Unbalanced-balanced converter as a mixer input circuit

Abstract: A mixer input circuit for converting a signal from a tuning section of an unbalanced type into a balanced signal. In the mixer input circuit (12;20,13-15), an RF signal from an input terminal (11) amplified by an RF amplifier (12) is transmitted to a tuning circuit section (20) being a distributed constant line. A central conductor (22) of the distributed constant line in the tuning circuit section (20) is electromagnetically combined with a central conductor (13), thereby constituting an unbalance-balance conversion section. A middle point of the central conductor (13) on an output side of the unbalance-balance conversion section is grounded, and a balanced output signal from both ends transmitted to a mixer (16), so as to be converted into an IF signal. With the present mixer input circuit (12,20,13-15), unbalance-balance conversion can be carried out without using a transformator, and a reduction in size can be realized due to absence of deterioration of performance.

Description: This invention relates to a mixer input circuit, and particularly to a mixer input circuit for converting signals from an unbalanced-type tuning section into balanced signals and inputting the signals to a mixer. In order to optimize noise figure for an input circuit for inputting a signal to a mixer of a so-called UHF tuner, it is conventionally known to carry out balanced input to the mixer. However, a radio frequency (RF) tuning section of a conventional tuning circuit is of an unbalanced type. Fig.1 of the accompanying drawings, for instance, shows an example of the conventional mixer input circuit. Referring to Fig.1, an RF signal from an antenna is transmitted via an RF input terminal 51 to an RF amplifier 52 so as to be amplified. The amplified RF signal is then transmitted to one of the input terminals of an unbalance-balance conversion circuit 53 made up of an inductance element such as a coil. The one input terminal is grounded via a trimmer capacitor 54 while the other input terminal is grounded directly. An output signal from one of output terminals of the unbalance-balance conversion circuit 53 is transmitted via a capacitor 55C and a coil 55L to one of input terminals of a mixer 61. An output signal from the other output terminal of the unbalance-balance conversion circuit 53 is transmitted via a capacitor 56C and a coil 56L to the other input terminal of the mixer 61. The pair of output terminals of the unbalance-balance conversion circuit 53 are connected with each other through a trimmer capacitor 58 between them. An oscillation output signal is supplied from a local oscillator 62 to the mixer 61, so as to be frequency-converted into an intermediate frequency (IF) signal and then to be taken out from an output terminal 63. Fig.2 shows another example of the conventional mixer input circuit. Referring to Fig.2, an RF signal obtained from an input terminal 51 via an RF amplifier 52 is transmitted via a tuning circuit section 65 made up of a capacitor and coils to an unbalance-balance conversion circuit 66 employing a so-called balun transformer. Similar to the arrangement shown in Fig.1, the RF signal of this example is frequency-converted into an IF signal by a signal from a local oscillator 62, so as to be outputted from an output terminal 63. The conventional mixer input circuit as described above requires an unbalance-balance conversion circuit employing a transformer, and thus is likely to generate inconvenience, such as, a rise in production costs due to an increase in the number of parts and deterioration of performance due to interference of the conversion coil. Accordingly, it is an object of the present invention to provide a mixer input circuit in a device such as a tuner in which an unbalance-balance conversion circuit employing a transformer is not required, thereby rendering it possible to carry out unbalance-balance conversion with an inexpensive arrangement and to prevent deterioration of performance. According to the present invention, there is provided a mixer input circuit for converting a signal from a tuning section into a balanced signal and outputting the signal, said mixer input circuit comprising: a tuning circuit section comprising electromagnetically coupled central conductors made of a distributed constant line to which an RF signal is input, an unbalance-balance conversion section comprising a central conductor made of distributed constant line which is electromagnetically coupled to one of said central conductors, and a mixer to which a balanced output signal from said unbalance-balance conversion section is supplied; characterised in that: said unbalance-balance conversion section central conductor is selected from a plurality of conducting plates each having three legs, the middle leg of each plate being in a position different to the others, and wherein the central conductor is selected such that, in use, the balanced output signals are at the same level. It is preferable that with the central conductor on the output side of the unbalance-balance conversion section, the level of balanced output signals be adjusted by moving a grounding centre in accordance with positions of parts. It is also preferable that the phase and level of the balanced signals to the mixer be adjusted by inserting and connecting an impedance element between the middle point and the grounding point of the central conductor on the conversion output side, or by connecting the impedance elements to both ends of the central conductor, respectively. With the mixer input circuit according to the present invention, since the unbalance-balance conversion section made up of the distributed constant line is employed, it is possible to carry out inexpensive unbalance-balance conversion without using expensive parts such as a transformer. Also, deterioration of performance due to the conventional unbalance-balance conversion circuit using a coil or a transformer may be prevented, and such an arrangement is suitable for a reduction in size. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which: - Fig.1 is a block circuit diagram showing a schematic arrangement of an example of a conventional mixer input circuit. Fig.2 is a block circuit diagram showing a schematic arrangement of another example of the conventional mixer input circuit. Fig.3 is a block circuit diagram showing a schematic arrangement of an example of a mixer input circuit according to the present invention. Fig.4 is a block circuit diagram showing essential portions of the mixer input circuit shown in Fig.3. Figs.5A & B are diagrams showing an example of a specific arrangement of a central conductor employed in the mixer input circuit according to the present invention. Fig.6 is a block diagram showing a concrete example of essential portions of the mixer input circuit according to the present invention. Fig.7 is a block circuit diagram showing another concrete example of the essential portions of the mixer input circuit according to the present invention. Referring to Fig.3, a radio frequency (RF) signal from an antenna is supplied to an input terminal 11. The RF signal is amplified by an RF amplifier 12, and is then transmitted to a distributed constant tuning circuit section 20 employing a distributed constant transmission line. The distributed constant circuit section 20 is constituted by central conductors 21, 22 of the distributed constant line and by trimmer capacitors 23, 24. The central conductor 22 in the distributed constant tuning circuit section 20 is electromagnetically linked with another central conductor 13 of the distributed constant line, thereby constituting an unbalance-balance conversion section. A middle point or an intermediate tap of the central conductor 13 is grounded, and both ends of the line are connected to a pair of input terminals of a mixer 16 via capacitors 14, 15, respectively. To the mixer 16, an oscillation output signal is supplied from a local oscillator, not shown, similarly to the aforementioned circuits shown in Figs.1 and 2. The input RF signal is mixed with the local oscillation signal, so as to be frequency-converted into a so-called IF signal and then to be outputted from an output terminal 17. In the unbalance-balance conversion section made up of the central conductors 22, 13 of the distributed constant line, for adjusting each of the pair of balanced input signals to the mixer 16 to the same level when grounding the mid point of the central conductor 13, it is preferable to adjust an ideal position of the middle point in accordance with positions of parts. That is, the position of the middle point of the central conductor 13 is variably adjusted as indicated by arrows in Fig. 4. Specifically, several central conductors 13A and 13B, each having a middle leg portion in a position different from another, are prepared and inserted into a print base plate 18 one at a time, as shown in Fig. 5(A) and 5(B), so as to select the central conductor which adjusts the balanced input signals to the same level. Next, impedance elements 26, 27 of impedance Z1, Z2, respectively, are inserted and connected between both ends of the central conductor 13 and the pair of input terminals of the mixer 16, as shown in Fig. 6. This allows adjustment of phase and amplitude or level of the balanced input signals, such that the phases of the balanced input signals are inverted relative to each other, while the amplitudes are adjusted to be equal to each other. It is also possible to adjust the level of the balanced input signals by inserting and connecting an impedance element 28 of impedance Z3 between the middle point of the central conductor 13 and the grounding point, as shown in Fig. 7. The impedance elements 26, 27, and 28 are constituted by using resistance and a capacitor. With the above-mentioned constitution, the unbalance-balance conversion section is constituted by the distributed constant line. Accordingly, the unbalance-balance conversion circuit employing a conventional transformer is not required, and the mixer input circuit of the balanced type which is inexpensive, free from deterioration of performance, and suitable for a reduction in size, can be constituted. The present invention is not limited to the above embodiments. For instance, the tuning circuit section 20 employing the distributed constant line is not limited to the example shown, but may be constituted in various manners, such as, one having a trimmer capacitor and a central conductor. As is apparent from the above description, according to the mixer input circuit according to the present invention, the tuning circuit section to which an RF signal is inputted is constituted by using the distributed constant line, and the unbalance-balance conversion section is constituted by electromagnetically coupling the central conductor in the tuning circuit section with the central conductor for balanced output, so that the balanced output signal is inputted from the conversion section to the mixer. Therefore, inexpensive unbalance-balance conversion can be carried out without using expensive parts such as a transformer, and there is no need, in designing, to consider deterioration of performance due to the conversion circuit, thereby contributing to a reduction in size.

Claims: A mixer input circuit for converting a signal from a tuning section into a balanced signal and outputting the signal, said mixer input circuit comprising: a tuning circuit section (20) comprising electromagnetically coupled central conductors (21, 22) made of a distributed constant line to which an RF signal is input, an unbalance-balance conversion section comprising a central conductor (13) made of distributed constant line which is electromagnetically coupled to one of said central conductors (22), and a mixer (16) to which a balanced output signal from said unbalance-balance conversion section is supplied; characterised in that: said unbalance-balance conversion section central conductor (13) is selected from a plurality of conducting plates each having three legs, the middle leg of each plate being in a position different to the others, and wherein the central conductor is selected such that, in use, the balanced output signals are at the same level. The mixer input circuit as claimed in claim 1, wherein the phase and amplitude of the balanced output signal are adjusted by connecting impedance elements between both ends of said central conductor and a pair of input terminals of the mixer. The mixer input circuit as claimed in claim 1, wherein the amplitude of the balanced output signal is adjusted by connecting an impedance element between a middle point of said central conductor and a grounding point.

Inventors: ONO JUN, SUDA TORU, ONO, JUN, SUDA, TORU, ONO, JUN, SONY CORPORATION, SUDA, TORU, SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: H01P5
IPC codes: H04N5, H01P5, H03D7