1. Field of the Invention
The present invention relates to non-reciprocal circuit elements and in particular relates to non-reciprocal circuit elements such as isolators and circulators preferably for use in microwave bands.
2. Description of the Related Art
In the related art, non-reciprocal circuit elements such as isolators and circulators have a characteristic that they transmit a signal only in a specific predetermined direction and do not transmit a signal in the opposite direction. For example, isolators employ this characteristic when used in a transmission circuit section of a mobile communication apparatus such as a cellular phone.
As described in Japanese Patent No. 4197032 and Japanese Patent No. 4155342, examples of a two-port isolator used as such a non-reciprocal circuit element are known and have a basic configuration in which a first central electrode and a second central electrode are arranged on the surface of a ferrite so as to cross each other while being insulated from each other, a resistor is connected between one end of the first central electrode connected to an input port and one end of the second central electrode connected to an output port, and a capacitor is connected in parallel with the resistor. In both examples, improvements in insertion loss and isolation characteristics are achieved.
Meanwhile, in recent years, communication in a plurality of frequency bands using a single cellular phone has become possible. In order to achieve this, in the related art, a single isolator has been used for each frequency band and therefore the number of components has increased. Consequently, a non-reciprocal circuit element that can be used in a plurality of frequency bands has been demanded. In short, a non-reciprocal circuit element that has a single input but at least two outputs for outputting signals of a plurality of frequency bands has been demanded.
The present inventors conceived of forming a single non-reciprocal circuit element to be used in a plurality of frequency bands by combining a pair of the two-port isolators described in Japanese Patent No. 4197032 or Japanese Patent No. 4155342. This two-port isolator is a high-pass-type isolator and as illustrated in FIG. 15 when a combination is formed that operates at frequencies f1 and f2, a harmonic band of the frequency f2 is superposed with the frequency f1 and therefore the quality of communication is poor.
Furthermore, the present inventors proposed the non-reciprocal circuit element described in Japanese Unexamined Patent Application Publication No. 2011-176668 that is intended to suppress increases in the number of components and insertion loss as much as possible while being capable of achieving good operation in a plurality of frequency bands.
This non-reciprocal circuit element, as illustrated in FIG. 16, is formed by combining a first isolator 100 and a second isolator 200 and each of the isolators 100 and 200 is configured as a high-pass isolator that is provided with a first central electrode 135 (inductors L1H and L1L respectively) and a second central electrode 136 (inductors L2H and L2L respectively) arranged on a ferrite 132, which is applied with a direct-current magnetic field from a permanent magnet (not illustrated), so as to cross each other while being insulated from each other. The pass frequency band of the first isolator 100 is set so as to be higher than the pass frequency band of the second isolator 200 and inputs of the first isolator 100 and the second isolator 200 are electrically connected to each other to form a single input terminal IN and their outputs form output terminals OUT1 and OUT2. In addition, a low-pass filter LPF is provided between the input terminal IN and the input of the second isolator 200.
The non-reciprocal circuit element illustrated in FIG. 16 has the single input terminal IN formed by electrically connecting the inputs of the first isolator 100 and the second isolator 200 and functions as a single non-reciprocal circuit element. Furthermore, since the low-pass filter LPF is provided between the input terminal IN and the input of the second isolator 200, a harmonic band of the second isolator 200, which has a low pass frequency band, is attenuated and interference with the first isolator 100, which has a high pass frequency band, is prevented. In addition, the low-pass filter LPF is provided at a single location between the input terminal IN and the input of the second isolator 200 and therefore an increase in insertion loss and an increase in the number of components are suppressed.
In more detail, in the isolators 100 and 200, in order to reduce insertion loss, one end of each of the first central electrodes 135 forms an input port P1 and the other end of each of the first central electrodes 135 forms an output port P2, one end of each of the second central electrodes 136 also forms the output port P2 and the other end of each of the second central electrodes 136 forms a ground port P3, a resistor R1H and a capacitor C1H, which are connected in parallel with each other, and a resistor R1L and a capacitor C1L, which are connected in parallel with each other, are connected between the respective input ports P1 and output ports P2, and capacitors C2H and C2L are connected in parallel with the respective second central electrodes 136. The first central electrode 135 and the capacitor C1H or C1L form a resonant circuit and the second central electrode 136 and the capacitor C2H or C2L form a resonant circuit. In addition, capacitors CS1H and CS2H and CS1L and CS2L are connected to sides of the input port P1 and the output port P2 in order to adjust impedance.
The non-reciprocal circuit element composed of the isolators 100 and 200 is incorporated into a transmission circuit of a cellular phone. That is, the input terminal IN is connected to a transmission-side power amplifier PA via matching circuits 60 and 70 and the output terminals OUT1 and OUT2 are connected to an antenna via a duplexer or the like.
Usually, the output impedance of the power amplifier PA is low at around 5Ω and the input impedance of the isolators 100 and 200 is high at around 50Ω. It is possible to reduce the input impedances of the isolators 100 and 200 by reducing the crossing angle of the first central electrode 135 and the second central electrode 136, but there is a limit to how much the crossing angle can be reduced (how much the input impedance can be reduced) due to the demand for size reduction of the isolators 100 and 200.
Consequently, the matching circuit 60, which is composed of an inductor L13 and a capacitor C14, and the matching circuit 70, which is composed of an inductor L14 and a capacitor C15, are interposed between the isolators 100 and 200 and the power amplifier PA to make the impedances gradually increase to 25Ω and then 50Ω to match the impedance of the isolators 100 and 200. However, interposing the matching circuits 60 and 70 leads to the insertion loss being increased and also the number of components and the cost of the transmission circuit being increased. Regarding the insertion loss, as illustrated in FIG. 16, in the isolator 100, an insertion loss of the matching circuits 60 and 70 of 0.8 dB is added to the insertion loss of the isolator 100 of 0.7 dB, resulting in a total of 1.5 dB. For the isolator 200, an insertion loss of the low-pass filter LPF of 0.3 dB and an insertion loss of the matching circuits 60 and 70 of 0.8 dB are added to the insertion loss of the isolator 200 of 0.7 dB, giving a total of 1.8 dB.