JPS61131902A - Frequency modulator - Google Patents

Abstract

PURPOSE:To output a signal to be frequency-modulated by providing a frequency divider dividing the frequency of an output of a variable oscillator to which an analog input signal is applied and a switched capacitor filter to which an output of an output of the variable oscillator is fed as a clock signal and to which a frequency-division output is fed. CONSTITUTION:A digital input signal is fed to a voltage controlled variable oscillator 3 from a terminal 1 via a D/A converter 2. The oscillator 3 outputs a rectangular wave signal in response to a level of an analog input signal to a frequency divider 4. The rectangular wave signal from the frequency divider 4 is fed to a switched capacitor filter 5 and an output from the oscillator 3 is applied as a clock signal. Thus, a signal to be frequency-modulated is obtained at an output of the filter 5, the operation is stable for a long time operation and a frequency modulator in which the possibility of the mixture of the ternary harmonic is precluded is obtained.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to frequency modulators.

[Conventional technology]

Conventional frequency modulators include those using variable reactance oscillators.

[Problem that the invention seeks to solve]

Such a conventional frequency modulator using a variable reactance oscillator has the disadvantage that it has poor stability during long-term operation, and that the third-order high-order wave of the modulated signal is mixed into the frequency-modulated signal.

The present invention seeks to propose a frequency modulator free of such drawbacks.

[Means for solving problems]

The frequency modulator according to the present invention comprises a variable oscillator (3) to which an analog input signal is supplied and whose frequency is controlled, a frequency divider (4) to which the output of the variable oscillator (3) is supplied, and a variable oscillator (4) to which the output of the variable oscillator (3) is supplied. (3) is supplied as a clock signal, and the output of the frequency divider (4) is supplied and filtered. A frequency modulated signal can be obtained from a filter (5).

[Effect]

According to the present invention, the cutoff frequency of the output capacitor filter of the variable oscillator whose frequency is controlled by being supplied with an analog input signal is determined by the output of the variable oscillator which is supplied as a clock signal.

〔Example〕

(2) A digital input signal (for example, a still image signal) is supplied to the converter from an input terminal and is subjected to D/A conversion. (This is a three-way voltage controlled variable oscillator, to which an analog input signal from the D/A converter (2) is supplied, and the oscillation frequency is controlled according to the level of its positive half cycle.

A variable oscillator (3) outputs a rectangular wave signal with a frequency that corresponds to the level of the analog input signal. This rectangular wave signal is supplied to a frequency divider (4)K having a division ratio of 1/20 to 1/100, for example 1150, and is frequency-divided.

(No. 5 is a switched-capacitor delta filter, which is supplied with the rectangular wave from the frequency divider (4) and is converted into a low-pass F wave, which is then output to the output terminal (6) for frequency modulation with a sine wave as a carrier wave. A signal is output.The output from the variable oscillator (3) is used as a clock signal to the switched canopy filter (5).
and its cutoff frequency is set to, for example, 1150 of the clock frequency.

Next, the present invention will be explained in detail with reference to FIG. FIG. 2 shows a transmission system for transmitting still image signals using telephone lines. η and (2) indicate the transmitting side and the receiving side, respectively, which include a CPU (central processing unit) (2), a memory (capable of storing two frames of still image signals), and a monitor. It includes a television receiver (to) and an external storage device (for example, a floppy disk drive device for storing still image signals) (11).

Transmitting side 01) From the magnetic disk of an electronic still camera (not shown) that projects the subject image from the optical system onto a photoelectric conversion element and records the far-sea signal from the photoelectric conversion element on a magnetic disk. The reproduced signal is written in the memory a4, read out, and supplied to a digital frequency modulator incorporating the present invention for modulation. Then, the analog frequency modulated signal is supplied to the telephone (to) via an acoustic coupler (not shown), and the transmitted signal is transmitted to the receiving side (2) telephone (4)K via the telephone line. . Telephone (7)
) is transmitted through an acoustic coupler (not shown).
It is supplied to a digital frequency demodulator to obtain the original still image digital signal and stored in the memory 04. Then, the signal read from the memory α→ is supplied to a monitor receiver (to), an external storage device (g), etc. for monitoring and storage.

〔Effect of the invention〕

According to the present invention described above, the operation is stable even during long-time operation, there is little risk of the third harmonic of the modulating signal being mixed into the frequency-modulated signal, there is no beat between the modulating signal and the clock signal, and the circuit A frequency modulator with a simple configuration can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing a preferred still image transmission system to which the present invention is applied. (1) is an input terminal, (2) is a D/A converter, (3) is a variable oscillator, (4) is a frequency divider, (5) is a switched capacitor filter, and (6) is an output terminal. Figure 1 Figure 2

Claims (1)

[Claims] a variable oscillator to which an analog input signal is supplied and whose frequency is controlled; a frequency divider to which the output of the variable oscillator is supplied; an output from the variable oscillator is supplied as a clock signal, and the frequency divider A frequency modulator comprising a switched capacitor filter to which an output of the filter is supplied and filtered, and a frequency modulated signal is obtained from the switched capacitor filter.