JPH03110908A - High frequency balance mixer - Google Patents

Abstract

PURPOSE:To reduce the leakage of a local oscillation signal by providing a transformer converting an unbalanced input signal into a balanced signal, 1st and 2nd capacitors connected in series between output terminals of the transformer and whose connecting point receives the local oscillation signal, a mixture circuit connecting to the output terminal pair of the said transformer and a circuit extracting a prescribed intermediate frequency signal from the output of the mixer circuit. CONSTITUTION:When a frequency of a local oscillation signal is sufficiently higher than a frequency of an input signal, the impedance of capacitors 12, 13 is high with respect to the input signal and low with respect to the local oscillation signal. When the capacitance of the capacitors 12, 13 is nearly equal, the local oscillation signal appears at each output terminal of a transformer 10 to an imaginary midpoint (connecting point between capacitors 12, 13) as the signal of the same phase because of the capacitors of the same capacitance. Diodes 5-8 mix the input signal and the local oscillation signal inputted to them respectively. Since the local oscillation signal is of the same phase, the local oscillation signal is cancelled by the transformer 9 and does not appear in the intermediate frequency signal at the output terminal 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an improvement of a high frequency balanced mixer using a balanced-unbalanced conversion transformer.

(Prior Art) Frequency conversion circuits in high-frequency communication equipment use balanced or double-balanced mixers with high isolation between the local oscillation output and the intermediate frequency output, because leakage of the local oscillation frequency causes various disturbances. is used.

A balanced mixer introduces an unbalanced R input signal to the mixer via a transformer that converts it from balanced to unbalanced, and mixes it with a local oscillation signal. Figure 4 shows an example of a conventional double-balanced mixer and details. In Fig. 4,
Input terminal 1 receives an unbalanced input signal, and terminal 2 receives an unbalanced local oscillation output.

An input port consisting of the input terminal 1 and the ground terminal is connected to one end of one of the two windings of the unbalanced-balanced conversion transformer 10, which has two windings wound around a toroidal core. One end of the other winding of the transformer 10 is grounded.

A primary winding L1 of the transformer 4 wound in the same toroidal manner is connected between each winding of the transformer 10. transformer 4
The secondary winding L2 is provided with an intermediate tap, and the local oscillation output from the terminal 2 is supplied to this intermediate tap.

Each end of the secondary winding L2 is connected to the anode of the diode 5, the cathode of the diode 70, the anode of the diode 6, and the cathode of the diode 8, respectively, which constitute a double-balanced diode circuit. In addition, the above diode circuit includes the cathode of diode 5 and the diode 8.
The anode of diode 6 is connected to the anode of diode 7, and the cathode of diode 6 and the anode of diode 7 are connected. These diodes 5
and the cathode and anode connection point of 8 and the diode D6,
The cathode and anode connection point of 7 is connected to an output port having an output terminal 3 and a ground terminal via a frequency selection circuit 16 that selects an intermediate frequency signal, and via lances 1 to 9. The transformer 9 performs balanced-unbalanced conversion in the opposite way to the transformer 10.

In the frequency conversion circuit, an input signal converted into a balanced signal by the transformer 10 is U-combined with a local oscillation signal converted into a balanced signal by the transformer 4 by the diodes 5 to 8. The intermediate frequency signal generated by the mixing has a sum component and a difference component, and the frequency selection circuit 16 usually selects the difference component. The transformer 9 returns the balanced intermediate frequency signal from the frequency selection circuit 16 to an unbalanced state.

In the above, a problem arises in that the local oscillation signal that cannot be removed by the frequency selection circuit 16 leaks into the intermediate frequency signal to the transformer 9. This leakage of the local oscillation signal is solved by the transformer 4. That is, since the transformer 4 extracts the incoming horn signal in the opposite phase and derives the local oscillation signal in the same phase, the local oscillation signal is canceled by the action of the transformer 9 and does not appear at the output terminal 3. ing.

In this way, the circuit shown in FIG. 4 achieves isolation between the input terminal 2 and output terminal 3 of the local oscillation signal.

By the way, the transformer 4 is required to have balanced characteristics over a wide range from the input signal frequency conversion band to the local oscillation signal band, making it difficult and expensive to design. That is, the transformer 4 functions as a balun transformer, and the transformer 3
It has a toroidal structure in which parallel wires are wound around a core, and the number of turns and the material of the core are appropriately selected depending on the frequency used. Winding with three parallel wires tends to cause unraveling or lifting from the core, and the characteristics change, making it difficult to obtain a transformer with good characteristics over a wide band.

(Problems to be Solved by the Invention) As explained above, the conventional balanced mixer requires a wideband transformer 4 to replace a balun transformer, and such a transformer 4 is difficult to manufacture and does not have good quality. It was difficult to mass-produce products with good balance characteristics.

It is an object of the present invention to eliminate the above-mentioned problems and provide a high frequency balanced mixer that does not use a transformer with good characteristics and has less leakage of local oscillation signals.

[Structure of the Invention] (Means for Solving the Problems) This invention provides a transformer that converts an unbalanced input signal into a balanced signal, and a transformer that is connected in series between the output terminals of the transformer and a local oscillator at the connection point. The first signal is introduced. It has a second capacitor, a U metal circuit connected to the pair of output terminals of the transformer, and a circuit for extracting a predetermined intermediate frequency signal from the output of the mixing circuit.

(Function) Generally, in a frequency conversion circuit, the frequency of the local oscillation signal is higher than that of the input signal, and the first aspect of the present invention. The second capacitor has a small impedance to the local oscillation signal and a high impedance to the input signal. Therefore, the local oscillation signal passes through the capacitor and is input to the mixer circuit as a signal of the same phase, where it is mixed with the input signal. That is, ', 1st. The second capacitor functions as a conventional balun transformer, providing a wideband, low-cost frequency conversion circuit.

(Example) Hereinafter, the present invention will be explained in detail with reference to illustrated examples.

FIG. 1 is a circuit diagram showing an embodiment of a high frequency balanced mixer according to the present invention, and elements common to those in FIG. 4 are given the same reference numerals.

In FIG. 1, an unbalanced input signal introduced into an input terminal 1 is converted into a balanced signal by a transformer 10. The transformer 10 has a structure in which an input signal is input/output to one winding as in the case of FIG. 4, one end of the other winding is connected to the ground point, and the other end derives a signal due to induction. ing.

First and second capacitors 12 and 13 are connected in series between the output terminal of one winding of the transformer 10 and the output terminal of the other winding. The unbalanced local oscillation signal from the terminal 2 is guided to the connection point of the series connection.

Note that the output from the circuit of the transformer 10 and the capacitors 12, 13 is supplied to the transformer 9 via the capacitors 14, 15, the diodes 5 to 8, and the frequency selection circuit 16, as in the conventional case. In contrast to the transformer 10, the transformer 9 converts balanced to unbalanced signals and outputs a mixed intermediate frequency signal to the output terminal 3.

In such a configuration, if the frequency of the local oscillation signal is sufficiently higher than the frequency of the input signal, the capacitor 12.
13 has a high impedance with respect to the input signal,
Impedance becomes smaller for local oscillation signals. Here, assuming that the capacitances of capacitors 12 and 13 are approximately the same,
Since the capacitors have the same capacitance, the local oscillation signals appear at each output terminal of the transformer 10 as signals having the same phase with respect to the virtual midpoint (the connection point of the capacitors 12 and 13). Also, the input signal balanced by the transformer 10 is
Because the impedance of capacitor 12.13 is high,
Directly pass through capacitors 14 and 15 to diode 5~
8. The diodes 5 to 8 can mix the respective input signals and local oscillation signals as described above.

Local oscillation signal components leak into the intermediate frequency signal after mixing, but in this case as well, since the local oscillation signals have the same phase, they are canceled by the transformer 9, and the local oscillation signal components leak into the intermediate frequency signal at the output terminal 3. does not appear.

Figure 2 shows the input terminal 2 of the local oscillation signal and the diode 5.
Insertion loss characteristics up to 8 are shown. The horizontal axis represents frequency and the vertical axis represents loss. As shown in this figure, the loss level (dB) of the input signal band is extremely large, and the loss level of the local oscillation signal is approximately 3 dB. It can be seen that sufficiently good characteristics can be obtained in the local oscillation signal band.

In this way, sufficient isolation for the local oscillation frequency band can be achieved between the input terminal 2 and the output terminal 3 of the local oscillation signal. Note that the local oscillation signal leaking to the input terminal 1 through the transformer 10 is canceled and eliminated by the transformer 10 in the same manner as the transformer 9.

According to this balanced mixer, there is no need for the transformer 4 (in the case of Fig. 4) that winds three parallel wires in a toroidal manner, and manufacturing is simplified, and the isolation performance between boats is superior to that of the double balanced mixer. No leakage,
A frequency conversion circuit having good characteristics can be provided.

Although the above embodiment is a double balanced mixer using diodes, the present invention can also be applied to a balanced mixer using two diodes (5, 6) as shown in FIG. In this case as well, isolation between the output terminal 3 and the input terminal 1 with respect to the local oscillation signal is established.

It should be noted that the mixer circuit is not limited to the above diode circuit, but may also be a transistor circuit.

[Effects of the Invention] As explained above, according to the present invention, frequency conversion characteristics with less leakage of the local oscillation signal to the intermediate frequency signal can be achieved without using a difficult-to-manufacture transformer such as a balun type transformer. effective.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an embodiment of a high frequency balanced mixer according to the present invention, Fig. 2 is a characteristic diagram showing the insertion loss from the local oscillation input to the intermediate frequency output in the circuit of Fig. 1, and Fig. 3 is a diagram showing the insertion loss of the circuit of Fig. 1. A circuit diagram showing another embodiment, FIG. 4 is a circuit diagram showing a conventional high frequency balanced mixer. 1...Input terminal, 2...Local oscillation signal input terminal, 3
...Output terminal, 5-8...Diode, 9.10.
...Trance, 12.13...1st. Second capacitor.

Claims (1)

[Claims] A transformer that converts an unbalanced input signal into a balanced signal, and first and second capacitors connected in series between the output terminals of this transformer and into which a local oscillation signal is introduced. A high frequency balanced mixer comprising: a mixer circuit connected to a pair of output terminals of the transformer; and a circuit for extracting a predetermined intermediate frequency signal from the output of the mixing circuit.