JP2006013733A - Voltage controlled oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a two-frequency selective voltage-controlled oscillator wherein the quality of an output signal at each oscillated frequency is not deteriorated. <P>SOLUTION: In the two-frequency voltage-controlled oscillator that includes two circuits of voltage-controlled oscillators each comprising a series connection of: a band pass filter for determining an oscillated frequency; matching circuits each provided to the pre-stage and the post-stage of the band pass filter; a fixed phase shifter for determining the oscillation condition and the oscillated frequency; a voltage-controlled variable phase shifter for controlling the oscillated frequency by an external voltage signal; and a distributor for determining the distribution to an external output and an in-loop output, and includes a power supply switching circuit of the voltage-controlled oscillators and a coupling circuit for coupling output signals of the voltage-controlled oscillators, the coupling circuit adopts a circuit configuration of a 2-input low pass filter comprising capacitors, inductors, and a resistor. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a series feedback voltage controlled oscillator using a band-pass filter, and more particularly to a two-frequency selective voltage controlled oscillator that selectively outputs only an output signal from one signal generator among a plurality of signal generators. Is.

  In a wireless communication device using a plurality of frequency bands or an optical communication device having a plurality of reference frequencies, in order to obtain a desired frequency from a plurality of different frequencies, a plurality of oscillators are arranged and an output signal is switched by a switching circuit. The method is used. However, in this case, an oscillator that is not involved continues to operate, so that power consumption increases and unnecessary spurious noise is generated. In addition, a method of switching including the power supply circuit of the oscillator is also employed, but in this case, the circuit scale is increased, and thus the mounting area is increased, which is disadvantageous.

Under these circumstances, an oscillator that can oscillate at a plurality of frequencies and can select a frequency to be output has been used in a field where a demand for further miniaturization is severe. As a frequency switching method in the series feedback voltage controlled oscillator, there are a plurality of oscillation circuits built-in and their power supply and output are switched at the same time, and a filter in the oscillation circuit is switched. Since the former is completed by itself, the oscillation circuit itself is basically the same as a single-frequency oscillator, but the latter is often unstable due to multiple feedback paths in the feedback loop. It causes oscillation and is very difficult to handle. For this reason, the former type incorporating a plurality of oscillation circuits is used in a system with high reliability requirements.
FIG. 2 shows an example of a conventional two-frequency selective oscillator in which the power source and the output are simultaneously switched to switch the oscillation frequency. In this circuit configuration, since the outputs of the two oscillators are only bundled by capacitor coupling, the circuit scale can be suppressed to the smallest among the two-frequency oscillators incorporating two oscillation circuits.
Similarly, FIG. 3 is an example of a type in which the oscillation frequency is switched by simultaneously switching the power source and the output, but the impedance from the operating oscillator output circuit to the stopped oscillator output circuit is increased by switching the diode. . Such an example of the conventional two-frequency selective oscillator is characterized in that the leakage of the oscillation signal to the stopped oscillator is suppressed to be small, and the decrease in the output signal strength is suppressed. Further, by switching the diode using the bias of the oscillation circuit in operation, it is also possible to suppress an increase in circuit scale relatively small (see, for example, Japanese Patent Laid-Open No. 2000-59142).

  FIG. 4 shows a conventional two-frequency selective oscillator that switches the oscillation frequency by switching the band-pass filter in the oscillation circuit. In this circuit, a voltage-controlled oscillator capable of oscillating at a plurality of oscillation frequencies is configured by selecting a band-pass filter having a desired frequency by a diode switch, for example. In addition, as a method of switching the band pass filter in the oscillation circuit, there is also a method of switching a filter contributing to oscillation by switching the phase of the feedback loop by a voltage control phase shifter without using a switch circuit such as a diode. (See JP 2001-127548 A). In these methods, since an amplifier circuit and a voltage control phase shift circuit used for oscillation are shared, there is almost no drift before and after switching, which is advantageous for applications in which switching frequently occurs.

Next, a basic circuit of a feedback type voltage controlled oscillator using a band-pass filter, for example, a surface acoustic wave filter, is known, for example, from Japanese Patent Laid-Open No. 59-158106. As an example of a conventional voltage controlled oscillator of this type, FIG. 5 shows a block diagram of a series feedback surface acoustic wave oscillator. A surface acoustic wave filter (SAW filter) 1 is provided as a frequency selection element, and a matching circuit 2 is disposed in the front and rear stages of the filter. The amplifier circuit 3 is for compensating for the loss of the feedback loop to ensure an appropriate loop gain, and the distributor 4 distributes the signal to the oscillation output and the feedback loop. The fixed phase shifter 6 is for adjusting the oscillation frequency by correcting the phase error due to the manufacturing variation and mounting variation of the SAW filter. The voltage controlled variable phase shifter 5 is the oscillation set by the fixed phase shifter 6. The oscillation frequency is controlled in accordance with a frequency control input within an appropriate frequency variable band from the frequency. The matching circuit 2 may be omitted depending on the matching condition between the input / output impedance of the SAW filter and the circuits before and after the SAW filter. The fixed phase shifter 6 has a variable band only by the voltage controlled variable phase shifter 5. It may be omitted if it can be secured.

  The oscillation condition of this type of oscillator is that the phase condition of the feedback loop is n × 360 degrees (n = 1, 2,... N) and the loop gain is 0 dB or more, and usually passes through a SAW filter. It is designed to satisfy the phase condition within the band.

In the voltage controlled variable phase shifter 5, a variable capacitance diode is generally used as a voltage variable element. Specifically, an all-pass filter type voltage control phase shifter, a high-pass filter type, a low-pass filter type, a 90-degree hybrid, or the like is applied.
JP 59-158106 A JP 2001-127548 A JP 2000-59142 A

In the conventional two-frequency selective oscillator shown in FIG. 2, the output lines of two independent oscillators are combined by a capacitor and bundled into one line. Therefore, there is not much leakage of the signal from the active oscillator to the stopped oscillator, and the output signal strength is reduced or waveform distortion due to impedance mismatch occurs with respect to the oscillation signal extracted by a single oscillation circuit There was a problem of doing.
In the example of the conventional two-frequency selective oscillator shown in FIG. 3, since the impedance from the output circuit of the operating oscillator to the output circuit of the stopped oscillator is increased by switching the diode, the oscillation to the stopped oscillator is performed. While signal leakage can be suppressed to a small level and output signal strength can be prevented from decreasing, the output waveform is distorted by the nonlinear effect of the diode, generating harmonics twice or three times the fundamental wave. When used as a clock source, there is a problem that the duty ratio is deteriorated and an error rate is increased.
The conventional two-frequency selective oscillator shown in FIG. 4 has a plurality of feedback paths in an important feedback loop that determines the oscillation condition, so that it is likely to be an unstable oscillator depending on the layout design such as mounting and is very difficult to handle. . In addition, switching circuit components such as diode switches and choke coils are generally more expensive than components such as transistors, chip resistors, and chip capacitors used in amplifier circuits, and the total cost cannot be reduced despite the small number of components. There was a problem.

  The present invention has been made to solve such problems of the prior art, and the object of the present invention is to perform two-frequency selective voltage control that does not degrade the quality of the output signal at each oscillation frequency. It is to provide an oscillator. Here, the quality of the output signal mainly relates to the signal strength and the spurious level.

  The dual-frequency selective voltage controlled oscillator according to claim 1 determines a band-pass filter, a matching circuit provided before and after the band-pass filter, an amplifier circuit for compensating for the loss of the feedback loop, an oscillation condition and an oscillation frequency. Fixed phase shifter, voltage-controlled variable phase shifter for controlling the oscillation frequency with an external voltage signal, and a distributor that determines the distribution to the external output and the output in the loop. In a two-frequency selection voltage controlled oscillator having two voltage controlled oscillators, and further comprising a power supply switching circuit for each voltage controlled oscillator and a coupling circuit for bundling the output signals of each voltage controlled oscillator, It is characterized by using a coupling circuit with a filter function, which reduces the decrease in output signal strength at the output terminal. And, it is possible to realize a high voltage-controlled oscillator with small quality spurious level.

  The oscillator according to claim 2 is characterized in that, by using a surface acoustic wave filter as a bandpass filter, a compact circuit layout can be realized by taking advantage of the size advantage of the surface acoustic wave filter. This is advantageous when considering various parasites, and has the advantage that the total size can be kept small.

  According to the present invention, in a two-frequency selective voltage controlled oscillator configured by combining two voltage controlled oscillators having different oscillation frequencies, a small space voltage controlled oscillator can be obtained without degrading the electrical characteristics of a single voltage controlled oscillator. Can be provided.

The best mode for carrying out the present invention will be described below.
FIG. 1 is a block diagram showing a configuration of an embodiment according to claim 1 of the present invention. Hereinafter, the two-frequency selective voltage controlled oscillator 10 will be described with reference to FIG.

  As shown in FIG. 1, the band pass filter 1, the matching circuit 2 provided before and after the band pass filter 1, the amplifier circuit 3 for compensating for the loss of the feedback loop, the oscillation condition and the oscillation frequency are determined. A fixed phase shifter 6, a voltage control variable phase shifter 5 for controlling the oscillation frequency by an external voltage signal, and a distributor 4 for determining distribution to the external output and the output in the loop are arranged in series. Constitutes a feedback loop. The outputs of the two voltage controlled oscillators are fed to a coupling circuit having the characteristics of a low-pass filter, and are collected at one output terminal.

  As the band-pass filter, a filter using a dielectric resonator can be used in addition to the SAW filter. Although the matching circuit is realized by using L, C, and R lumped constant devices, a matching circuit using a distributed constant circuit such as a microstrip line can be applied. An LC filter circuit is used for the fixed phase shifter. In addition, an RC filter circuit, of course, literally a delay line, and a voltage-controlled variable phase shifter are used to enable external adjustment. You can also. The voltage control variable phase shifter can be applied to a variable capacitance diode as a voltage variable element and an all-pass filter type voltage control phase shifter, a high-pass filter type, a low-pass filter type, or a 90-degree hybrid.

  In such a configuration, when a power supply voltage is applied, the amplifying circuit 3 amplifies noise generated by each element, and a distributor, a fixed phase shifter, a voltage controlled variable phase shifter, a matching circuit, a band pass filter, Although it circulates in the loop in the order of the matching circuit, only the frequency components necessary for passing through the SAW filter are extracted and fed back to the amplifier circuit. The oscillation condition at this time is that the phase condition of the feedback loop is n × 360 degrees (n = 1, 2,... N), the loop gain is 0 dB or more, and the phase is within the pass band of the SAW filter. A stable oscillation output can be obtained by designing to satisfy the conditions.

  The coupling circuit is a low-pass filter with two inputs and one output composed of L, C, and R as shown in FIG. 1, and an optimum cutoff at fc = 1 / √LC according to each oscillation frequency. A frequency is selected. When there is a discrepancy between the two oscillation frequencies, the capacitor and inductor on the input side of the coupling circuit can be set for each oscillation frequency, and an appropriate cut-off frequency can be prepared for each. Further, as the L, C, and R components used in the coupling circuit, a distributed constant circuit formed of a microstrip line can be used.

An embodiment according to claim 1 of the present invention will be described below.
Table 1 below shows the oscillation output level and the harmonic level for each oscillation frequency of the two-frequency selective voltage controlled oscillator manufactured according to the first embodiment. For comparison, each characteristic value in the circuit configuration of FIG. 2 is shown as Conventional Example 1, and each characteristic value in the circuit configuration of FIG. In addition, typical characteristic values of a single voltage-controlled oscillator for a single frequency circuit separated in the same oscillation circuit configuration are shown. This oscillator is designed to have an oscillation frequency of 622.08 MHz and an output level of 6 dBm.

  As can be seen from Table 1, in the two-frequency selective voltage controlled oscillator manufactured with the circuit configuration of Conventional Example 1, the output is reduced by about 4 to 5 dB with respect to the output level of one stage, and the spurious characteristics The harmonic signal intensity has deteriorated by 6 dB. Next, in the circuit configuration of Conventional Example 2, the output drop is suppressed to about 1.5 dB with respect to the output level for one stage, but the harmonic signal intensity is similarly deteriorated by about 4 dB.

  On the other hand, in the two-frequency selective voltage controlled oscillator manufactured with the circuit configuration according to the embodiment, the output attenuation is about 1.5 dB, which is the same as that of the conventional example 2, but the harmonic intensity is -31 dBc. It can be confirmed that the harmonics are suppressed more than the characteristics of the oscillator. This is considered to be the result of extracting the oscillation output with high signal purity due to the attenuation of the harmonic spectrum because the function of the low-pass filter is incorporated in the coupling circuit.

It is a functional block diagram which shows Example 1 of this invention. It is a functional block diagram which shows the prior art example 1. FIG. It is a functional block diagram which shows the prior art example 2. It is a functional block diagram which shows the prior art example 3. This is a basic circuit of a series feedback voltage controlled oscillator.

Explanation of symbols

1 Band pass filter (SAW filter)
2 Matching circuit 3 Amplifying circuit 4 Divider 5 Voltage controlled variable phase shifter 6 Fixed phase shifter 7 Resistor 8 Capacitor 9 Inductor 10 Surface acoustic wave voltage controlled oscillator according to claim 1

Claims (2)

  A band-pass filter for determining the oscillation frequency, a matching circuit provided in the preceding stage and the subsequent stage thereof, an amplifier circuit for compensating for the loss of the feedback loop, a fixed phase shifter for determining the oscillation condition and the oscillation frequency, Two voltage controlled oscillators configured by arranging in series a voltage controlled variable phase shifter for controlling the oscillation frequency by an external voltage signal and a distributor for determining distribution to the external output and the output in the loop. And a two-frequency selection voltage controlled oscillator having a power switching circuit for each voltage controlled oscillator and a coupling circuit for bundling the output signals of each voltage controlled oscillator, wherein the coupling circuit has two inputs including a capacitor, an inductor, and a resistor. A two-frequency selective voltage controlled oscillator characterized by having a circuit configuration of a low-pass filter.   2. The two-frequency selective voltage controlled oscillator according to claim 1, wherein a surface acoustic wave filter is used as the band-pass filter.