JPS6345082Y2 - - Google Patents

Description

[Detailed explanation of the invention] (1) Technical field of the invention The invention is based on a local oscillator signal injection circuit for a crystal oscillator that supplies a local oscillator signal from a crystal oscillator to a mixing circuit. The present invention relates to a local oscillator signal injection circuit that sufficiently attenuates spurious signals near the tuning frequency by coupling and facilitates peak output adjustment.

(2) Background of the Technology It has been known in the art to provide a double-tuned circuit to attenuate unnecessary harmonics in overtone crystal oscillators and the like to achieve that purpose. For example, FIG. 1 shows the configuration of a conventional circuit of this type, which is equipped with, for example, a Colpitts type crystal oscillation circuit as an oscillation circuit, 1 is a crystal oscillator, 2 is an oscillation transistor, and 3 is a tuning transformer whose primary On the side, a tuned circuit is configured with coil L ₁ and capacitor C ₁ , and on the secondary side, a similar tuned circuit is configured with coil L ₂ and capacitor C ₂ , and the output of the secondary side is connected to coupling capacitor C ₃ . It is connected to the base side of the frequency mixing transistor 4 via. 5 is a high frequency amplification transistor, 6 is a variable inductor, _C4 is a tuning capacitor, and -B is a negative voltage application terminal. Note that R ₁ to _{R 9} are resistors, and C ₅ to C ₁₁ are other capacitors.

In the circuit shown in Figure 1, the crystal oscillator is, for example, 3
The oscillation frequency of the next overtone is ₀ = 39MHz
Assuming that, the high _{frequency 30 (} 39MHz x 3 =
117MHz) is taken out via the tuning transformer 3 and injected into the base of the frequency mixing transistor 4.

(3) Problems with the Prior Art However, since many high frequencies are generated in the oscillation transistor 2, the base of the frequency mixing transistor 4 generates the fundamental frequency itself and unnecessary high frequencies other than the fundamental frequency, such as 39MHz, 78MHz, 156MHz, 195MHz,
234MHz... etc. must be sufficiently attenuated.
For this purpose, a tuned transformer 3 having double tuned circuits L ₁ , C ₁ , L ₂ , C ₂ is used.

However, in a double-tuned transformer having such a tuning circuit, it is difficult to reduce the coupling coefficient (coupling degree) in order to bring the coupling coefficient close to the critical coupling, and its frequency characteristics are shown in Figure 2. As shown in , the result is a bimodal characteristic with two peak values (the bandwidth of which is approximately 10 MHz).

There are certainly several ways to reduce the degree of coupling between the two tuned circuits C ₁ , L ₁ and C ₂ , L ₂ , such as reducing the secondary inductance L ₂ or reducing the primary inductance in the tuned transformer. Although there are means to increase the coupling distance between the inductances L ₁ and L ₂ on the secondary side and the secondary side, the frequency characteristics or band characteristics shown in FIG. 2 are not substantially improved.

(4) Purpose of the invention The present invention solves the above-mentioned problems.In this invention, the inductance on the secondary side is reduced without using a double-tuned transformer equipped with a conventional double-tuned circuit. The purpose of this invention is to provide a local oscillation signal injection circuit which is divided into two parts, and by reducing the coupling inductance, the coupling of the tuned circuit is brought close to the critical coupling, and a sufficient injection voltage can be extracted. .

(5) Embodiments of the invention Next, embodiments of the invention will be described with reference to the attached drawings.

FIG. 3 shows an embodiment of the injection circuit of the present invention, and FIG. 4 shows the frequency characteristics of the tuning circuit portion of FIG. 3 according to the present invention.

In Fig. 3, the main components are: 1 a crystal oscillator, 2 an oscillation transistor, 3
is a tuned transformer and its primary side tuned circuit
The configuration of C ₁ and L ₁ is the same as that in Fig. 1, but unlike the former, the secondary side is only a coil L ₂ ' with a small number of turns (small inductance), and one end of the coil L ₂ ' is used. It is connected to a new variable inductor 10 provided separately. C ₃ ′ is the tuning capacitor,
4 is a frequency mixing transistor, 5 is a high frequency amplification transistor, 6 is a variable inductor,
The other resistors _R1 to _R9 and capacitors _C4 to _C11 are the same as those in FIG.

That is, in the present invention shown in FIG _{. 3} , _the secondary capacitor C ₂ of the tuning transformer shown in FIG. The circuit is the same as the circuit shown in FIG. 1 except for the fact that the second variable inductor 10 is provided in the form of .

In any case, in the present invention, as described above, the secondary side of the tuning transformer 3 is made only of the coil L ₂ ' with small inductance, and a separate second variable inductor 10 is provided, and the tuning capacitor C ₃ ' and the input capacitance of the frequency mixing transistor 4 constitute a tuning circuit corresponding to L ₂ and C ₂ shown in FIG. In operation, as can be seen from FIG. 3, since the inductance of the secondary coupling coil L ₂ ' coupled to the primary coil L ₁ of the tuned transformer 3 is made small, the coupling between the two can be made close to the critical coupling. The frequency characteristics are as shown in FIG. 4, and the peak value can be easily adjusted. Therefore, a sufficient injection voltage can be injected into the base of the frequency mixing transistor 4, and spurious waves can be sufficiently attenuated.

FIG. 4 shows the frequency characteristics of the tuning circuit portion of the present invention. As can be seen from the figure, 3 ₀ = applied to the base of the frequency mixing transistor 4.
The bandwidth expressed at a position 3 dB lower than the center frequency (resonant frequency) of 117 MHz is approximately 1 MHz, which is a sharp characteristic with a significantly smaller bandwidth than the frequency characteristic with bimodal characteristics in the conventional circuit shown in Figure 2. .

(6) Effects of the invention As described above, in the circuit of the invention, critical coupling can be realized by coarse coupling in the tuning circuit section, and furthermore, peak adjustment of frequency characteristics is facilitated. Furthermore, since critical coupling is obtained as described above, it is possible to sufficiently attenuate spurious signals other than local oscillation signals. Furthermore, since a sufficiently large injection voltage can be applied to the frequency mixing circuit, variations in injection voltage are reduced compared to conventional circuits.

[Brief explanation of drawings]

FIG. 1 shows a conventional oscillation signal injection circuit, FIG. 2 shows the frequency characteristics of the tuned circuit section in the injection circuit of FIG. 1, FIG. 3 shows an embodiment of the oscillation signal injection circuit according to the present invention, and FIG. It is a frequency characteristic of the tuning circuit part in the injection circuit of FIG. 3. In the figure, 1 is a crystal oscillator, 2 is an oscillation transistor, 3 is a tuning transformer, 10 is a second variable inductor, C ₃ ' is a tuning capacitor, 4 is a frequency mixing transistor, 5 is a high frequency amplification transistor, 6 denote the first variable inductors, respectively.

Claims (1)

[Scope of utility model registration request] In a local oscillation signal injection circuit that includes a crystal oscillator and a tuning circuit that is connected to the output side of the oscillator and extracts a signal having a frequency that is a predetermined multiple of the oscillation frequency ₀ , a tuning transformer that constitutes the tuning circuit tunes the primary side. The secondary side is formed of a winding with a small inductance, and a variable inductance and a tuning capacitor are connected in series to one end of the winding to form a series tuned circuit. A local oscillation signal injection circuit characterized in that a first tuning circuit comprising a tuning transformer and a second tuning circuit including the variable inductance are coupled through critical coupling.