JPS62195902A - Variable frequency oscillator circuit - Google Patents

Abstract

PURPOSE:To stabilize the frequency in using the titled oscillator as a fixed frequency oscillator by applying a bias voltage over a normal bias voltage of a varactor diode at the fixed frequency oscillation to the varactor diode connected in series with a crystal oscillator. CONSTITUTION:A series circuit comprising a coil 2 and a varactor diode 3 connected in series with a crystal oscillator 1 is connected to an active circuit A, and bias voltage to the varactor diode 3 is given by throwing a changeover switch 4 to the frequency variable (normal reception state) position R. In fixing the frequency, the switch 4 is thrown to the position T to give a bias voltage higher than the maximum variable bias voltage at the position R to the circuit A through an RFC 7 and also capacitor 8 is given in parallel with the varactor diode 3 to compensate the reduction of the capacitance of the varactor diode 3 due to a high bias voltage improved thereupon. Since the capacitance and its temperature coefficient of the varactor diode 3 are decreased with the high bias voltage, the effect of temperature is reduced and the frequency stability at the fixed frequency approaches the stability of the fixed crystal oscillator.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a circuit configuration of a variable frequency crystal oscillator (hereinafter abbreviated as VXO) mainly used in wireless communication equipment and the like.

[Prior art and problems] VXO is capable of arbitrary frequency changes within a certain frequency range while maintaining the characteristics of a crystal oscillator with excellent frequency stability. It is widely used.

Originally, the oscillation frequency of a crystal oscillator is determined by the mechanical dimensions of the crystal oscillator, so it has extremely good frequency stability compared to a normal self-excited oscillator, and as a fixed frequency oscillator, it has an excellent practical quantity and system. Fine adjustment of the oscillation frequency can be done by adding or subtracting a small capacitor in parallel or series with the crystal oscillator, but the change is less than 1 kHz, and is several kHz to several tens of kHz.
It is not applicable to VXO whose purpose is to change the frequency of .

The operating principle of VXO is described in handbooks related to communication engineering, etc., but
SSB Handbook" (first edition published on December 15, 1960), pages 212 to 216 contain the operating principle and experimentally measured values in the section "Points for producing vXO that obtains variable frequency with crystal oscillation." This is the basis for the cited numerical values in this specification.

Figure 2 shows the basic configuration of a VXO, in which a coil L and a capacitor Cv are connected in series to a crystal oscillator Y, and Lt is equal to Cv.
By changing the vO value, a frequency change of up to 0.5% of the oscillation frequency is obtained. Usually, L is fixed and a variable capacitor is used for Cv to change the frequency. L
The larger the value of , the greater the frequency stability, but if it exceeds the limit value, self-oscillation or parasitic oscillation will occur, so 100μH
Often before or after. Also Cvf'1100. Above F, the frequency change slows down, and 20. Below F, the oscillation becomes unstable, so use the formula 10. Although the range of F is appropriate, such variable capacitors are too large for current small devices and have restrictions on placement, so currently voltage-controlled variable capacitance diodes (hereinafter abbreviated as capacitance diodes) CDs are used. is often constructed as shown in Figure 3.

A capacitor diode is an electronic component that has the same shape as a small diode, and when a reverse bias voltage is applied between the electrodes, the capacitance between the spots changes depending on the bias voltage value.

The relationship between the bias voltage and the interelectrode capacitance is approximately hyperbolic as illustrated in FIG. Therefore, a large capacitance change can be obtained when the bias voltage is near 0, but high frequency loss also increases, so it is usually not used at a bias voltage below IV.

Further, as the bias voltage becomes higher, the capacitance and the capacitance change cap become smaller, so it is appropriate that the bias voltage be set in the range of 1 to IOV.

This bias voltage is set by the potentiometer VR, and the voltage is set by the high frequency power supply coil RFC or by passing the voltage through the high resistance.
D and adjust VR to change the frequency.

Part 4 in FIGS. 2 and 3 is an active circuit composed of electron tubes and transistors, and most crystal oscillation circuits are used.

A preferred application of VXO is to use it in a local oscillator of a radio to finely adjust the operating frequency. In the case of a transceiver, there are functions called ``Receive Fine Tuning'' and ``Receive Shift'' that allow you to avoid interference by changing only the receiving frequency while keeping the transmitting frequency fixed, and check the usage status of adjacent channels. used for
As shown in Figure 5, the circuit for this purpose switches the CD bias circuit in conjunction with the transmitting/receiving switch S, and in the receiving state, the VXO
In the transmitting state, a bias voltage approximately at the center frequency of the variable range may be applied. In this case, the stability of the bias voltage is important, but the stability of L+CD with respect to temperature and humidity cannot be ignored.

When L is on the μH order, it is easy to obtain an air core with good Renoid stability, and on the order of several 10 μH, a high frequency magnetic core can be reduced and good stability can be obtained by selecting the magnetic core material and shape. Since it is an inter-electrode capacitance, the influence of temperature cannot be avoided, and an example thereof is shown in FIG.

Of course, the amount of variation in the VXO frequency due to the temperature coefficient of the CD is extremely small, so it does not reduce the effect of the vXO, but when using a fixed frequency, it is desired to operate with the same stability as a crystal oscillator.

[Purpose of the invention]

This invention uses a capacitive diode °CD as a frequency adjustment element.
An object of the present invention is to provide a VXO circuit that improves the decrease in frequency stability caused by the temperature coefficient of CD capacitance when the VXO circuit is used as a fixed frequency oscillator.

[Summary of the invention]

This invention relates to a crystal oscillator circuit in which the oscillation frequency is changed by adjusting the bias voltage of a capacitive diode connected in series with a crystal oscillator, in which a bias voltage higher than the normal bias voltage of the capacitive diode is applied during fixed frequency operation, and A VXO circuit is characterized in that changes in oscillation frequency caused by the temperature characteristics of the capacitor diode are reduced by installing a correction capacitor in parallel with the capacitor diode, and the outline thereof will be explained with reference to FIG. Then, in a vXO circuit that connects the crystal oscillator 1 in series with the coil 2 and capacitive diode 3 to the active circuit,
The bias voltage of the capacitive diode 3 is determined by setting the selector switch 4 to the variable frequency (normal reception state) R side to apply a variable bias to the potentiometer 6 through RFC5, and by setting the changeover switch 4 to the fixed frequency (normal transmission state) T side to apply a variable bias to the potentiometer 6.
At the same time, a capacitor 8 is placed in parallel with the capacitor diode 3 to compensate for the decrease in capacitance due to the high bias voltage of the capacitor diode 3. 8 is conveniently set as semi-fixed to accurately set the fixed frequency.

The switch 4 may be an electronic switch using a diode or the like since a mechanical switch is greatly affected by restrictions on installation location and stray capacity.

When the capacitance diode 3 is in a high bias voltage state, the capacitance wrinkle value is small as shown in Fig. 4, and the temperature coefficient of capacitance is also small as shown in Fig. 6, so overall the influence of S degrees becomes extremely small. , it is possible to bring the frequency stability at a fixed frequency close to that of a fixed crystal oscillator.

In FIG. 6, since Cv has a positive temperature coefficient whose capacitance increases as the temperature rises, it is possible to further improve frequency stability by using a suitable negative temperature coefficient for the correction capacitor 8.

[Embodiments of the invention]

An example of a practical circuit in which the present invention is applied to a local oscillator of a transceiver is shown in FIG. Note that some symbols and explanations of parts in the drawings that are not directly related to the configuration and operation of the present invention are omitted.

Q is an oscillation transistor (NPN type/Gipolar type is shown in the column shown, but PNP type or FET type is also applicable), and the capacitor C1 between the emitter and ground and the capacitor C2 between the pace emitter and Together with the crystal oscillator Y, a fourteen-inch Colpits oscillation circuit is constructed, and the output is supplied to one stage of buffer through the emitter capacitor C3.

Crystal oscillator Y includes coil L for VXO and capacitance diode C
The set frequency is finely adjusted by a semi-fixed capacitor TC connected in series with the frequency setting capacitor 7'#/S C4 between the base and ground, and in parallel with the capacitor C4.

The switch S operates in conjunction with the transmission/reception conversion circuit, and is closed to the T side during transmission to fix the oscillation frequency, and opened to the R side during reception to vary the oscillation frequency as a VXO. When receiving, variable voltage +2v~8 by potentiometer VR
v is applied as a bias voltage to the capacitive diode CD via the reverse current prevention diode D2/RPCl, and operates as a variable capacitor of the vXO circuit. At this time, the diode DI in parallel with CD is in a reverse bias state and is non-conducting.If a high frequency diode is used, the internal capacitance is less than 1 pF, which is several tenths of the capacitance of co, so it is not practical for practical purposes. The impact is negligible.

During transmission, S is closed to the T side, and the +12V stabilized power supply voltage is applied to the PFCl, Dl, and PFCl-Rol circuits,
Pass Dl and Ro through the capacitance diode CD +】2■
Since a high bias voltage is applied, the capacitance of CD is close to the minimum value, and due to the conduction of Dl, the capacitor Cs is connected in parallel to CD, and it operates at a fixed frequency setting value. Alternatively, the capacity of a portion thereof may be semi-fixed and variable.

Since the bias voltage during transmission is applied to the diode D2 as a reverse voltage, it does not flow to the VR circuit.

Capacitors in the circuit Cot, Co2, cos Ir! ,
Approximately 0.01 to 0.1 μF is appropriate for bypass.
Resistor R0, ・not・Rol is for discharge and has a number of Ω to several 10
I'm taking it to Ω.

〔Effect of the invention〕

The VXO circuit sacrifices some frequency stability in order to make the oscillation frequency of the crystal oscillator variable, and especially when a capacitance diode is used as a variable capacitance fit element for frequency adjustment, the temperature inherent to the semiconductor capacitance As the effects of There is.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of the XO circuit of the present invention, Figure 2 is a VXO circuit diagram.
Figure 3 is the basic circuit of vXO using a capacitance diode as a variable cable, Figure 4 is an example of special notes for the capacitance between reverse bias ridges and poles of WM diodes, Figure 5 is the variable frequency ■XOX0 circuit used for both fixed frequency and fixed frequency applications FIG. 6 shows an example of the characteristic of capacitance diode versus ambient temperature versus capacitance change rate, and FIG. 7 shows an example of a circuit implementing the present invention. 1.Y...Crystal oscillator, 2.L...Coil, 3.C
D...Capacitance diode, 4.S...Sending/receiving switch, 5.7.RFC...High frequency 1/2 coil, 8.c
s...Correction capacitor, Δ...Active circuit, c
l・C2・C3・C4...Capacitor, DI' D
I...diode, Q...transistor, VR...
- Potentiometer. Patent applicant: Yaesu Musen Co., Ltd. Figure 1 Figure 2 Figure 3 Figure 4 Figure 4

Claims (1)

[Claims] In a crystal oscillator circuit that changes the oscillation frequency by adjusting the bias voltage of a voltage-controlled variable capacitance diode connected in series with a crystal oscillator, during fixed frequency operation, the bias voltage must be higher than the normal bias voltage of the voltage-controlled variable capacitance diode. In addition, a correction capacitor is inserted in parallel with the voltage-controlled variable capacitance diode to reduce changes in oscillation frequency caused by temperature characteristics of the voltage-controlled variable capacitance diode.