JPH088454B2 - Frequency modulation circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a frequency modulation circuit capable of removing a harmonic component of a constant level 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, many odd harmonic components are included. When such a frequency modulation circuit is used in, for example, a VTR FM audio system, high-order harmonics contained in the FM output enter the video system and cause a decrease in fidelity. For example, as shown in FIG. 3, 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. 4 is used.

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 A of FIG. 5, the original FM output V _FM centered on the reference center frequency f ₀ is obtained, and the primary and secondary harmonic 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. 5, 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 center frequency (carrier frequency) depends on the attenuation gains G ₁ and G _2.
It can be suppressed to a low level that does not cause inconvenience as compared with the component of f ₀ . V _FM0 is the final FM with harmonic components removed
Represents the output.

[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. For this reason, it is difficult to obtain a sufficient attenuation ratio for the harmonic components, and when the cutoff frequency f _C changes due to variations in the element characteristics, the attenuation ratio for the harmonic components changes and the harmonic components are sufficiently attenuated. I can't let you do it.

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,
The purpose of the present invention is to realize a frequency modulation circuit capable of setting an optimum cutoff frequency f _C corresponding to f ₀ and removing unnecessary harmonic components irrespective of the deviation 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 configured with a controlled oscillator controlled by voltage or current, and is provided with a capacitor for setting a center frequency, so as to change the amplitude of an input signal. A modulator (MOD22) that obtains a frequency-modulated output with a corresponding frequency change,
An analog variable filter (26) in which a plurality of cutoff frequencies capable of removing harmonic frequency components contained in the frequency modulation output of this modulator are set and the cutoff frequencies are controlled according to changes in the center frequency And a characteristic control unit (28) for detecting a frequency deviation representing the fluctuation of the center frequency from the frequency modulation output of the modulator, and selectively switching the cutoff frequency of the analog variable filter according to the frequency deviation. It is characterized by having.

[Action]

The modulator (MOD22) generates a frequency modulation output V _FM having a frequency change corresponding to the amplitude change of the input signal V _i with reference to an arbitrary center frequency f ₀ . By controlling the cutoff frequency f _C of the filter (LPF26) by changing the center frequency f ₀ of this modulator (MOD22), it is possible to make the center frequency f ₀ and the cutoff frequency f _C have a correlation. become,
Optimal cutoff frequency f _C according to changes in center frequency f ₀
The cutoff frequency f _C corresponding to the center frequency f ₀ is constantly optimized.

〔Example〕

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

The input signal V _i to be frequency-modulated is generated by a frequency modulator (MOD) 22 from a signal source 20 provided on the upstream side of the frequency modulation circuit.
Is added to

The MOD22 is composed of a controlled oscillator controlled by voltage or current and has a frequency shift according to an input signal V _i from the signal source 20 with a specific center frequency f ₀ as a reference.
FM output V _FM is obtained. The capacitor 24 added to this MOD 22 has a capacitance C ₀ for setting the center frequency f ₀ .

The low-pass filter (LPF) 26 has a cutoff characteristic that can remove the components of the harmonic frequencies f ₁ and f ₂ contained in the FM output V _{FM of the} MOD 22, using an active filter or the like. It is configured so that _C can be controlled externally.

Then, the characteristic control unit 28 detects the frequency deviation ± Δf (= f _{m to} f ₀ ) representing the fluctuation of the center frequency f ₀ from the MOD 22, and generates the control output V _C according to the frequency deviation ± Δf. This control output V _C controls the cut-off frequency f _C of the LPF 26, so that a correlation is obtained between the center frequency f ₀ and the cut-off frequency f _C, and the optimum cut that always corresponds to the center frequency f ₀ is obtained. The off frequency f _C is set.

In this frequency modulation circuit, for example, in FIG.
As shown in, when the center frequency f ₀ changes to f ₀ ′ or f ₀ ″ due to variations in element characteristics in the MOD22,
The harmonic frequencies f ₁ and f ₂ similarly change to f ₁ ′, f ₁ ″, f ₂ ′, and f ₂ ″. In the characteristic control unit 28, the frequency deviation (variation width) ± Δf of the center frequency f ₀ is detected, and the control output V _C having a level according to its magnitude is obtained.

This control output V _C is applied to the LPF 26 as a control input of the cutoff frequency f _C , and corresponds to the fluctuation of the center frequency f ₀ , and as shown in B of FIG. 2, the cutoff frequencies f _C ′ and f _C ″. Controlled by.

As a result, the LPF 26 is set with the cutoff frequency f _C corresponding to the fluctuation of the center frequency f ₀ , and the harmonic frequencies f ₁ and f ₂
Corresponding to, constant attenuation gains G ₁ and G ₂ are set. 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 ″ attenuation gain
G ₁ ′, G ₁ ″, G ₂ ′, G ₂ ″ are set, and the harmonic frequency f ₁ ,
The FM output V _FM0 from which the components of f ₂ , f ₁ ′, f ₁ ″, f ₂ ′ and f ₂ ″ have been reliably removed at the same level is taken out from the output terminal 30.

〔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. The frequency can be adjusted by using the relationship between the center frequency and the cutoff frequency. For example, the frequency output corresponding to the system with different center frequencies such as threo frequency modulation, PAL system, SECAM system, etc. can also be extracted. You can

[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 block showing a conventional frequency modulation circuit. 4 and 5 are circuit diagrams showing an active low-pass filter used in a conventional frequency modulation circuit, and FIG. 5 is a diagram showing operating characteristics of the frequency modulation circuit shown in FIG. 20 …… Signal source 22 …… Frequency modulator (modulator) 26 …… Low-pass filter (filter) 28 …… Characteristic control block V _i …… Input signal V _FM , V _FM0 … FM output

Claims (1)

[Claims] 1. A modulation which comprises a controlled oscillator controlled by a voltage or a current and which is provided with a capacitor for setting a center frequency to obtain a frequency modulation output having a frequency change according to an amplitude change of an input signal. And a variable analog filter in which a plurality of cutoff frequencies capable of removing harmonic frequency components included in the frequency modulation output of the modulator are set and the cutoff frequency is controlled according to the change of the center frequency. And a characteristic control unit that detects a frequency deviation representing a variation of a center frequency from the frequency modulation output of the modulator, and selectively switches the cutoff frequency of the analog variable filter according to the frequency deviation. A frequency modulation circuit characterized in that