A transformer

Abstract: The invention relates to a frequency selective transformer circuit integrated on a multilayer printed board and operating at radio frequencies. To the input (1) of the transformer circuit there is coupled a resonant circuit realized by microstrip conductors (8, 9) operating as a two-pole filter. With the aid of the microstrip transformer according to the invention it is also possible to realize a mixer which attenuates the image frequency.

Description: The invention relates to an integrated filter and transformer circuit operating at radio frequencies realized on a multilayer printed circuit board by stripline techniques. A high frequency filter, e.g. a ceramic or SAW filter has previously been used in connection with a transformer or a mixer to filter out interference signals or undesired mixing results. Figure 1 shows a transformer construction according to the conventional approach. The transformer is realized on a multilayer printed circuit board so that the primary 1 and the secondary 2 and 3 are located in different conductor layers. The coupling between the primary and the secondary is effected through a magnetic field. The transformer circuit frequency band is narrowed with the aid of capacitors 4 and 5, whereby the construction forms a resonant circuit. Figure 2 shows the frequency response of a transformer according to the conventional approach. The graph 7 represents the power distribution from port 1 into ports 2 and 3. The graph 6 illustrates the return loss of the input port. In figure 2 the frequency (MHz) is on the horizontal axis and the attenuation (dB) on the vertical axis. The power is equally divided between the ports 2 and 3; therefore the graph 7 comprises two superimposed curves. Due to the construction of the transformer, however, the signals have a phase difference of 180 degrees. Figure 2 shows that it is a rather wideband transformer arrangement. A separate high frequency filter entails costs and it consumes space on the printed board, which poses a problem when aiming at smaller total areas. US-A-4330868 discloses a microwave frequency converter including a single mixer of the diode-quad bridge type which circulates generated image frequency within the bridge. The bridge is connected between balun coupled RF microstrip conductors and unbalanced LO microstrip conductors. IEEE Transactions on consumer electronics Vol.27, No.3, August 1981, New York US, pages 410-415 includes an article by T.Uwamo et al entitled "An MDS down converter of thick-film MIC for 2 GHz pay TV". This article discloses a mixer comprising mixing diodes having an IF amplifier coupled to the output thereof. The output resonating circuit of the amplifier includes a narrow width stripline element. EP-A2-419756 discloses a printed circuit comprising: a substrate including a planar portion of insulating material; at least one conductive planar region disposed on said substrate to provide a ground plane; and a balun transformer including a pair of first and second coextensive elongated conductors disposed on said planar portion to form a strip transmission line having first and second end portions, said strip transmission line being disposed in a loop, said second conductor having a connection to said ground plane at said first end portion of said strip transmission line, and said first conductor having a connection to said ground plane at said second end portion of said strip transmission line, whereby a balanced signal is provided from said first conductor at said first end portion and said second conductor at said second end portion. The object of the present invention is to provide a transformer with which a high frequency filter can be combined. According to a first aspect of the present invention there is provided a frequency selective transformer realized on a multilayer printed circuit board, the transformer primary being formed by a stripline on one layer of the printed circuit board and the secondary being formed by a stripline on another layer of the printed circuit board, characterized in that a resonant circuit realized by striplines is directly connected to the transformer primary input to enable filtering of the input signal. According to a second aspect of the present invention there is provided a diode mixer having interfaces for the local oscillator signal, for the intermediate frequency signal and for the high frequency signal, in which the signals are mixed at a common mixing point, characterized in that between the high frequency interface and the mixing point there is a transformer as described above wherein the high frequency interface is coupled to the transformer input port and the transformer output is coupled in a balanced way to the mixing point through the diode pair. The invention obviates the need for a separate high frequency filter in mixer applications. Embodiments of the invention are described in detail below, by way of example, with reference to the accompanying drawings in which; Figure 1 shows a transformer construction in accordiance with a conventional approach; Figure 2 shows the frequency response of a transformer according to Figure 1; Figure 3 shows the transformer construction in accordance with the invention; Figure 4 shows the frequency response of the transformer according to Figure 3; Figure 5 shows an alternative transformer construction in accordance with the invention; Figure 6 shows the frequency response of the alternative transformer according to Figure 5; and Figure 7 the construction of a mixer in accordance with the invention which attenuates the image frequency. Figures 1 and 2 were described above. The approach in accordance with the invention is described below with reference to figures 3 - 7. Figure 3 shows the construction of the transformer in accordance with the invention. A resonant circuit realized by microstrip conductors is connected to the input of the transformer circuit. Thus a two-pole filter is produced which also has a transmission zero or notch at the upper stop band. The resonant circuit comprises in parallel with the input port a stripline or a microstrip conductor 9 shorted at one end, and in parallel with it a series connection of a capacitor 10 and a microstrip conductor 8 shorted at one end. Figure 4 shows the frequency response of the combined transformer/resonant circuit in accordance with the invention. The graph 12 represents the power distribution from port 1 into ports 2 and 3. The graph 12 illustrates the return loss of the input port. In figure 4 the frequency (MHz) is on the horizontal axis and the attenuation (dB) on the vertical axis. It is seen in the figure that the circuit has become highly frequency selective. In the desired operating range (800...950 MHz in the figure) the circuit operates as a transformer should. The circuit strongly attenuates input signals outside the operating range. The desired frequency response of the arrangement is obtained by changing the construction of the resonant circuit connected to the transformer. Figure 5 shows an alternative arrangement of the transformer approach in accordance with the invention, in which the shorted strip is replaced by the capacitor 13. Then the transmission zero is moved below the operating range. Figure 6 shows the frequency response of the alternative transformer arrangement in accordance with the invention. The graph 15 represents the power distribution from port 1 into ports 2 and 3. The graph 14 illustrates the return loss of the input port. In figure 6 the frequency (MHz) is on the horizontal axis and the attenuation (dB) on the vertical axis. Using a simpler resonant circuit, e.g. only a parallel resonant circuit, the two-pole filter construction can be realized without a transmission zero or notch. With the invention it is possible to integrate the separate rf-filter in connection with the transformer. In this way a frequency selective transformer or power divider can be realized. The frequency selective transformer circuit can be incorporated in a mixer which attenuates the image frequency. Such a mixer is shown in figure 7. A high frequency signal 16 is in a balanced way supplied through the filtering transformer to the diode pair 17 and 18. The local oscillator signal 19 is supplied through the high-pass filter 20 to the middle point of the diode pair, and an intermediate frequency signal 21 is obtained from this point, filtered by the low-pass filter 22. The components 8, 9 and 10 of the resonant circuit are dimensioned so that the notch frequency equals the image frequency. This mixer arrangement can be used in the receiver of a radiotelephone to replace the commonly used series connection of an rf-filter and a mixer. The diode mixer has interfaces for the local oscillator signal, for the intermediate frequency signal and for the high frequency signal. The signals are mixed in a common mixing point. Between the high frequency interface and the mixing point there is a microstrip transformer realized on a multilayer printed board with the first circuit connected in a balanced way to the mixing point through the diode pair, and the second circuit connected to the high frequency interface, whereby to the second circuit is connected a resonant circuit realized by strip lines operating as a two-pole filter. The radio frequency signal is supplied to the second circuit of the transformer, the transformer operating to transfer a radio frequency signal to the intermediate frequency. The filtering mixer circuit could also be used in connection with up-converting, whereby the input signals are supplied to the connectors 19 and 21 (figure 7) and the output signal is 16. Then this circuit filters out from the output signal undesired frequency components, which are i.a. the input signals and their harmonic components and other mixing results formed as a result of mixing. The intermediate frequency signal and the local oscillator signal are mixed and then supplied through said diodes to the first circuit of the transformer, the mixer operating to transfer the intermediate frequency signal to the radio frequency. With the invention it is possible to remove the need to use a separate high frequency filter, or substantially to alleviate the demands made upon it. The desired frequency response of the combined filter/resonant circuit is obtained by changing the construction of the resonant circuit. Due to their simplicity, the presented implementations are preferably used in radio telephones. In the specification and claims the term stripline is intended to cover a stripline or a microstrip conductor.

Claims: A frequency selective transformer realized on a multilayer printed circuit board, the transformer primary being formed by a stripline (1) on one layer of the printed circuit board and the secondary being formed by a stripline (2,3) on another layer of the printed circuit board, characterized in that a resonant circuit realized by striplines (8,9) is directly connected to the transformer primary input to enable filtering of the input signal. A transformer according to claim 1, wherein the resonant circuit is formed by striplines (8,9) between the transformer primary (2,3) and ground, the whole circuit acting as a two-pole filter. A transformer according to claim 1 or claim 2, wherein the resonant circuit is formed on the same surface of the printed circuit board as the transformer primary (1). A transformer according to any preceding claim wherein the resonant circuit is formed by a stripline (9) in parallel with the transformer input port, the stripline being shorted at a first end opposite the transformer input, and a capacitor (10) and a stripline (8) in parallel with the transformer input and in series with stripline (9), the stripline (8) being shorted at a first end opposite the transformer input. A transformer according to claim 4 wherein the resonant circuit comprises a capacitor (13) in parallel with the transformer input port, and a capacitor (10) and a stripline (8) in parallel with the transformer input port and in series with the capacitor (13), the stripline (8) being shorted at a first end opposite the transformer input port. A diode mixer having interfaces for the local oscillator signal (19), for the intermediate frequency signal (21) and for the high frequency signal (16), in which the signals are mixed at a common mixing point, characterized in that between the high frequency interface and the mixing point there is a transformer as claimed in any of the preceding claims wherein the high frequency interface (16) is coupled to the transformer input port and the transformer output is coupled in a balanced way to the mixing point through the diode pair. A diode mixer as claimed in claim 6, wherein the radio frequency signal is supplied to the second circuit (2,3) of the transformer, the mixer operating to convert a radio frequency signal to an intermediate frequency signal. A diode mixer as claimed in any of claims 6 to 7 wherein the components (8, 9 and 10) of the resonant circuit are chosen so that the notch frequency equals the image frequency of the input RF signal.

Inventors: HUUSKO RISTO, HUUSKO, RISTO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1992
Main classification: H01P5
IPC codes: H01F19, H03D7, H03D9, H01P5