MIXER CIRCUIT

Abstract: In order to be able to use a selected one of two local oscillation signals with respective input terminals (2, 3), a microwave mixer has a radio frequency input terminal connected by way of a transmission line section (5) to one terminal of a mixer diode (6). The other diode terminal is connected to a quarter wavelength line section (7) and to an intermediate frequency output terminal (4) by way of a line section incorporating a low pass filter (10). Transmission line sections connected to the local oscillation terminals (2, 3) are coupled to the quarter wavelength section (7) by respective distribution coupling type bandpass filters (8, 9) tuned to the two different local oscillation frequencies.

Description: BACKGROUND OF THE INVENTION This invention relates generally to a microwave mixer which receives an input microwave signal and converts it to an intermediate frequency (IF) signal of a lower frequency. More particularly, the present invention relates to a novel microwave mixer which is capable of receiving two local oscillation signals having different frequencies. In a conventional single mixer circuit, such as disclosed in Japanese Patent JP-A-5710507 against which Claim 1 is delimited, containing one mixer diode, one of the terminals of the mixer diode is connected to the end of a main line which receives an input microwave signal, and the other end of the mixer diode is connected to a line for taking out an intermediate signal. In this case, a local oscillation signal is supplied to the main line through a bandpass filter. If two kinds of local oscillation signals are to be alternatively supplied to the single mixer circuit of the kind described above in order to expand a reception frequency band, it is necessary to employ a circuit construction in which two local oscillation signal supply lines are aligned in parallel with each other on one side of the main line or two local oscillation signal supply lines are disposed on both sides of the main line. However, in such circuit construction where the two local oscillation signal supply lines are coupled to the main line together, more conversion loss is derived and a conversion characteristic deteriorates. Further , a larger circuit space is necessary. For these reasons, the mixer circuit having such circuit construction is not practically used. A mixer cicuit which is capable of receiving two different local frequencies is known from "A 12GHz Low Noise Convertor for Two Band Reception", by H. Minra et al., IEEE Transactions on Consumer Electronics, vol CE-30, no. 3, August 1984, pp 259-264, see particularly Figure 7. However, its circuit configuration requires two mixer diodes and a branch-type hybrid, and so is not adapted to providing a compact and low cost mixer circuit. SUMMARY OF THE INVENTION According to the present invention, there is provided a microwave mixer comprising a first transmission line means for receiving an input radio frequency signal at one end, a single mixer diode with one terminal thereof coupled to the other end of the first line means, second line means for coupling a local oscillation signal from a first terminal to the mixer diode by way of a distribution-coupling type bandpass filter, an output line means for delivering an intermediate frequency signal, a second terminal for an alternative local oscillation signal having a different frequency to the first said local oscillation signal, a third line means for coupling the alternative local oscillation signal to the mixer diode, a fourth line means coupled to the other terminal mixer diode and having a length one quarter of a wavelength at the input radio frequency, and two distribution-coupling type bandpass filters flanking the fourth line means and passing the local oscillation signals from the second and third line means respectively, which are adjacent the two bandpass filters respectively, and in which the output line means is coupled to the end of the fourth line means which is coupled to the said other terminal of the mixer diode. The invention, a preferred embodiment of which will be described in more detail below provides a microwave mixer capable of receiving two local oscillation signals which is free from conversion characteristic deterioration and which does not require a large circuit space. In this mixer, the leakage of local oscillation signal to an input terminal is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of the invention will now be described with reference to the drawings in which: Fig. 1 is a circuit diagram of a single mixer circuit which receives one local oscillation signal according to the prior art; Fig. 2 is a block diagram showing a first embodiment of the present invention; Fig. 3 is a block diagram showing a second embodiment of the present invention; and Fig. 4 is a diagram showing the characteristics of a notch filter used in the second embodiment of the present invention. DESCRIPTION OF THE PRIOR ART A prior art single mixer circuit (JP-A-57 10 507) will be first described prior to the description of the preferred embodiments in order to help understanding the present invention. In a conventional single mixer shown in Fig. 1, only one local oscillation frequency has been used. In the Figure, a microwave signal inputted from an input terminal 1 passes through a main line 5 and is applied to a mixer diode 6. On the other hand, a local oscillation signal inputted from a terminal 2 passes through a distribution-coupling type bandpass filter (hereinafter referred to as "BPF") 8 coupled with the main line 5 and is applied to the diode 6. An intermediate frequency signal (hereinafter referred to as the "IF signal") is generated upon the application of the input microwave signal and the local oscillation signal to the mixer diode 6 and picked up through a low bandpass filter (hereinafter referred to as "LPF") 10 to an IF output terminal 4. The LPF 10 removes the input microwave signal and the local oscillation signal. In this case, if another local oscillation signal having a different frequency is supplied in order to receive another microwave signal so as to make this mixer circuit operable in a wider reception band, another local oscillation signal supply line and another BPF should be afresh disposed, in principle, on the opposite side of the BPF 8 with respect to the main strip line 5, or in parallel with the BPF 8. However, as described above, this circuit arrangement can not be fitted for practical use. DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a first embodiment of the present invention will be described with reference to Fig. 2. In this Figure, an input microwave signal from an input terminal 1 is applied through a main line 5 to a mixer diode 6. A distribution-coupling line 7 having a length about 1/4 of the wavelength with respect to the input microwave signal is connected to the other lead of the mixer diode 6. Therefore, the line 7 is disposed on the opposite side to the main line 5 with respect to the mixer diode 6. On the other hand, first and second local oscillation signal supply lines and corresponding BPFs 8 and 9 are disposed along both sides of the distribution-coupling line 7 as shown in Fig. 2. Thus, the first and second local oscillation signals supplied from terminals 2 and 3 are alternatively supplied through BPFs 8 and 9 and through the distribution-coupling line 7 to the mixer diode 6. The BPFs 8 and 9 have a length of 1/2 λ (λ: wavelength) with respect to respective local oscillation frequencies. As the performance required for these BPFs 8 and 9, the damping characteristics of at least 10 dB with respect to the input microwave signal and to the counter-part local oscillation signal, and the insertion loss of up to 3 dB are necessary. When these requirements are satisfied, the local oscillation signal can be applied efficiently to the mixer diode 6. Therefore, the signal power of the local oscillation signal may be of a low level, and the leakage of the input microwave signal to the local oscillation signal input terminals 2 and 3 can be reduced. Furthermore, as shown in Fig. 2, the present invention can reduce a necessary area for the circuit configuration. In the embodiment, the IF signal is delivered from the tip of the coupling line 7 at the side where the diode 6 is connected. The delivered IF signal is supplied through a LPF 10 to an output terminal 4. This LPF 10 consists of a tip-open stub having a length of about 1/4 λ with respect to a cut-off frequency in an image band, and a high impedance line. A stub 11 functions as a matching circuit to reduce the conversion loss in the mixer diode 6, and as a matching circuit for the local oscillation signal. A grounded high impedance line 12 is provided as part of a current feedback loop for the mixer diode 6, and its length is selected to be about 1/4 λ of the input microwave signal. A resistor 15 is disposed to limit the current in the current feedback loop of the mixer diode 6. One of the leads of the resistor 15 is connected to an impedance line 13 and the other is grounded. In order to prevent a undesirable influence upon the IF signal derived by the resistor 15, a capacitor 14 is disposed in parallel to the resistor 15. The entire length through the high impedance line 13 and the resistor 15 (or the capacitor 14) is set to be about 1/4 λ of the IF signal. It is assumed in this embodiment that the input microwave frequency band is 1 GHz from 11.7 to 12.7 GHz and the intermediate frequency band is 500 MHz from 1.O to 1.5 GHz. Then, the first local oscillation frequency is selected to 10.70 GHz and the second local oscillation frequency, 11.20 GHz. In this case, the low bandpass filter 10 having a 1/4 λ stab has the characteristic of the cut-off frequency of 9.0 GHz in the image band. In the microwave band receiver capable of using two local oscillation signals, as shown in Fig. 2, a frequency gap getween the two local oscillation signals is as relatively small as 500 MHz, for example, and an isolator cannot be utilized therein because of structural requirement of the receiver, i.e., requirement of a more compact receiver. Furthermore, the number of stages of bandpass filters (not shown) to be interposed in a transmission line before the mixer circuit is small such as three stages, for example. In this case, a local oscillation frequency close to a cut-off range of the bandpass filter (this signal will be hereinafter referred to as a "first local oscillation signal") can be easily damped by about 20 dB, but a local oscillation frequency close to a passing range (this signal will be hereinafter referred to as a "second local oscillation signal") can be damped only by about 5 dB. Therefore, the leakage of the second local oscillation signal to the microwave input terminal occurs and it becomes a drawback. In order to eliminate such drawback, a second embodiment of the present invention is provided by including two notch filters in order to trap the second local oscillation signal. One of the two notch filters is formed by disposing a tip-open type high impedance line having a 1/4 λ of a frequency of (where n: an integer of 2 or more and f e : a frequency to be damped) on one side of the main line between the microwave input terminal and the frequency conversion portion, and the other is formed by disposing another tip-open type high impedance line having a 1/4 λ of the frequency of about on the opposite side of the main line. In Fig. 3 showing the block diagram of the second embodiment, a first notch filter 16 and a second notch filter 17 are disposed in such a manner as to interpose the main line 5 therebetween. Except for these notch filters 16 and 17, the circuit configuration of the second embodiment is the same as the first embodiment shown in Fig. 2. In Fig. 3, elements 12, 13, 14 and 15 shown in Fig. 2 are omitted. The notch filters 16 and 17 reduce the leakage of the second local oscillation signal (11.2 GHz) to the input terminal 1. If a notch filter is realized by a line of 1/4 λ of the second local oscillation frequency itself, a damping curve shifting from a damping range to a passing range becomes gentle when the passing range is relatively as narrow as about 500 MHz, and hence the passing range cannot be expanded. In this embodiment, a 1/5 (n = 2) frequency is selected and the notch filter utilizing its higher-order (harmonic) mode is realized. Thus, the passing range can be expanded. In practice, a tip-open line having a 1/4 λ of 1/5 of the second local oscillation frequency is disposed to the main line as the first notch filter 16. This tip-open line consist of a thin high impedance line (e.g., about 150 ohms when an ethylene tetrafluoride/glass cross/copper plated substrate is used). Furthermore, in order to further expand the passing range, the second notch filter 17 is disposed on the opposite side of the main line 5. In the second notch filter 17, the tip-open line has a length of 1/4 λ of a 1/5 frequency of 6/5 frequency with respect to the second local oscillation frequency, i.e., 6/25 of the second local oscillation frequency (n = 2). Since the second notch filter 17 is added, the damping curve becomes sharp, and, thus, the passing range can be expanded. When the single mixer circuit shown in this embodiment is used for a frequency conversion unit, a matching impedance to the mixer diode 6 must be adjusted while the notch filters are included. Next, the case of Figs. 3 in which the ethylene tetrafluoride/glass cross/copper plated substrate is used will be explained. It is assumed in the same way as in the first embodiment that the input microwave signal is in the 1 GHz band of 11.7 to 12.7 GHz, the intermediate frequency is in the 500 MHz band of 1.0 GHz to 1.5 GHz, the first local oscillation frequency is 10.70 GHz and the second local oscillation frequency is 11.20 GHz. In this case, the 11.2 GHz of the second local oscillation frequency should be damped. Therefore, the first notch filter 16, consists of the tip-open line having the 1/4 λ of 22.24 GHz (11.2 x 1/5) and having characteristic impedance of 150 Ω, and the second notch filter 17 consists of the tip-open line having the 1/4 λ of 2.688 GHz (11.2 x 6/5 x 1/5) and having characteristic impedance of 150 Ω. As a result, damping of 17.5 dB at 11.2 GHz and an insertion loss of about 1 dB at 11.9 to 13.0 GHz can be obtained as shown in Fig. 4. Thus, the second embodiment can provide the effect of limiting the leakage of the second local oscillation signal to the microwave input terminal. As described above, in the single mixer circuit in accordance with the present invention, in order to supply the two local oscillation signals, the distribution-coupling line having a about 1/4 λ of the input microwave signal is connected to the mixer diode and two distribution-coupling type bandpass filters are disposed on both the sides to the distribution-coupling line. Therefore, the present invention provides the following effects: (1) The conversion loss in the mixed circuit can be reduced. (2) The local oscillation signal power may be small. (3) The single mixer having a small circuit area can be accomplished.

Claims: A microwave mixer comprising a first transmission line means (5) for receiving an input radio frequency signal at one end (1), a single mixer diode (6) with one terminal thereof coupled to the other end of the first line means, second line means for coupling a local oscillation signal from a first terminal (2) to the mixer diode by way of a distribution-coupling type bandpass filter (8), and an output line means for delivering an intermediate frequency signal, characterised by a second terminal (3) for an alternative local oscillation signal having a different frequency to the first said local oscillation signal, a third line means for coupling the alternative local oscillation signal to the mixer diode (6), a fourth line means (7) coupled to the other terminal of the mixer diode and having a length one quarter of a wavelength at the input radio frequency, and two distribution-coupling type bandpass filters (8 and 9) flanking the fourth line means (7) and passing the local oscillation signals from the second and third line means respectively, which are adjacent the two bandpass filters respectively, the output line means being coupled to the end of the fourth line means (7) which is coupled to the said other terminal of the mixer diode (6). A mixer as claimed in claim 1, further comprising a filter (10) disposed in the output line means for cutting off an image frequency. A mixer as claimed in claim 1 or 2, further comprising a filter coupled to the first line means (5) for trapping the local oscillation signals. A mixer as claimed in claim 1, 2 or 3, further comprising first and second notch filters (16, 17) coupled to the two sides of the first line means (5) for trapping the local oscillation signals, the first notch filter (16) consisting of a line having a length of ¼ wavelength with respect to f/(2n + 1) and the second notch filter (17) consisting of a line having a length of ¼ wavelength with respect to 2f(n+1)/(2n+1)² where f is the frequency of the respective local oscillation signal and n is an integer greater than 1.

Inventors: SHIZUME, YASUMICHI C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1986
Main classification: H03D9
IPC codes: H03D9, H03D7