JPH0671189B2 - Frequency control circuit - Google Patents

Description

The present invention relates to a frequency control circuit using a vibrating element, and in particular, the parallel capacitance of the vibrating element is varied by an equivalent reactance circuit to control the frequency. The present invention relates to a frequency control circuit to perform.

(B) Conventional technology In order to obtain a signal synchronized with an input reference signal, a PLL circuit has been widely used in recent years. The PLL circuit is usually composed of a phase comparator, a low-pass filter, a DC amplifier, a voltage (current) controlled oscillator, etc., and the phase of the input signal in the phase comparator and the output of the voltage controlled oscillator. The phase of the signal is compared, and the frequency of the voltage controlled oscillator is changed by the signal corresponding to the phase difference.

Japanese Unexamined Patent Publication No. 59-57515 discloses a frequency control circuit for an oscillator circuit as shown in FIG. In FIG. 2, the frequency control circuit includes a first and a second transistor (1) and (2) whose emitters are commonly connected to form a differential amplifier, and a first and a second transistor (1) and (2). A first current source transistor (3) connected to a common emitter; third and fourth transistors (4) and (5) having emitters connected in common to form a differential amplifier; and third and fourth transistors (4) ) And (5), a second current source transistor (6) connected to the common emitter, a capacitor (7) connected between the collector and base of the second transistor (2), and the first and fourth A resistor (8) connected between the common base of the transistors (1) and (5) and the common base of the second and third transistors (2) and (4), and the first to fourth transistors (1) This is from (5) Connected to the motor current mirror circuit (9)
And a vibrating element (10) such as a crystal unit, and an oscillation circuit (11)
And an equivalent reactance is generated in accordance with a control signal from the phase comparator applied to the bases of the first and second current source transistors (3) and (6), and the oscillating element (10) is connected in parallel. The equivalent frequency is changed to control the oscillation frequency. The equivalent reactance of the circuit is
Given by ± gmRC. Here, gm is the transconductance of the differential amplifier, R is the resistance value of the resistor (8), and C is the capacitance of the capacitor (7).

(C) Problems to be Solved by the Invention However, in the frequency control circuit as shown in FIG. 2, the absolute value of the equivalent capacitance increases as the mutual conductance gm increases, but as the gm increases, the real number of the equivalent reactance increases. Doubled, the reactance characteristics are lost,
It was difficult to expand frequency control. That is, when the equivalent admittance looking at the reactance circuit is Y and the positive reactance acts, Therefore, the real part of the second term of the equation (1) increases as gm increases, and the advance of the current with respect to the voltage decreases (the current does not have a phase difference angle of 90 degrees compared to the voltage), and the reactance characteristics Is lost. Therefore, there is a region where the equivalent capacity does not increase even if gm is increased, and there is a limit to the control amount. The same applies when a negative reactance acts.
Therefore, the capture range, which is an important characteristic in PLL circuits and the like, is narrowed, and high-precision vibration elements are required. Further, when the pilot signal level becomes low, there is a possibility that a problem of unlocking may occur.

(D) Means for Solving the Problems The present invention has been made in view of the above points, and has a positive or negative equivalent depending on a control signal of a phase comparator or the like of a frequency control circuit using an oscillating element. A variable reactance circuit that operates as a reactance and a negative reactance circuit that operates as a negative bias reactance that is fixedly biased are provided, and a frequency is generated by applying a control signal to the variable reactance circuit while the negative reactance circuit is operating. Is characterized in that it is controlled.

(E) Operation According to the present invention, the equivalent parallel capacitance of the vibrating element can be reduced by operating the negative reactance circuit that is fixedly biased and operates as a negative equivalent reactance. In this state, if you control the variable reactance circuit that operates as a positive or negative equivalent reactance,
Large control is possible with a small capacity.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention. ( 12 ) is a variable reactance circuit operating as a positive or negative equivalent reactance, and ( 13 ) is operating as a negative equivalent reactance. Negative reactance circuit, ( 14 ) is an oscillation circuit to which the output terminals A of the variable and negative reactance circuits ( 12 ) and ( 13 ) are commonly connected, and (15) is an output terminal A of the oscillation circuit ( 14 ) and ground. It is a vibrating element such as a crystal oscillator connected between and.

Therefore, the variable reactance circuit ( 12 ) includes first and second transistors (16) and (17) whose emitters are commonly connected, and a common emitter of the first and second transistors (16) and (17). A first current source transistor (18) connected to a variable current source, third and fourth transistors (19) and (20) having emitters commonly connected, and the third and fourth transistors (19) A second current source transistor (21) connected to the common emitters of (20) and serving as a variable current source, and a common base of the output terminal A and the second and third transistors (17) and (19). Between a first capacitor (22) connected in between and a common base of the first and fourth transistors (16) and (20) and a common base of the second and third transistors (17) and (19). A first resistor (23) connected to the And a diode-connected transistor (24) and a transistor (25) connected to a common collector of the third transistors (16) and (19) and a common collector of the second and fourth transistors (17) and (20). And a first current mirror circuit ( 26 ), which comprises the first and second current source transistors (18)
A control voltage from a phase comparator or the like is applied to the bases of and (21). Further, the second reactance circuit ( 13 ) is
The fifth and sixth transistors (27) and (28) whose emitters are commonly connected, and the fifth and sixth transistors (2)
Constant current circuit (29) connected to the common emitters of 7) and (28), and diode-connected transistor (30) and transistor connected to the collectors of the fifth and sixth transistors (27) and (28) A second current mirror circuit ( 32 ) composed of (31), a second capacitor (33) connected between the output terminal A and the base of the fifth transistor (27), the fifth and the fifth capacitors (33). The second resistor (34) is connected between the bases of the six transistors (27) and (28).

Now, in the variable reactance circuit ( 12 ), when the predetermined control voltage is applied to the base of the first current source transistor (18) and the control voltage is not applied to the base of the second current source transistor (21), the output A positive equivalent reactance is obtained when viewed from the end A. Then, since the collector current of the first current source transistor (18) changes according to the magnitude of the control voltage applied to the base of the first current source transistor (18), an equivalent capacitive reactance from zero to a predetermined positive value is generated. Further, when the control voltage is not applied to the base of the first current source transistor (18) and the predetermined control voltage is applied to the base of the second current source transistor (21), when viewed from the output terminal A, A negative equivalent capacitive reactance is obtained. And
Since the collector current of the second current source transistor (21) changes according to the magnitude of the control voltage applied to the base of the second current source transistor (21), an equivalent capacitive reactance from zero to a predetermined negative value is generated. That is, the capacity of the first capacitor (22)
C ₁ , the resistance value of the first resistor (23) is R ₁ , the transconductance of each of the first and second transistors (16) and (17) and the third and fourth transistors (19) and (20) is
If gm, the equivalent capacitive reactance of the first reactance circuit ( 12 ) viewed from the output terminal A is ± gmR ₁ C ₁ . Since the mutual conductance gm changes according to the collector currents of the first and second current source transistors (18) and (21), the equivalent capacitive reactance becomes variable.

Further, in the negative reactance circuit ( 13 ), a constant current flows through the constant current circuit (29), so that a negative equivalent capacitive reactance is generated. That is, the capacity of the second capacitor (33) is C ₂ , the resistance value of the second resistor (34) is R ₂ , and the fifth and sixth
Transconductance of transistors (27) and (28)
If gm, the negative reactance circuit ( 1
The equivalent capacitive reactance of 3 ) is −gmR ₂ C ₂ .

Therefore, the first and second reactance circuits ( 12 ) and ( 1
3 ) is connected in parallel to the vibrating element (15), and as a result, the equivalent capacitive reactance is C ₀ −gmR ₂ C ₂ ± gmR ₁ C ₁ (2) (where C ₀ is the vibrating element (15) Equivalent parallel capacitance of), and by applying a control voltage to the bases of the first and second current source transistors (18) and (21),
By changing the third term of the equation (2), the oscillation frequency of the oscillator circuit ( 14 ) can be changed. Here, since the negative reactance circuit ( 13 ) acts as a negative capacitive reactance,
If the variable reactance circuit ( 12 ) is controlled in a state where the equivalent parallel contact capacitance (C ₀ −R ₂ C ₂ gm) of the vibrating element ( 15 ) is small, a small reactance capacitance can be obtained without increasing the variable width. Wide range control is possible.

Next, the change width of the equivalent capacitance is compared.

In conventional circuits, Therefore, the equivalent capacitance change of the vibrating element becomes the second term of the equation (3).

On the other hand, in the case of the example Therefore, the change in equivalent capacitance of the vibrating element becomes the second term of the equation (4). Taking the ratio of the amounts of change in equation (3) and equation (4), And the second term of the denominator of the equation (5) becomes the increment of the change amount.

_{Now, C 0 = 300PF, gm =} 1/50, R 2 = 1000Ω, when the C ₂ = 10 pF, the second term of the denominator of the equation (4), Therefore, the equation (5) becomes 1 / (1-0.66) ≈3,
A variable amount that is three times that of the conventional method can be obtained.

FIG. 3 is a diagram comparing the capture range characteristics of the conventional circuit and the circuit of the embodiment of the present invention. In FIG. 3, the horizontal axis represents the frequency shift and the vertical axis represents the pilot voltage. ) Is wider than the conventional frequency range (dotted line) and extends to a place where the pilot voltage is low.

The current flowing through the constant current circuit (29) of the negative reactance circuit ( 13 ) may be selected so that the equivalent parallel capacitance of the vibrating element (15) can be reduced.

(G) Effect of the Invention As described above, according to the present invention, frequency control is performed in a state where the equivalent capacitive reactance of the vibration element is reduced by the negative reactance circuit, so that the control range can be expanded, and the PLL
The capture range of the circuit can be expanded, and the vibration element having a large initial frequency tolerance can be used. In addition, the lock range can be expanded, and even if the pilot level decreases, the lock does not come off.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional frequency control circuit, and FIG. 3 is a comparison of capture range characteristics between a conventional circuit and an embodiment circuit of the present invention. FIG. ( 12 ) …… Variable reactance circuit, ( 13 ) …… Negative reactance circuit, ( 14 ) …… Oscillation circuit, (4) …… Vibration element.

Claims (1)

[Claims] 1. An oscillating circuit, an oscillating element that determines an oscillating frequency of the oscillating circuit, and a variable reactance circuit that is connected in parallel to the oscillating element and operates as a positive or negative equivalent reactance. In a frequency control circuit for obtaining an oscillation frequency according to an applied control signal, a negative reactance circuit that is connected in parallel to the vibrating element and that operates as a fixed biased negative equivalent reactance is provided. Control circuit.