JP2014197752A - FM demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an FM demodulator that can be used as an FM demodulator for a narrow band, is excellent in the S/N ratio of a FM modulated signal, and also can reduce the distortion of the FM demodulated signal.SOLUTION: An FM demodulator comprises: a switching signal generation circuit 10 that generates two-phased switching signals with a frequency corresponding to the frequency of an input FM modulation signal and with 180 degree phase difference from each other; a switched capacitor current output circuit 20 that generates an FM modulation current signal obtained by adding the center frequency current component and frequency deviation current component of the FM modulation signal by a switched capacitor circuit made up of a switch switched by the two-phased switching signals and a capacitor; an FM demodulation circuit 30 that takes out the frequency deviation current component by subtracting the center frequency current component from the FM modulation current signal; and a current/voltage conversion circuit 40 that converts the frequency deviation current component into a voltage signal.

Description

  The present invention relates to an FM demodulator that generates an FM demodulated signal by converting a frequency signal into a current signal by inputting the FM modulated signal into a switched capacitor current output circuit.

  There is a phase detection method as an FM demodulation method used in a narrow band field such as a specific low-power radio. In this phase detection method, if the Q of the phase shifter circuit is increased, a phase signal whose phase changes sharply with respect to the frequency deviation of the FM modulation signal can be obtained. Therefore, the phase signal is converted into a voltage. Thus, even when the frequency deviation of the FM modulated signal with respect to the center frequency is small, an FM demodulated signal having a good S / N ratio can be obtained (Patent Document 1).

  In general, a phase shifter is composed of a resonance circuit composed of a coil and a capacitor, and uses a phase change of an output signal when an FM modulation signal is input, but operates with an IF frequency of 400 kHz to several MHz. If a device is to be constructed, a coil having an inductance value of about several hundred μH is required, which makes integration difficult, and the coil must be externally attached.

  When the phase change is obtained by using, for example, the phase change point of the low-pass filter, the phase change amount of the phase shifter is not linear with respect to the frequency change amount. The demodulated signal may be distorted. In addition, a high-order filter is inevitably large in circuit scale, which is disadvantageous in terms of current consumption and cost.

  On the other hand, there is a pulse count method as another example of the FM demodulation method. In this method, an input FM modulation signal is converted into a voltage and then integrated by a low-pass filter. In this pulse count method, since there is no circuit element that generates nonlinearity, the linearity is good and distortion generated in the FM demodulated signal is small. In addition, no coil is required and a high-order filter is not required, and there is no problem caused by the phase detection method.

JP 2000-232323 A

  However, in a field such as narrowband radio where the frequency deviation with respect to the center frequency of the FM modulated signal is small, the level of the FM demodulated signal obtained tends to be low and the S / N ratio tends to be poor. The reason for the poor S / N ratio is that the level of the FM demodulated signal is determined by the ratio between the center frequency and the frequency deviation. In general, as a method for solving this problem, there is a method of using a wideband radio (radio or the like) having a large frequency deviation or increasing a ratio with the frequency deviation by lowering the center frequency. The latter method of lowering the center frequency can be applied to the phase detection method, but has a problem that it is easily affected by 1 / f noise of the active element.

  An object of the present invention is to provide an FM demodulator that can be used for a narrow band such as a specific low-power radio, has a good S / N ratio of an FM demodulated signal, and can reduce distortion of the FM demodulated signal. It is.

In order to achieve the above object, an FM demodulator according to a first aspect of the present invention is a switching signal generator that generates a two-phase switching signal having a phase difference of 180 degrees at a frequency corresponding to the frequency of an input FM modulation signal. A switched capacitor that generates an FM modulated current signal obtained by adding a center frequency current component and a frequency deviation current component of the FM modulated signal by a switched capacitor circuit composed of a circuit, a switch switched by the two-phase switching signal, and a capacitor A current output circuit; and an FM demodulation circuit that subtracts the center frequency current component from the FM modulated current signal to extract the frequency deviation current component.
The invention according to claim 2 is the FM demodulator according to claim 1, further comprising a current / voltage conversion circuit for converting the frequency deviation current component into a voltage signal.
According to a third aspect of the present invention, in the FM demodulator according to the second aspect, the current / voltage conversion circuit includes a voltage signal corresponding to the center frequency current component and a voltage signal corresponding to the frequency deviation current component. A differential voltage is output.
According to a fourth aspect of the present invention, in the FM demodulator according to the first, second, or third aspect, the switched capacitor current output circuit includes a first low-pass filter that removes switching noise.
According to a fifth aspect of the present invention, in the FM demodulator according to the first, second, third, or fourth aspect, the FM demodulating circuit includes a second low-pass filter that extracts the center frequency current component from the FM modulated current signal. And a current subtracting means for subtracting the center frequency current component obtained by the second low-pass filter from the FM modulated current signal.

  According to the present invention, the capacitance value of the switched capacitor of the switched capacitor current output circuit can be set as appropriate, and the level of the FM demodulated signal can be set freely. For this reason, it is possible to realize a high S / N ratio that can be used for narrowband radio with a small frequency deviation, and the degree of freedom for the user is increased. Further, since the frequency deviation current component of the FM modulation signal is extracted, an FM demodulated signal with small distortion can be obtained if the current capacity of the active element is sufficient. Furthermore, although the capacitance value of the capacitor of the switched capacitor current output circuit may be 1000 pF or more, since the drive capability of the operational amplifier used can be set low, there is an advantage that the mounting area can be reduced. .

1 is a circuit diagram of an FM demodulator according to a first embodiment of the present invention. It is a circuit diagram of the FM demodulator of the 2nd Example of this invention. FIG. 3 is an operation waveform diagram of the FM demodulator of FIG. 2. It is a circuit diagram of the FM demodulator of the 3rd example of the present invention. It is a S-shaped curve waveform diagram.

<First embodiment>
FIG. 1 shows an FM demodulator according to a first embodiment of the present invention. In FIG. 1, 10 is a switching signal generation circuit for generating two-phase rectangular waves CLK and CLKB having a phase difference of 180 degrees from an input FM modulation signal (F ₀ ± F _DEV ), and 20 is a two-phase rectangular wave CLK, A switched capacitor current output circuit that receives the CLKB and generates an FM modulated current signal corresponding to the frequency, 30 is an FM demodulation circuit, and 40 is a current / voltage conversion circuit. Of the FM modulation signal, F ₀ is the center frequency, and F _DEV is the deviation frequency.

In the switched capacitor current output circuit 20, a switched capacitor circuit is constituted by the capacitor C1, the switch SW1 connected in series to the capacitor C1 and switched by the rectangular wave CLK, and the switch SW2 connected in parallel to the capacitor C1 and switched by the rectangular wave CLKB. Has been. An operational amplifier 21 that controls the base voltage of the transistor Q1 is provided so that the voltage generated in the equivalent resistor composed of the switched capacitor circuit matches the reference voltage V _REF .

  The FM demodulating circuit 30 includes transistors Q2 and Q3 whose base voltages are controlled by the output voltage of the operational amplifier 21, an averaging circuit 31 that averages the collector current of the transistor Q2, and transistors Q4 and Q5 that are current mirror connected. ing. Transistors Q3 and Q5 constitute current subtracting means.

  The current / voltage conversion circuit 40 inputs a current obtained by subtracting the collector current of the transistor Q5 from the collector current of the transistor Q3, and converts it into a voltage signal.

Now, the FM modulation current I flowing through the collector of the transistor Q1 of the switched capacitor current output circuit 20 is such that the frequency of the rectangular wave CLK is F _CLK (= F ₀ ± F _DEV ), and the capacitance value of the capacitor C1 is centered on C1 and I _F0 . If the frequency current component, _IFDEV is the frequency deviation current component,
I = V _REF × F _CLK × C1
I = (V _REF × F ₀ × C1) ± (V _REF × F _DEV × C1)
= I _F0 ± I _FDEV (1)
This FM modulation current I is mirrored by the transistors Q2 and Q3 and becomes the collector current thereof.

A center frequency current component I _F0 flows through the collector of the transistor Q4 by the averaging circuit 31 and is mirrored from the transistor Q4 to the collector of the transistor Q5. Therefore, the current / voltage conversion circuit 40 has a frequency deviation current which is a current obtained by subtracting the center frequency current component I _F0 flowing in the collector of the transistor Q5 from the FM modulation current “I _F0 ± I _FDEV ” flowing in the collector of the transistor Q3. The component ± I _FDEV flows.

From the above, when the capacitance value C1 of the capacitor C1 of the switched capacitor current output circuit 20 is increased, the level of the frequency deviation current component I _FDEV can be increased, thereby _improving the S / N ratio of the FM demodulated signal. Can do. Further, since the frequency change of the FM modulation signal is converted into a current change and taken out, an FM demodulated signal with a small distortion can be obtained if the current capacity of the active element is sufficient, that is, operated in a linear region. Furthermore, since it does not have a resonance frequency unlike the phase detection method, it can cope with any frequency and has versatility. Furthermore, by adjusting the capacitance value C1 of the capacitor C1 of the switched capacitor current output circuit 20, an FM demodulated signal at a desired level can be obtained, so that the degree of freedom for the user is increased. Furthermore, although the capacitance value of the capacitor C1 of the switched capacitor current output circuit 20 may be 1000 pF or more, since the drive capability of the operational amplifier 21 to be used can be set low, the mounting area when integrated is reduced. Can be smaller

<Second embodiment>
FIG. 2 shows a second embodiment of the FM demodulator according to the present invention. Here, in the switched capacitor current output circuit 20, a low-pass filter 22 composed of a resistor R1 and a capacitor C2 is inserted in a feedback path from the collector of the transistor Q1 to the non-inverting input terminal of the operational amplifier 21, and the low-pass filter 22 causes the switch SW1 to switch. , SW2 is removed. Further, in the FM demodulation circuit 30, the averaging circuit 31 is constituted by a low-pass filter including a resistor R2 and a capacitor C3, and the current component I _FDEV is removed from the collector current of the transistor Q2.

The signal waveforms at the nodes N1 to N6 in FIG. 2 are shown in FIG. It can be _{seen that} the level of the frequency deviation current component I _{FDEV at} the node N6 increases as the frequency of the input FM modulation signal increases, and the level of the frequency deviation current component I _FDEV decreases as the frequency decreases.

<Third embodiment>
FIG. 4 shows a third embodiment of the FM demodulator according to the present invention. Here, the current / voltage conversion circuit 40 includes an operational amplifier 41 and a resistor R4. Further, the transistor Q6 is connected to the transistor Q6 which is current-mirror connected, and the resistor R3 is connected to the collector thereof. The collector common connection point of the transistors Q3 and Q5 is connected to the inverting input terminal of the operational amplifier 41, and the common connection point of the collector of the transistor Q6 and the resistor R3 is connected to the non-inverting input terminal.

In this embodiment, the voltage V ₀ which depends on the center frequency current component I _F0 to the resistor R3 is generated, the voltage is generated that depends on the frequency deviation current component I _FDEV to the resistor R4, the output voltage V _OUT of the operational amplifier 41 (FM demodulated signal) can have an S-curve characteristic as shown in FIG. This output voltage V _OUT is obtained from the equation (1):
V _OUT = V ₀ ± I _FDEV × R4
_{= (V REF × F 0 ×} C1 × R3) ± (V REF × F DEV × C1 × R4) (2)
It becomes. The first term on the right side is the demodulated DC voltage, and the second term is the demodulated signal voltage.

Thus, since the S-shaped curve characteristic can be obtained, the center frequency F ₀ of the FM modulation signal can be easily detected.

10: switching signal generation circuit 20: switched capacitor current output circuit, 21: operational amplifier, 22: low-pass filter, 30: FM demodulation circuit, 31: low-pass filter, 40: current / voltage conversion circuit, 41: operational amplifier

Claims (5)

A switching signal generation circuit that generates a two-phase switching signal having a phase difference of 180 degrees from each other at a frequency corresponding to the frequency of the input FM modulation signal;
A switched capacitor current output circuit for generating an FM modulated current signal obtained by adding a center frequency current component and a frequency deviation current component of the FM modulated signal by a switched capacitor circuit composed of a switch and a capacitor that are switched by the two-phase switching signal. When,
An FM demodulation circuit that subtracts the center frequency current component from the FM modulated current signal to extract the frequency deviation current component;
An FM demodulator characterized by comprising: The FM demodulator according to claim 1,
An FM demodulator comprising a current / voltage conversion circuit for converting the frequency deviation current component into a voltage signal. The FM demodulator according to claim 2,
The FM demodulator characterized in that the current / voltage conversion circuit outputs a voltage difference between a voltage signal corresponding to the center frequency current component and a voltage signal corresponding to the frequency deviation current component. The FM demodulator according to claim 1, 2, or 3,
2. The FM demodulator according to claim 1, wherein the switched capacitor current output circuit includes a first low-pass filter that removes switching noise. The FM demodulator according to claim 1, 2, 3 or 4,
The FM demodulation circuit includes a second low-pass filter that extracts the center frequency current component from the FM modulated current signal, and a current that subtracts the center frequency current component obtained by the second low-pass filter from the FM modulated current signal. An FM demodulator comprising a subtracting unit.

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