JP7793399B2 - Frequency modulation device and frequency modulation method - Google Patents

Description

This disclosure relates to a high-precision frequency modulation device and frequency modulation method.

Frequency modulation (FM) is a method of transmitting information by varying the frequency of a carrier wave, which is the modulated signal. It is used in FM radio broadcasting, amateur radio, wireless microphones, fire department radios, taxi radios, and other applications.

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 amplitude noise is added along the transmission path, it can be removed by the receiver's limiter circuit (amplitude limiter), preventing degradation of the signal-to-noise ratio (SNR) due to amplitude noise.

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

The circuit configuration of a frequency modulator and demodulator is shown in Figure 1. The frequency modulator (100) receives an electrical signal from a modulation signal source (10) as an input signal (50). The voltage-controlled oscillator (101) modulates the modulated signal into a frequency-modulated signal 51, which is then transmitted 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), which then 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" (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.

The voltage-controlled oscillator (101) is an electronic oscillator circuit that outputs a frequency signal whose frequency is proportional to an input voltage (hereinafter referred to as a 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 ( _Vc ) and frequency ( _fvco ) of the voltage-controlled oscillator are expressed by the following equation (1):
Here, _Kv is the VCO gain, which is a coefficient that expresses the rate of change in the output frequency _fvco relative to a change in the control voltage _Vc in Hz/V (or (rad/sec)/V using angular velocity). Also, _f0 is the offset frequency, which is the value of the output frequency _fvco when _Vc = 0.

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

The frequency-modulated signal input to the 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) along with a delayed frequency-modulated signal (54), which is the output signal of the delay circuit. The exclusive OR (203) outputs a delayed detection pulse signal with a pulse width τ, starting from the change point of the pulse signal (51), to the output terminal (33). This delayed 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.

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

[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).
If the modulating signal is m(t) and the carrier signal is x _c (t)=A cos(2πf _c t), the frequency modulated signal S(t) is expressed by equation (2).
That is, the frequency-modulated signal S(t) can be understood as a phase-modulated signal in which the phase of the carrier signal _xc (t) is changed by an amount proportional to the time integral of the modulated signal m(t) with a proportionality constant _Kf .

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.

The pulse density modulated signal generated by the pulse count detector is a pulse signal of a fixed duration, output starting from the time when the phase of the frequency modulated signal S(t) changes by π [rad]. Therefore, since the time interval between pulse occurrences 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, demodulation can be achieved by converting it 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.

Frequency modulation converts the time integral of the modulated signal m(t) into the phase of the carrier signal _xc (t), and demodulation is the operation of time-differentiating the phase of _xc (t). Therefore, 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.

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 greater the amplitude. Therefore, the frequency spectrum of the noise after demodulation is smaller at lower frequencies and becomes larger as the frequency increases. This is known as triangular noise, and because noise is smaller in the low-frequency range, a high SNR can be achieved by extracting the low-frequency frequency band using a low-pass filter.

JP 2006-324734

Toho University Media Net Center/Virtual Laboratory/Information and Communication Theory - Hypertext of Information and Communication/Frequency Modulation, URL: https://www.mnc.toho-u.ac.jp/v-lab/yobology/frequency_modulation/frequency_modulation.htm (Retrieved December 28, 2021)

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 amount of noise increases, resulting in a deterioration of the SNR.

In particular, when frequency modulation is performed using a voltage-controlled oscillator, the phase noise of the voltage-controlled oscillator and the nonlinearity of the VCO gain ( _Kv ) cause degradation of 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).

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, when the control voltage and oscillation frequency of a voltage-controlled oscillator are nonlinear, i.e., when the VCO gain ( _Kv ) changes depending on the control voltage ( _Vc ), the waveform of the demodulated signal becomes distorted, resulting in the generation of unwanted harmonic signals.

To solve the above problems, the present invention aims to provide a frequency modulation device and a frequency modulation method that can reduce noise generated within the frequency modulation device and noise contained in the demodulated signal due to the nonlinearity of the voltage-controlled oscillator, and suppress the increase in unnecessary harmonic signals during demodulation.

To achieve the above objective, 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 whose signal amplitude increase/decrease rate changes depending on the frequency of the input signal.

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 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;
Equipped with.

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;
The electrical signal obtained by subtracting the demodulated signal from the input signal is passed 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 by the first filter is set as the control voltage.

Figure 4 is a diagram illustrating the principle of the frequency modulation device and method according to the present invention. In Figure 4, X is the input signal (55) and Y is the output signal (56). The signal obtained by subtracting Y from X is input to the 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 signal frequency. 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 the feedback signal for subtracting Y from X at the feedback point (300), forming a loop.

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

In equation (4), when the signal frequency band of X is _ωx and H(f) is sufficiently larger than 1 at _ωx , the coefficients of the terms X and Q in equation (4) are as follows:
Therefore, equation (4) becomes Y=X, and X passes through the first filter (302), and only the noise Q is attenuated.

In Figure 4, if we consider the summing point (301) to be the VCO, the noise Q can be reduced, and even if the VCO gain (Kv) is nonlinear, the increase in noise and unwanted harmonic signals contained in the demodulated signal during demodulation can be suppressed. Therefore, the present invention can provide a frequency modulation device and frequency modulation method that can reduce noise generated within the frequency modulation device and noise contained in the demodulated signal due to the nonlinearity of the voltage-controlled oscillator, and suppress the increase in unwanted harmonic signals during demodulation.

The feedback circuit of the frequency modulation device according to the present invention is characterized by having 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 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 this 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.

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.

By using a pulse count detector as the demodulator that demodulates the frequency modulated signal and feeding back its output, the first filter can achieve a noise suppression effect even on noise generated by the pulse count detector.

The feedback circuit of the frequency modulation device according to the present invention is characterized by having a second filter that attenuates frequency bands contained in the pulse density modulated signal other than the frequency band of the input signal. This increases the amplification factor of the first filter in the required frequency band, thereby enhancing the noise reduction effect.

The above inventions can be combined as much as possible.

The present invention provides a frequency modulation device and frequency modulation method that can reduce noise generated within the frequency modulation device and noise contained in the demodulated signal due to the nonlinearity of the voltage-controlled oscillator, and suppress the increase in unnecessary harmonic signals during demodulation.

FIG. 2 is a diagram illustrating the circuit configuration of a frequency modulator and a demodulator. FIG. 2 is a diagram illustrating the circuit configuration of a pulse count detector. 1 is a diagram illustrating the operation principle of demodulating a frequency modulated signal by a pulse count type FM demodulator. 1 is a diagram illustrating the principle of a frequency modulation device and method according to the present invention; 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. 1A and 1B are diagrams illustrating an embodiment of a first filter of a frequency modulation device according to the present invention. 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. 1 is a diagram illustrating the principle of a frequency modulation device and method according to the present invention; 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention. 1 is a diagram illustrating a circuit configuration of a frequency modulation device according to the present invention.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and drawings are considered to be identical to each other.

(Embodiment 1)
5 is a diagram illustrating the circuit configuration of a frequency modulation device (501) according to this embodiment.
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.

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) for converting 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).

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, a 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 passband of the BPF and increases gain for signals in other frequency bands.

[Supplementary explanation 2]
A supplementary explanation will be given of 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 formula below, the output voltage signal (V _OUT ) is the value obtained by integrating the input voltage signal (V _IN ) and multiplying it by a coefficient (-1/CR). C is the capacitance, and R is the resistance value.
V ₊ is the zero reference for integration. In Figure 6, V ₊ is GND, so voltages _V- higher than GND are integrated as positive, and voltages _V- lower than GND are integrated as negative. By setting this zero reference V ₊ to Vref (any voltage value), voltages _V- higher than Vref can be treated as positive, and voltages _V- lower than Vref can be treated as negative.

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 source of noise Q in Figure 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 pulse count detector (111) in the signal passband where H(f) >> 1, in accordance with the frequency characteristics of the first filter (302).

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 due to 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 types of demodulators.

(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 frequency bands contained in the pulse density modulated signal (52) other than the frequency band of the input signal (55). The transfer function of the second filter (304) is G(f), where f is the signal frequency.
In FIG. 7, X is the input signal (55), Y is the pulse density modulated signal (52), and Z is the frequency modulated signal (58) output from the frequency modulation device (502).

In the case of a frequency modulation device (501), if the fed-back pulse density modulated signal (52) is directly subtracted from the input signal (55) at the feedback point (300), the amplitude of the electrical signal (50) after subtraction becomes large, and it is necessary to increase the output dynamic range of the first filter (302), where 1<<H(f). It is not desirable to realize a first filter (302) with a large output dynamic range using an electronic circuit, and as a result, the amplification factor of the first filter (302) must be limited.

As shown in Figure 7, the unnecessary frequency band of the pulse density modulated signal (52), which serves as the feedback signal, is attenuated by a second filter (304) placed in the feedback circuit (115). By subtracting this signal from the input signal (55) as a local FM demodulated signal (61), the unnecessary band of the pulse density modulated signal (52) can be removed, and the fed-back local FM demodulated signal (61) becomes a signal that is very similar in shape to the input signal (55). Because the amplitude of the difference between the local FM demodulated signal (61) and the input signal (55) is reduced, it is possible to increase the amplification factor of the first filter (302) in the required frequency band.

8 is a diagram for explaining the relationship between the input X and output Y of the frequency modulation device 502. The relationship between the input X and the output Y is expressed by equation (7).

The second filter (304) has an amplification factor of 1 (G(f) = 1) in the signal band of the input signal X, i.e., the frequency band where H(f) >> 1, and has the characteristic of attenuating signals in other unnecessary frequency bands (frequency bands where H(f) is small). By giving the second filter (304) such characteristics, the impact of placing the second filter (304) can be ignored in the signal band of the input signal X and the frequency band where noise suppression is desired.

(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-modulated signal (51) output from a voltage-controlled oscillator (101) in a feedback circuit (115) and demodulates the frequency-modulated 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).

In the frequency modulation device (501) of Figure 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 method and may use other demodulation methods.

The frequency modulation device (503) does not require the 1-bit counter (303) provided in the frequency modulation device (501) of Figure 5, and the output (51) of the VCO (101) can be used directly as the frequency modulation signal (58).

(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.

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

The integrator circuit (402) ideally has 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. For this reason, measures to prevent this oscillation are required in the integrator circuit (402).

Since the output signal (50) of the analog subtraction circuit (401) is a signal with extremely small amplitude, an amplifier circuit (403) is used to increase the gain in the passband of the modulation signal integrator circuit (402).

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

The other of the two branched pulse density modulated signals (52) is passed through a low-pass filter (112). The low-pass filter (112) has a characteristic that the gain is 1 (0 dB) in the signal band (DC to _ωs ) of the frequency modulated signal (51) and attenuates in the band above _ωs . By passing the pulse density modulated signal (52) through the low-pass filter (112), noise components in unnecessary frequency bands are attenuated to produce a demodulated signal (53).

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 this demodulated signal (53) after passing through the low-pass filter (112) are determined by the signal level (high/low voltage values 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).

The amplifier circuit (404) and DC offset adjustment circuit (405) are placed 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 1x (0 dB), and the DC offset adjustment circuit (405) adjusts the DC offset value.

The signal that passes 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 the analog subtraction circuit (401) as a feedback signal and subtracted from the input signal (55).

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 bandpass filter or the like depending on the required band of the modulation signal. When using a bandpass filter 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 passband of the modulation signal and attenuates the signal in unwanted bands.

(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.

The present invention enables low-noise frequency modulation and is used in industries that require highly accurate frequency-modulated signals.

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 modulation signal 53: Demodulated signal 54: Delayed frequency modulation signal 55: Input signal 56: Output signal 57: Noise signal 58: Signal obtained by converting pulse density modulation signal to frequency modulation signal using a 1-bit counter 59: Integrator output signal 60: Amplifier circuit output signal 61: Local FM demodulation signal 63: Amplifier circuit output signal 64: Integration circuit output signal 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 point 301: Addition point 302: First filter 303: 1-bit counter 304: Second filter 401: Analog subtraction circuit 402: Integration circuit 403: Amplification circuit 404: Amplification circuit 405: DC offset adjustment circuit 501 to 504: Frequency modulation device

Claims (4)

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. 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. 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. 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.