JPS6248801A - High frequency oscillation circuit - Google Patents

Abstract

PURPOSE:To provide a sure oscillation start characteristic and to attain the oscillation with high reliability by providing two diodes connected in anti-parallel between a feedback circuit inserted in series with a surface acoustic wave resonator and ground. CONSTITUTION:Coupling capacitors 17, 18 are inserted in series across the surface acoustic wave resonator 3 and two diodes 19, 20 connected in anti- parallel are provided between a beedback circuit of an amplifier circuit comprising a FET 2 and ground. Then no current flows to the diodes 19, 20 at oscillation start to attain a sufficiently high impedance and the bias applied to the FET 2 is adjusted to minimize the insertion loss of the feedback circuit and to maximize the mutual conductance. Thus, the loop gain is much increased, sure oscillation start is applied and when exciting power is increased after the oscillation start, a current flowing to the diodes 19, 20 is increased to increase the loss. Thus, the loop gain is controlled to the unity by the negative feedback to make the oscillation output of a high frequency oscillation circuit 1 constant thereby improving the reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to improvement of a high frequency oscillation circuit using a surface acoustic wave resonator.

(Prior art) A local oscillator is an important circuit part that determines the reliability of a superheterodyne receiver, and the higher the selectivity characteristics of the receiver, the higher the frequency of the local oscillation number 43 output from the local oscillator. Must be stable.

In recent years, with the rapid development of microfabrication technology, it has become possible to manufacture surface acoustic wave resonators having a resonant frequency in the UHF band. Therefore, local oscillation circuits that utilize the extremely excellent frequency stability of surface acoustic wave resonators have been in use for a long time.

Here, FIG. 3 shows an example of a conventional high frequency oscillation circuit using a surface acoustic wave resonator used as a local oscillation circuit.

In FIG. 3, the high frequency oscillation circuit 1 is a field effect transistor (hereinafter referred to as FET)? , ionic surface wave resonator (
(hereinafter referred to as SAW)3. Bypass capacitor 4, 5, 6
, bias resistor 7, 8, 9° 10, phase correction filter 11
．． Consists of 12. 13 is a power supply terminal. Then, the local oscillation signal is applied from the high frequency oscillation circuit 1 to the mixer transistor 15 of the mixing circuit via the injection capacitor 14 and mixed with the signal at the signal input terminal 16.

The above-mentioned high-frequency oscillation circuit 1 consists of an amplifier circuit composed of FET 2 and a 5AW3 connected in series to the feedback circuit of this amplifier circuit, and the resonance frequency of 5AW3 is strongly attributed to it. oscillate. And 5A
Since W3 has an extremely sharp selectivity Q of several thousand to tens of thousands, an oscillation wave with sharp frequency characteristics is output from the high frequency oscillation circuit 1, and a superheterodyne receiver with excellent selectivity characteristics is created. can get.

(Problems to be Solved by the Invention) By the way, 5AW3 is manufactured using microtechnology, and its excitation electrode is extremely thin. Therefore, the standing wave excited by the excitation power is too large and the excitation electrode is likely to be destroyed.

In conventional high-frequency oscillation circuits, F is set at a high bias in order to increase the loop gain in order to ensure reliable oscillation startup.
The problem is that when the ET2 is operated, the excitation power increases until it is limited by the amplitude limiting characteristic due to the saturation of the FET2, and the excitation power that exceeds the limit is applied to the 5AW3, causing the 5AW3 to be easily destroyed and lacking in reliability. was there. If the FET 2 is operated with a low bias in order to limit this excitation power, the problem arises that the loop gain decreases, the startup characteristics deteriorate, and a sufficient oscillation output level cannot be obtained. Therefore, it has not been possible to sufficiently satisfy both reliability and oscillation start-up characteristics at the same time.

The purpose of the present invention is to solve the problems of the conventional high frequency oscillator circuit described in J. The purpose of the present invention is to provide a high frequency oscillation circuit.

(Means for Solving the Problem) In order to achieve the above object, the high frequency oscillation circuit of the present invention includes a transistor as an amplifying element and a surface acoustic wave resonator interposed in series in the feedback circuit. 2 connected in parallel with opposite polarity between the feedback circuit and ground.
It is constructed by interposing several diodes.

(Function) Since two diodes connected in parallel with opposite polarities are inserted between the feedback circuit in which the surface acoustic wave resonators are inserted in series and the ground, the equivalent resistance of the diodes decreases when oscillation starts. is large, the equivalent capacitance is small, and the impedance is sufficiently high, resulting in a large loop gain and reliable oscillation startup characteristics.Moreover, after oscillation starts, the excitation power to the surface acoustic wave resonator causes the diode's equivalent resistance to be small, and the equivalent capacitance to be As the impedance becomes larger and the impedance becomes lower, the loss in the feedback circuit increases, the loop gain decreases, the excitation power becomes constant, and reliable oscillation operation is obtained.

(Description of Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 1 and 2. FIG. 1 is a circuit diagram of an embodiment of a high frequency oscillation circuit of the present invention using a surface acoustic wave resonator used as a local oscillation circuit, and FIG. 2 is an equivalent circuit diagram of the high frequency oscillation circuit of FIG. It is a circuit diagram. In FIG. 1, circuit elements that are the same as those in FIG. 3 are given the same reference numerals and redundant explanations will be omitted.

First, in Fig. 1, the difference from Fig. 3 is that coupling capacitors 17 and 18 are connected in series on both sides of 5AW3 in the feedback circuit, and are connected in parallel with opposite polarity between the feedback circuit and the ground. The reason is that two diodes F19 and F20 are interposed.

Next, to explain the equivalent circuit in FIG. 2, 21 is F
The equivalent circuit on the output side of ET2, 22 is the equal input circuit of the mixed circuit, 23 is the equivalent circuit of the diode 19.20, 24 is the equivalent circuit of 5AW3, 25 is the equivalent circuit on the input side of FET2, ED is the FET Output open terminal voltage, RD is FE
T is the equivalent output resistance, CD is the FET output equivalent capacitance, LD is the inductance of the output side phase correction coil [2], CM is the mixed circuit equivalent input resistance, RM is the mixed circuit equivalent input resistance,
C1 is the capacitance of the output side coupling capacitor 17, Rd is the diode equivalent resistance, Cd is the diode equivalent Furuzuke, Cpl,
Cp2 is SAW equivalent parallel capacitance, Cs is SAW equivalent series capacitance, Ls is SAW equivalent series inductance, Rs is SA
W is the equivalent series resistance, C2 is the capacitance of the input end coupling capacitor 18, LG is the inductance of the input side phase correction coil 11, CG is the FET equivalent human capacity, RG is the FET equivalent input resistance, and EG is the FET feedback voltage.

Here, let IL be the insertion loss of the feedback circuit of FET2.

and the mutual conductance of FET2 is gm.

If the input voltage of FET2 is EG', and the voltage amplification degree of FET is G.

ED = gm 1IRD 1IEG ・−・・−(2
). Therefore, from equations (1) and (3), if the loop gain is GL, then the following equation is obtained.

At the time of oscillation startup, if this loop gain GL is GL>>■, reliable oscillation startup characteristics can be obtained, and after oscillation startup, if the loop gain GL is controlled to 1, the excitation power is amplified and FET2 is saturated. , the excitation power becomes constant, stable oscillation output is obtained, and the excessive excitation power causes 5AW3
will not be destroyed.

Therefore, in the present invention, when starting one oscillation, no current flows through the diode 19.20, the diode equivalent resistance Rd is large, the diode equivalent capacitance Cd is small, and the impedance is sufficiently high. Loss is small. Then, the bias applied to the FET 2 is adjusted using the sufficiently high impedance of the diodes 19 and 20 so that the insertion loss IL of the feedback circuit is minimized and the mutual conductance gm is maximized. Therefore, the loop gain GL is extremely large and oscillation can be started reliably.

Furthermore, as the oscillation is started and the excitation power increases,
The current flowing through the diode 19,20 increases. For this reason, the diode equivalent resistance Rd decreases, the diode equivalent capacitance lcd increases, the impedance decreases, and the loss due to the diode 19, 20 increases. For this reason, the loop gain GL is controlled to 1 by a negative feedback effect before the FET 2 is saturated with respect to an increase in excitation power. As a result, the oscillation output of the high frequency oscillation circuit 1 becomes constant, and there is no excessive increase in excitation power, so the 5AW3 is not destroyed and is highly reliable.

Here, the coupling container 17 provides a large actance to the feedback circuit with respect to impedance changes of the diodes lθ, 20, reducing the influence on the amplifier circuit including the FET 2, and also reduces the influence on the voltage injected into the mixer transistor 15. Acts as an isolator to reduce the impact.

In the above embodiment, the FET 2 is used as the multilayer transistor, but the present invention is not limited to this, and it goes without saying that a transistor other than a field effect transistor may be used.

In addition, as in the above embodiment, two
The present invention is not limited to the one in which a set of diodes is interposed between the feedback circuit and ground. A plurality of sets of diodes may be interposed in series or in parallel between the feedback circuit and ground.

(Effects of the Invention) As explained above, the high frequency oscillation circuit of the present invention has two diodes connected in parallel with opposite polarities between the feedback circuit in which surface acoustic wave resonators are interposed in series and the ground. Since the diode has a large equivalent resistance, a small equivalent capacitance, and a high upward impedance at the time of starting oscillation, the loop gain is large and reliable oscillation starting characteristics can be obtained.
Moreover, after oscillation starts, the excitation power to the surface acoustic wave resonator reduces the equivalent resistance of the diode and increases the equivalent resistance, resulting in a low impedance, which increases the loss in the feedback circuit and decreases the loop gain, so that the excitation power remains constant. This has the excellent effect of providing reliable oscillation operation.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of a high frequency oscillation circuit of the present invention using a surface acoustic wave resonator used as a local oscillation circuit, and FIG. 2 is an equivalent circuit diagram of the high frequency oscillation circuit of FIG. FIG. 3 is a circuit diagram of an example of a conventional high frequency circuit using a surface acoustic wave resonator used as a local oscillation circuit. 1: High frequency oscillation circuit, 2: Field effect transistor, 3: Surface acoustic wave resonator, 19, 20: Diode.

Claims (1)

[Claims] In a high frequency oscillation circuit in which a transistor is used as an amplification element and a surface acoustic wave resonator is inserted in series in a feedback circuit, two diodes connected in parallel with opposite polarities are inserted between the feedback circuit and ground. A high frequency oscillation circuit characterized by: