JPH082008B2 - Frequency modulation circuit - Google Patents

Description

The present invention relates to a frequency conversion circuit using a voltage controlled oscillator, which is applied to a PLL circuit, an FM transmitter and the like.

[Conventional technology]

FIG. 2 is a circuit diagram showing a conventional frequency modulation circuit of this type. In the figure, 1 is an LC oscillator, and an LC tuning circuit 2
And an emitter common differential pair 3. The common emitter differential pair 3 is a pair of common emitters connected to each other.
It consists of PNP transistors 4,5, the base of NPN transistor 4 is connected to the collector of NPN transistor 5, and the base of NPN transistor 5 is connected to the collector of NPN transistor 4, respectively.
The LC tuning circuit 2 is connected. The output terminal 6 of the LC oscillator 1 is connected to the collector of the NPN transistor 5, and the power supply voltage V is supplied to the collector of the NPN transistor 4.
_A power supply terminal 7 for _{supplying CC} is connected. Reference numeral 8 is a voltage-current converter, which is composed of a PNP transistor 9 and a resistor 10 whose one end is connected to its emitter. The other end of the resistor 10 is connected to a power supply terminal 7, and the base of the PNP transistor 9 is a modulation signal. Is connected to the input terminal 11 for inputting. 12 is
It is a current relay for flowing a current equal to the collector current of the PNP transistor 9 to the common emitter of the common emitter differential pair 36, and includes a diode 13 and an NPN transistor 14 forming a current mirror with the diode 13. Diode 13
Is connected to the collector of the PNP transistor 9 and the base of the NPN transistor 12, and the cathode is connected to the ground terminal 15. The collector of the NPN transistor 14 is connected to the common emitter of the common emitter differential pair 3 and the emitter is connected to the ground terminal.
15 connected to each. Reference numeral 16 is a bias circuit for applying a bias to the voltage-current converter 8 and is composed of a bias power supply 17 and a resistor 18 having one end connected to the positive electrode side, and the other end of the resistor 18 is connected to the base of the PNP transistor 9. The negative electrode of the bias power supply 17 is connected to the ground terminal 15.

Next, the operation of the above frequency modulation circuit will be described. Let I be the output current of the voltage-current converter 8 when the modulation signal is input to the input terminal 11, However, I ₀ : Output DC current when there is no modulation signal I _m : Modulation current according to the modulation signal V _CC : Power supply voltage provided from power supply terminal 7 V ₁ : Voltage of bias power supply 17 V _BE : Base of PNP transistor 9 Emitter voltage V _m : Modulation signal level R ₁ : Resistor 10 resistance value. The term corresponding to the modulation current I _m is −V _m / R ₁ because the output current I of the voltage-current converter 8 decreases as the modulation signal level V _m increases. Then, by the action of the current relay 12, the voltage-current converter 8
Output current I = I ₀ + I _m is equal to NPN transistor 1
It is supplied to the 4th side, that is, the common emitter of the common emitter differential pair 3.

On the other hand, the output signal frequency f _osc extracted from the output terminal 6 of the LC oscillator 1 is However, L: inductance value of tuning circuit 2 C: capacitance value of tuning circuit 2 C ': equivalent input capacitance value of common emitter differential pair 3. By the way, the equivalent input capacitance value C ′ of the common emitter differential pair 3 is equal to the current I ₀ supplied to the common emitter differential pair 3.
It changes according to + I _m , and is given by C ′ = f (I ₀ + I _m ) ... (3). Therefore, when the modulation current I _m changes according to the change of the modulation signal level V _m input to the input terminal 11, the equivalent input capacitance value C ′ changes, and according to the change, it is given by the equation (2). The oscillation frequency f _osc will be modulated. That is, the oscillation output is taken out from the output terminal 6 after being subjected to frequency modulation according to the modulation signal level V _m . The center frequency of the oscillation output is determined by the direct current component I _{0 of the} current I ₀ + I _m supplied to the common emitter differential pair 3.

[Problems to be Solved by the Invention] Since the conventional frequency modulation circuit is configured as described above, the power supply voltage V _CC given from the power supply terminal 7 and the voltage V _{1 of the} bias power supply 17 may fluctuate, and the diffusion resistance If the resistance value R ₁ of the resistor 10 formed by the product varies among individual products, or if the resistance value R ₁ of the resistor 10 or the base-emitter voltage V _{BE of the} transistor 9 fluctuates due to ambient temperature changes, direct current The component I ₀ fluctuates and the center frequency of the oscillation output fluctuates. Therefore, due to this, for example, PL
In the L circuit, the free-run frequency fluctuates to cause lock range asymmetry, and in the FM transmitter, the reception frequency shifts.

The present invention has been made to solve the above problems, and is stable regardless of fluctuations in the power supply voltage or the voltage of the bias power supply, variations in the resistance value of the voltage-current converter, changes in the ambient temperature, and the like. An object is to obtain a frequency modulation circuit that can obtain an oscillation output having a center frequency.

[Means for solving the problem]

A frequency modulation circuit according to the present invention, an LC oscillator composed of a common emitter differential pair and an LC tuning circuit, a constant current source for generating a constant DC current that determines the center frequency of the oscillation output of the LC oscillator, , A first voltage-current converter that receives as input signals a modulation signal and a DC bias that biases the modulation signal, and has the same characteristics as the first voltage-current converter and receives only DC bias as an input signal Second
Voltage-current converter, a bias circuit for applying a common DC bias to the first and second voltage-current converters as the both DC biases, an output current of the first voltage-current converter, and A differential current generating circuit that generates a differential current from the output current of the second voltage-current converter, and superimposes the differential current from the differential current generating circuit on the constant DC current from the constant current source. What is done is supplied to the said differential pair.

[Action]

In the present invention, the differential current generation circuit uses the difference between the output current of the first voltage-current converter and the output current of the second voltage-current converter, that is, as the input signal to the first voltage-current converter. A current (that is, only the modulation current component) corresponding to the change in the voltage level of the applied modulation signal is generated as the differential current. On the other hand, the constant current source generates a constant DC current that determines the center frequency of the oscillation output of the LC oscillator. And, the one obtained by superimposing the differential current on the direct current is LC
It is supplied to the common emitter differential pair of the oscillator. This
The LC oscillator oscillates at a frequency centered on the frequency determined by the DC current and at a frequency according to the differential current, and frequency modulation is performed.

〔Example〕

FIG. 1 is a circuit diagram showing a frequency modulation circuit according to the present invention, and the structure of the LC oscillator 1 is the same as the conventional circuit described above. Reference numeral 19 is a first voltage-current converter, which is composed of a PNP transistor 20 and a resistor 21 whose one end is connected to its emitter, and the other end of which is connected to a power supply terminal 7 which supplies a power supply voltage V _CC. The base of the PNP transistor 20 is connected to the input terminal 11 for inputting the modulation signal. Reference numeral 22 is a second voltage-current converter, which is composed of a PNP transistor 23 and a resistor 24 whose one end is connected to its emitter.
The other end of is connected to the power supply terminal 7, and the collector of the PNP transistor 23 is connected to the common emitter of the emitter common differential pair 3 of the LC oscillator 1. As the PNP transistor 23 and the resistor 24 of the second voltage-current converter 22, those having the same characteristics and the same resistance value as those of the PNP transistor 20 and the resistor 21 of the first voltage-current converter 19 are used. ing.

_{Reference numeral} 25 is a current relay for supplying a current equal to the output current I _m1 of the first voltage-current converter 19 to the connection point N ₁ between the collector of the PNP transistor 23 and the common emitter of the common emitter differential pair 3. It is composed of a diode 26 and an NPN transistor 27 that forms a current mirror with the diode 26. The anode of the diode 26 is the collector of the PNP transistor 20 and the NPN transistor 27.
At the connection point with the base of the PN transistor 27, the cathode is connected to the ground terminal 15, and the NPN transistor 27
Has a collector at the above-mentioned connection point N ₁ and an emitter at the ground terminal
15 are connected to each other. Therefore, the current relay 25 emits a difference current I _m1 −I _m2 obtained by subtracting the output current I _m2 of the second voltage-current converter 22 from the output current I _{m1 of} the first voltage-current converter 19. It functions as a differential current supply circuit that supplies the common emitter of the common differential pair 3.

28 is a constant current circuit, one end of which is connected to the power supply terminal 7, a current source 29, the other end of which is connected to the anode 30 of the diode 30 It consists of NPN transistors 31 and 32.
The bases of the NPN transistors 31 and 32 are connected to the anode of the diode 30, the cathode of the diode 30 and the emitters of the NPN transistors 31 and 32 are connected to the ground terminal 15, and the collector of the NPN transistor 32 is a common emitter differential pair 3. Connected to the common emitter of.

33 is each PNP of the first and second voltage-current converters 19, 22
A bias circuit for applying a common bias to the bases of the transistors 20 and 23. One end is connected to the power supply terminal 7 and the other end is connected.
A resistor 34 connected to the collector of the NPN transistor 31,
A resistor whose base is connected to the resistor 34 and the NPN transistor 31 and whose collector is connected to the power supply terminal 7 and whose one end is connected to the emitter of the NPN transistor 35 and whose other end is connected to the ground terminal 15. 36, one end is NP
At the connection point between the N-transistor 35 and the resistor 36, the other end is each PNP transistor of the first and second voltage-current converters 19, 22.
It is composed of resistors 37 and 38 connected to the bases of 20 and 23, respectively.

Next, the operation of the frequency modulation circuit according to this embodiment will be described. In the constant current circuit 28, the constant current I ₀ flows through the NPN transistors 31 and 32 due to the current supplied from the current source 29. Further, in the bias circuit 33, a constant voltage I ₀ flowing through the resistor 34 applies a constant voltage to the base of the NPN transistor 35, whereby the NPN transistor 35 becomes conductive, and the voltage V _B generated at the resistor 36 at this time is biased. Are given to the bases of PNP transistors 20 and 23 via resistors 37 and 38, respectively. That is, at this time the bias voltage
V _B is V _B = V _CC −I ₀ × R ₃ −V _BE (4) However, V _CC : Power supply voltage given from power supply terminal 7 V _BE : Base-emitter voltage of NPN transistor 35 R ₃ : Resistor 34 It becomes the resistance value of.

On the other hand, when the modulated signal is input to the input terminal 11, the first
The output current I _m1 of the voltage-current converter 19 is the same as the expression (1) in the conventional circuit, However, V _BE1 : Base-emitter voltage of PNP transistor 20 R ₁ : Resistor 21 resistance value. In addition, the output current I of the second voltage-current converter 22
_m2 is However, V _BE2 : base-emitter voltage of PNP transistor 23 R ₂ : resistance of resistor 24

In the current relay 25, a current equal to the current I _m1 flowing in the diode 26 flows as the collector current of the NPN transistor 27, so that the current flowing from the common emitter of the common emitter differential pair 3 toward the connection point N ₁ is I _m . Then, the relationship of I _m = I _m1 −I _m2 (7) holds. By the way, as described above, the PNP transistor 2 of the first and second voltage-current converters 19 and 22 is used.
Since 0 and 23 and resistors 21 and 24 have the same characteristics and the same resistance value, respectively, the relationship of V _BE1 = V _BE2 (8) R ₁ = R ₂ (9) is established. Therefore, from equations (5) to (9) The relationship is established. That is, the first and second voltages −
A current converter 19 and 22, current I _m which is supplied by a current relay 25 to the common emitter of the common-emitter differential pair 3 consists only current component corresponding to the voltage level V _m of the modulation signal. Further, since the constant current I ₀ flows as a collector current in the NPN transistor 32 of the constant current circuit 28, from this and the formula (10), the common emitter of the common emitter pair 3 of the LC oscillator 1 is connected to the common emitter. Will be supplied. In the equation (11), the first term is a current component when the modulation signal is not input, that is, the direct current component, and the second term -V _m / R ₁ corresponds to the modulation current component. Then, the oscillation output having the frequency f _osc of the equation (2) shown in the case of the conventional circuit is the output terminal 6 of the LC oscillator 1.
The center frequency is determined by the direct current component I ₀ . This direct current component I ₀ is the constant current circuit 28
Is determined by the current flowing through the current source 29 of the resistor 21, the resistance values R ₁ and R ₂ of the resistors 21 and 24 due to the fluctuation of the power supply voltage V _CC , the fluctuation of the bias voltage V _B , and the ambient temperature change,
The fluctuations of the base-emitter voltages V _BE1 and V _BE2 of 20, 23 are all canceled. Further, even if the absolute resistance values R ₁ and R ₂ of the resistors 21 and 24 due to diffusion resistance vary among individual products, the ratio of the resistance values R ₁ and R ₂ for each product hardly varies. It does not appear as a change in center frequency.

〔The invention's effect〕

As described above, according to the present invention, the second voltage-current conversion that receives only the output current and the DC bias of the first voltage-current converter that receives the modulation signal and the DC bias as the input signal. The differential current with the output current of the container is generated by the differential current generation circuit. This differential current corresponds to a current corresponding to only the change in the voltage level of the modulation signal (that is, only the modulation current component). On the other hand, a constant direct current that determines the center frequency of the oscillation output of the LC oscillator is generated by a constant current source. Then, by supplying the DC current superposed with the differential current to the common emitter pair of the LC oscillator, the LC oscillator oscillates at a frequency centered on the frequency determined by the DC current and at a frequency corresponding to the differential current. It is possible to do. In this way, of the current supplied to the common emitter differential pair of the LC oscillator, the DC current component that determines the center frequency of the oscillation output is configured to be supplied only from the constant current source. Thus, it is possible to obtain a frequency modulation circuit having a stable output center frequency that is not affected by variations in resistance value of the voltage-current converter and changes in ambient temperature.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a frequency modulation circuit according to the present invention, and FIG. 2 is a circuit diagram showing a conventional frequency modulation circuit. In the figure, 1 is an LC oscillator, 2 is an LC tuning circuit, 3 is a common emitter differential pair, 19 is a first voltage-current converter, 22 is a second voltage-current converter, 25 is a current relay, 28 is a constant current circuit and 33 is a bias circuit. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. An LC oscillator comprising an emitter common differential pair and an LC tuning circuit, a constant current source for generating a constant direct current for determining a center frequency of an oscillation output of the LC oscillator, and a modulation signal. A first voltage-current converter that receives, as an input signal, a DC bias that biases the modulation signal; and a second voltage that has the same characteristics as the first voltage-current converter and that receives only a DC bias as an input signal. -A current converter, and the first and second voltages as the both DC biases-
A bias circuit that applies a common DC bias to the current converter, and a differential current generation circuit that generates a differential current between the output current of the first voltage-current converter and the output current of the second voltage-current converter. A frequency modulation circuit comprising: a constant DC current from the constant current source and the differential current from the differential current generation circuit superposed on the constant DC current to the differential pair.