JPH088453B2 - Frequency modulation circuit - Google Patents

Description

The present invention relates to a frequency modulation circuit capable of removing an unnecessary high frequency component regardless of a change in center frequency of a frequency modulation (FM) output.

[Conventional technology]

Conventionally, a multi-vibrator type frequency modulation circuit is known, but since the FM output of this frequency modulation circuit is usually a rectangular wave output, it contains many odd-order high frequency components. If such a frequency modulation circuit is used, for example, in a VTR FM audio system, the high-order harmonics contained in the FM output will enter the video system and cause a decrease in fidelity. For example, as shown in FIG. 6, a low pass filter (LP) is provided on the output side of the frequency modulator (MOD) 2.
F) 4 is installed, and a π-type filter is used for LPF4. With such a π-type filter,
It is difficult to build it in an IC, and LPF4
For example, the secondary active LPF shown in FIG. 7 is used for this.

In this secondary active LPF, resistors 8, 10 and a capacitor 12 are installed in the preceding stage of the operational amplifier 6, and a capacitor 14 is installed between the input side and the output side of the operational amplifier 6,
Configured as a circuit that can be built into the IC. R is the resistance value,
C and 2C represent capacitance values.

Then, in MOD2, as shown in FIG. 8A, the original FM output V _FM centering on the reference center frequency f ₀ is obtained, and the primary and secondary high frequency frequencies f ₁ and f ₂ are centered. The FM output of the higher harmonic component is obtained.

Further, in the LPF4, as shown in B of FIG. 8, since the filter characteristic having the cutoff frequency f _C corresponding to the center frequency f ₀ is set, as shown by the solid line in C of FIG. The components of the first and second harmonic frequencies f ₁ and f ₂ are
The attenuation gains G ₁ and G ₂ can be suppressed to a low level that does not cause inconvenience as compared with the component of the center frequency f ₀ . V
_FM0 represents the final FM output with harmonic components removed.

[Problems to be Solved by the Invention]

By the way, the cutoff frequency f _C of the LPF 4 is set to a frequency sufficiently separated from the center frequency f ₀ in consideration of variations in element characteristics. Therefore, it is difficult to obtain a sufficient attenuation ratio for the harmonic component, and moreover, if the cutoff frequency f _C changes due to variations in element characteristics,
The attenuation ratio with respect to the harmonic component changes, and the harmonic component cannot be attenuated sufficiently.

When the cutoff frequency f _C is fixed,
Systems with different center frequencies f ₀ , such as PAL and SECAM,
Alternatively, in stereo frequency modulation with different center frequencies f _{0 in} the left and right channels, the optimization of the cutoff frequency f _{C with} respect to the center frequency f ₀ is broken, and the removal of harmonic components is eliminated by the center frequency f _0.
Could be insufficient. For example, the fifth
When the frequency is changed to f ₀ ′ and f ₀ ″ shown in the figure C, the harmonic frequencies f ₁ and f ₂ are also changed to f ₁ ′, f ₁ ″, f ₂ ′ and f ₂ ″. The relationship between f ₀ and the cutoff frequency f _C is broken, and the attenuation gains G ₁ and G ₂ also change like G ₁ ′, G ₁ ″, G ₂ ′ and G ₂ ″, and at the attenuation gain G ₁ ′, This means that the component of the harmonic frequency f ₁ cannot be sufficiently removed.

Therefore, the present invention allows the center frequency f ₀ and the cutoff frequency f _C to have a certain correlation,
It sets the optimal cut-off frequency f _C corresponds to f _0, it is an object to realize a frequency modulation circuit which enables independent removal of unwanted harmonic components in any of the center frequency f _0.

[Means for solving the problem]

As illustrated in FIG. 1, the frequency modulation circuit of the present invention is a modulation input current I _0i (= I ₀ + i) obtained by superimposing an input signal current (signal current i) on a reference constant current (I ₀ ). ), A current mirror circuit (transistors 158, 21, 41) that receives the modulated input current obtained by the input control unit and distributes it as a plurality of output currents, and a current mirror circuit of this current mirror circuit. A modulator (MOD20) that receives the modulation input current through an output and generates a frequency modulation output having a center frequency determined by the constant current contained in the modulation input current and a frequency change corresponding to an amplitude change of the input signal current. ,
The modulation input current is received through another output of the current mirror circuit, and the frequency modulation has a center frequency determined by the constant current included in the modulation input current and a frequency change according to an amplitude change of the input signal current. An analog variable filter (LPF40, bandpass filter, high-pass filter) with a cutoff frequency that removes harmonic components of the output is provided to change the frequency modulation output and the cutoff frequency proportionally. It is characterized by having done.

[Action]

The input control unit 15 receives the input signal at the reference current (constant current I ₀ ).
Modulated input current composed of a reference current (constant current I ₀ ) and a signal current i by superimposing a signal current i obtained by converting V _i into a current.
Generate I _oi .

The modulator (MOD20) has a frequency composed of a center frequency f ₀ corresponding to the reference current (constant current I ₀ ) in the obtained modulation input current I _oi and a frequency change corresponding to the amplitude change of the signal current i. The modulation output V _FM is generated, and the filter (LPF40) controls the cutoff frequency f _C by the modulation input current I _oi ,
_The cutoff frequency f _C set by _oi removes unnecessary frequency components from the frequency modulation output V _FM .

Therefore, the center frequency f ₀ of the modulator (MOD20) and the cutoff frequency f _C of the filter (LPF40) are the modulation input current I
_The cut-off frequency f _C corresponding to the change of the center frequency f ₀ is optimized with a certain correlation through _oi , and the unnecessary harmonic components are removed regardless of the change of the center frequency f ₀ .

〔Example〕

FIG. 1 shows an embodiment of the frequency modulation circuit of the present invention.

The input control unit 15 includes a reference input terminal 151 and a signal input terminal 152. The reference input terminal 151 has a constant current I ₀ as a reference current, and the signal input terminal 152 has an input signal V from the signal source 16 such as an audio signal. _i is added. The constant current I ₀ flows through the resistor 153, the input signal V _i is converted into the signal current i through the capacitor 154 and the resistor 155, and the constant current I ₀ is converted to the signal current i _0.
Is superposed through the total feedback amplifier composed of the operational amplifier 156 and the transistor 157, and the modulated input current I _oi (= I
₀ + i) is obtained. In this case, the operational amplifier 156 and the transistor 157 constitute a conversion means for adding the constant current I ₀ and the signal current i to obtain the modulation input current I _oi .

The modulation input current I _oi thus obtained is used for the frequency modulator (MOD) 20 and the low-pass filter (LPF) 40, and the transistor 21 forming a current mirror circuit with a transistor 158 having a common base and collector. , Through 41,
The MOD20 is the modulation input, and the LPF40 is the control input for the cutoff frequency f _C.

The MOD 20 is constituted by, for example, a controlled oscillator controlled by a current as a control input, and when the modulation input current I _oi is applied, the center frequency f corresponding to the constant current I ₀ is
FM with ₀ and frequency shift corresponding to signal current i
The output V _FM is obtained. Capacitor added to this MOD20
In 22 is set a capacity necessary to obtain the center frequency f ₀ .

The LPF 40 is composed of an active low-pass filter and the like, and has a controllable cutoff characteristic for removing the components of the high frequency frequencies f ₁ and f ₂ included in the FM output V _{FM of the} MOD 20. The cutoff frequency f _C is set as a control input according to the modulation input current I _oi .

In this frequency modulation circuit, when the constant current I ₀ in the modulation input current I _oi changes to I ₀ ′ or I ₀ ″, the FM output V _{FM of the} MOD 20 changes, for example, as shown in A of FIG. ,
The center frequency f ₀ changes to f ₀ ′ or f ₀ ″, and the harmonic frequency f ₁ changes to f ₁ ′ or f ₁ ″ and the harmonic frequency f ₂ changes to f ₂ ′ or f ₂ ″ in a fixed proportional relationship. To do.

Further, since the modulation input current I _oi is added to the LPF 40 as a characteristic control input, the modulation input current I _oi is I _oi ′ or
When it changes to I _oi ″, a cutoff characteristic corresponding to the change is obtained, and as shown in B of FIG.
f _C changes to f _C ′ or f _C ″. Therefore, the LPF 40 has a cutoff frequency f _C corresponding to the fluctuation of the center frequency f _0.
Is set, and constant attenuation gains G ₁ and G ₂ are set corresponding to the harmonic frequencies f ₁ and f ₂ . Therefore, as shown in C of FIG. ₂ , as with the attenuation gains G ₁ and G ₂ of the harmonic frequencies f ₁ and f ₂ due to the cutoff frequency f _C , the changed harmonic frequencies f ₁ ′ and f ₁ ″ , F ₂ ′, f ₂ ″ have damping gain G ₁ ′,
G ₁ ″, G ₂ ′, G ₂ ″ are set, and harmonic frequencies f ₁ , f ₂ ,
The FM output V _FM0 from which the components of f ₁ ′, f ₁ ″, f ₂ ′ and f ₂ ″ have been reliably removed at the same level is taken out from the output terminal 50.

Next, FIG. 3 shows a specific circuit configuration example of the frequency modulation circuit of the present invention.

The modulation input current I _oi generated by the input controller 15 is applied to the MOD 20 through the transistor 158 and the transistor 158
Is drawn from the emitter side of the transistors 201 and 202 through the transistor 211 forming the current mirror circuit. Since the bases of the transistors 201 and 203 and the transistors 202 and 204 are commonly connected, a base current corresponding to the emitter current drawn by the transistor 211 flows through the transistors 201 and 202. Transistors 203, 204, resistors
205 and 206 and the constant current source 207 form a differential amplifier, and the transistors 201 and 203 or the transistors 208 and 209 added to the transistors 202 and 204 form a feedback circuit, and between the emitters of the transistors 208 and 209. Center frequency f ₀
A capacitor 22 for setting is set.

The resistors 213 and 214 form a voltage dividing circuit, and the reference voltage ΔV is formed on the resistor 213 side. This reference voltage ΔV is applied to the base of the transistor 215 forming the voltage correction circuit, and the modulation input current I _oi is drawn from the emitter side of the transistor 215 by the transistor 158 and the transistor 212 forming the current mirror circuit. Therefore, the reference voltage ΔV corrected by the modulation input current I _oi is obtained on the emitter side of the transistor 215. The outputs of both terminals of the capacitor 22 together with the reference voltage ΔV are applied to the comparators 216 and 217 for comparison. The comparison output is added to the set input S and the reset input R of the SR-flip-flop circuit (SR-FF) 218, and the non-inverted output Q and the inverted output obtained by the set input S and the reset input R are obtained. . The non-inverted output Q and the inverted output are fed back to the base sides of the transistors 203 and 204 and are added to the bases of the transistors 219 and 220 of the output buffer amplifier. An operating current flows through the transistors 219 and 220 by the constant current source 221, and the FM output shown in FIG. 4 is passed through the diode 222 and the resistor 223 installed on the collector side of the transistors 219 and 220.
V _FM is obtained.

Here, if the capacitance of the capacitor 22 is C ₀ , the constant current is
With I ₀ and the reference voltage ΔV, the center frequency f ₀ is And has a direct proportional relationship with the constant current I ₀ as the reference current. However, T is the period of the FM output V _FM , ΔV is the amplitude voltage, and 2ΔV is the peak-to-peak voltage.

Next, the LPF 40 is constituted by two variable resistance circuits 42 and 44 and capacitors 46 and 48, and the FM output V _FM is added to the base of the transistor 421 in the previous variable resistance circuit 42. In the transistor 421, a constant current is drawn by the constant current source 422 installed on the emitter side.
A current corresponding to the M output V _FM is extracted from the emitter. Transistors 423, 424, resistors 425, 426 and constant current source 247
Are transistors 428 and 429, a diode 430 and a resistor 431 that form a differential amplifier and form a current mirror circuit as a load.
Is installed. On the base side of the transistor 424,
For feedback input, a transistor 432 and a constant current source 433 are installed corresponding to the transistor 421 and the constant current source 422. The collector outputs of the transistors 423 and 424 are added to the bases of the transistors 434 and 435 installed in the next stage. Transistors 436 and 437 forming a current mirror circuit are installed as loads on the collector side of the transistors 434 and 435 to form a differential amplifier. Each transistor
The emitters of 434 and 435 are provided with a transistor 411 that forms a current mirror circuit with the transistor 158, and its operating current is set by the modulation input current I _oi .

The output of the variable resistance circuit 42 on the front side is the transistor 435.
Is taken out from the collector side of and is fed back to the base of the transistor 432, and at the same time, the variable resistance circuit 44 on the subsequent stage side.
Is added to the base of transistor 441 at. The transistor 441 includes a constant current source 422 installed on the emitter side.
Since a constant current is drawn by, the current corresponding to the variable resistance output on the preceding stage side is taken out from the emitter side. Transistors 443, 444, resistors 445, 446 and constant current source 447 constitute a differential amplifier, and transistors 448, 449 forming a current mirror circuit as a load, diode 450 and resistor 451 are installed. For feedback input, a transistor 452 and a constant current source 453 are installed on the base side of the transistor 444 corresponding to the transistor 441 and the constant current source 442. The collector outputs of the transistors 443 and 444 are added to the bases of the transistors 454 and 455 installed in the next stage. Transistors 456 and 457 forming a current mirror circuit are installed as loads on the collector side of the transistors 454 and 455 to form a differential amplifier. Each transistor 45
Transistors 412 that form a current mirror circuit with the transistor 158 are installed on the emitter side of 4, 455, and the operating current thereof is set by the modulation input current I _oi .

Then, the output of the variable resistance circuit 44 on the latter stage side is taken out from the collector side of the transistor 455 and fed back to the base of the transistor 452, and at the same time, the transistors 461, 4
Added to the base of 62. The transistors 461 and 462 and the constant current sources 463 and 464 are installed as an output buffer circuit, and the emitter of the transistor 461 and the transistor 441.
A capacitor 46 is connected between the bases of the two, and a capacitor 48 is connected between the base of the transistor 461 and the ground. Each transistor 461, 462 is a constant current source installed on the emitter side.
Operating current flows by 463 and 464. The resistors 159, 2
24, 225, 458 and 459 are inserted in order to compensate the characteristics of the transistors 158, 211, 212, 411 and 412 which form the current mirror circuit.

As described above, the LPF 40 is configured by the two variable resistance circuits 42 and 44 and the capacitors 46 and 48, and since the variable resistance circuits 42 and 44 are controlled by the modulation input current I _oi drawn through the transistors 411 and 412, The modulation input current I _{oi is} applied to the capacitors 46 and 48 composed of fixed elements.
A resistance value corresponding to is obtained equivalently.

FIG. 5 shows an equivalent circuit of this LPF40. This equivalent circuit is composed of buffer circuits 401, 402, 403, resistors 404, 405 and capacitors 46, 48, and the input voltage of terminal 43 is V ₁ ,
The input voltage of the buffer circuit 402 is V ₂ , the output voltage obtained from the terminal 50 through the buffer circuit 403 is V ₀ , each resistor 404, 405
If the resistance value of R is R and the capacitances of capacitors 46 and 48 are C ₁ and C ₂ , There satisfied, the equation _{(2), V 1 -V 2} = SC 2 R (V 2 -V 0) ...... (4) , and the equation (4) V _{1 from, V 1 = (1 + SC} 2 R) V ₂ −SC ₂ RV ₀ (5) Further, from the formula (3), V ₂ −V ₀ = SC ₁ RV ₀ …… (6), and from the formula (6), V ₂ becomes V ₂ = (1 + SC ₁ R) V ₀ …… (7) Become.

Therefore, from the equations (5) and (7), V ₁ is V ₁ = (1 + SC ₂ R) (1 + SC ₁ R) V ₀ −SC ₂ RV ₀ = (1 + SC ₁ R + S ² C ₁ C ₂ R ² ) V ₀ ... (8), and from equation (8), From equation (9), the cutoff angular frequency ω ₀ and the coefficient Q are Assuming that the Butterworth filter is C ₂ = 2C ₁ ≡C, the cutoff frequency f _C and the coefficient Q can be calculated from the equation (10).
Is Becomes

By the way, the added resistance value of the resistors 425 and 426 and the resistor 44
Assuming that the added resistance value of 5 and 446 is R _E and the constant current from the constant current sources 427 and 447 is I ₁ , the resistance value R is Substituting this into equation (12), the cutoff frequency f _C becomes It can be seen that, like the center frequency f ₀ , the cutoff frequency f _C also has a direct proportional relationship with the constant current I ₀ .

Thus has a constant relationship with respect to the center frequency f ₀ of MOD20 as mediating the cut-off frequency f _C is a constant current I ₀ of the LPF 40, as shown in FIG. 2, when the constant current I ₀ changes,
The center frequency f ₀ and the cutoff frequency f _C was relatively changed, the output terminal of the FM output V _FM0 unrelated harmonic component to a change in the center frequency f ₀ is removed with high accuracy at a constant level
You can take out from 50.

Although the LPF 40 has been described using the second-order active low-pass filter, the LPF 40 can be configured with a filter corresponding to the harmonic component to be removed.

Further, although the case where the LPF 40 is used as a filter has been described in the embodiment, the present invention can be applied to a case where a bandpass filter or a high-pass filter is used as necessary.

〔The invention's effect〕

As described above, according to the present invention, since the center frequency of the modulator and the cutoff frequency of the low-pass filter can have a certain correlation, an optimum cutoff frequency corresponding to any center frequency can be obtained. At the same time, an optimum cutoff frequency can be set for the center frequency, a sufficient attenuation ratio can be set, the center frequency fluctuates due to variations in element characteristics, etc., and harmonic components can be reliably removed at the same level. , It is also possible to adjust the frequency by using the relationship between the center frequency and the cutoff frequency. For example, stereo frequency modulation, PAL system, SECAM system, etc.
It is also possible to extract the frequency output corresponding to the system having different center frequencies.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a frequency modulation circuit of the present invention, FIG. 2 is a diagram showing operating characteristics of the frequency modulation circuit shown in FIG. 1, and FIG. 3 is a frequency modulation shown in FIG. FIG. 4 is a circuit diagram showing a concrete circuit configuration example of the circuit, FIG. 4 is a diagram showing an FM output of a modulator in the frequency modulation circuit shown in FIG. 3, and FIG. 5 is a diagram showing the frequency modulation circuit shown in FIG. FIG. 6 is a circuit diagram showing an equivalent circuit of a low-pass filter, FIG. 6 is a block diagram showing a conventional frequency modulation circuit, FIG. 7 is a circuit diagram showing an active low-pass filter used in a conventional frequency modulation circuit, and FIG. FIG. 7 is a diagram showing operating characteristics of the frequency modulation circuit shown in FIG. 6. 15 …… Input controller 20 …… Frequency modulator (modulator) 21, 41,158 …… Transistor (current mirror circuit) 40 …… Low pass filter (filter) V _i …… Input signal I _oi …… Modulation Input current (I ₀ + i) V _FM , V _FM0 …… Frequency modulation output

Claims (1)

[Claims] 1. An input control unit for generating a modulation input current by superimposing an input signal current on a reference constant current, and a modulation input current obtained by the input control unit, and receiving a plurality of output currents. A current mirror circuit to be distributed, and receives the modulation input current through the output of the current mirror circuit, and has a center frequency determined by the constant current included in the modulation input current and a frequency change corresponding to an amplitude change of the input signal current. A modulator that generates a frequency modulation output, receives the modulation input current through another output of the current mirror circuit, and changes the amplitude of the input signal current together with the center frequency determined by the constant current included in the modulation input current. An analog variable filter having a cutoff frequency for removing high frequency components of the frequency modulation output having a frequency change according to It comprises a filter, a frequency modulation circuit which is characterized in that so as to vary the cut-off frequency and the frequency modulation output relatively.