Harmonic mixer

[0001] This application is a Continuation of PCT International Application No. PCT/JP2022/011465, filed on Mar. 15, 2022, which is hereby expressly incorporated by reference into the present application.

[0002] The present disclosure relates to a harmonic mixer.

[0003] Switching-type harmonic mixers are known as a technology of performing low-loss frequency conversion in a sub-terahertz band. For example, a frequency converter in which the source terminals of N switch transistors are connected to an input terminal, and the drain terminals of the N switch transistors are connected to an output terminal is described in Patent Literature 1. Here, N is an integer greater than or equal to 2. When N local oscillation signals whose phases differ from one another by 360/N degrees are inputted to the gate terminals and a high frequency signal is inputted to the input terminal, the frequency converter functions as an N-th harmonic mixer which outputs a signal whose frequency is shifted with respect to that of the input signal from the output terminal.

[0004] Patent Literature 1: International Publication Number WO/2015/001924

[0005] In the harmonic mixer described in Patent Literature 1, when the high frequency signal which is the input signal has an RF frequency close to the maximum oscillation frequency of the transistors, the isolation between the source and drain terminals of the switch transistors degrades. Therefore, a problem with the harmonic mixer is that the influence of the characteristic of an IF matching circuit at the RF frequency upon the impedance seen from the RF terminal toward the output terminal causes degradation in the noise figure (referred to as “NF” hereinafter) characteristic of the harmonic mixer.

[0006] The present disclosure is made to solve the above-mentioned problem, and it is therefore an object of the present disclosure to provide a harmonic mixer which provides a low NF even when the isolation of transistors degrades.

[0007] According to the present disclosure, there is provided a harmonic mixer including: multiple transistors having respective first terminals, respective second terminals, and respective third terminals; an RF terminal which is connected to the second terminals, to receive a high frequency signal as input; multiple LO terminals which are connected to the respective first terminals, to receive local oscillation signals as input; an IF terminal which is connected to the third terminals, and from which an intermediate frequency signal is outputted; a series resonant circuit which is disposed between the IF terminal and a ground terminal, and whose resonance frequency is an RF frequency of the high frequency signal; and an IF matching circuit disposed between the IF terminal and an output terminal, in which, when the RF frequency is identical or close to a maximum oscillation frequency of the transistors, impedance seen from the IF terminal toward the output terminal at the RF frequency becomes low, and impedance seen from the IF terminal toward the output terminal at an IF frequency of the intermediate frequency signal is a combination of impedance of the series resonant circuit and impedance of the IF matching circuit.

[0008] According to the present disclosure, when the RF frequency of the high frequency signal is identical or close to the maximum oscillation frequency of the transistors, the impedance seen from the IF terminal toward the output terminal at the RF frequency becomes low, and the impedance seen from the IF terminal toward the output terminal at the IF frequency is the combination of the impedance of the series resonant circuit and the impedance of the IF matching circuit. Even when the RF frequency of the high frequency signal is identical or close to the maximum oscillation frequency of the transistors, and thus the isolation of the transistors degrades, the impedance seen from the IF terminal toward the output terminal at the RF frequency becomes low and the impedance seen from the IF terminal toward the output terminal at the IF frequency becomes one at which power matching can be established, so that the harmonic mixer according to the present disclosure can implement a low NF.

[0014] FIG. 1 is a circuit diagram showing an example of the configuration of a harmonic mixer 1 according to Embodiment 1. The harmonic mixer 1 shown in FIG. 1 receives an input signal which is a high frequency signal and local oscillation signals, and outputs a signal which is a result of performing frequency conversion on the input signal. For example, when the RF frequency of the input signal is F_RF, the IF frequency of an intermediate frequency signal is F_IF, and the local oscillation frequency of the local oscillation signals is F_LO, the harmonic mixer 1 is an N-th one that outputs a signal having the IF frequency F_IF (=F_RF=N×F_LO). Here, N is an integer greater than or equal to 2.

[0015] The harmonic mixer 1 includes N transistors 2-n, an RF terminal 3, N LO terminals 4-n, an IF terminal 5, a series resonant circuit 6, an output terminal 7, and an IF matching circuit 8. n is an integer greater than or equal to 1 and less than or equal to N. Each of N transistors 2-n is, for example, a field effect transistor (FET) having a gate terminal G which is a first terminal, a source terminal S which is a second terminal, and a drain terminal D which is a third terminal. The N source terminals S are connected to one point and the N drain terminals D are connected to one point, so that the N transistors 2-n are connected in parallel.

[0016] The RF terminal 3 is an input terminal connected to the N source terminals S, and a high frequency signal having a high frequency (RF frequency F_RF) is inputted, as the input signal, to the RF terminal. The N LO terminals 4-n are connected to the respective N gate terminals G of the N transistors 2-n, and the local oscillation signals having the local oscillation frequency (LO frequency F_LO) are inputted to the N LO terminals. For example, the local oscillation signals each having a phase difference of 360(n−1)/N degrees are inputted to the respective N LO terminals 4-n. The IF terminal 5 is connected to the N drain terminals D, and an intermediate frequency signal having the intermediate frequency (IF frequency F_IF) is outputted from the IF terminal.

[0017] The series resonant circuit 6 is disposed between the IF terminal 5 and a ground terminal GND, and resonates at a preset frequency. The ground terminal GND has a ground potential which is a reference potential. For example, the series resonant circuit 6 is an LC resonance circuit including a capacitor C1 and an inductor L1 which are connected in series. In the series resonant circuit 6, the IF terminal 5 is connected to one terminal of the capacitor C1 and the inductor L1 is connected to the other terminal of the capacitor, and the ground terminal GND is connected to the inductor L1. For example, in the series resonant circuit 6, the resonance frequency is set to the RF frequency F_RF of the high frequency signal.

[0018] The IF matching circuit 8 is disposed between the IF terminal 5 and the output terminal 7, and matches the impedance of the IF terminal 5 to that of the output terminal 7. For example, the IF matching circuit 8 includes a capacitor C2 and an inductor L2. In the IF matching circuit 8, the IF terminal 5 is connected to one terminal of the capacitor C2, and the output terminal 7 is connected to the other terminal of the capacitor. The IF terminal 5 is connected to one terminal of the inductor L2, and the other terminal of the inductor is grounded.

[0019] The high frequency signal having the RF frequency F_RF is inputted to the RF terminal 3. The local oscillation signals having a phase of 0 degrees, . . . , a phase of 360(n−1)/N degrees, . . . , and a phase of 360(N−1)/N degrees are inputted, via the LO terminals 4-1, . . . , 4-n, . . . , and 4-N, to the gate terminals G of the transistors 2-1, . . . , 2-n, . . . , and 2-N. The transistors 2-1, . . . , 2-n, . . . , and 2-N perform switching of electrically connecting the source terminals S and the drain terminals D according to the local oscillation signals inputted to the gate terminals G. As a result, each of the transistors performs frequency conversion on the high frequency signal inputted to the RF terminal 3, to generate an intermediate frequency signal having the IF frequency F_IF. The intermediate frequency signal is outputted to the IF terminal 5, passes through the IF matching circuit 8, and is outputted from the output terminal 7.

[0020] FIG. 2 is a Smith chart showing an example of a result of a simulation of the minimum noise figure (referred to as “NF_min” hereinafter) when the impedance at the RF frequency F_RF of the IF matching circuit 8 is varied, and shows NE_min which is calculated by performing a load pull simulation of the IF terminal 5 at the RF frequency F_RF.

[0021] This simulation is performed on a fourth harmonic mixer which is configured using a 45 nm SOI CMOS process to have the same circuit configuration as the harmonic mixer 1, and which is designed in such a way that the RF frequency F_RF is 300 GHz, the LO frequency F_LO is 70 GHZ, and the IF frequency F_IF is 20 GHz. An RF matching circuit is separately disposed at the RF terminal of the above-mentioned fourth harmonic mixer, the design is performed in such a way that the impedance seen from the IF terminal at the RF frequency F_RF is 50Ω, and a 50 Ω port is disposed at the input terminal of the above-mentioned fourth harmonic mixer.

[0022] In addition, a port is disposed at the IF terminal of the above-mentioned fourth harmonic mixer, and NF_min of the fourth harmonic mixer at each impedance is calculated by sweeping the impedance of this port.

[0023] As shown in FIG. 2, NF_min varies by the impedance at the IF terminal 5 at the RF frequency F_RF, and NF_min is improved as the impedance becomes lower. An NF optimal point shown in FIG. 2 is the impedance at the IF terminal at the RF frequency F_RF at which NF_min is the smallest in this load pull simulation.

[0024] Here, when the RF frequency F_RF is identical or close to the maximum oscillation frequency F_max of the transistors 2-n, the isolation between the source terminals S and the drain terminals D in the transistors 2-n degrades. As a result, the impedance at the IF terminal 5 becomes visible from the RF terminal 3, and the NF_min characteristic degrades.

[0025] Accordingly, in the harmonic mixer 1, the impedance at the IF terminal 5, i.e. the impedance seen from the IF terminal 5 toward the output terminal 7 is made to be a low impedance at the RF frequency F_RF, and to be impedance at which it is possible to establish power matching at the IF frequency F_IF. As a result, even when the isolation of the transistors 2-n degrades, the harmonic mixer 1 for reception having a low NF is implemented.

[0026] FIG. 3 is a view showing the impedance Z_IF seen from the IF terminal 5 toward the output terminal 7 at the RF frequency F_RF in the harmonic mixer 1. As shown in FIG. 3, the impedance seen from the IF terminal 5 toward the output terminal 7 at the RF frequency F_RF is denoted by Z_IF. Because the resonance frequency of the series resonant circuit 6 is the RF frequency F_RF, the series resonant circuit 6 is virtually short-circuited via the ground terminal GND, and the impedance of the series resonant circuit 6 becomes zero. Because the impedance of the IF matching circuit 8 becomes invisible from the IF terminal 5, as shown by a broken line in FIG. 3, the impedance Z_IF seen from the IF terminal 5 toward the output terminal 7 at the RF frequency F_RF becomes zero, i.e. sufficiently low.

[0027] FIG. 4 is a view showing the impedance seen from the IF terminal 5 toward the output terminal 7 at the IF frequency F_IF in the harmonic mixer 1. As shown in FIG. 4, the impedance seen from the IF terminal 5 toward the output terminal 7 at the IF frequency F_IF is denoted by Z_IF. The impedance of the series resonant circuit 6 is not zero at the IF frequency F_IF, and the impedance of the IF matching circuit 8 becomes visible from the IF terminal 5. More specifically, the impedance Z_IF seen from the IF terminal 5 toward the output terminal 7 at the IF frequency F_IF becomes one which is the combination of the impedance of the series resonant circuit 6 and the impedance of the IF matching circuit 8.

[0028] For the impedance Z_IF which is the combination of the impedance of the series resonant circuit 6 and the impedance of the IF matching circuit 8, the circuit constants of the series resonant circuit 6 and the IF matching circuit 8 are set in such a way that power matching can be established.

[0029] In the harmonic mixer 1, when the RF frequency F_RF is identical or close to the maximum oscillation frequency F_max of the transistors 2-n, the impedance Z_IF seen from the IF terminal 5 toward the output terminal 7 at the RF frequency F_RF becomes low, and the impedance Z_IF seen from the IF terminal 5 toward the output terminal 7 at the IF frequency F_IF is the combination of the impedance of the series resonant circuit 6 and the impedance of the IF matching circuit 8. As a result, the harmonic mixer 1 can implement a low NF.

[0030] Although the case where in the harmonic mixer 1 the IF matching circuit 8 includes the capacitor C2 and the inductor L2 is shown above, this embodiment is not limited to this example, and the IF matching circuit 8 should just have a circuit configuration in which power matching can be established at the IF frequency.

[0031] Further, although the harmonic mixer 1 in which the RF terminal 3 is connected to the N source terminals S of the N transistors 2-n, and the IF terminal 5 is connected to the N drain terminals D of the N transistors 2-n is shown, this embodiment is not limited to this example. For example, the harmonic mixer may have a configuration in which the RF terminal 3 is connected to the N drain terminals D of the N transistors 2-n, and the IF terminal 5 is connected to the N source terminals S of the N transistors 2-n.

[0032] As mentioned above, the harmonic mixer 1 according to Embodiment 1 includes: the N transistors 2-n; the RF terminal 3 to which a high frequency signal is inputted; the N LO terminals 4-n to which local oscillation signals are inputted; the IF terminal 5 from which an intermediate frequency signal is outputted; the series resonant circuit 6 which is disposed between the IF terminal 5 and the ground terminal GND, and whose resonance frequency is the RF frequency F_RF of the high frequency signal; and the IF matching circuit 8 disposed between the IF terminal 5 and the output terminal 7.

[0033] In the harmonic mixer 1, when the RF frequency F_RF of the high frequency signal is identical or close to the maximum oscillation frequency F_max of the transistors 2-n, the impedance Z_RF seen from the IF terminal 5 toward the output terminal 7 at the RF frequency F_RF becomes low, and the impedance Z_IF seen from the IF terminal 5 toward the output terminal 7 at the IF frequency F_IF is the combination of the impedance of the series resonant circuit 6 and the impedance of the IF matching circuit 8.

[0034] As a result, because even when the isolation of the transistors 2-n degrades, the impedance Z_RF at the RF frequency F_RF becomes low and the impedance Z_IF at the IF frequency F_IF becomes one at which power matching can be established, the harmonic mixer 1 can implement a low NF.

[0035] In the harmonic mixer 1 according to Embodiment 1, the local oscillation signals each having a phase difference of 360(n−1)/N degrees are inputted to the respective N LO terminals 4-n. As a result, the harmonic mixer 1 can perform frequency conversion on the high frequency signal inputted to the RF terminal 3 in each of the transistors.

[0036] FIG. 5 is a circuit diagram showing an example of the configuration of a harmonic mixer 1A according to Embodiment 2. In FIG. 5, the harmonic mixer 1A receives an input signal which is a high frequency signal and local oscillation signals, and outputs a signal which is a result of performing frequency conversion on the input signal. For example, when the RF frequency of the input signal is F_RF, the IF frequency of an intermediate frequency signal is F_IF, and the local oscillation frequency of the local oscillation signals is F_LO, the harmonic mixer 1A is an N-th one that outputs a signal having the IF frequency F_IF (=F_RF=N×F_LO). Here, N is an integer greater than or equal to 2.

[0037] The harmonic mixer 1A includes N transistors 2A-n, an RF terminal 3A, N LO terminals 4A-n, an IF terminal 5A, a series resonant circuit 6A, an output terminal 7A, and an IF matching circuit 8A. n is an integer greater than or equal to 1 and less than or equal to N. Each of the N transistors 2A-n is, for example, a field effect transistor (FET) having a gate terminal G which is a first terminal, a source terminal S which is a second terminal, and a drain terminal D which is a third terminal. The N source terminals S are connected to one point and the N drain terminals D are connected to one point, so that the N transistors 2A-n are connected in parallel.

[0038] The RF terminal 3A is an input terminal connected to the N source terminals S, and a high frequency signal having an RF frequency F_RF is inputted, as the input signal, to the RF terminal. The N LO terminals 4A-n are connected to the respective N gate terminals G of the N transistors 2A-n, and the local oscillation signals having the LO frequency F_LO are inputted to the N LO terminals. For example, the local oscillation signals each having a phase difference of 360(n−1)/N degrees are inputted to the respective N LO terminals 4A-n. The IF terminal 5A is connected to the N drain terminals D, and an intermediate frequency signal having the IF frequency F_IF is outputted from the IF terminal.

[0039] The series resonant circuit 6A is disposed between the IF terminal 5A and a ground terminal GND, and resonates at a preset frequency. The series resonant circuit 6A is an LC resonance circuit including a capacitor C1 and a parasitic inductance L1′ of a wire 9. In the series resonant circuit 6A, the IF terminal 5A is connected to one terminal of the capacitor C1, and the wire 9 is connected to the other terminal of the capacitor. The wire 9 in which the parasitic inductance L1′ is formed is connected to the ground terminal GND. The capacitor C1 is grounded via the parasitic inductance L1′ of the wire 9. In the series resonant circuit 6A, the resonance frequency is set to the RF frequency F_RF of the high frequency signal.

[0040] In the harmonic mixer 1A, when the RF frequency F_RF is identical or close to the maximum oscillation frequency F_max of the transistors 2A-n, the impedance Z_IF seen from the IF terminal 5A toward the output terminal 7A at the RF frequency F_RF becomes low, and the impedance Z_IF seen from the IF terminal 5A toward the output terminal 7A at the IF frequency F_IF is the combination of the impedance of the series resonant circuit 6A and the impedance of the IF matching circuit 8A. As a result, even when the isolation of the transistors 2A-n degrades, the harmonic mixer 1A can implement a low NF.

[0041] Because in the harmonic mixer 1A the series resonant circuit 6A includes the capacitor C1 and the wire 9, the inductor L1 which the series resonant circuit 6 includes is unnecessary. As a result, the series resonant circuit 6A is downsized, and a downsizing of the harmonic mixer 1A can also be achieved.

[0042] Further, because in the series resonant circuit 6A the capacitor C1 is connected as a shunt, the inductance value of the parasitic inductance L1′ of the shunt which is used in the IF matching circuit 8A in order to establish power matching at the IF frequency F_IF can be reduced. As a result, a downsizing of the inductor which constitutes the series resonant circuit 6A can be achieved. In general, in IF matching circuits, the inductor has a larger size than the capacitor. Therefore, a downsizing of the inductor results in a downsizing of the series resonant circuit 6A, and, in turn, can cause a downsizing of the harmonic mixer 1A.

[0043] Because the parasitic inductance L1′ of the wire 9 increases at the RF frequency F_RF, the capacitance value of the capacitor C1 for setting the resonance frequency of the series resonant circuit 6A to the RF frequency F_RF can also be made small, and thus the capacitor C1 can be downsized. Also as a result of this downsizing, the series resonant circuit 6A is downsized, and, in turn, the harmonic mixer 1A can be downsized.

[0044] Further, although the harmonic mixer 1A in which the RF terminal 3A is connected to the N source terminals S of the N transistors 2A-n, and the IF terminal 5A is connected to the N drain terminals D of the N transistors 2A-n is shown, this embodiment is not limited to this example. For example, the harmonic mixer may have a configuration in which the RF terminal 3A is connected to the N drain terminals D of the N transistors 2A-n, and the IF terminal 5A is connected to the N source terminals S of the N transistors 2A-n.

[0045] As mentioned above, in the harmonic mixer 1A according to Embodiment 2, the series resonant circuit 6A includes the capacitor C1 connected, via the wire 9, to the ground terminal GND, and the parasitic inductance L1′ formed in the wire 9. In the series resonant circuit 6A, the inductor L1 which the series resonant circuit 6 includes is unnecessary. As a result, a downsizing of the harmonic mixer 1A can be achieved.

[0046] It is to be understood that a combination of embodiments can be made, a change can be made to any component in each of the embodiments, or any component in each of the embodiments can be omitted.

[0047] The harmonic mixer according to the present disclosure can be used for, for example, frequency converters in a sub terahertz band.

[0048] 1, 1A harmonic mixer, 2-1 to 2-N, 2A-1 to 2A-N transistor, 3, 3A RF terminal, 4-1 to 4-N, 4A-1 to 4A-N LO terminal, 5, 5A IF terminal, 6, 6A series resonant circuit, 7, 7A output terminal, 8, 8A IF matching circuit, and 9 wire.