JPS59117848A - Fm modulator - Google Patents

Abstract

PURPOSE:To apply sufficiently modulation even to the low frequency component of an input signal by using an input signal and a signal component passing the input signal through an LPF, and applying again modulation to a voltage controlled oscillator. CONSTITUTION:The phase of an output of a variable frequency divider 2 selecting an output frequency of the voltage controlled oscillator 1 is compared 3 with a reference frequency fO in phase and the compared output is outputted to a synthesis circuit 8. The circuit 8 synthesizes the compared output with a signal integrating a modulating input signal vin at an integration circuit 7, and the synthesized output is inputted to a synthesis circuit 5 via an LPF4. The circuit 5 synthesizes the signal vin with an output of the LPF4, controls the oscillator 1 so as to apply the FM modulation by the PLL system. Since the signal vin and a low frequency signal component obtained by passing the signal vin through the LPF4 apply modulation to the oscillator 1, the modulating characteristic is not deteriorated to the low frequency component of the signal vin.

Description

[Detailed description of the invention] This invention relates to an FM modulator that utilizes an islock roof.

Generally, when an FM modulator is configured using the PLL method, the conventional method of adding a baseband signal to the input side of the voltage controlled oscillator has the disadvantage that the transmission characteristics of the low frequency components of the baseband signal deteriorate. there were. For this reason, for example, when transmitting an NRZ signal as a baseband signal, this causes interference in the amount of code, leading to a deterioration of the error rate in signal transmission.

A conventional example of this type of FM modulator will be explained with reference to the blocks in FIG. 1. 1 is a voltage controlled oscillator.

2 is the specified frequency division ratio a I Ha 27..., a
A variable frequency divider that can change the divided frequency according to k; 6 is a reference frequency f; 3 is a phase comparator that converts the phase difference between the output X2 of the frequency divider 2 and the reference frequency fo into a voltage.

Further, 4 is a low-pass filter, and this output X4 and the input base punt signal are added to the addition circuit 5 to form a PLL modulator. In this example, when the input vin becomes high level, the output fout of the voltage controlled oscillator 1 becomes fout
ut ” fc+Δf (Δf〉0), the two-phase comparator 5 outputs an oscillation output f.The voltage X3 that lowers ut
= outputs −Δτ. Here, due to the relationship between phase and frequency, there is a relationship of Δυ=αfΔfdt.

α is a constant for converting from phase to voltage.

So, when the high level of vio continues for a long time.

The output X3 of the phase comparator 3 is f. In the low pass filter 4, in order to continue outputting a voltage that lowers ut.

This voltage cannot be removed and is passed into output X4. Then, the output fout is finally pulled back to the center frequency fc. This is the same even when the inputs υ and n continue to be at a low level, and modulation by the low frequency component of υ1n is difficult to avoid.

This phenomenon can be improved to some extent by lowering the cutoff of the low-pass filter 4 so as not to pass the phase ratio diaphragm output Δτ generated due to the signal component of the input υ, but NRZ Depending on the signal pattern, it may contain a small amount of low frequency component energy, so it cannot be completely eliminated. moreover.

Lowering the cutoff of the low pass filter.

Not only does the circuit constant become large, but there is a limit due to reasons such as the 9th order. In other words, in a wireless system that requires switching control between two frequency channels, when using a modulator with zero tightness, if the low-pass filter power and to-off are lowered, PLL lock-in will take longer. When switching frequency channels, even if the frequency division ratio of the variable frequency divider is changed, it is not possible to immediately switch to another channel, which poses a major obstacle in setting up a wireless line. Furthermore, the PLL locks when the power is turned on. Since it takes a long time to complete the process, it is difficult to use it in a signal transmission system that needs to send out a burst signal instantly after the power is turned on.

The object of the present invention is to eliminate the drawbacks of the prior art mentioned above,
Not only the input signal, but also the low frequency components of the input signal are sufficiently modulated by re-modulating the voltage controlled oscillator using the signal component of the input signal passed through a low-pass filter. The object of the present invention is to provide an FM modulator that can perform FM modulation.

According to the present invention, there is provided a voltage controlled oscillator, a variable frequency divider that selectively divides the output frequency of the voltage controlled oscillator, and a phase comparison that compares the phase of the output of the variable frequency divider with respect to a reference frequency. The output of the phase comparator is passed through a low-pass filter, and then in a two-synthesis FM modulator, the synthesis means further adds a low frequency component separately extracted from the input signal for modulation. The FM modulator is characterized in that the voltage controlled oscillator is controlled by the combined output.

Next, we will give two examples of FM modulators according to the present invention.
This will be explained with reference to the drawings.

FIG. 2 is a block diagram showing a first embodiment of the present invention. In this figure, numerals 1 to 6 should be understood to have the same functions, as shown by the same symbols as in the conventional example of FIG. In this example, an integrator circuit 7 that integrates the input vin and a circuit 8 that doubles the output of the integrator circuit 7 (K is a constant) and synthesizes it are added. In other words, a frequency deviation Δf corresponding to the input v1n occurs and "out
= fc + Δf, the phase comparator 5 outputs v3-v
When c - Δτ is output, the output of the 1-synthesizing circuit 8 is.

xB =x3 +K f 'U1ndt-υ. −Δv
+K f v, ndt=vc−αfΔfdt+Kfυi
n dt. Here, since Δf is approximately proportional to the input τ, +n, K should be chosen appropriately.

−αfΔf d t+Kfl11ndt:0 can be set. As a result, the output of the low-pass filter 4 hardly contains the influence of Δf caused by the input 'D1n. Therefore, even if the input υ remains constant and many patterns containing DC components occur, the voltage controlled oscillator 1 must not have this effect on the output of the low-pass filter 4.
The frequency of the output of f. ut is f. without being drawn back to.

It is possible to continue constant oscillation with fc+Δf(7). As a result, even the low frequency components of the input V can be easily modulated.

In the above embodiment, an addition and synthesis circuit is provided as the two circuits 5, so that when the input a is at a high level and out=fc+Δf, the output of the two-phase comparator 3 becomes X3−vc−Δυ. I tried to respond.

Another method is to use a subtractive synthesis circuit instead of the addition synthesis circuit 5°.

X3−υ+Δυ for two-phase comparator 3 when 1Δf
If you use one that shows the response, -Kfυ1ndt is synthesized by the two-synthesizing circuit 8, and the output is X8-υ. ten Δ
υ−K f? '1ndt-=vc+αfΔfdt-Kf
The same objective can be achieved by obtaining v1ndt.

FIG. 3 is a block diagram showing a second embodiment of the present invention. In this figure, numerals 1 to 4 and 6 have the same functions as in the first embodiment, as indicated by the same symbols. In this example, as other elements, 9 three-man power addition and synthesis circuits, 10 integration circuits, and 11 low-pass filters are used.

Here, the integrator circuit 10 has the same performance as the integrator circuit 7 of the first embodiment, and the low-pass filter 1 has the same characteristics as the low-pass filter 4, and has a transfer function twice as large. things are used. by this.

The influence of the input υ1n on the input of the voltage controlled oscillator 1 is equivalent to that of the first embodiment shown in FIG. 2, and the same modulation characteristics can be obtained. In addition, in this embodiment, the same characteristics can be obtained by changing the connection order of the integrating circuit 10 and the low-pass filter 4, adding the input υ to the low-pass filter 4, and then passing it through the integrating circuit 10. It's not even 1.

In the first and second embodiments described above, the DC drift of the inputs τ and n directly causes the drift of the transmission frequency, but this can be avoided if the DC drift is suppressed to a sufficiently small range compared to the input υ1n. There are no practical problems. By using only a voltage controlled oscillator, the center of its oscillation frequency is from fc to f. Even if the deviation is +Δfc, the output compared and detected by the 1-phase comparator directly passes through the low-pass filter to control the voltage controlled oscillator, so that Δf can be sufficiently suppressed.

As is clear from the above description, according to the present invention, the input signal and the low-pass signal component obtained by passing the input signal through a low-pass filter are used to modulate the PLL type voltage controlled oscillator. By multiplying.

Since the modulation characteristics do not deteriorate even with respect to low frequency components of the transmission signal input, it can be applied not only to NRZ signals but also to various signal systems in which code amount interference is a problem. In addition, because the cutoff of the low-pass filter used in the PLL roots can be selected high, the response of the entire PLL loop is fast, and the switching time required when switching one frequency channel can be shortened. This has a significant effect on the reliability of communication systems.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional configuration example of an FM modulator using a PLL system, FIG. 2 is a block diagram showing a configuration example of a first embodiment according to the present invention, and FIG. It is a professional diagram showing the configuration of the embodiment. In the figure, ■ is a voltage controlled oscillator, 2 is a variable frequency divider, and 3 is a voltage controlled oscillator.
is a phase comparator, 4.11 is a low-pass filter, 5 and 9 are addition and synthesis circuits, 6 is a reference frequency oscillator, 7.10 is an integration circuit, and 8 is a synthesis circuit.

Claims (1)

[Claims] (1) A voltage controlled oscillator, a variable frequency divider that selectively divides the output frequency of the voltage controlled oscillator, and a phase that compares the phase of the output of the variable frequency divider with respect to a reference frequency. a comparator, and the output of the phase comparator is passed through a low-pass filter and then synthesized with a modulation input signal by a synthesizing means to control the oscillation frequency of the voltage controlled oscillator.
In the M modulator, the synthesis means further adds and synthesizes a low frequency component separately extracted from the input signal for modulation, and controls the voltage controlled oscillator with the synthesized output. FM modulator. 2. The FM modulator according to claim 1, further comprising a second combining means for integrating the input signal and combining the integrated output with the output of the phase comparator; By applying the output of the combining means of No. 2 to the low-pass filter. An FM modulator, characterized in that a low frequency component of the input signal is added to the first combining means. 3. In the FM modulator according to claim 1, an integrating circuit and a second low-pass filter are added, and these are connected in cascade without being restricted in their order. An FM modulator, characterized in that the input signal is applied to the front stage side, and the output from the latter stage is applied to the synthesis means.