Diode mixer

Abstract: A simplified diode mixer in which the transformer supplying the local oscillator signal is replaced by circuit means which prevents build-up of a d.c. voltage at the terminals of the mixer diodes (3, 4). The circuit means can be e.g. a coil (5), a strip line, or a diode pair. The filters (1, 2) for the signals to be mixed are realized as band-pass filters. The mixer is suitable for use in a mobile telephone.

Description: This invention relates to a diode mixer, particularly a radio frequency diode mixer for use in radiotelephones. Diode mixers are well known in the art. Figure 1 schematically shows a typical embodiment of a diode mixer 100, in which the mixer comprises an r.f. transformer 105, two diodes 103 and 104, a high-pass filter 101 and a low-pass filter 102. A diode mixer of this type is particularly well suitable for cases where the frequency RF to be mixed and the mixing result IF are not close to each other, whereby the signals can be separated with filters. The local oscillator signal LO is isolated with the transformer. The r.f. transformer functions to generate equally high signals of opposite phase for the diodes 103, 104 whereby the local oscillator signal is considerably attenuated in the common mixing point of the diodes. The attenuation is necessary, because otherwise the local oscillator signal would pass through the low-pass or high-pass filters to other stages connected to the mixer. The transformer also functions to prevent d.c. voltage build-up at the diode terminals. Such an effect occurs as a result of the rectification in the diodes and due to the diode capacitances. In the development of radiotelephones it is vigorously sought to reduce the size of the structures. In presently known mixers the transformer takes the largest part of the space required by the mixer and also constitutes the greatest part of the mixer total price. Thus the object of the invention is to find a simpler circuit design, without impairing the electrical characteristics of the mixer. The present invention is defined in the appended claims. According to an embodiment of the invention a circuit arrangement is connected in parallel with the mixer diodes, the circuit arrangement conducting a current in the local oscillator circuit so that no d.c. potential is formed in the mixing point between the diodes. The circuit arrangement can be realized either with a coil or with diodes. Then the filters preferably are band-pass filters. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 shows a prior art diode mixer realized with a transformer; Figure 2 shows an embodiment of a simplified diode mixer in accordance with the invention using a coil; Figure 3 shows the detailed circuit diagram of the diode mixer according to figure 2; and Figure 4 shows a second embodiment of the invention using four diodes in all. Figure 2 shows the basic diagram of the diode mixer according to the invention. The r.f. transformer 105 of figure 1 is replaced by a coil 5, and the low-pass and high-pass filters 101 and 102, respectively, are replaced by band-pass filters 1 and 2. The local oscillator signal LO is supplied directly to the diodes 3 and 4 (D1, D2) by coupling the signal to the anode of diode 3. Because the diodes are in series, they are conducting and nonconducting at the same time, in the same way as in the known circuit of figure 1 realized with a transformer. Build-up of a d.c. voltage at the diode terminals is prevented by coupling the coil 5 (L1) from the anode of diode 3 to the cathode of diode 4, or in parallel with the diodes. The attenuation of the local oscillator signal LO to the i.f. output IF and to the r.f. input RF is attenuated by replacing the high-pass and low-pass filters 101, 102 of figure 1 by band-pass filters 1 and 2, respectively. At high frequencies the coil L1 could be realized as a strip line. The band-pass filters 1 and 2 can be realized in any filter technique, but a simple series resonant circuit formed by a coil and a capacitor is preferable. Figure 3 shows a mixer realized in practice. The r.f. input signal RF is supplied to the mixing point MP between the mixer diodes 3 and 4 through the series resonant circuit formed by the coil L160 and the capacitor C161. The intermediate frequency IF is correspondingly obtained at the mixing point of the diodes 3, 4 via the series resonant circuit formed by the coil L161 and the capacitor C162. The diodes 3, 4 are realized in one component U160. The local oscillator signal is supplied substantially directly to the anode of the diode 3 through the capacitor C160, which isolates the d.c. circuits from each other. This design functions well when the frequencies to be mixed are far from each other. On the other hand, if the frequencies to be mixed are close to each other it is necessary to use a dual-balanced mixer comprising e.g two transformers and four diodes. In most practical applications the frequencies to be mixed, RF, IF and the local oscillator frequency LO, are far from each other, whereby the very simple mixer of figure 3 functions as well as a mixer realized with a transformer. In the solutions shown in figure 2 and 3 the function of the circuit arrangement, i.e. the coil, the inductor, or the strip line, is generally to keep the biasing of the diodes at zero, in other words to prevent a d.c. voltage to be formed due to the local oscillator signal and an asymmetric load impedance and the diode capacitances. The coil, the inductor, or the strip line could thus represent e.g. an infinite impedance at the local oscillator frequency, and thus it will not take an active part in conducting the signal to ground. If the circuit arrangement comprises two diodes the mixer's local oscillator port impedance is symmetric on both half-cycles, whereby no d.c. voltage is formed at the mixer diode terminals, and thus the diodes will not drift into a constantly conducting state. The component values shown in figure 3 are exemplary values used for a certain application. With the guidelines presented here a person skilled in the art can naturally change the component values to suit the actual application, utilizing design methods known per se. For the sake of clarity it should be noted that the presented diode mixer according to the invention provides a smaller size than the prior art solution (figure 1). Correspondingly the invention lowers the costs. Further the requirements on filtering out-band signals from the i.f. signal are reduced, because a band-pass filter is used in front of the diodes, i.e. at the r.f. input. Figure 4 shows as a circuit the principle of a second embodiment of the invention, where the coil 2 is replaced by two diodes 5' and 5". During the second half-cycle of the local oscillator signal LO the diodes 3 and 4 conduct the signal to the mixing point MP and further to the signal ground GND; correspondingly, during the second half-cycle the diodes 5' and 5'' conduct the signal in the opposite direction. This prevents rectification of the r.f. signal and build-up of a d.c. voltage at the diode terminals. The diode solution according to figure 4 is attractive because it is easy to realize as an integrated circuit. At present this preferred solution has a slight disadvantage in that half of the local oscillator signal in a sense is lost in the diodes 5' and 5'' (D3 and D4). The diodes 3 and 4 (D1, D2) shown in figures 2, 3 and 4 could also be connected in the opposite direction, i.e. so that they conduct the signal from the signal ground GND to the local oscillator input LO. Then of course also the diodes 5' and 5'' (D3, D4) in figure 4 are connected in the opposite direction. A person skilled in the art can modify the presented examples in numerous ways, e.g. the filters, as was mentioned above. Further it is conceivable to realize the circuit means 5 by other components than those presented here in the examples. It is essential that the local oscillator signal is connected substantially directly to the mixer diodes, without an intermediary transformer. A diode mixer in accordance with the invention is particularly suitable for use in a radiotelephone, for example a radiotelephone intended for use in the pan-European digital cellular radio system known as Groupe Spéciale Mobile (GSM).

Claims: A diode mixer having a mixing point (MP) to which an r.f. signal from a first input (RF) via a first filtering means (1), and an oscillator signal from a second input (LO) via a circuit means are applied; the circuit means comprising two diodes (3, 4) connected in the same conducting direction and in series between the oscillator signal input (LO) and the signal ground (GND) such that the junction between the diodes serves as the mixing point (MP), and a transformerless circuit arrangement (5; 5', 5") forming a substantially zero potential in relation to the mixing point and being connected in parallel with the diodes (3, 4), the arrangement being operable to prevent build-up of d.c. voltage at the terminals of the diodes; wherein the signal at the mixing point (MP) is supplied through a second filtering means (2) to an output (IF). A diode mixer according to claim 1, wherein the circuit arrangement is formed by a coil (5). A diode mixer according to claim 2, wherein the circuit arrangement is formed by a strip line. A diode mixer according to claim 1, wherein the circuit arrangement is formed by a second diode pair (5', 5"), conducting in a direction opposite to that of said two diodes (3, 4). A diode mixer according to claim 4, wherein the local oscillator signal (LO), which is connected substantially directly to the terminal of one of said two diodes (3), is conducted to the signal ground (GND) during one half-cycle through the said two diodes (3, 4) and during the other half-cycle through the second diode pair (5', 5"). A diode mixer according to any previous claim, wherein said filtering means are band-pass filtering means (1, 2). Diode mixer according to any previous claim, wherein the filtering means are realized by a series resonant circuit comprising a coil and a capacitor (L160, C161 resp. L161, C162). The use of a diode mixer according to any previous claim in a radiotelephone, particularly in a GSM-telephone.

Inventors: VAEISAENEN RISTO, VAEISAENEN, RISTO, Väisänen, Risto
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1992
Main classification: H03D7
IPC codes: H03D7, H03D9