JPH0318761B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Application) The present invention relates to a VCO circuit (voltage controlled oscillation circuit) that does not require adjustment of free-running oscillation frequency by using a high Q resonance circuit such as a crystal oscillator.

(Prior technology) Conventional MPX IC (integrated circuit for stereo signal demodulation)
As shown in Figure 1, the VCO circuit used was a system in which the free-running oscillation frequency was determined by the charging and discharging time constants of the capacitor and resistor.

In the circuit of FIG. 1, consider now the case where the control voltage Vcon is 0. Here, the control voltage Vcon is a voltage obtained as the output of the phase comparator, and is a voltage that increases or decreases depending on the phase difference between the two frequencies. Usually, when this phase difference is 0, Vcon=0.

Now, when the control voltage Vcon=0, the differential amplifier including transistors 1 and 2 is balanced,
1/2 of the current I 1 determined by the transistor 3 and the resistor 14 flows through each of the transistors 1 _and 2. At this time, a current of I ₁ /2 flows through one collector of the transistor 5, and a control current Icon corresponding to the current I ₁ /2 flows through the other collector.

When the control voltage Vcon≠0, transistor 1,
2, the current _I1 is divided and flows according to Vcon.
When the current flowing through the transistor 2 increases, the control current Icon also increases, and when the current flowing through the transistor 2 decreases, the control current Icon decreases.

Therefore, the control current Icon increases or decreases depending on the control voltage Vcon.

Control current Icon output from transistor 5
flows through resistor R, variable resistor Rv, and capacitor C. Then, capacitor C is charged and discharged. Charging and discharging are performed using capacitor C, resistor R, and variable resistor.
This is done with a time constant determined by Rv.

Now, the base of transistor 6 is transistor 7
When the potential is higher than the base of , that is, V _6B > V _7B
Considering the case of , transistor 6 is on and transistor 7 is off, so transistor 10,
and 11 are cut off. However, since a discharge circuit consisting of resistors R, Rv and capacitor C is connected to the base of transistor 6, if control current Icon is small, transistor 6
The base voltage of V _6B will definitely drop.
And the base voltage of transistor 7 V _7B (=Vref)
At the moment when it crosses the line, transistor 7 is turned on, and transistors 10 and 11 become active. Then, due to the emitter current of transistor 11, the base voltages of transistors 6 and 7 V _6B ,
V _7B will rise.

At this time, the base voltage V _7B is limited by the resistors 19, 20 and 21, but the base voltage V _6B
can be charged to a potential V ₁ . That is, at a certain point in time, transistor 6 is turned on and transistor 7 is turned off.

When transistor 7 is turned off, transistors 10 and 11 are turned off as described above, and capacitor C is connected to capacitor C, resistor R, and variable resistor.
Discharges with a time constant determined by Rv. Then, when the base voltage V _6B of the transistor 6 decreases and becomes smaller than the base voltage V _7B determined by the voltage division between the resistors 20 and 21, the transistor 7 becomes conductive again. Then, transistors 10 and 11 are turned on.

Since such operations are repeated, the transistors 6 and 7 are repeatedly turned on and off according to the discharge time constant determined by the capacitor C, the resistor R, and the variable resistor Rv.

Here, if the charging/discharging currents of C and R are changed, the oscillation frequency of the signal obtained at the output terminal Vout will change.
This can be done by controlling the control voltage Vcon.

In other words, the circuit shown in FIG. 1 operates as a VCO circuit.

However, if multiple VCO circuits with such a configuration are created, the oscillation frequency of each circuit will vary due to variations in Vref (due to variations in resistors 19, 20, 21 and constant voltage potential V ₁ ), h _FE of transistor 11, Since it varies due to variations in the resistor 18, R, Rv, capacitor C, etc., the free-running oscillation frequency when the control voltage Vcon=0 also varies greatly. Therefore, in conventional VCO circuits, each VCO is controlled by a variable resistor Rv.
The disadvantage was that the free-running frequency f ₀ of the circuit had to be adjusted.

(Objective) An object of the present invention is to provide a VCO circuit that uses a high-Q resonant circuit such as a crystal oscillator and can have a wide frequency variable range without adjusting the free-running oscillation frequency.

(Summary) The present invention is characterized by using a high-Q resonant circuit such as a crystal oscillator in order to obtain an accurate free-running frequency, and by increasing the variable amount of the variable phase circuit in the oscillation loop. The point is that the variable width is increased.

(Embodiment) An embodiment of the present invention will be described below with reference to the block diagram of FIG.

A high Q element such as an oscillator, that is, a resonant circuit 3
The output signal h〓 of 0 is input to a phase shifting circuit 31 that shifts the phase by 45 degrees and a second adder 33. The output signal h〓 of the phase shift circuit 31 is input to the inversion circuit 34, and its phase is inverted. The output signal -k〓 of the inverting circuit 34 enters the second adder 33;
3, it is added to the signal h〓. Therefore, the second
The adder 33 outputs a signal h〓−k〓.

The signal k 〓 output from the phase shift circuit 31 is further input to a first adder 32 controlled by the control voltage Vcon. Further, the output h〓−k〓 of the second adder 33 is input to the first adder 32. Then, the first adder 32 performs the following operation.

h〓′=pk〓+q(h〓−k〓) However, h〓′ is the output signal of the first adder 32, p
+q=1.

Since p and q in the above equation are values determined by the control voltage Vcon, as is clear from the vector diagram in Figure 3, the output h〓' varies between the signal k〓 and h〓−k〓, and the phase difference can be changed by ±45° around h〓.

The phase shift amount between h〓' and h〓 is the amount of phase shift required for the oscillator, that is, the resonant circuit 30, and at a frequency corresponding to this amount of phase, the frequency oscillation condition for one round is satisfied, and the oscillator oscillates. It turns out.

FIG. 4 shows a specific example of the circuit shown in FIG. In the figure, 101-119 are transistors, 120-119 are transistors, and 120-119 are transistors.
133 is a resistor, 134 is a capacitor, and 140 is an oscillator.

Transistors 101, 102, resistors 120, 1
21 constitutes a differential amplifier, which operates as a DC amplifier that amplifies the control voltage Vcon. The collector outputs of transistors 101 and 102 are applied to the base of a first adder consisting of a differential amplifier pair and transistors 105-103. This first adder functions to add a mixture ratio of h〓−k〓 and k〓, which will be described later, in accordance with the collector outputs of the transistors 101 and 102. The output of this first adder becomes h〓'.

The transistors 118, 119 and the resistor 128 constitute a Darlington-connected emitter follower circuit, and the input impedance is sufficiently large, and in the circuit shown in FIG.
determined by the low pore value of Now, assuming that the input of the emitter follower circuit is h〓, the output h〓 whose impedance has been converted by the emitter follower is connected to the base of the transistor 113 of the transistor pair 112 and 113 constituting the second adder. Along with this, it is added to a phase shifter consisting of a low pass filter circuit including a low resistor 129 and a capacitor 134. A phase shift of 45° is then given to the oscillation frequency.

The resulting h〓 is applied to the base of the transistor 112, and the sum of the signal h〓 in phase with h〓 applied to the base of the transistor 113 together with −k〓 is applied to the collector of the transistor 112, that is, h〓−k 〓
is obtained. On the other hand, the transistor pair 110, 11
1, a signal k〓 is obtained at the collector of the transistor 110.

Transistors 104, 109, resistors 123, 1
24 are transistors 118, 119 and resistor 12
8 and 131, so that no offset occurs in the base potentials of the transistors 110 to 113.

Transistors 114, 115, 117, resistor 1
25, 126, and 127 constitute a constant current circuit.

FIG. 5a shows an equivalent circuit of the crystal oscillator 140, and FIG. 5b shows its impedance and phase characteristics.
According to FIG. 5b, the phase is 0° at the resonant frequency f _r and the anti-resonant frequency f _ar .

In this embodiment, the crystal oscillator 140 is used within a range centered around the resonant frequency f _r , and the phase changes to positive or negative depending on the deviation from f _r .

Returning to FIG. 4 again, the operation of the circuit shown in FIG. 4 will be explained. When the control voltage Vcon is 0, p and q in the above equation are equal, so the phase shift between h〓' and h〓 is 0. Therefore, the oscillator 140 oscillates around the resonant frequency _fr . In other words, the free-running oscillation frequency is f _r .

On the other hand, when the control voltage Vcon≠0, the control voltage
The operating point of the oscillator 140 changes in response to the phase change due to Vcon. The operating point corresponding to a phase change of ±45° is when the impedance of the oscillator becomes the same as the resistor 131 receiving the oscillator output in Figure 4, and this frequency range is variable by the control voltage. Become. The output of the circuit of this embodiment is output from the collector of the transistor 111 as Vout.

From the above, according to this embodiment, the free-running oscillation frequency is determined by the crystal oscillator, and since the Q is high, stable oscillation is achieved without being affected much by variations in h _FE , V _BE, etc. of the transistors. You can get the frequency.

Further, a maximum phase change of ±45° can be obtained by controlling the control voltage Vcon, and a correspondingly large frequency variable range can be obtained.

FIG. 6 shows a block diagram when the VCO circuit of the present invention is applied to an MPX IC (FM demodulation circuit) for stereo broadcasting. In the figure, 150 is a phase comparator;
151 is a low-pass filter, 152 is the VCO circuit according to the invention described above, and 153 and 154 are 1/2 frequency dividing circuits.

When using FM MPX, input signal a is 19kHz.
The pilot signal is input. Phase comparator 150
sends an output to the low-pass filter 151 according to the phase difference between the input signal a and the output signal b of the 1/2 frequency divider circuit 154. The output signal of the low-pass filter 151 is the control voltage Vcon, and the VCO circuit 1
52 oscillates at 76kHz with this control voltage Vcon. This 76kHz signal is output from Vout.
The output of the VCO circuit 152 is a 1/2 frequency divider circuit 153.
38kHz, and further 19kHz with 1/2 divider circuit 154
The frequency is divided into 1 and fed back to the phase comparator.

(Effects) According to the present invention, since it is not necessary to adjust the free-running oscillation frequency, the step of adjusting the free-running oscillation frequency is unnecessary, which is economical. In addition to this,
Performance is improved. Further, according to the present invention, there is an advantage that the frequency variable range can be widened.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional VCO circuit, Fig. 2 is a block diagram of an embodiment of the present invention, Fig. 3 is a vector diagram, and Fig. 4 is a circuit diagram showing a specific example of the above embodiment. 5a and 5b respectively show an equivalent circuit diagram and an impedance characteristic diagram of the oscillator used in the above embodiment, and FIG. 6 shows a block diagram of an example of application of the present invention. 30...Resonance circuit, 31...Phase shift circuit, 3
2...first adder, 33...second adder, 3
4... Inverting circuit, 140... Oscillator, 150...
Phase comparator, 151...Low pass filter, 15
2...VCO circuit, 153,154...1/2 frequency divider circuit.

Claims (1)

[Claims] 1. A resonant circuit with a high quality index Q, a phase shift circuit to which the output of the resonant circuit is input, a second adder to which the output of the resonant circuit and a signal obtained by inverting the output of the phase shift circuit are input, and The output of the second adder and the output of the phase shift circuit are input,
A first adder is provided that mixes the two input signals at a ratio according to the external control voltage and supplies the output to the resonant circuit, and the oscillation frequency is controlled according to the external control voltage. A VCO circuit characterized in that it is designed to