JPH0730332A - Fm modulator - Google Patents

Abstract

PURPOSE:To make a modulated frequency characteristic flat, to remove a low frequency noise, and to prevent the deterioration of an S/N by providing a compensating circuit which mainly compensates the low pass components of a modulated frequency band between an input terminal and an attenuator. CONSTITUTION:A compensating circuit 11 is equipped with the frequency characteristic of the logarithm scale, an inclination in a direction increasing in the low pass, a reverse characteristic for offsetting the attenuation of the modulated frequency characteristic of a VCO 5 in the low pass. Therefore, a demodulated output signal 16 of a demodulator 15 can be equipped with the flat frequency characteristic. Also, the inclination of the modulated frequency characteristic of a reference voltage control oscillator SVCO 1 in the low pass can be also compensated by the circuit 11. The frequency characteristic of the circuit 11 only in the necessary band can also compensated, and the low frequency noise can be removed by decreasing the unnecessary low level as well.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM modulator, and more particularly to an FM modulator of a transmitter for digital data transmission communication.

[0002]

2. Description of the Related Art In a conventional FM modulator, as shown in FIG. 3, a voltage controlled oscillator (hereinafter referred to as VCO) 5 for oscillating a frequency of a transmission output signal and a part of a transmission output signal are branched by a coupler 14. Frequency divider 3 that divides the output signal at a specified frequency division ratio for each desired transmission frequency, the output signal of frequency divider 3 and a voltage-controlled reference oscillator (hereinafter referred to as SVCO)
The phase comparator 2 that performs a phase comparison with the output of 1 and outputs a DC voltage signal corresponding to the phase difference, and removes unnecessary harmonic components and noise included in the DC voltage of the output of the phase comparator 2, There is a loop filter 4 that outputs a control voltage signal of the VCO 5. On the other hand, the modulation signal from the modulation input terminal 12 is branched, one modulation signal is passed through the attenuator 6, combined by the combiner 15 with the output signal of the loop filter 4, and applied to the VCO 5. The other modulation signal passes through the attenuator 7 and the SVCO 1
Is applied as a control signal.

Here, the amplitude frequency characteristics when the FM-modulated transmission signal from the transmission output terminal 13 is demodulated by the reception side demodulator 15 are shown in FIGS. 4 (a) to 4 (c) and 4 (d).
It shows in (f). FIG. 4A shows the frequency characteristics of the demodulated output signal V1 of only the SVCO 1, where the characteristics in the low frequency band 8A are flat and the characteristics in the high frequency band 8B are drooping. FIG. 4B shows the frequency characteristic of the demodulation output signal V5 of only the VCO5,
The characteristics of the high frequency region 9B become flat, and the characteristics of the low frequency region 9A become drooping characteristic. Therefore, when the frequency characteristic of the total demodulated output signal V _t modulated by both SVCO 1 and VCO 5 is displayed on a logarithmic scale, it becomes as shown in FIG.
If the level adjustment of 7 and 7 is bad, low range 10A, high range 10B
At the frequencies f1 and f
If 2 does not match, the characteristics will be inclined. Further, even if the level adjustment was performed properly, discontinuity occurred in the characteristic during Δf as shown in FIG. 4 (f).

[0004]

In this conventional FM modulator, the influence of the loop band due to the PLL circuit and the VC
Since the modulation characteristics of O and SVCO are attenuated in the low range or the high range, it is difficult to level-adjust the modulation frequency characteristics due to the difference in frequency and voltage sensitivity between the two. Further, there is a drawback that a ripple of the modulation frequency characteristic is generated at a combination point of the low frequency band and the high frequency band even if the level adjustment is completely performed.

[0005]

The FM modulator of the present invention comprises:
A voltage controlled oscillator that oscillates a transmission output signal, a frequency divider that divides a part of the transmission output signal at a predetermined ratio, and a reference voltage controlled oscillator that oscillates a reference frequency of an output frequency signal of the frequency divider. A phase comparator that outputs a DC voltage corresponding to the phase difference between the frequency divider output signal and the reference frequency;
A loop filter for removing unnecessary harmonic components and low frequency noise included in the DC voltage, a combiner connected between the loop filter and the voltage controlled oscillator, the combiner and the reference voltage. In an FM modulator having a control oscillator and a terminal for inputting a modulation signal to be FM-modulated via an attenuator for adjustment, a low frequency component of a modulation frequency band is mainly compensated between the terminal and the attenuator. It has a compensation circuit.

[0006]

The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention, and a part different from the conventional example is a compensating circuit 11 that makes a frequency characteristic variable.
Have been added. The compensating circuit 11 has a slope in the rising direction in the low frequency band 12A as shown in FIG. 2 (a), which is a frequency characteristic of a logarithmic scale, and is mainly VC shown in FIG. 2 (b).
It has the reverse characteristic of canceling the attenuation in the low frequency range of the modulation frequency characteristic of O5. Here, the modulation frequency characteristic of the SVCO 1 can be compensated including the slope in the low frequency range 13c of FIG. 2 (b). Therefore, the demodulation output signal 16 of the demodulator 15
Has a flat frequency characteristic as shown in FIG. Further, by making the frequency characteristic of the compensating circuit 11 as shown by the characteristic 12D of FIG. 2D, only the necessary band of FIG. 2D is compensated, and finally, the unnecessary low frequency shown in FIG. It is also possible to lower the band level to remove low frequency noise and prevent S / N deterioration.

[0007]

As described above, the present invention can flatten the modulation frequency characteristic by providing the compensation circuit 11. Further, by changing the frequency characteristic of the compensation circuit, there is an effect that low frequency noise can be removed and S / N deterioration can be prevented.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a modulation frequency characteristic of the embodiment of FIG.

FIG. 3 is a block diagram of a conventional FM modulator.

FIG. 4 is an explanatory diagram of frequency characteristics of a conventional example.

[Explanation of Codes] 1 voltage control type reference oscillator (SVCO) 2 phase comparator 3 frequency divider 4 loop filter 5 voltage controlled oscillator (VCO) 6, 7 attenuator 11 compensation circuit 12 modulation input terminal 13 transmission output terminal 14 coupling vessel

Claims (2)

[Claims] 1. A voltage-controlled oscillator that oscillates a transmission output signal, a frequency divider that divides a part of the transmission output signal at a predetermined ratio, and a reference frequency of an output frequency signal of this frequency divider. A reference voltage controlled oscillator, a phase comparator that outputs a DC voltage corresponding to the phase difference between the frequency divider output signal and the reference frequency, and unnecessary harmonic components and low frequency noise included in the DC voltage. , A combiner connected between the loop filter and the voltage-controlled oscillator, and a modulation signal to be FM-modulated in each of the combiner and the reference voltage-controlled oscillator via an adjusting attenuator. In the FM modulator having a terminal for inputting, an FM modulator mainly having a compensation circuit for compensating for a low frequency component of a modulation frequency band between the terminal and the attenuator. 2. The FM modulation according to claim 1, wherein the compensation circuit attenuates the frequency characteristic of the low frequency noise region of the low frequency component of the modulated wave signal applied to the reference voltage controlled oscillator. vessel.