JPH01183903A - Surface acoustic wave oscillator - Google Patents

Abstract

PURPOSE:To obtain an oscillator output with low noise by equipping with a matching circuit in which the impedance to consider a surface acoustic wave element from an amplifier terminal shows the value close to the impedance to give the noise index minimum of the amplifier, between the amplifier terminal and the surface acoustic wave element. CONSTITUTION:Matching circuits 8a and 8b are inserted between a surface acoustic wave element 1 and an amplifier 2. An input side matching circuit 8a is set so that an impedance ZMi to consider the surface acoustic wave element 1 side from an input terminal 3a of the amplifier 2 can become the value close to an impedance Zm to give the noise index minimum of the amplifier 2. An output side matching circuit 8b is set in the same way as the input side matching circuit 8a. The figure shows an impedance characteristic when the matching circuit 8 is added to the surface acoustic wave oscillator. At this time, the impedance ZMi of the surface acoustic wave element 1 is away from a matching point ZAi of the amplifier 2, a mis-matching loss occurs. However, the reducing quantity of a noise index is large than a mis-matching loss increasing quantity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a surface acoustic wave oscillator that oscillates a sine wave signal using a surface acoustic wave element and an amplifier.

[Conventional technology]

A conventional surface acoustic wave oscillator will be explained using FIGS. 5 to 7. A conventional surface acoustic wave oscillator is described in the document MICROW
AVES & RF-March 1984. pp, 1
53-185.

In FIG. 5, 1 is a surface acoustic wave element, and an amplifier 2
This forms a positive feedback loop and causes oscillation. The input terminal 3a of the amplifier 2 is connected to the output terminal 5a of the surface acoustic wave element 1 via an input matching circuit 4a.2. amplifier 2
The output terminal 3b of is connected to the input terminal 5b of the surface acoustic wave element 1 via an output side matching circuit 4b. 6 is a load resistance.

The matching circuit 4 is set so that mismatch loss between the amplifier 2 and the surface acoustic wave element 1 is reduced.

Next, the operation will be explained. Particular emphasis is placed on the input side of the amplifier 2. The oscillation frequency of the surface acoustic wave oscillator is mainly determined by the surface acoustic wave element 1.

The surface acoustic wave element 1 uses an element that has a minimum loss near the oscillation frequency and a large ratio of change in passing phase to a slight change in frequency, that is, a large Q value, which is a quantity representing the sharpness of resonance. .

For this reason, a surface acoustic wave delay line or a surface acoustic wave resonator is often used for the surface acoustic wave element 1. The noise figure of the surface acoustic wave oscillator with the oscillation frequency F (Hz) largely depends on the Q value of the surface acoustic wave element 1 in the range below the detuning frequency (F/2Q).

The limit of the Q value of the surface acoustic wave element 1 is approximately determined by the substrate material used. In the range above the detuning frequency (F/2Q), the noise figure of the surface acoustic wave oscillator largely depends on the loss of the surface acoustic wave element 1 and the noise figure of the amplifier 2. A large proportion of the loss in the surface acoustic wave element 1 is due to impedance mismatch. Therefore, in the surface acoustic wave oscillator, a matching circuit 4 has been inserted between the surface acoustic wave element 1 and the amplifier 2 in order to reduce this mismatch loss.

That is, the action of the input-side matching circuit 4a is such that if the impedance looking from the input terminal 3a of the amplifier 2 toward the amplifier 2 side is ZAi, and the impedance looking into the surface acoustic wave element 1 side is ZMi, ZAi and ZMi are mutually This is to convert the impedance Z of the surface acoustic wave element 1 so that it becomes complex conjugate. The output side matching circuit 4b is also similar to the input side matching circuit 4a. FIG. 6 shows an example of the impedance characteristics of the surface acoustic wave element 1 when the matching circuit 4 is not inserted. The figure also shows an example of a matching point Z that is the complex conjugate of the input impedance ZAi of the amplifier 2. The impedance ZMi of the surface acoustic wave element l and the matching point ZA of the amplifier 2 are quite far apart, and the mismatch loss is large.

FIG. 7 shows an example in which a matching circuit 4 is added to the surface acoustic wave oscillator to perform impedance conversion.

The impedance Z of the surface acoustic wave element 1 is extremely close to the matching point ZX of the amplifier 2, and mismatch loss can be considerably reduced.

[Problem to be solved by the invention]

A conventional surface acoustic wave oscillator is configured as described above, and the matching circuit is determined with attention only to reducing mismatch loss between the surface acoustic wave element and the amplifier. However, the noise figure generated by the amplifier depends on the impedance of the circuit connected to the terminals of the amplifier, and the impedance Zm of the circuit that minimizes the noise figure is different from the impedance Zxi that minimizes the mismatch loss. Therefore, when a matching circuit having an impedance that minimizes mismatch loss is connected, the noise figure of the amplifier becomes larger than the minimum noise figure. Therefore, in conventional surface acoustic wave oscillators, even if the mismatch loss between the amplifier and the surface acoustic wave element is minimized, the noise figure of the amplifier increases, making it impossible to obtain an oscillator output with sufficiently low noise. was there.

The present invention has been made to solve the above-mentioned problems, and its purpose is to reduce the noise figure of the amplifier and obtain a surface acoustic wave oscillator that provides an oscillator output with lower noise.

[Means to solve the problem]

In the surface acoustic wave oscillator according to the present invention, the impedance between the amplifier terminal and the surface acoustic wave element terminal, looking into the surface acoustic wave element from the amplifier terminal, is close to the impedance that provides the minimum noise figure of the amplifier. It is equipped with a matching circuit that indicates the value.

[Effect]

The surface acoustic wave oscillator of the present invention uses a matching circuit inserted between the amplifier and the surface acoustic wave element to adjust the impedance Z when looking from the terminal of the amplifier to the surface acoustic wave element side.
Impedance Zm that minimizes the noise figure of the amplifier
The noise figure of the amplifier is reduced.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 4. Components that are the same as those in FIGS. 5 to 7 are designated by the same reference numerals, and the description thereof will be omitted. In FIG. 1, matching circuits 8a and 8b are inserted between the surface acoustic wave element 1 and the amplifier 2. In FIG. The input matching circuit 8a is set so that the impedance ZMi looking from the input terminal 3a of the amplifier 2 toward the surface acoustic wave element 1 has a value close to the impedance Zm that provides the minimum noise figure of the amplifier 2. The output side matching circuit 8b is also set in the same manner as the input side matching circuit 8a.

The noise figure NF of the amplifier 2 is described in the document PROCEEDINGS OF THE IRE, 19
60. As also shown in PP, 69-74,
It is calculated using the following formula.

NF (dB) = 10 Log (NFmin+
%14(G-Gm)'+ (B-Bm)")
--(1) Here, NFm1n is the minimum noise figure, Rg is the equivalent noise resistance, and (Gm, Bm) are admittances that provide the minimum noise figure and are constants determined by the amplifier 2.

(G, B) is the admittance of the circuit connected to terminal 3 of amplifier 2, and the admittance of this circuit is (G m +
The noise figure NF7 of amplifier 2 is equal to
'l(minimum)

That is, the matching circuit 8a is added such that the impedance ZMi seen from the input terminal 3a of the amplifier 2 has a value close to the impedance Zm (=1/(Gm+jBa+)) that provides the minimum noise figure of the amplifier 2.

FIG. 2 shows the impedance characteristics when a matching circuit 8 is added to the surface acoustic wave oscillator. At this time, the impedance ZMi of the surface acoustic wave element l is the matching point zA of the amplifier 2.
mismatch loss occurs due to the distance from but,
Since the improvement in the noise figure is greater than the increase in mismatch loss, the surface acoustic wave oscillator with the matching circuit 8a that minimizes the noise figure is better than the conventional surface acoustic wave oscillator. The noise is lower than that of

FIG. 3(a) is a graph in which the noise figure in the amplifier 2 of the surface acoustic wave oscillator is calculated using the above formula (1),
FIG. 3(b) is a graph showing mismatch loss in amplifier 2 of the same surface acoustic wave oscillator. In FIG. 3(a), A is a matching circuit 8 having a value close to the impedance Zm that provides the minimum noise figure for the surface acoustic wave oscillator.
B is the case of a conventional surface acoustic wave oscillator in which a matching circuit 4a that minimizes mismatch loss is added to the surface acoustic wave oscillator. Minimum noise figure NFm1 of the above amplifier 2
n,, equivalent noise resistance Rg, admittance (Gm, Bm
) is the impedance Zm in Figure 2 and the matching point Z in Figure 7.
The measured value of amplifier 2 which showed k was used. In FIG. 3(a), the surface acoustic wave oscillator A of this embodiment has 1.5 to 3.5 dB lower noise than the conventional surface acoustic wave oscillator B over all detuned frequencies. be. Also,
In Figure 3(b), A is 0.5 to 2.5 larger than B.
The dB mismatch loss is large, but if you also consider the noise figure, it is 1 dB lower for all detuned frequencies than for B.
The noise is moderately low. That is, the surface acoustic wave oscillator of this embodiment can realize impedance characteristics with even lower noise than the conventional surface acoustic wave oscillator.

In this embodiment, the impedance ZMi of the surface acoustic wave element 1 via the matching circuit 8 is the impedance Z that provides the minimum noise figure of the amplifier 2 near the oscillation frequency.
However, as shown in FIG. 4, the impedance Zm of the amplifier 2 is equal to
The impedance ZMi may be set near the center of the trajectory. Further, the output side matching circuit 8b may not be used. Furthermore, the output side matching circuit 8b may include a phase adjustment mechanism.

The number of the amplifiers 2 described above is not limited to one, but two or more amplifiers 2 may be used, and each amplifier 2 may include elements having amplification capabilities such as a plurality of transistors.

〔Effect of the invention〕

As described above, according to the present invention, between the amplifier terminal and the surface acoustic wave element terminal, the impedance looking into the surface acoustic wave element from the amplifier terminal is the impedance that provides the minimum noise figure of the amplifier. Since a matching circuit that exhibits close values is provided, the noise level of the amplifier is reduced and an oscillator output with lower noise is obtained.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a surface acoustic wave oscillator according to an embodiment of the present invention, Fig. 2 is an impedance characteristic diagram of an embodiment, and Fig. 3 (
a) is a characteristic diagram of the noise figure of the amplifier, FIG. 3(b) is a characteristic diagram of mismatch loss of the amplifier, FIG. 4 is an impedance characteristic diagram of another embodiment, and FIG. 5 is a conventional surface acoustic wave oscillator. The circuit diagrams of FIGS. 6 and 7 are shown in conventional impedance characteristic diagrams. 1-Surface acoustic wave element, 2-Amplifier, 8a, 8
b - matching circuit; Agent: Masuo Oiwa (and 2 others), 30 years old, male, figure 7, procedural amendment (voluntary): Hiroki Agency Director's Palace 1, case indication: Patent Application No. 1983-008G43, 3;
Person making the amendment No. 4 Representative Moriya Shiki No. 3 5 Column for detailed explanation of the invention subject to amendment. a Contents of the amendment (1) The phrase "resonance" in the 17th line of page 2 of the specification is amended to read "resonance."that's all

Claims (1)

[Claims] In a surface acoustic wave oscillator comprising a feedback circuit using a surface acoustic wave element and an amplifier, the elastic A surface acoustic wave oscillator comprising a matching circuit such that the impedance looking into the surface wave element exhibits a value close to the impedance that provides the minimum noise figure of the amplifier.