JPS6359008A - Fm modulation circuit - Google Patents

Abstract

PURPOSE:To always bring a reference DC level of a modulation input signal to be at the center of an output frequency by changing the frequency division ratio of an output of a voltage controlled oscillator inputted to a phase comparator in response to a mean voltage of the modulation input signal by means of a variable frequency divider. CONSTITUTION:A mean voltage detection circuit 7 detects a mean voltage of a modulation signal inputted to the voltage controlled oscillator 1. Then the detected analog mean voltage is converted into a digital signal by an A/D converter 8 and the result is inputted to a variable frequency divider 6. Then the frequency dividing ratio of the variable frequency divider 6 is subject to variable control so as to be proportional to the mean voltage of the modulation signal. The phase comparator 2 compares the phase of the output of the variable frequency divider 6 with that of the output of the frequency divider 4 and controls the output (modulated signal) frequency of the said voltage controlled oscillator 1 linearly by feeding back the difference signal voltage in response to the phase difference to a control terminal of the voltage controlled oscillator 1. Thus, in modulating a modulation signal having asymmetry, the reference DC level of the modulation signal is the center of the output frequency.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an FM modulation circuit, and in particular to an FM modulation circuit that suppresses fluctuations in DC level during demodulation of asymmetric modulation signals such as pulse signals or television signals. Regarding.

(Prior Art) No. 2150 shows the configuration of a general conventional FM modulation circuit, which is a voltage controlled oscillator (VCO).
ge Controlled 0scillator)
1. Phase comparator (PD: Phase Dete)
ctor) 2, frequency dividers 3 and 4, reference signal generator 5
is equipped.

In the FM modulation circuit shown in FIG. 2, the output of the voltage controlled oscillator is divided into two parts during the modulation process in which the modulated signal input from the input terminal Tin is modulated through the voltage controlled oscillator 1 and outputted to the output terminal T out as a modulated signal. The frequency is divided by the frequency divider 3 with a fixed frequency ratio (1/N) and inputted to the phase comparator 2, and the reference signal outputted from the reference signal generator 5 is also input to the frequency divider 3. Similarly, after being frequency-divided by a frequency divider 4 with a fixed frequency division ratio (1/M), it is input to the phase comparator 2 as a comparison input for the output of the frequency divider 3.

The phase comparator 2 compares the phases of the output of the frequency divider 3 and the output of the divider 4, and returns a difference signal voltage corresponding to the phase difference to the control terminal of the voltage controlled oscillator 1.
By this, the frequency of the modulated signal is stabilized.

In this type of FM modulation circuit, when a symmetrical signal is input as a modulation signal, the frequency of the output (modulated signal) increases or decreases in response to the level of fluctuation from the center level of the signal. It operates so that the center level of the signal is the center of the output frequency.

On the other hand, when an asymmetric signal containing a DC component such as a pulse signal or a television signal is input as a modulation signal, the DC component of the signal, that is, the average voltage, operates to be the center of the output frequency.

Therefore, when the output (modulated signal) is demodulated on the receiving side, the demodulated output has a DC level fluctuation corresponding to the average pressure of the modulated signal.

(Problems to be Solved by the Invention) As described above, in the above-mentioned conventional FM modulation circuit, when a modulation signal having asymmetry such as a pulse signal or a television signal is input, the average voltage of the modulation signal is higher than that of the modulated signal. Since it operates as much as possible at the center of the frequency, there is a problem in that the DC level of the demodulated output fluctuates when it is demodulated on the receiving side, making it impossible to obtain a demodulated output that matches the modulated signal.

(Means for Solving the Problems) The FM modulation circuit of the present invention includes a voltage controlled oscillator which inputs a modulation signal and outputs a modulated signal, and an average voltage of the modulation signal input to the voltage controlled oscillator. and a variable frequency divider that variably sets a frequency division ratio according to the detection output of the average voltage detection circuit and divides the output of the voltage controlled oscillator according to the set frequency division ratio. , a reference signal generator that generates a reference signal, a divider that divides the reference signal generated from the reference signal generator at a fixed frequency division ratio, and an output of the variable frequency divider and an output of the divider. Compare the phases,
The oscillator is configured to include a phase comparator that feeds back a difference signal voltage corresponding to the phase difference to a control terminal of the voltage controlled oscillator.

(Function) In the FM modulation circuit of the present invention, the frequency division ratio of the output of the voltage controlled oscillator that is input to the phase comparator is changed according to the average voltage of the modulation input signal by the variable frequency divider. The operation is made such that the reference DC level of the input signal is always at the center of the output (modulated signal) M wave number.

(Example) Hereinafter, an example of the present invention will be described in detail based on the accompanying drawings.

FIG. 1 shows an embodiment of the FM modulation circuit according to the present invention, and parts that perform the same functions as the conventional FM modulation circuit shown in FIG. 2 are designated by the same reference numerals. It is marked.

In FIG. 1, a variable frequency divider 6 is newly connected between the output side of the voltage controlled oscillator 1 and the phase comparator 2.

Furthermore, an average voltage detection circuit 7 and an A/D converter 8 are connected between the input side of the voltage controlled oscillator 1 and the variable frequency divider 6 as means for variably controlling the frequency division ratio of the variable frequency divider 6. is connected.

In this FM modulation circuit, the modulation signal inputted from the input terminal Tin is modulated through the voltage controlled oscillator 1, and during the modulation process to be outputted to the output terminal T out as a modulated signal, the average voltage detection circuit 7 modulates the modulated signal inputted to the voltage controlled oscillator 1. The average voltage of the input modulation signal is detected, and the detected average voltage is further converted from an analog signal to a digital signal by the A/D converter 8, and then input to the variable frequency divider 6. The frequency division ratio of the frequency divider 6 is variably controlled so as to be proportional to the average voltage of the modulation signal.

Therefore, the output of the voltage controlled oscillator 1 is the output of the variable frequency divider 6
As described above, the signal is divided by the frequency division ratio set according to the output of the A/D converter 8, and then input to the phase comparator 2.

On the other hand, the reference signal output from the reference signal generator 5 is input to the phase comparator 2 after being frequency-divided by a fixed frequency division ratio (1/M) via a frequency divider 4.

Then, the phase comparator 2 compares the phases of the output of the variable frequency divider 6 and the output of the frequency divider 4, and feeds back a difference signal voltage corresponding to the phase difference to the control terminal of the voltage controlled oscillator 1. By doing so, the output (modulated signal) frequency of the voltage controlled oscillator 1 is linearly controlled.

Note that when the average voltage detection circuit 7 and the A/D converter 8 variably control the frequency division ratio of the variable frequency divider 6 in proportion to the average voltage of the modulation signal, the average voltage level of the modulation signal is changed. In this case, care must be taken so that the reference DC level of the modulation signal always operates at the center of the output frequency.

Through such control, the FM modulation circuit of the present invention always maintains the DC level of the demodulated signal when the modulated signal is demodulated on the receiving side, even when modulating an asymmetric signal such as a pulse signal or a television signal. Can be maintained at a constant value.

Furthermore, when a symmetrical signal is modulated, the average voltage of the modulated signal is 0 level, so the center level of the modulated signal operates as the center of the output frequency, and the conventional modulation process is performed. be able to.

(Effects of the Invention) As explained above, according to the FM modulation circuit of the present invention, the frequency division ratio of the variable frequency divider connected to the output of the voltage controlled oscillator is variably controlled according to the average voltage of the modulation signal, Even when modulating a modulated signal with asymmetry, the reference DC level of the modulated signal is operated as much as possible at the center of the output frequency, so when the modulated signal at this time is demodulated,
This has an excellent effect in that fluctuations in the DC level of the demodulated output can be suppressed and a demodulated output that matches the modulated signal can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an FM modulation circuit according to the present invention, and FIG. 2 is a block diagram showing the configuration of a conventional FM modulation circuit. 1... Voltage controlled oscillator (VCO), 2... Phase comparator (PD), 3, 4... Frequency divider, 5... Reference signal generator, 6... Variable frequency divider, 7... Average voltage detection circuit, 8... A/D converter.

Claims (1)

[Claims] a voltage-controlled oscillator that receives a modulation signal as an input and outputs a modulated signal; an average voltage detection circuit that detects an average voltage of the modulation signal that is input to the voltage-controlled oscillator; a variable frequency divider that variably sets a frequency division ratio and divides the output of the voltage controlled oscillator according to the set frequency division ratio, and a reference signal generator that generates a reference signal;
A frequency divider that divides the reference signal generated from the reference signal generator at a fixed frequency division ratio, and a phase difference between the output of the variable frequency divider and the output of the frequency divider, and according to the phase difference. and a phase comparator that feeds back a differential signal voltage to a control terminal of the voltage controlled oscillator.