Microwave mixing circuit and down-converter using the same

Abstract: A micro-wave mixing circuit produces a stable frequency conversion characteristic against a dispersion of output voltage of a local oscillation signal tapped off from a local oscillator (2). A micro-wave signal fed into a micro-wave input signal (1) can be converted into an intermediate frequency signal of a 1 GHz bandwidth where a stable frequency conversion characteristic is maintained against a dispersion of an output voltage of a local oscillation signal, by supplying bias current from a bias supply circuit (12) to an anode of a Schotkky barrier diode (5) responsive to a local oscillation output which is supplied from a local oscillator (2).

Description: Field of the Invention The present invention relates to a micro-wave mixing circuit used in a down-flow converter for satellite communication or a satellite broadcasting. Background of the Invention As the satellite broadcasting becomes popular, (CS) communication satellite broadcasting which uses a communication satellite is now commercially available. Thus, an ordinary house has more opportunities to receive broadcasting from plural satellites. A receiving antenna which is smaller in size and lower at cost is therefore desirable. Fig. 3 depicts a conventional micro-wave mixing circuit and an intermediate frequency amplifier. A micro-wave signal having a 12 GHz bandwidth fed into a micro-wave signal input part 1 and a local oscillation signal (e.g. 10.678 GHz), supplied from a local oscillator 2 via a band pass filter 3 (BPF), are mixed by a frequency converting diode, such as Schottky barrier diode 5 (SBD). SBD 5 is coupled to a micro strip line 4 (MSL), and then the mixed signal is converted into an intermediate frequency signal having a 1 GHz bandwidth. When a local oscillation signal voltage at SBD 5 is less than an expected value, loss due to frequency conversion is increased. In order to prevent the frequency conversion loss from being increased, a bias voltage is applied to the frequency converting diode. In order to produce this bias voltage, a specified bias control voltage is applied to a bias current supply terminal 6. The intermediate frequency signal passed through a low pass filter 7 (LPF) is amplified by intermediate frequency amplifiers 8 and 9, and then tapped off from an intermediate frequency signal output part 10. However, according to the above structure, the local oscillation signal voltage is not constant but varies widely due to individual frequency characteristics of each device (e.g. metal semiconductor [MESEFT]) used in the local oscillator 2 and also dispersion caused by mounting those devices. Thus, a rectifying voltage of the local oscillation signal voltage of SBD 5 has a dispersion responsive to the dispersion of the local oscillation signal voltage applied to SBD 5. As a result, even if a constant bias control voltage is applied to the bias supply terminal 6, the bias current travelling through SBD 5 does not become an expected value, but generates a wider dispersion. Accordingly, it is very difficult to obtain a certain level of performance from the conventional micro-wave mixing circuit when volume production thereof has been started. Reference may also be made to EP-A-0317642 relative to which the present invention is characterised. Summary of the Invention The present invention is defined in the claims. An advantage of the invention is that the bias current supplied to the diode may be responsive to an output voltage of the local oscillation signal (as detected at the detection terminal), thereby narrowing the dispersion of conversion loss due to mixing. In a preferred embodiment of the invention, a microwave mixing circuit comprises; a microwave signal input part formed on a dielectric substrate with a micro stripline (MSL); SBD 5 of which an anode is coupled to a terminal of the above MSL, and of which a cathode is coupled (indirectly) to ground, and a bias supply circuit. The above bias supply circuit includes a bias current terminal which applies a bias voltage to an anode of a mixer diode, and a detection terminal which detects a dc voltage generated at the anode. The bias supply circuit applies a bias control voltage to the bias supply terminal which supplies bias current responding to a dc voltage generated at the detection terminal. Even if an output voltage of a local oscillation signal which is supplied from the local oscillator has a dispersion, conversion loss due to the mixing can be reduced to a small value by applying the bias current responding to the dc voltage to the anode of SBD 5. The present invention addresses the problems which the conventional micro-wave mixing circuit has, and provides a micro-wave mixing circuit as well as a down-flow converter with a smaller dispersion and at an inexpensive cost. Brief Description of the Drawings Fig. 1 is a circuit pattern of a micro-wave mixing circuit used in Exemplary Embodiment 1 of the present invention. Fig. 2 is a circuit pattern of a down-flow converter incorporating a micro-wave mixing circuit which is used in Exemplary Embodiment 1. Fig. 3 is a circuit pattern of a conventional micro-wave mixing circuit. Preferred Exemplary Embodiments Exemplary Embodiment 1 Fig. 1 is a circuit pattern of a micro-wave mixing circuit used in Exemplary Embodiment 1 of the present invention. The circuit comprises the components below : a micro-wave signal input part 1, a local oscillator 2, BFP 3 for passing a local oscillation frequency, MSL 4, SBD 5, a bias current supply terminal 6 which supplies bias current to SBD 5, LPF 7 for passing an intermediate frequency, intermediate amplifiers 8 and 9, an output part 10 of the intermediate frequency signal, a detection terminal 11 for detecting a dc voltage generated at SBD 5, a bias supply circuit 12 comprising a computation circuit, resistors 13 and 14 for dividing a supply voltage (not shown in Fig. 1). The divided voltage is applied to a lead of an input terminal of the bias supply circuit as a predetermined reference voltage, thereby minimizing the conversion loss at SBD 5. The micro-wave mixing circuit and the intermediate frequency amplifier so structured as above are now detailed in their operation. A micro-wave signal having a 12 GHz bandwidth, for example, is fed into the -micro-wave signal input part 1 and a local oscillation signal (e.g. 10.678 GHz) supplied from the local oscillator 2 via BPF 3 travel together to MSL 4. On the anode of SBD 5 which is coupled to MSL 4, a dc voltage is produced by the following two factors : a dc voltage generated by an applied voltage from the bias current supply terminal 6, and a dc voltage generated by being rectified by the local oscillation signal supplied from the local oscillator 2. The produced dc voltage is detected by the detection terminal 11 as a detection voltage. This bias supply circuit 12 compares the detection voltage with the reference voltage, and then applies a bias control voltage to the bias current supply terminal 6. The bias supply circuit 12 is thus controlled so that the detection voltage may become equal to the reference voltage. Therefore, SBD 5 is not influenced by a dispersion of the output voltage of the local oscillation signal supplied from the local oscillator 2. As a result, conversion loss is maintained at a reasonable level. The micro-wave signal, which is converted by SBD 5 into the intermediate frequency signal having a 1 GHz bandwidth, for example, passes through LPF 7, and is then amplified to a desirable level by the intermediate amplifiers 8 and 9. Finally, the micro-wave signal is tapped off from the intermediate frequency signal output part 10. Exemplary Embodiment 2 Fig. 2 is a circuit pattern of a down-flow converter incorporating a micro-wave mixing circuit which is used in Exemplary Embodiment 1. A probe 15 converts a micro-wave signal having a 12 GHz bandwidth, for example, radiated from a satellite into a quasi transverse electromagnetic (TEM) wave. Low noise amplifiers 16 and 17 comprise low noise elements such as high electron mobility transistors. BPF 3 passes a local oscillation frequency. MSL 4 and SBD 5 are the same as in Fig. 1. A bias current supply terminal 6 applies a bias current to SBD 5. LPF 7 passes an intermediate frequency signal. Other elements included in Fig. 2 are intermediate amplifiers 8 and 9, an intermediate frequency signal output part 10, a detection terminal 11 for detecting dc current generated at SBD 5, a bias supply circuit 12 comprising a computation circuit, resistors 13 and 14 which divide a supply voltage (not shown in Fig. 2). The divided voltage is applied to a lead of an input terminal of the bias supply circuit as a predetermined reference voltage, thereby minimizing a conversion loss at SBD 5. The down flow converter thus structured as above is now detailed in its operation. The micro-wave signal radiated from the satellite is converted by the probe 15 into the quasi TEM wave which can travel through a micro strip line. The micro-wave signal is amplified by the low noise amplifiers 16 and 17. The amplified micro-wave signal and the local oscillation signal (e.g. 10.678 GHz) travel together to MSL 4. On the anode of SBD 5 which is coupled to MSL 4, a dc voltage is produced by the following two factors : a dc voltage generated by an applied voltage from the bias current supply terminal 6, and a dc voltage generated by being rectified by the local oscillation signal supplied from the local oscillator 2. The produced dc voltage is detected by the detection terminal 11 as a detection voltage. The bias supply circuit 12 compares the detection voltage with the reference voltage, and then applies a bias control voltage to the bias current supply terminal 6. The bias supply circuit 12 is thus controlled so that the detection voltage may become equal to the reference voltage. Therefore, SBD 5 is not influenced by a dispersion of the output voltage of the local oscillation signal supplied from the local oscillator 2. As a result, conversion loss is maintained at a reasonable level. The micro-wave signal, which is converted by SBD 5 into the intermediate frequency signal having a 1 GHz bandwidth, for example, passes through LPF 7, and is then amplified to a desirable level by the intermediate amplifiers 8 and 9. Finally, the micro-wave signal is tapped off from the intermediate frequency signal output part 10. According to the present invention, the following three elements are added to the conventional micro-wave mixing circuit : (1) the detection terminal which detects the dc voltage generated at the anode, (2) the bias current supply terminal with which the bias current is applied to the anode of SBD 5, (3) the bias supply circuit which supplies the bias control voltage to the bias current supply terminal responding to the DC voltage generated at the detection terminal. As a result, the present invention realizes an inexpensive and a small size micro-wave mixing circuit as well as a down flow converter both of which have stable characteristics against a dispersion of the local oscillation output tapped off from the local oscillator.

Claims: A microwave mixing circuit for use with a micro-wave input signal comprising: (a) a local oscillation circuit (2); (b) a diode (5) which converts a frequency by using said micro-wave input signal and a local oscillation signal supplied from said local oscillation circuit (2); (c) a bias current supply terminal (6) to which a bias control voltage is applied to provide a bias current to said to said diode (5); characterised by : (d) a detection terminal (11) at which a dc voltage generated at said diode is detected; and (e) a bias supply circuit (12) responsive to the voltage at the detection terminal (11) for controlling the bias control voltage supplied to said bias current supply terminal. A microwave mixing circuit according to claim 1, wherein said bias supply circuit (12) controls said bias control voltage so that the dc voltage at said detection terminal (11) becomes a predetermined voltage. A microwave mixing circuit according to claim 1 or 2, further comprising a microwave signal input part (1, 4) for receiving said input signal, and wherein an anode of said diode (5) is coupled to a terminal of said microwave signal input part (1), and a cathode of said diode (5) being coupled indirectly to ground. A microwave mixing circuit according to claim 3, wherein said microwave signal input part comprises a micro stripline (4). A down flow converter for use with a microwave signal, comprising: a probe (15) which converts said microwave signal into a transverse electromagnetic wave; a low noise amplifier (16, 17); and a microwave mixing circuit according to claim 4, said micro stripline of the microwave mixing circuit being coupled to an output of said low noise amplifier.

Inventors: KASHIMA YUKIRO, KASHIMA, YUKIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H03D7
IPC codes: H03D7, H04B1, H03D9