MIXER

Abstract: The mixer uses a diode circuit and a balun transformer. The transformer (5) is fixed to a support plate (10), and the terminals of the transformer are connected to stems (11) of the plate. Air-core coils (12, 13) are connected between balancing terminals and the grounding terminals of the support plate.

Description: The present invention relates to a mixer used in a secondary frequency convertor in a receiver for satellite broadcasting. BACKGROUND ART In satellite broadcasting, radio waves in the 12-GHz band width are transmitted from geostationary broadcasting satellites located in a stationary orbit approximately 36,000 km above the equator, and the radio waves are received by high gain parabola antennas on the ground. The waves collected by the parabola antenna are frequency converted to the 1-GHz band width by a low noise convertor, and then carried indoors by a coaxial cable. This frequency conversion is conventionally referred to as primary conversion. The 1-GHz band television signal conducted indoors is again frequency converted to a 400-MHz second intermediate frequency signal. This secondary frequency signal is signal processed for FM demodulation to obtain the video and audio signals. The secondary frequency convertor effects a tuning function, selecting one wave from the 1 GHz primary frequency signal to obtain the secondary frequency signal. Devices conventionally used for secondary frequency signal convertors include transistor mixers, which use transistors, and diode mixers, which use diodes and balun-type transformers. While the transistor mixer offers the advantage of conversion gain, diode mixers are mere commonly used due to their low intercept point. US-A-4,119,914 defining the closest prior art from which the invention proceeds discloses a double balanced mixer which includes a balun transformer comprising a ferrite core and wires wrapped around said core and forming a pair of unbalanced terminals, a pair of balanced terminals and a ground terminal, and first and second diodes in which the first diode has its anode connected to a cathode of the second diode and its cathode connected to one of said pair of balanced terminals of the balun transformer, and the second diode has its anode connected to the other of said pair of balanced terminals of the balun transformer. JP-A-6028331 discloses a double conversion tuner in which coils for intermediate frequency tuning in common use for balance adjustment are provided respectively to a double balanced mixer so as to attenuate the leak of local oscillation output and attain ideal balancing without causing theoretically a loss. A tuning circuit comprising capacitors for intermediate frequency tuning and the coils for tuning is provided after a ring circuit because any inductance in the ring circuit constituting a mixing circuit causes a loss to attain the broad band tuning form tuned to the intermediate frequency. A further conventional device is described below with reference to the accompanying diagrams. Fig. 1 is an equivalent circuit diagram for a frequently used double balun transformer mixer. In Fig. 1, 4 and 5 are balun transformers, and 6, 7, 8 and 9 are diodes. There is a degree of inherent isolation in this mixer because the ports are balanced, and the design of the isolation filter is simplified because the leakage to the intermediate frequency output terminal 2 of the local oscillation signal applied to terminal 3 and the high frequency input signal applied to terminal 1 is low. Fig. 2 is an equivalent circuit diagram for a single balun transformer mixer. In Fig. 2, 5 is a balun transformer, and 6 and 7 are diodes. The features of the single balun transformer mixer include a simpler construction and lower cost than a double balun transformer mixer, and low conversion loss because of the smaller number of diodes and balun transformers. On the other hand, because the high frequency input terminal 1 and the intermediate frequency output terminal 1 are not isolated, a filter with better attenuation characteristics is required (Ref.: "Koushuha Kairo no Sekkei to Jissai (Design and practice of high frequency circuits)", by Yukihiko Miyamoto, issued October 1987). A balun transformer is generally comprised of a compact toroidal core and a so-called "spectacle" core, constructed by wrapping a 0.1 - 0.3 mm diameter wire around the core 5′ as shown in Fig. 3 (Ref.: "Minseiyou Koushuha Device Handbook (Handbook of high frequency devices for consumer applications)", NEC Co., Ltd, June 1986). In Fig. 3, 30 and 31 are unbalanced terminals, 32 and 33 are balanced terminals, and 34 is a ground terminal. An example of a mixer comprised of the balun transformer and diodes as thus described mounted on a printed circuit board is shown in Figs. 4 a and b. The frequency characteristic of the conversion loss, which is the most important characteristic of a mixer circuit, is approximately 6 - 7 dB as shown in Fig. 5, which figure is slightly lower than the 8 - 10 dB conversion loss in a double balanced mixer. The operation of a single balun mixer constructed as described above is discussed hereinbelow. The function of the balunce transformer 5 in Fig. 2 is to unbalance-balance convert the local oscillator output signal applied to the local oscillator input terminal 3, and insert the converted signal to diodes 6 and 7. What is important here is that the balun transformer 5 is balanced, which means that the phase of the local oscillator output component at points A and C is each ±90° to point B (a center tap of the coil). If the transformer is thus balanced, the separated local oscillator output signal is canceled respectively at the high frequency input terminal and the intermediate frequency output terminal 1, i.e., isolation is good. A Schottky barrier diode is normally used for the diodes 6 and 7 because a sufficiently high speed switching characteristic is required for high frequency signals. As these diodes are switched on/off by the local oscillator signal, an equivalent circuit as shown in Fig. 6 can be constructed. (Ref.: ("Toroiguru koa Katsuyou Hyakka (Encyclopedia of toroidal core applications)", p, 270, Hideo Yamamura, issued January 1983). Referring to Fig. 6, because the high frequency input signal applied to terminal 1 is switched by the local oscillator signal, an output with the components as shown in the following formula appears at the intermediate frequency output terminal 1. f IFm,n = ±mf RF ±nf LO where m, n = 0, 1, 2, 3... f IF : intermediate frequency f RF : high frequency input frequency f LO : local oscillator signal frequency. In the above equation, m=n=0 is the DC component which is normally blocked. In addition, a high output is removed by the LPF connected to the intermediate frequency output terminal, and the difference component of the frequency of the local oscillator signal and the high frequency input signal is extracted as the intermediate frequency signal, thereby effecting a frequency convertor function. However, in a conventional mixer as shown in Fig. 4, it is necessary to electrically connect by means of soldering the wires which act as the input/output terminals of the balun transformer 5 after mounting the balun transformer 5 to the printed circuit board 14 by inserting the wires into holes provided in the printed circuit board 14. The wires used for the balun transformer 5 are light copper wires 0.1 - 0.3 mm diameter coated with a polyurethane or other protective coating. These soft wires are hard to insert in the circuit board holes during mounting and can be easily bent, thereby resulting in variations in conversion gain and other performance characteristics. SUMMARY OF THE INVENTION With respect to the above problems, the present invention provides a mixer with excellent high frequency characteristics, minimal variation in performance resulting during mass production, and which can be easily mounted on a printed circuit board. To achieve the aforementioned objective, a mixer according to the present invention comprises a balun transformer comprising a ferrite core and wires wrapped around said core and forming a pair of unbalanced terminals, a pair of balanced terminals having a ground terminal positioned on the wire between the two balanced terminals, and first and second diodes in which the first diode has its anode connected to a cathode of the second diode and its cathode connected to one of said pair of balanced terminals of the balun transformer, and the second diode has its anode connected to the other of said pair of balanced terminals of the balun transformer, and is characterized in that a support block secures the balun transformer and comprises plural conductive stems to which are connected the terminals of the balun transformer, hollow coils are connected from each of said balanced terminals of the balun transformer to ground terminal, and said pair of unbalanced terminals are connected to terminals of a local oszillator, and the terminals of the diodes other than said connecting terminals connected to the balanced terminals of the balun transformer are both a high frequency input terminal and an intermediate frequency output terminal. In a mixer constructed according to the present invention, the local oscillator signal is input to the input terminal of the balun transformer wherein one terminal of the pair of unbalanced terminals is the ground terminal and the other terminal is the input terminal. The input local oscillator signal is unbalance-balance converted by the balun transformer, forming between the balanced terminal and ground terminal a balanced signal the phase of which differs 180° from the original input signal. This balanced local oscillator signal switches the same-phase diode connected serially between the balanced terminals ON at one half of the cycle of the local oscillator signal, and switches the diode OFF at the other half of the cycle. Thus, the amplitude of the input local oscillator signal must be sufficiently large to make the diodes used continuous. Because the high frequency signal is input to the contacts of the diodes, the high frequency signal is switched by the diode at the cycle of the local oscillator signal. This switching action produces a signal comprising an intermediate frequency signal and a high frequency component at the intermediate frequency output terminal by means of mixing the high frequency signal and local oscillator signal. If this signal is extracted through a low pass filter (LPF) connected to the intermediate frequency output terminal, an intermediate frequency signal only can be obtained. Considering the impedance from the high frequency input terminal side to the diode switched by the local oscillator signal, the mixer operation is executed efficiently because the impedance between a ground state and open state even if the lead length from the balun transformer balanced terminal to the diode contact is sufficiently short. However, in the case of the present invention, a support block with stems is used to fix the balun transformer, and the length of the lead between the balanced terminals of the balun transformer and the contacts with the diode cannot be ignored, and the conversion loss of the mixer deteriorates greater the higher the frequency of the signal processed. This is because the impedance from the high frequency input terminal to the diode is not switched between the ground and open states ideal for mixer operation. Thus, if the impedance is reduced by connecting a hollow coil between the ground terminals and the contacts of the stems, which are the balanced terminals of the support block, and the diodes, the deterioration in the conversion loss of the mixer can be recovered. It is to be noted that while the amplitude of the local oscillator signal is attenuated slightly by the use of a hollow coil, this is no particular problem for local frequency conversion as occurs in a satellite broadcast receiver. Furthermore, by using identical hollow coils, the loss of balance in the balanced terminals of the balun transformer can be prevented, and leakage of the local oscillator signal to the high frequency input terminal does not become worse. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an equivalent circuit diagram for a conventional double balanced mixer; Fig. 2 is an equivalent circuit diagram for a conventional single balanced mixer; Fig. 3 is an oblique view of a balun transformer; Figs. 4 a and b are plane views and side views, respectively, of a conventional single balanced mixer; Fig. 5 is a frequency characteristics diagram of conversion loss in a conventional single balanced mixer; Fig. 6 is an equivalent circuit diagram for a conventional single balanced mixer; Figs. 7 a and b are plane views and side views, respectively, of a mixer according to a preferred embodiment of the present invention; Fig. 8 is an equivalent circuit in a preferred embodiment of the present invention; and Fig. 9 is a frequency characteristics diagram of conversion loss in a mixer according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION A preferred embodiment of the present invention is described hereinbelow with reference to the accompanying diagrams. Figs. 7 a and b show a mixer according to a preferred embodiment of the present invention wherein reference number 1 is a high frequency input terminal and an intermediate frequency output terminal, 3 is a local oscillator input terminal, 5 is a balun transformer used for unbalanced-balanced conversion of a local oscillator signal, 6 and 7 are Schottky barrier diodes switched by a local oscillator signal, and 10 is a support block which secures the balun transformer 5 and facilitates insertion of the balun transformer 5 to a printed circuit board 19. Additionally, 11 is a support block stem connected to a balun transformer 5 terminal, 12 and 13 are hollow coils, one end of each being connected to the pair of balanced terminals of the balun transformer 5 via the support block stem 11, and the other end of each being connected to a ground terminal (ground through-holes) 14 and 15. The equivalent circuit of Fig. 7 is shown in Fig. 8. In Fig. 8, the inductance component of the support block 10 stem 11 is expressed by inductance 20, 21, 22, 23 and 24. The local oscillator signal input from the local oscillator input terminal 3 is unbalance-balance converted by the balun transformer 5 and supplied to diodes 6 and 7, thus switching the diodes 6 and 7. At this time, the impedance from the high frequency input terminal 1 to the diode is not switched by the inductance components 20, 21 and 22 resulting at the support block stem 11 to the ideal ground and open states for mixer operation. Specifically, the conversion loss from the mixer when the hollow coil is not connected is in the high frequency band as shown in Fig. 9, and the frequency characteristics are poor. To compensate for this, if the impedance at the contact of the diodes 6 and 7 and support block 10 is lowered by the hollow coils 12 and 13, deterioration of mixer conversion loss can be corrected as shown in Fig. 9. A hollow coil made from 0.6 mm diameter wire wrapped 1.5 turn to a 3 mm diameter is sufficient to provide this compensation, and there is minimal change in the characteristics of the mixer even if the inductance of the hollow coil is changed slightly. While conversion loss recovery is improved with a lower inductance, this increases the load of the VCO (local oscillator circuit). This makes it easier for oscillation to stop, and caution is therefore required in designing the inductance characteristic. It is to be noted that while the hollow coils 12 and 13 are used in the present embodiment to compensate for the inductance component of the support block 10, an equivalent effect can be achieved by forming a microstrip line 0.3 mm wide and approximately 25 mm long on the printed circuit board in place of the hollow coil. This substitution, however, is applicable when the printed circuit board is manufactured from a glass epoxy material and is 1.0 mm thick. If a microstrip line is used, the change in mixer characteristics is minimal if the length and/or width is changed slightly. Furthermore, if the diodes 6 and 7 are surface mountable diodes, then virtually all of the components comprising the mixer can be assembled in a surface mounting process. Furthermore, while the most basic relay transformer wrapping method using a so-called "spectacle" core is shown for the balun transformer, when a normal relay transformer or a balun transformer using a toroidal core is used and fixed to the support block, high frequency conversion loss deteriorates, but this deterioration can be recovered by means of a hollow coil. According to the present invention, a mixer with outstanding high frequency characteristics, minimal variation in performance resulting from mass production, and which can be easily mounted on a printed circuit board can be provided by fixing a balun transformer on a support block, and compensating for the inductance component arising from the support block stem by means of a hollow coil.

Claims: A mixer comprising: a balun transformer (5) comprising a ferrite core and wires wrapped around said core and forming a pair of unbalanced terminals, a pair of balanced terminals having a ground terminal positioned on the wire between the two balanced terminals and first and second diodes (6, 7) in which the first diode (6) has its anode connected to a cathode of the second diode (7) and its cathode connected to one of said pair of balanced terminals of the balun transformer (5), and the second diode (7) has its anode connected to the other of said pair of balanced terminals of the balun transformer (5); characterized in that a support block (10) secures the balun transformer (5) and comprises plural conductive stems (11) to which are connected the terminals of the balun transformer (5); hollow coils (12, 13) are connected from each of said balanced terminals of the balun transformer (5) to ground; and said pair of unbalanced terminals are connected to terminals of a local oscillator (3), and the terminals of the diodes (6, 7) other than said connecting terminals connected to the balanced terminals of the balun transformer (5) are both a high frequency input terminal (1) and an intermediate frequency output terminal (1).

Inventors: OMOTO NORIAKI, OMOTO, NORIAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: H03D7
IPC codes: H03D9, H03D7