Frequency modulation device and frequency modulation method

Abstract

To provide a frequency modulation device and frequency modulation method, capable of reducing noise generated in the frequency modulation device and noise contained in a demodulation signal due to non-linearity of a voltage-controlled oscillator, and capable of suppressing increase of unwanted harmonic wave signals at demodulation.SOLUTION: A frequency modulation device according to the invention further includes: a pulse count detector (111) for demodulating a frequency modulated signal (51) and outputting a pulse density modulated signal (52); and a one-bit counter (303) for re-converting the pulse density modulated signal (52) to a frequency modulated signal (58), a feedback circuit (115) takes the pulse density modulated signal (52) as a demodulation signal (53).SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to a high-precision frequency modulation device and a frequency modulation method. [Background technology]

[0002] Frequency modulation (FM) is a method of transmitting the modulated signal, which is the information signal to be transmitted, by changing the frequency of the carrier wave. It is used in FM radio broadcasting, amateur radio, wireless microphones, fire department radio, taxi radio, and other applications.

[0003] In frequency modulation, the modulating signal is converted into a change in the frequency of the carrier wave and transmitted, so the amplitude of the carrier wave does not contain any information and information can be transmitted even if the amplitude is constant.Therefore, even if noise in the amplitude direction is added in the transmission path, etc., it can be removed by the receiver's limiter circuit (amplitude limiter), preventing deterioration of the signal-to-noise ratio (SNR) due to noise in the amplitude direction.

[0004] Furthermore, if the change in frequency used for modulation (frequency deviation) is set large, the dynamic range and occupied bandwidth will be widened, which has the advantage of increasing the SNR.

[0005] The circuit configurations of the frequency modulator and demodulator are shown in Figure 1. The frequency modulator (100) receives an electrical signal from a modulation signal source (10) as an input signal (50), modulates the modulation signal into a frequency-modulated signal 51 using a voltage-controlled oscillator (101), and transmits it over a transmission line 40. The demodulator (110) converts the frequency-modulated signal (51) into a pulse-density-modulated signal (52) using a pulse count detector (111), and passes the signal through a low-pass filter (112). The output signal (53) of the low-pass filter (112) becomes the original signal from the modulation signal source (10). In the following description, the voltage-controlled oscillator may be referred to as a "VCO" (Voltage Controlled Oscillator). The operation of frequency modulation and the principles of demodulation using a VCO are disclosed, for example, in Non-Patent Document 1.

[0006] The voltage-controlled oscillator (101) is an electronic oscillator circuit that outputs a frequency signal whose frequency is proportional to the input voltage (hereinafter referred to as the control voltage), and the frequency is determined by the control voltage. Frequency modulation can be achieved by inputting a modulation signal as a control voltage to the voltage-controlled oscillator and changing the frequency of the output signal using the modulation signal. The control voltage (V c ) and the frequency of the output signal (f vco ) is expressed by the following equation (1).

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[0007] On the other hand, demodulation of a frequency modulated signal can be achieved by pulse count detection (see, for example, Patent Document 1). The operating principle of demodulation of a frequency modulated signal by a pulse count type FM demodulator using a pulse count detector will be explained using Figures 2 and 3.

[0008] A frequency-modulated signal input to an input terminal (23) is converted into a pulse signal with a constant amplitude by a limiter circuit (201). This pulse signal is branched into two, and one pulse signal is input to a delay circuit (202) that delays it by a time τ. The other branched pulse signal is input to an exclusive OR (203) together with a delayed frequency-modulated signal (54), which is the output signal of the delay circuit. The exclusive OR (203) outputs a differential detection pulse signal with a pulse width τ, starting from the change point of the pulse signal (51), to an output terminal (33). This differential detection pulse signal is a pulse density modulation signal (52) whose output time interval becomes narrower the higher the frequency of the input frequency-modulated signal, and conversely, whose output time interval becomes wider the lower the frequency of the input frequency-modulated signal.

[0009] This differential detection pulse signal, i.e., pulse density modulated signal (52), passes through a low-pass filter (112) to obtain the original modulated signal (50).

[0010] [Supplementary explanation 1] The operation of the pulse count type FM demodulator will be explained (see, for example, Patent Document 1 and Non-Patent Document 1). The modulating signal is m(t) and the carrier signal is x c (t)=Acos(2πf c t), the frequency modulated signal S(t) is expressed by equation (2).

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[0011] Demodulation, which extracts the original modulated signal m(t) from a frequency-modulated signal S(t), involves determining the frequency of the modulated signal S(t) at any time. This can be achieved by detecting the amount of phase change per time of the frequency-modulated signal S(t) (equivalent to the angular velocity, or frequency, in units of rad / sec). This operation is equivalent to differentiating the phase of S(t) with respect to time.

[0012] The pulse density modulated signal generated by the pulse count detector is a pulse signal with a fixed duration, starting from the time when the phase of the frequency modulated signal S(t) changes by π [rad]. Therefore, the time interval between pulses in the pulse density modulated signal is proportional to the angular velocity of the phase of the frequency modulated signal S(t), i.e., the inverse of the frequency. Therefore, demodulation can be achieved by converting the pulses into a voltage or current signal that is inversely proportional to the pulse appearance interval time. This can be achieved by passing the differential detection signal through a low-pass filter.

[0013] Frequency modulation is the time integral of the modulating signal m(t) as the carrier signal x c (t) phase, and demodulation is x c Since this is an operation that time-differentiates the phase of (t), the time-integrated modulated signal m(t) is demodulated back to the original signal by time differentiation, but noise during transmission that affects the phase of the frequency-modulated signal S(t) is only subjected to time differentiation.

[0014] The frequency characteristics of the time derivative form a high-pass filter with a slope of 20 dB / dec, where the lower the frequency, the more the amplitude attenuates and the higher the frequency, the more the amplitude increases. Therefore, the frequency spectrum of the noise after demodulation is smaller at lower frequencies and increases as the frequency increases. This is known as triangular noise, and because the noise is smaller in the low-frequency region, a high SNR can be achieved by extracting the low-frequency frequency band with a low-pass filter. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Patent Publication No. 2006-324734 [Non-patent literature]

[0016] [Non-Patent Document 1] Toho University Media Net Center / Virtual Laboratory / Information and Communication Theory - Information and Communication Hypertext / Frequency Modulation, URL: https: / / www.mnc.toho-u.ac.jp / v-lab / yobology / frequency_modulation / frequency_modulation.htm (Retrieved December 28, 2021) Summary of the Invention [Problem to be solved by the invention]

[0017] Frequency modulation is a modulation method that is more resistant to noise than other modulation methods such as amplitude modulation and phase modulation, and can transmit signals with high accuracy. However, the noise that affects the phase added by a frequency modulation device exhibits triangular noise characteristics due to the effect of time differentiation, but the overall noise amount increases and the SNR deteriorates.

[0018] In particular, when frequency modulation is performed using a voltage-controlled oscillator, the phase noise and VCO gain (K v The nonlinearity of the phase noise (θ(t)) generated by the voltage-controlled oscillator causes a degradation in the accuracy of the demodulated signal during demodulation. The frequency-modulated signal including the phase noise (θ(t)) generated by the voltage-controlled oscillator is expressed by equation (3), which adds the phase noise (θ(t)) to equation (2).

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[0019] By performing time differentiation of the phase, which is a demodulation operation, θ(t) is time differentiated and becomes a noise component of dθ(t) / dt. Therefore, it can be seen that the frequency characteristics of this noise exhibit triangular noise characteristics due to the effect of time differentiation. In other words, the nonlinearity of the control voltage and oscillation frequency of the voltage controlled oscillator, i.e., the VCO gain (K v ) is the control voltage (V c ), there was a problem that the waveform of the demodulated signal was distorted and unwanted harmonic signals were generated.

[0020] Therefore, in order to solve the above problems, an object of the present invention is to provide a frequency modulation device and a frequency modulation method that can reduce noise generated inside the frequency modulation device and noise contained in the demodulated signal due to the nonlinearity of the voltage-controlled oscillator, and can suppress the increase of unnecessary harmonic signals during demodulation. [Means for solving the problem]

[0021] In order to achieve the above object, the frequency modulation device of the present invention feeds back the output of the VCO to the input side, and places a first filter before the VCO, the rate of increase / decrease of the signal amplitude of which changes depending on the frequency of the input signal.

[0022] Specifically, the frequency modulation device according to the present invention comprises: a first filter whose signal amplitude increase / decrease rate changes depending on the frequency of the electrical signal; a voltage-controlled oscillator that uses the voltage of the signal output by the first filter as a control voltage and outputs a frequency-modulated signal; a feedback circuit that subtracts a demodulated signal obtained by demodulating the frequency-modulated signal from an external input signal to obtain the electrical signal; Equipped with.

[0023] Further, the frequency modulation method according to the present invention comprises: generating a frequency modulated signal with a voltage controlled oscillator based on the control voltage; feedback the demodulated signal obtained by demodulating the frequency-modulated signal from an external input signal so as to subtract it from the demodulated signal; Passing the electrical signal obtained by subtracting the demodulated signal from the input signal through a first filter in which the rate of increase / decrease of the signal amplitude changes depending on the frequency; and The voltage of the signal output from the first filter is used as the control voltage. Do the following.

[0024] FIG. 4 is a diagram illustrating the principle of a frequency modulation device and method according to the present invention. In FIG. 4, X is an input signal (55) and Y is an output signal (56). A signal obtained by subtracting Y from X is input to a first filter (302). The first filter (302) is a filter whose rate of increase or decrease in signal amplitude changes depending on the frequency of the input signal. The transfer function of the first filter (302) is H(f), where f is the frequency of the signal. Noise Q (57) is added to the output of the first filter (302) to obtain Y. Y is branched into two, one of which becomes a feedback signal for subtracting Y from X at a feedback point (300), forming a loop.

[0025] The relationship between X and Y in FIG. 4 is expressed by equation (4).

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[0026] In equation (4), the signal frequency band of X is ω x Let H(f) be ω x When it is sufficiently larger than 1, the coefficients of the X and Q terms in equation (4) are

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[0027] 4, if we consider the summing point (301) to be a VCO, it is possible to reduce noise Q, and even if the VCO gain (Kv) is nonlinear, it is possible to suppress an increase in noise and unnecessary harmonic signals contained in the demodulated signal during demodulation. Therefore, the present invention can provide a frequency modulation device and a frequency modulation method that can reduce noise generated inside the frequency modulation device and noise contained in the demodulated signal due to the nonlinearity of the voltage-controlled oscillator, and can suppress an increase in unnecessary harmonic signals during demodulation.

[0028] The feedback circuit of the frequency modulation device according to the present invention is characterized in that it includes a demodulator that receives the frequency-modulated signal output by the voltage-controlled oscillator and demodulates the frequency-modulated signal to generate the demodulated signal. The signal that is fed back to the input side is a signal obtained by demodulating the frequency-modulated signal. If the noise generated by the demodulator that demodulates the frequency-modulated signal is small enough to have no practical effect, the demodulator can be placed in the feedback circuit, and the output of the VCO can be used as the output (frequency-modulated signal) of the frequency modulation device.

[0029] The frequency modulation device according to the present invention comprises: a pulse count detector that demodulates the frequency modulated signal and outputs a pulse density modulated signal; a one-bit counter for converting the pulse density modulated signal back into the frequency modulated signal; Furthermore, The feedback circuit uses the pulse density modulated signal as the demodulated signal.

[0030] By using a pulse count detector as the demodulator that demodulates the frequency modulated signal and feeding back its output, the first filter can also suppress noise generated by the pulse count detector.

[0031] The feedback circuit of the frequency modulation device according to the present invention is characterized by having a second filter that attenuates a frequency band other than the frequency band of the input signal, which is included in the pulse density modulated signal, and can increase the amplification factor of the first filter in the required frequency band, thereby enhancing the noise reduction effect.

[0032] The above inventions can be combined as much as possible. [Effects of the Invention]

[0033] The present invention can provide a frequency modulation device and a frequency modulation method that can reduce noise generated inside the frequency modulation device and noise contained in the demodulated signal due to the nonlinearity of the voltage-controlled oscillator, and can suppress the increase in unnecessary harmonic signals during demodulation. [Brief explanation of the drawings]

[0034] [Figure 1] FIG. 2 is a diagram illustrating the circuit configuration of a frequency modulator and a demodulator. [Figure 2] FIG. 2 is a diagram illustrating the circuit configuration of a pulse count detector. [Figure 3] 1 is a diagram illustrating the operation principle of demodulating a frequency modulated signal using a pulse count FM demodulator. FIG. [Figure 4] 1 is a diagram illustrating the principle of a frequency modulation device and method according to the present invention; [Figure 5] 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. [Figure 6] 3A and 3B are diagrams illustrating an embodiment of a first filter of a frequency modulation device according to the present invention. [Figure 7] 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. [Figure 8] 1 is a diagram illustrating the principle of a frequency modulation device and method according to the present invention; [Figure 9] 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. [Figure 10] 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0036] (Embodiment 1) FIG. 5 is a diagram illustrating the circuit configuration of a frequency modulation device (501) of this embodiment. The frequency modulation device (501) includes: a first filter (302) whose signal amplitude increase / decrease rate changes depending on the frequency of the electrical signal (50); a voltage-controlled oscillator (101) that uses the voltage of the signal (64) output from the first filter (302) as a control voltage and outputs a frequency-modulated signal (51); a feedback circuit (115) that subtracts a local FM demodulated signal (61) obtained by demodulating a frequency modulated signal (51) from an external input signal (55) to generate an electrical signal (50); Equipped with.

[0037] The frequency modulation device (501) a pulse count detector (111) that demodulates a frequency modulated signal (51) and outputs a pulse density modulated signal (52); a 1-bit counter (303) that converts the pulse density modulated signal (52) back into a frequency modulated signal (58); Furthermore, The feedback circuit (115) is characterized in that it converts the pulse density modulated signal (52) into a local FM demodulated signal (61). In FIG. 5, X is the input signal (55), Y is the pulse density modulation signal (52), and Z is the frequency modulation signal (58) output from the frequency modulation device (501).

[0038] For example, the first filter (302) can be configured with an integrator as shown in Figure 6. An integrator has infinite gain at DC and a frequency characteristic that attenuates as the frequency increases. Therefore, the first filter (302) configured with an integrator exhibits the characteristics of a high-pass filter, which attenuates lower-frequency noise and increases gain for higher-frequency signals. The first filter (302) can also be configured with a band-pass filter (BPF) and an amplifier. In this case, the first filter (302) exhibits the characteristics of a band-stop filter, which attenuates noise in the pass band of the BPF and increases gain for signals in other frequency bands.

[0039] [Supplementary explanation 2] We will provide additional explanation about the integrator circuit in Figure 6. The integrator in Figure 6 is a circuit configured using an operational amplifier (OP-AMP). As shown in the following formula, the output voltage signal (V OUT ) is the input voltage signal (V IN ) is integrated and multiplied by a coefficient (-1 / CR), where C is the capacitance and R is the resistance.

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[0040] The frequency modulation device (501) uses the VOC (101) which is a frequency modulator and the pulse count detector (111) which is a demodulator as the sources of noise Q in Fig. 4. The frequency modulation device (501) passes the input signal (55) as a modulated signal (51) and a pulse density modulated signal (52), and can suppress noise generated by the VOC (101) and the pulse count detector (111) in the signal pass band where H(f)>>1 in accordance with the frequency characteristics of the first filter (302).

[0041] The frequency modulation device (501) has a configuration in which a pulse count detector (111) which is a demodulator is placed immediately after the VCO (101). This configuration has the following advantageous effects. (Effect 1) The first filter (302) can also suppress noise that occurs when a frequency-modulated signal containing harmonic noise caused by the nonlinearity of the VCO (101) is demodulated by the pulse count detector (111). (Effect 2) The pulse density modulated signal (52) output from the pulse count detector (111) can be easily reconverted into a frequency modulated signal (58) by the 1-bit counter (303). In other words, the configuration can be simpler than that of other demodulators.

[0042] (Embodiment 2) 7 is a diagram illustrating a frequency modulation device (502) of this embodiment. The frequency modulation device (502) differs from the frequency modulation device (501) of FIG. 5 in that it includes a second filter (304) in the feedback circuit (115) that attenuates a frequency band other than the frequency band of the input signal (55) that is included in the pulse density modulated signal (52). The transfer function of the second filter (304) is G(f), where f is the frequency of the signal. In FIG. 7, X is the input signal (55), Y is the pulse density modulation signal (52), and Z is the frequency modulation signal (58) output from the frequency modulation device (502).

[0043] In the case of the frequency modulation device (501), when the fed-back pulse density modulation signal (52) is directly subtracted from the input signal (55) at the feedback point (300), the amplitude of the electrical signal (50) after subtraction increases, and it is necessary to increase the output dynamic range of the first filter (302) with 1 << H(f). It is not preferable to realize the first filter (302) with a large output dynamic range by an electronic circuit. As a result, the amplification factor of the first filter (302) has to be limited.

[0044] Therefore, as shown in FIG. 7, the pulse density modulation signal (52) serving as the feedback signal is attenuated in an unnecessary frequency band by the second filter (304) arranged in the feedback circuit (115). By subtracting this signal from the input signal (55) as the local FM demodulation signal (61), the signal in the unnecessary band of the pulse density modulation signal (52) can be removed, and the fed-back local FM demodulation signal (61) becomes a signal extremely similar in shape to the input signal (55). Since the amplitude of the difference between the local FM demodulation signal (61) and the input signal (55) becomes small, it becomes possible to increase the amplification factor in the required frequency band of the first filter (302).

[0045] FIG. 8 is a diagram for explaining the relationship between the input X and the output Y of the frequency modulation device (502). The input X and the output Y have the relationship shown by Equation (7).

Equation

[0046] The second filter (304) is a filter having a characteristic that the amplification factor of the second filter (304) is 1-fold (G(f) = 1) in the signal band of the input signal X, that is, in the frequency band where H(f) >> 1, and attenuates signals in other unnecessary frequency bands (frequency bands where H(f) becomes small). By making the second filter (304) have such characteristics, the influence of arranging the second filter (304) can be ignored in the signal band of the input signal X and in the frequency band where noise is to be suppressed.

[0047] (Embodiment 3) 9 is a diagram illustrating a frequency modulation device (503) of this embodiment. The frequency modulation device (503) differs from the frequency modulation device (501) of FIG. 5 in that it includes a demodulator (117) that receives a frequency modulation signal (51) output from a voltage-controlled oscillator (101) as input to a feedback circuit (115) and demodulates the frequency modulation signal (51) to generate a local FM demodulated signal (61). In FIG. 9, X is the input signal (55), Y is the frequency modulation signal (51), and Z is the frequency modulation signal (58) output from the frequency modulation device (501).

[0048] In the frequency modulation device (501) of Fig. 5, if the noise generated by the pulse count detector (111) is small enough to have no practical effect, it can be placed on the feedback circuit (115) side. The demodulator (117) is not limited to the pulse count type, and other demodulation types may be used.

[0049] The frequency modulation device (503) does not require the 1-bit counter (303) included in the frequency modulation device (501) of FIG. 5, and can directly use the output (51) of the VCO (101) as the frequency modulation signal (58).

[0050] (Embodiment 4) In this embodiment, a specific circuit constituting the frequency modulation device (502) explained in FIG. 7 will be explained. 10 is a diagram illustrating the circuit configuration of a frequency modulation device (504) of this embodiment. In the frequency modulation device (504), the feedback point (300) is realized by an analog subtractor (401). In the frequency modulation device (504), the first filter (302) is realized by an amplifier (403) and an integrating circuit (402). In addition, in the frequency modulation device (504), the second filter (304) is realized by a low-pass filter (112), an amplifier (404), and a DC offset adjustment circuit (405). This configuration is effective in suppressing noise in the low-frequency range.

[0051] An analog subtraction circuit (401) subtracts a local FM demodulated signal (61) from an input signal (55). An electrical signal (50) output from the analog subtraction circuit (401) is amplified in amplitude by an amplifier (403) for gain adjustment and input to an integrating circuit (402).

[0052] Ideally, the integrator circuit (402) would have infinite gain at DC, but the operational amplifier that makes up the integrator has a finite gain, which limits the DC gain. By connecting multiple integrators in series, it is possible to make the gain and noise attenuation characteristics steeper. However, since the phase of an integrator rotates by 90 degrees, connecting two or more integrators to form a feedback loop will cause oscillation. Therefore, measures to prevent this oscillation are required in the integrator circuit (402).

[0053] Since the output signal (50) of the analog subtraction circuit (401) is a signal with an extremely small amplitude, an amplifier circuit (403) is used to increase the gain in the pass band of the integrator circuit (402) of the modulated signal.

[0054] The output signal (64) of the integrator circuit (402) becomes a control signal for the VCO (101) to obtain a frequency-modulated signal (51). The frequency-modulated signal (51) is input to a pulse count detector (111) to become a pulse-density-modulated signal (52). The pulse-density-modulated signal (52) is split into two, one of which is converted into a frequency-modulated signal (58) by a 1-bit counter circuit (303) and output.

[0055] The other of the two branched pulse density modulated signals (52) passes through a low pass filter (112). The low pass filter (112) filters out the signal band (DC to ω) of the frequency modulated signal (51). s ) and the gain is 1 (0 dB) s The pulse density modulated signal (52) is passed through a low-pass filter (112) to become a demodulated signal (53) in which noise components in unnecessary frequency bands are attenuated.

[0056] The DC offset value and F / V gain (the rate at which the output signal increases or decreases when the frequency is increased or decreased) of the demodulated signal (53) after passing through the low-pass filter (112) are determined by the signal level (high / low voltage value of the pulse signal) of the pulse density modulation signal (52). For this reason, the characteristics of the demodulated signal (53) may not match those of the input signal (55).

[0057] An amplifier circuit (404) and a DC offset adjustment circuit (405) are arranged after the low-pass filter (112) so that the signal fed back to the analog subtraction circuit (401) has the same characteristics as the input signal (55). Specifically, the amplifier circuit (404) sets the gain of the feedback circuit (115) to 1 (0 dB), and the DC offset adjustment circuit (405) adjusts the DC offset value.

[0058] The signal that has passed through the low-pass filter (112) is subjected to gain adjustment and DC offset adjustment to become a local FM demodulated signal (61), which is input to an analog subtraction circuit (401) as a feedback signal and subtracted from the input signal (55).

[0059] The frequency modulation device (504) is an example in which an integrator circuit (402) is used for the first filter (302), and as mentioned above, the integrator can be replaced with a band-pass filter or the like depending on the required band of the modulation signal. When a band-pass filter is used for the first filter (302), the second filter (304) should also be changed to attenuate noise on the low-frequency side, and it is necessary to change it to a filter that multiplies the signal by 1 in the pass band of the modulation signal and attenuates the signal in unnecessary bands.

[0060] (Effects of the Invention) As is clear from the above explanation, the present invention can suppress the increase in noise and unnecessary harmonic signals contained in the demodulated signal during demodulation, which are caused by phase noise generated inside the frequency modulation device and nonlinearity of the voltage-controlled oscillator, and thereby realize a highly accurate frequency modulation device. [Industrial Applicability]

[0061] The present invention allows for low noise frequency modulation and is used in industries that require highly accurate frequency modulated signals. [Explanation of symbols]

[0062] 10: Modulation signal source 21: Frequency modulator input terminal 22: FM demodulator input terminal 23: Pulse detector input terminal 31: Frequency modulator output terminal 32: FM demodulator output terminal 33: Pulse detector output terminal 40: Transmission line 50: Electrical signal 51: Frequency modulation signal 52: Pulse density modulated signal 53: Demodulated signal 54: Delayed frequency modulated signal 55: Input signal 56: Output signal 57: Noise signal 58: A signal obtained by converting a pulse density modulated signal into a frequency modulated signal using a 1-bit counter 59: Integrator output signal 60: Amplification circuit output signal 61: Local FM demodulated signal 63: Amplification circuit output signal 64: Output signal of the integrator circuit 100: Frequency modulator 101: Voltage controlled oscillator (VCO) 110: Pulse count FM demodulator 111: Pulse count detector 112: Low-pass filter (LPF) 115: Feedback circuit 117: Demodulator 201: Limiter circuit 202: Delay circuit 203: Exclusive OR (XOR) 300: Feedback points 301: Addition point 302: First filter 303: 1-bit counter 304: Second filter 401: Analog subtraction circuit 402: Integral circuit 403: Amplification circuit 404: Amplification circuit 405: DC offset adjustment circuit 501-504: Frequency modulation device

Claims

1. a first filter whose signal amplitude increase / decrease rate changes depending on the frequency of the electrical signal; a voltage-controlled oscillator that uses the voltage of the signal output from the first filter as a control voltage and outputs a frequency-modulated signal; a feedback circuit that subtracts a demodulated signal obtained by demodulating the frequency-modulated signal from an external input signal to obtain the electrical signal; a pulse count detector that demodulates the frequency modulated signal and outputs a pulse density modulated signal; a one-bit counter for converting the pulse density modulated signal back into the frequency modulated signal; Equipped with The frequency modulation device is characterized in that the feedback circuit uses the pulse density modulated signal as the demodulated signal.

2. 2. The frequency modulation device according to claim 1, wherein the feedback circuit has a second filter that attenuates a frequency band included in the pulse density modulated signal other than the frequency band of the input signal.

3. generating a frequency modulated signal with a voltage controlled oscillator based on the control voltage; feedback the demodulated signal obtained by demodulating the frequency-modulated signal from an external input signal so as to subtract it from the demodulated signal; passing the electrical signal obtained by subtracting the demodulated signal from the input signal through a first filter in which the rate of increase / decrease of the signal amplitude changes depending on the frequency; a voltage of a signal output from the first filter is set as the control voltage; demodulating the frequency modulated signal into a pulse density modulated signal with a pulse count detector; converting the pulse density modulated signal back into the frequency modulated signal with a one-bit counter; and The pulse density modulated signal is used as the demodulated signal. A frequency modulation method comprising:

4. 4. The frequency modulation method according to claim 3, wherein, when said feedback is performed, a second filter attenuates frequency bands contained in said pulse density modulated signal other than the frequency band of said input signal.

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