circuit according to this invention upon receipt of a high band; Figure 3 shows an equivalent circuit of the antenna tuning circuit according to this invention upon receipt of a band ba a; Figure 4 illustrates a previous antenna tuning circuit; Figure 5 illustrates an equivalent circuit of the antenna tuning circuit prior to high band reception; Figure 6 illustrates an equivalent circuit of the antenna tuning circuit prior to receiving a low band; Figure 7 is an illustration of a transmission characteristic of the antenna tuning circuit upon receipt of a high band; and Figure 8 is an illustration of a transmission characteristic of the antenna tuning circuit upon reception of a low band.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY Referring now to Figures 1 to 3, a description will be made below of an antenna tuning circuit in accordance with an embodiment of the present invention. Figure 1 shows an antenna tuning circuit according to this invention, Figure 2 shows an equivalent circuit of the antenna tuning circuit according to this invention upon receipt of a high band, and Figure 3 shows a circuit Ll. equivalent of the antenna tuning circuit according to this invention to the reception of a low band. In these figures, the parts that are the same as the previous components are marked with the same numbers, and the description of them will be omitted for brevity. The difference of antenna tuning circuit 31 according to the embodiment of this invention of FIG. 4 of antenna tuning circuit 1 above is that a coil 32 is provided to be connected in series to a first switching diode 7 for connection between an input terminal 2 and a first high-band coil 9. Moreover, upon reception of a high band, this coil 32 is connected between the input terminal and the other end of the first high-band coil 9 when the first switching diode 7 arrives at a driving condition. Yet, this coil 32 has an impedance substantially equal to that of the adaptation coil 3 of the antenna tuning circuit 1 above. With the stipulation of this coil 32, the circuits equivalent to the reception of a high band and the reception of a low band are as shown in figures 2 and 3, respectively. More specifically, upon reception of a high band, when the first and second switching diodes 7, 8 reach the driving conditions, the second low band coil 10 in Figure 1 is connected in parallel to the newly added coil 32. This is shown as an equivalent coil 33 in Figure 2. Because the mductance of the second low-band coil 10 is greater than that of the coil 32, in the coil 33, the inductance of the coil 32 is under control. Accordingly, a first capacitor 14 is connected between the input terminal 2 and the output terminal 4, and a second capacitor 15 is connected between the other end of the first high band coil 9 and the output terminal 4. In addition, The coil 33, a second high-band coil 11 and a varactor diode 13 coupled in series with one another are connected in parallel to the first capacitor 14, thus establishing a first trap circuit 34, while the second high-band coil 11 and the Varactor diode 13 coupled in series with each other are connected in parallel to the second capacitor 15, thus establishing a second trap circuit 35. The second trap circuit 35 is made so that a trap frequency conforms to the UHF band as in the case of the previous trap circuit 21 shown in Fig. 5. The first trap circuit 34 also has the coil 33 as compared to the second trap circuit 35, and therefore, its resonance frequency is adjusted to be less than that of the second trap circuit 35. In this embodiment, the trap frequency of the first trap circuit 34 is adjusted to a closeness of an image frequency to the reception frequency, while the trap frequency of the second trap circuit 35 is adjusted to the band of the trap. UHF. That is, at the reception of 343.25 MHz of a high band, the first L3 circuit trap 34 is set to approximately 420 MHz, while the second trap circuit 35 is set to 750 MHz (European Specification). Accordingly, the transmission characteristic of the tuning circuit 31 is converted to what is indicated by a curve C in FIG. 7. As is obvious from the curve C in FIG. 7, when the image frequency is of 421.05 MHz (Cl in curve C), attenuation X2 relative to a reception frequency of 343.25 MHz (Al in curve 1) can reach approximately 30 dB, which shows greater-improvement compared to the previous XI attenuation (17 dB). In addition, apart from the image frequency, the frequency of a UHF band also attenuates, so that the attenuation of the disturbance signal is possible over a wide band from the image frequency to the UHF band. Upon reception of this high band, the first low band coil 6 is connected between the input terminal 2 and the ground, while in the case of the high band frequencies, its reactance is large so that there is no factor for adjusting the tuning frequency of the tuning circuit 31. Accordingly, as well as the matching coil 3 of the tuning circuit 1 above, the coil 33 has a function to achieve the adaptation between the antenna tuning circuit 31 and a antenna filter or similar being a preceding circuit. On the other hand, upon receipt of a low band, the first and second switching diodes 7, 8 reach the non-driving conditions, and the first low band coil 6, the second low band coil 10 and the second coil high band 11 are connected in series with each other, and the varactor diode 12 is coupled in parallel to the first low band coil 6, the second low band coil 10 and the second high band coil 11 connected in series with each other. In this case, the second high-band coil 11 is sufficiently smaller than the second low-band coil 10, and at a low band, its reactance is ignored, and thus the equivalent of the antenna tuning circuit 31 of Figure 1 to the reception of a band ba is expressed as shown in Fig. 3. The equivalent circuit of Fig. 3 is substantially the same as the equivalent circuit of the antenna tuning circuit 1 above of Fig. 5 upon receipt of a low band, except that the adaptation coil 3 shown in figure 6 is removed. Furthermore, the first capacitor 14 is connected in parallel to the second low band coil 10 and the variable capacitor 22 coupled in senes with each other to produce a circuit trap 23 similar to the previous trap circuit. The trap frequency of this trap circuit 23 is adjusted to the image frequency (approximately 240 MHz) as in the case of the previous circuit. In this case, the transmission characteristic is converted into what is indicated by a curve D in FIG. 8, so that, unlike the prior art, the reflected radiation B3 L5 does not appear. due to the adaptation coil 3 and the trap circuit 23, and the attenuation Y2 relative to the reception frequency (Bl) at a higher frequency than the image frequency B2 can be ensured by approximately 15 dB, which shows further improvement compared to the previous attenuation Yl (3 dB). Accordingly, it is possible to eliminate the disturbance due to the reflected radiation. As described above, in accordance with this invention, the above adaptation coil 3 is removed for the connection between the antenna tuning circuit 1 and an antenna filter and the like being a preceding coil, while the coil 32 with a mductance substantially equal to that of the adaptation coil 3 is connected in sequence to the first switching diode 7 for the connection between the input terminal 2 and the other end of the first high-band coil 9, and therefore, to the reception of a high band, two trap circuits 34, 35 can be established, and upon receipt of a low band, the so-called reflected radiation B3 does not rise due to the absence of a coil that creates resonance in sequence with the trap circuit 23. Incidentally, still if the reflected radiation B3 occurs upon receiving a low band, as long as the disturbance due to this reflected radiation does not create a problem, it is possible to place a coil similar to the bob 3 above adaptation between the antenna filter or the like being the preceding circuit and the input terminal 2 of the tuning circuit 31. In this case, at least when a high band is received, it is possible to eliminate both the disturbance due to the image frequency as the disturbance due to the frequency of the UHF band. As described above, since in the antenna tuning circuit according to this invention the coil is provided to be connected between the input terminal and the other end of the first high band coil upon reception of a high band and the first trap circuit including the first capacitor and this coil is formed between the input terminal and the output terminal and further the second trap circuit including the second capacitor and the second high band coil is established between them, upon receipt from a high band these two trap circuits can attenuate the disturbance signal over a wide band. Furthermore, in the antenna tuning circuit according to this invention, since the first trap circuit catches a closeness of an image frequency to the reception frequency of a high band while the second trap circuit catches a frequency of the band of UHF, in addition to the frequency of the UHF band it is possible to attenuate the image frequency which particularly tends to cause the disturbance, so that the elimination of the image disturbance becomes possible. In addition, in the antenna tuning circuit according to this invention, since this coil is placed between the input terminal and the other end of the first high band coil to be in sequence to the first switching diode adopting the condition of driving to the reception of a high band and one end of the first high band coil is connected to ground through the second switching diode which assumes the conduction condition to the reception of high band, the doe trap circuits can be established from it maintains that the first and second switching diodes are simply adjusted to the driving conditions upon reception of a high band. Furthermore, in the antenna tuning circuit according to this invention, since this coil is used as an adaptation coil to perform the impedance matching with the preceding circuit upon reception of a high band and the second low band coil which is to be connected to the reception of a low band is provided between the input terminal and the other end of the first high band coil, the so-called reflected radiation does not occur at a frequency higher than the image frequency at the low band frequency, and therefore no disturbance occurs due to the reflected radiation. It should be understood that the foregoing refers to only one preferred embodiment of the present invention, and that it should cover all changes and modifications of the embodiment of the invention used herein for the purpose of the description, which does not constitute departures from the spirit and scope of the invention.

Claims (4)

NOVELTY OF THE INVENTION CLAIMS

1. - An antenna tuning circuit with an input terminal and an output terminal, comprising: a first low band coil connected between said input terminal and a ground; a first high-band coil whose extr-emo is connected to ground at the reception of a high band; a second high-band coil placed between said output terminal and the other end of said first high-band coil; a first capacitor connected between said input terminal and said output terminal; and a second capacitor connected between said other end of said first high band coil and said output terminal, wherein a coil is provided to connect between said input terminal and said other end of said first high band coil upon receipt of a high band, and? n first trap circuit including said first capacitor and said coil is established between said input terminal and said output terminal and further a second trap circuit including said second capacitor and said second high band coil is made between rnisrnos.

2. An antenna tuning circuit according to claim 1, further characterized in that said first trap circuit captures a closeness of an image frequency to a reception frequency of a high band, while said second trap circuit catches a frequency. from a band < UHF.

3. An antenna tuning circuit according to claim 1, further characterized in that said coil is placed between said input terminal and said other end of said first high-band coil to be in sequence to a first switching diode that a conduction condition is adopted at the reception of the high band, and one end of said first high band coil e is connected to ground through a second switching diode which assumes a conduction condition at the reception of the high band.

4. An antenna tuning circuit according to claim 3, further characterized in that a preceding circuit is connected to said input terminal, and upon receipt of the high band, said coil is used as an adaptation coil to achieve the impedance matching with said preceding circuit, and a second low band coil for connecting to the reception of a low band is placed between said input terminal and said other end of said first high band coil.