JPH01252011A - Frequency modulating circuit - Google Patents

Abstract

PURPOSE:To fetch a frequency output to correspond to a system, in which a central frequency is different, by generating a frequency modulating output to have a frequency change, which corresponds to the amplitude change of an input signal, with the arbitrary central frequency as a reference from a modulator and controlling the cut-off frequency of a filter according to the change of the central frequency in this modulator. CONSTITUTION:An input signal Vi to be frequency-modulated is inputted from a signal source 20, which is provided in a front step, to a frequency modulator 22 of a frequency modulating circuit and an FM output VFM to have frequency transition, which corresponds to the input signal Vi, is outputted with a specified central frequency fO as the reference. The central frequency fO is set by a capacitor 24 to be added to this modulator 22 and frequency deviation + or - f, which expresses the fluctuation of the central frequency fO in the modulator 22, is detected by a characteristic control part 28. The FM output VFM to be outputted from the modulator 22 is inputted to an LPF26 and a harmonic frequency component to be included in the FM output is cut off by a cut-off frequency fC from the characteristic control part 28. Then, a VFMO is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a frequency modulation circuit that makes it possible to remove harmonic components at a constant level regardless of changes in the center frequency of a frequency-variable iA (FM) output.

[Conventional technology]

Conventionally, multi-high break type frequency modulation circuits are known, but since the FM ratio output of this frequency modulation circuit is usually a rectangular wave output, it contains many odd-order harmonic components. If such a frequency modulation circuit is used, for example, in the FM audio system of a VTR, high-order harmonics included in the FM ratio output will enter the video system, causing a reduction in fidelity. For example, as shown in Figure 3,
A low pass filter (LPF) 4 is installed on the output side of the frequency modulator (MOD) 2, and a π-type filter is used for the LPF 4. If such a π-type filter is used, it is difficult to incorporate it into the tC, so for example, a secondary active LPF shown in FIG. 4 is used as the LPF 4 on the assumption that it will be integrated into an IC.

This secondary active LPF has a resistor 8, 10 and a capacitor 12 installed before the operational amplifier 6', and a capacitor 14 between the input side and the output side of the operational amplifier 6, and can be built into an IC. Constructed as a circuit. R represents a resistance value, and Cl2C represents a capacitance value.

In MOD2, as shown in A of FIG. 5, the reference center frequency f. The original FM ratio output F14 centered on is obtained, and the -th and second harmonic frequencies f,
, an FM ratio output of harmonic components centered on f2 can be obtained.

Further, in the LPF 4, as shown in B of FIG. 5, the center frequency f. Cutoff frequency f corresponding to f.

Since the filter characteristic is set as C in Fig. 5,
As shown by the solid line in , the −th and second harmonic frequencies r
+, fz components have a center frequency (key + rear frequency) [.

It can be kept to a low level that does not cause any inconvenience compared to the components of ■,. is the final F with harmonic components removed
Represents M ratio output.

[Problems to be Solved by the Invention] By the way, the cutoff frequency f of the LPF4. is the center frequency f in consideration of variations in element characteristics. The frequency is set sufficiently far from the For this reason, it is difficult to obtain a sufficient attenuation ratio for harmonic components, and if the cutoff frequency f changes due to variations in element characteristics, the attenuation ratio for harmonic components changes and the harmonic components are attenuated sufficiently. I can't.

Moreover, when the cutoff frequency fc is set to a fixed value,
Center frequency f for PAL system, SECAM system, etc. is different, or the center frequency f is different for the left and right channels.
. In the stereo frequency modulation of , the center frequency f. The optimization of the cutoff frequency fo for the center frequency f collapses, and the removal of harmonic components becomes impossible at the center frequency f. There was a risk that it would be insufficient.

For example, f shown in C of FIG. When changing to J, foH, the harmonic frequencies f+, fz also change to fl', f
l ′, f z ′, f z ” tonal (DT:,
Center frequency f. The relationship between and the cutoff frequency fc collapses, and the attenuation gain G, ,G2 also becomes G,',G,'',G2
', G2', and attenuation gain Gl',
This means that the component of harmonic frequency f cannot be removed sufficiently.

Therefore, the present invention provides a center frequency f. and cutoff frequency r. to have a certain correlation, and an arbitrary center frequency f. The object of the present invention is to realize a frequency modulation circuit that can set an optimal cutoff frequency f in accordance with the f, and can remove unnecessary harmonic components regardless of the deviation of the center frequency fo.

[Means to solve the problem]

As shown in FIG. 1, the frequency modulation circuit of the present invention has an arbitrarily set center frequency f. Frequency modulation (FM) with a frequency change according to the amplitude change of the input signal V, with reference to
A modulator (MOD22) that obtains an output VFM, and a cutoff frequency f that removes unnecessary frequency (rt, rt) components from the frequency modulated output VFM of this modulator (MOD22). is set, and the center frequency r. A filter (LPF26) whose cutoff frequency fc is controlled according to changes in
It is equipped with the following.

[For production]

The modulator (MOD 22) generates a frequency modulated output VFM having a frequency change according to an amplitude change of the input signal 2, with an arbitrary center frequency fo as a reference.

The center frequency f of this modulator (MOD22). By controlling the cutoff frequency fc of the filter (LPF26) by changing the value of , it becomes possible to have a correlation between the center frequency r0 and the cutoff frequency fc, and the center frequency [. Optimal cutoff frequency rc according to changes in
and always center frequency r. Optimization of the cutoff frequency fc corresponding to is attempted.

〔Example〕

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

The input signal (2) to be frequency modulated is sent to a frequency modulator (MOD) from a signal source 20 provided at the front stage of the frequency modulation circuit.
) is added to 22.

The MOD 22 is constructed with a controlled oscillator controlled by voltage or current, and has a specific center frequency f. An FM output VFM having a frequency shift according to the input signal 2 from the signal source 20 is obtained based on . A capacitor 24 added to this MOD 22 has a center frequency f. It has a capacity of CO for setting.

The low-pass filter (LPF) 26 has a cutoff characteristic that can remove components of harmonic frequencies f, , f, contained in the FM output of the MOD 22, using an active filter or the like, and has a cutoff frequency f, is configured to be controllable from the outside.

Then, the characteristic control unit 28 receives the center frequency f from the MOD 22. Frequency deviation ±Δf (=f, ~fo) representing the fluctuation of
and control output VC according to the frequency deviation ±Δf.
occurs. This control output■. Since the cutoff frequency f of the LPF 26 is controlled by
A correlation is obtained between 0 and the cutoff frequency r,
Always center frequency f. An optimal cutoff frequency re corresponding to the above is set.

In this frequency modulation circuit, for example, as shown in A in FIG. 2, the center frequency f varies due to variations in element characteristics in M and OD 22. is f0' or f. When the harmonic frequencies f+ and fz change to #, the harmonic frequencies f+ and fz also change to fl'
, fl 9, r2', f2''. Characteristic control unit 2
8, the frequency deviation (fluctuation width) of the center frequency f0 ±
Δf is detected, and the control output ■ has a level corresponding to its magnitude. is obtained.

This control output■. is the cutoff frequency fc for LPF26
The cutoff frequency fc is added as a control input, and corresponds to the fluctuation of the center frequency fo, as shown in Fig. 2B.
', fc#.

As a result, a cutoff frequency f corresponding to fluctuations in the center frequency f0 is set in the LPF 26, and constant attenuation gains Cz and Gz are set corresponding to harmonic frequencies f1 and f2. Therefore, as shown in FIG. 2C, the harmonic frequency f+ due to the cutoff frequency f, the attenuation gain G+ of fg, and the varied harmonic frequency fl' as well as Gz.
, fl'', f2', ft'' have attenuation gains G l ', c
, ″, G2 ′, Gz″ are set, and the harmonic frequency f
1, fg, fl', fl', f2'sf2# are reliably removed at the same level, and FM ratios F and 4o are taken out from the output terminal 30.

〔Effect of the invention〕

As explained above, according to the present invention, there is a certain correlation between the center frequency of the modulator and the cutoff frequency of the low-pass filter, so an optimal cutoff frequency corresponding to any center frequency can be obtained. At the same time, it is possible to set a sufficient attenuation ratio by setting the optimal cutoff frequency for the center frequency, and even if the center frequency varies due to variations in element characteristics, harmonic components can be reliably removed at the same level. It is also possible to perform frequency adjustment using the relationship between the center frequency and the cutoff frequency, and for example, it is also possible to extract frequency outputs corresponding to systems with different center frequencies, such as stereo frequency modulation, PAL system, SECAM system, etc. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the frequency modulation circuit of the present invention, FIG. 2 is a diagram showing the operating characteristics of the frequency modulation circuit shown in FIG. 1, and FIG. 3 is a block diagram showing a conventional frequency modulation circuit. 4 is a circuit diagram 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. 3. 20...Signal source 22...Frequency modulator (modulator) 26...Low pass filter (filter) 28...Characteristics control unit ■,...Input signal V8, VFMO...' F M Output □f □f j”7 Fig. 4-f c f □f Fig. 5

Claims (1)

[Claims] A modulator that obtains a frequency modulated output having a frequency change according to the amplitude change of an input signal based on an arbitrarily set center frequency, and a modulator that removes unnecessary frequency components from the frequency modulated output of this modulator. 1. A frequency modulation circuit comprising: a filter having a cutoff frequency set therein, and a filter having a cutoff frequency controlled according to a change in the center frequency.