JPH0794949A - Digital frequency modulator - Google Patents

Abstract

PURPOSE:To obtain effective spurious characteristic and signal-to-noise characteristic in a digital frequency modulator capable of applying an input signal as digital data in each certain sampling period. CONSTITUTION:A filter 5 having characteristics allowing a component in a range between original frequency modulation wave components (f0-fm), (f0+fm) to pass and interrupting a component generated in the vicinity of the components (f0-fm), (f0+fm) by a component to be a spurious component reflected from the sampling frequency fS of an input signal 11 is connected to the post stage of a direct digital synthesizser (DDS) 3 to attain effective spurious characteristic and signal-to-noise characteristic or the like.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency modulator, and more particularly to a digital frequency modulator in which an input signal is given as a digital code.

[0002]

2. Description of the Related Art A block diagram of a conventional digital frequency modulator is shown in FIG.

Sample period (ie, sample frequency) f
The input signal 11 of _s is multiplied by the multiplication data 12 that determines the maximum frequency deviation f _m in the multiplication circuit 1 and output as data indicating the instantaneous frequency deviation amount. Next, the data in the adding circuit 2, is added to the addition data 13 to determine the center frequency f _o of the frequency modulation, it is outputted as data indicative of the frequency to be output instantly. The direct digital synthesizer (hereinafter abbreviated as DDS) 3 is
Since the data is integrated for each cycle of the output of the reference oscillator 4 (oscillation frequency _fr ) and used as a phase amount to obtain a sine wave output, frequency modulation in which the instantaneous oscillation frequency changes as the input signal 11 changes Waves are obtained. LPF (low pass filter) 6 is included in the output of DDS3, cage-back component from the oscillation frequency f _r of the reference oscillator 4 (in FIG. 3 2
3 components, which is equal to f _r −f _o ) and is output 14
Is a filter that outputs as, and its characteristic is 32 in FIG.
It is as indicated by the broken line.

[0004]

In this conventional configuration,
A return component from the oscillation frequency f _r of the reference oscillator 4 included in the output of the DDS 3 (component f _r − of 23 in FIG. 3).
f _o) is be removed, generated for only change in the cycle of the sample frequency f _s of the frequency of the output is the input signal 11, due to cage-back component from the sample frequency f _s of the input signal 11 component (in FIG. 3 a component of 22, f _o -f equal to _s or f _o + f _s) can not be removed, good spurious characteristic, has a drawback that the signal-to-noise characteristics can not be obtained.

An object of the present invention is to provide a digital FM modulator which eliminates the above-mentioned drawbacks and can obtain good spurious characteristics and signal-to-noise characteristics.

[0006]

According to the present invention, in a digital frequency modulator to which an input signal is given with a digital code for each sampling period f _s , the input signal and the data for determining the maximum frequency deviation f _m are a multiplier circuit for multiplying the center frequency f _o of the output and the frequency modulation of the multiplication circuit
The adder circuit that adds the data that determines, the synthesizer that integrates the output of the adder circuit for each cycle of the output of the reference oscillator to obtain a sine wave with the phase amount, and the output of that synthesizer Modulated wave f _o −
The components in the range between f _m and f _o + f _m are allowed to pass, and the components generated in the vicinity of f _o −f _s and f _o + f _s by the return component from the sampling frequency f _s of the input signal are cut off. A digital frequency modulator comprising: a filter.

[0007]

Embodiments of the present invention will now be described with reference to the drawings.

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

Direct digital synthesizer (D
The configuration is the same as that of the conventional example until the frequency modulated wave is obtained at the output of DS) 3, but as a filter at the subsequent stage,
A BPF (band pass filter) 5 having a characteristic as shown by a broken line in FIG. This BPF5 the ingredient in the range between f _o -f _m and f _o + f _m which is an original frequency modulation component (21 components in FIG. 3) is passed, the spurious components, a sample of the input signal 11 by removing the component (22 components in FIG. 3) generated in the vicinity of f _o -f _s and f _o + f _s by cage-back component from the frequency f _s, the output 14
Output as.

[0010]

The present invention described above, according to the present invention, the filter components that occur in the vicinity of f _o -f _s and f _o + f _s by cage-back component from the sample frequency f _s of the input signal provided by the digital code Since it is removed in step 1, there is an effect that a frequency modulator having good spurious characteristics and signal-to-noise characteristics can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram of a digital frequency modulator according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional digital frequency modulator.

FIG. 3 is a diagram for explaining the operation of FIGS. 1 and 2.

[Explanation of symbols]

 1 multiplication circuit 2 addition circuit 3 direct digital synthesizer (DDS) 4 reference oscillator 5 BPF 6 LPF 11 input signal 12 multiplication data 13 addition data 14 output 31 frequency characteristic of BPF 5 32 frequency characteristic of LPF 6

Claims (3)

[Claims] 1. A digital frequency modulator, to which an input signal is given by a digital code every sample period f _s , a multiplication circuit for multiplying the input signal and data for determining a maximum frequency deviation f _m , and a multiplication circuit of the multiplication circuit. an adder circuit for adding the data for determining the center frequency f _o of the output and the frequency modulation, the synthesizer to obtain a sine wave output of the addition circuit, it is integrated in each period of the output of the reference oscillator as a phase amount, the of the output of the synthesizer, which is the original frequency modulated wave f _o -f _m and f _o + components in the range between f _m is passed, f _o by cage-back component from the sample frequency f _s of the input signal - and a filter that cuts off components generated in the vicinity of f _s and f _o + f _s . 2. The digital frequency modulator according to claim 1, wherein the filter is a bandpass filter. 3. The digital frequency modulator according to claim 1, wherein the synthesizer is a direct digital synthesizer.