JPH09200072A - Direct conversion receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably demodulate a signal even when a received signal has a low modulation index or a frequency deviation is caused by providing a local oscillator receiving a frequency-modulated signal and eliminating a local oscillation signal component from a mixer output signal consisting of a local oscillation signal and an antenna input signal. SOLUTION: The receiver is provided with a local oscillator 12 receiving a frequency-modulated signal from a signal source 13. Output signals from mixers 11 receiving the local oscillation signal, the local oscillation signal phase- shifted by 90 deg. and an input signal from an antenna 10 are given to capacitors 16 and low-pass filters 17, in which a DC component and an adjacent channel component are eliminated and the result is fed to an elimination means 14. The elimination means 14 eliminates the effect of spread spectrum by the modulation signal from the local oscillator 12 and provides an output of the result to a demodulator 15. Thus, in the case of receiving a binary FSK signal, the spread spectrum is formed flat and the demodulator 15 demodulates the received signal stably even when a modulation index is small and the local oscillation frequency is deviated from the received frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio communication apparatus such as a selective call receiver, a cordless remote controller, and a cordless telephone, and more particularly to a receiver using a direct conversion reception system, that is, a homodyne system as a reception demodulation system.

[0002]

2. Description of the Related Art A direct conversion receiver has many advantages over a heterodyne receiver. In the heterodyne method, a filter for removing an image component is necessary in order to convert a received high-frequency signal into an intermediate frequency before processing. This filter is usually a ceramic or quartz filter,
These filters are expensive and large in shape. Since the direct conversion receiver directly converts the received high-frequency signal into a low-frequency signal near the baseband, no image component is generated. Therefore, such a filter as described above is unnecessary. Generally, a filter for removing components of adjacent channels is required. However, since the signal to be processed has a low frequency, an active filter having a low-pass characteristic can be used.
Conversion is also easy. That is, since the expensive and large-shaped filter is unnecessary, the direct conversion receiver can be realized at low cost and downsized.

FIG. 11 is a block diagram showing the structure of a conventional direct conversion receiver. In FIG. 11, 10 is an antenna, 11 is a mixer, 12 is a local oscillator, 15 is a demodulator,
16 is a capacitor, 17 is a low-pass filter, 18 is 9
It is a 0 degree phase shifter. The received signal from the antenna 10 is divided into two and input to the two mixers 11, respectively. Also,
The output of the local oscillator 12 is also divided into two and input to the mixer 11, one of which is phase-shifted by the 90-degree phase shifter and then input to the mixer 11. Here, the frequency of the local oscillator 12 is set to be the same as the center frequency of the received signal, so that the received signal is frequency-converted by the mixer 11 into a baseband signal. The output of the two mixers 11 is the capacitor 1
6 and the low-pass filter 17 and then input to the demodulator 15. The demodulator 15 performs demodulation. The above is the basic operation of the direct conversion receiver.

In the direct conversion receiver, since the frequency of the local oscillator is set to be substantially the same as the center frequency of the received signal as described above, signal leakage of the local oscillator, that is, local leakage, causes an unnecessary DC component in the mixer output. Occurs. Further, in the direct conversion receiver, the mixer needs a large conversion gain, and the DC offset voltage of the mixer output caused by this is also a problem. Demodulation is difficult unless this DC voltage component is removed. Therefore, a capacitor or the like is inserted in the output of the mixer to remove the direct current component and then input to the demodulator.

[0005]

However, in the above-mentioned conventional direct conversion receiver, since the removed DC component also includes the component of the received signal, the signal after removing the DC component contains the necessary information. Will be partially lost. Then, demodulation using this signal cannot obtain sufficient demodulated data. This is especially a problem when the frequency modulation signal has a small frequency displacement, that is, when the modulation index is small. Further, in the case of a modulation signal such as a binary FSK signal which has no peak of the spectrum component near the center frequency,
Although the influence of information loss due to the removal of the DC component is relatively small, the influence becomes large when the frequency of the local oscillator deviates from the center frequency of the received signal. Then, when this frequency shift is about the same as the frequency displacement amount of the modulated wave, the influence becomes serious. The present invention is to solve the above-mentioned problems, and suppresses the influence due to the loss of the DC component, and can perform stable demodulation in the case of a modulated signal with a small modulation index or even if a frequency deviation occurs in the local oscillator. The purpose is to provide a machine.

[0006]

In the direct conversion receiver of the present invention, a local oscillator frequency-modulated by a signal from a signal source, a mixer for inputting an output from this local oscillator and a signal from an antenna, A removing means for removing the frequency component of the signal source from the output of the mixer and a demodulating means for inputting the output of the removing means are provided. According to the present invention, there is an effect that stable demodulation is performed even when the modulation index of the received signal is small or the frequency of the local oscillator is deviated.

[0007]

To achieve this object, a direct conversion receiver according to the present invention comprises a signal source, a local oscillator frequency-modulated by a signal from the signal source, an output of a local oscillator and an antenna. A mixer that inputs the input signal of
A removing means for removing the frequency component of the signal source from the output of the mixer and a demodulating means for inputting the output of the removing means are provided.

Also, a signal source, a local oscillator frequency-modulated by a signal from the signal source, a mixer for inputting the output of the local oscillator and an input signal from the antenna, demodulation means for inputting the output of the mixer, and demodulation. The removing means is provided for removing the voltage change component corresponding to the voltage change of the signal source from the output of the means.

Further, by adjusting the phase and amplitude of the output from the demodulating means and the signal from the signal source to obtain the difference,
The voltage change component of the signal source is canceled from the output of the demodulation means.

Further, the removing means is provided with a low pass filter or a band stop filter for attenuating the frequency component of the signal source.

A signal source, a first local oscillator frequency-modulated by a signal from the signal source, a first mixer for inputting an output of the first local oscillator and an input signal from the antenna, and a signal source Second frequency-modulated by the signal from
The local oscillator, the second mixer for inputting the output of the first mixer and the output of the second local oscillator, and the demodulation means for inputting the output of the second mixer.

Further, a signal source, a first local oscillator frequency-modulated by a signal from the signal source, a second local oscillator, an output of the first local oscillator and an output of the second local oscillator are input. A first mixer, a second mixer for inputting the output of the first mixer and an input signal from the antenna, and a second mixer
A third mixer for inputting the output of the mixer and the output of the first local oscillator, and demodulation means for inputting the output of the third mixer.

Further, the reception state or the type of the input signal is identified using the input signal from the antenna or the signal from the demodulation means, and the frequency or amplitude of the signal source is changed according to the identification result.

Further, the reception state or the type of the input signal is identified by using the input signal from the antenna or the signal from the demodulation means, and whether to perform frequency modulation on the local oscillator is selected according to the identification result.

At the same time as mixing the frequency-modulated signal to convert the received signal to the baseband signal frequency,
By spreading the spectrum of the received signal in frequency, it is possible to suppress the loss of information of the signal due to the removal of the DC component, and after that, the spreading effect is removed by the removing means before demodulation.

Further, the signal spread as described above is input to the demodulator, and thereafter, the effect of spreading is removed from the output of the demodulator by the removing means.

Further, the effect of diffusion is removed by subtracting the voltage change of the signal source which is the source of frequency modulation from the output of the demodulator.

Further, the effect of diffusion is removed from the output of the demodulator by a low pass filter. In addition, the spectrum is spread by the first mixer at the same time as the frequency conversion, and the effect of the spread is removed by the second mixer.

Further, the type or state of the input signal is identified by the control circuit, and the state of frequency modulation is changed accordingly.

Further, the type or state of the input signal is identified by the control circuit, it is judged whether or not frequency modulation is performed, and the switch is turned on to perform frequency modulation.

Embodiments of the present invention will be described below with reference to the drawings. First, the principle that the direct conversion receiver according to the present invention suppresses the loss of signal information due to the removal of the direct current component will be described, and then the embodiment of the direct conversion receiver according to the present invention will be described.

FIG. 1 is a block diagram showing the principle of a direct conversion receiver according to an embodiment of the present invention. Here, a case of a binary FSK signal will be considered as an example of the received signal. In FIG. 1, 1 is a binary FSK signal source, 2 is a local oscillator, 3 is a mixer, 4 is a capacitor, 5 is a removing means, and 6 is a demodulator. The signal from the binary FSK signal source 1 is input to the mixer 3. The signal from the local oscillator 2 is also input to the mixer 3. Since the center frequency of the FSK signal source 1 and the center frequency of the local oscillator 2 are set to be substantially the same,
The signal from the FSK signal source 1 is converted into a baseband signal frequency by the mixer 3. The output of the mixer 3 is input to the capacitor 4 for removing the DC component.

FIG. 2 is a diagram showing the spectrum of a binary FSK signal. In FIG. 2, (a) shows a typical spectrum of a binary FSK signal. At the center frequency f0, the amount of frequency displacement is about 1.5 kHz. Frequencies f1 and f2
A spectrum peak can be seen at. In this case, the level near the center frequency f0 is 15 dB lower than these peaks.

First, consider the case where an unmodulated wave of frequency f0 is output from the local oscillator 2 in FIG. In this case, the signal from the FSK signal source 1 is sent to the mixer at the center frequency 0
It is converted to a Hz signal, but the shape of the spectrum does not change. Then, by passing through the capacitor 4, as shown in FIG.
The frequency components in the area shown in (a) of (a) are removed. Here, the component of 0 to 300 Hz is removed by the capacitor. Therefore, the signal passing through the capacitor 4 has some information lost. When the frequency of the local oscillator 2 is f0, the information that is lost is relatively small, so demodulation is possible in practice, but when the frequency of the local oscillator 2 shifts to f1 or f2, (a) in FIG. )
The area shown in is removed. In this case, demodulation becomes difficult because the amount of information lost is very large.

Next, consider the case where frequency modulation is applied to the local oscillator 2. The signal of the binary FSK signal source 1 is mixed with the signal from the local oscillator 2 to spread the spectrum. That is, the output of the mixer 3 is as shown in FIG. As can be seen from comparison with FIG. 2A, the sharp peak of the spectrum disappears and the shape is flat. The component lost by the capacitor 4 is in the region (a) of FIG. 2B when the center frequency of the local oscillator 2 is f1, and in the region (a) of f2. It can be seen that in any case, the amount of information lost is smaller than that in (a) of FIG. Therefore, it is possible to sufficiently demodulate using this signal.

However, if the above output is demodulated as it is, the original information cannot be obtained. That is, it is necessary to provide the removing means 5 for removing the effect of spreading the spectrum by the modulation signal of the local oscillator 2. The removing means 5 removes the above effects, and then the demodulator 6 performs demodulation.

(Embodiment 1) FIG. 3 is a block diagram showing the configuration of a first embodiment of a direct conversion receiver according to the present invention. In FIG. 3, 10 is an antenna, 11 is a mixer, 1
2 is a local oscillator, 13 is a signal source, 14 is removing means, and 15
Is a demodulator, 16 is a capacitor, 17 is a bandpass filter, and 18 is a 90-degree phase shifter.

The received signal from the antenna 10 is divided into two,
It is input to each of the two mixers 11. Further, the local oscillator 12 outputs a signal frequency-modulated by the signal from the signal source 13. Here, the signal of the signal source 13 can be a sine wave, but an arbitrary signal can be used. Further, the frequency and amplitude of the signal source 13 can be set arbitrarily. However, in order to spread the spectrum effectively, there is an optimum value corresponding to the modulation frequency and frequency displacement of the received signal. The output of the local oscillator 12 is divided into two and is input to the mixer 11. One of the outputs is phase-shifted by a 90-degree phase shifter and then input to the mixer 11. Here, the center frequency of the local oscillator 12 is set to be the same as the center frequency of the received signal, so that the received signal is frequency-converted by the mixer 11 into a baseband signal. The output of the mixer 11 is input to the removing means 14 through the capacitor 16 and the low pass filter 17. The low pass filter 17 is inserted in order to remove the component of the adjacent channel. The removing means 14 is provided to remove the effect of mixing the signal of the local oscillator 12 modulated by the signal source 13. As described above, the signal of the signal source 13 can be arbitrarily selected, and since this is known in advance, the removing means 14 can be set so as to remove the above effect.
The output of the removing means 14 is input to the demodulator 15 and demodulated.

(Embodiment 2) FIG. 4 is a block diagram showing the configuration of a second embodiment of the direct conversion receiver according to the present invention. 4, the same components as those in FIG. 3 are given the same numbers.

The difference from the first embodiment is that the output of the demodulator 15 is input to the removing means. In the first embodiment, the operation of removing the effect of the signal source 13 is performed using the output of the mixer, that is, the signal before demodulation, but in the second embodiment, the removal operation is performed after demodulation. The output of the demodulator 15 has a voltage change corresponding to the frequency change of the input of the demodulator 15. The original signal can be obtained by removing the effect of the signal source 13 from this voltage change. In this embodiment, since the removing operation is performed based on the change in the power of the demodulator output, there is an advantage that the circuit of the removing means can be easily configured.

(Third Embodiment) FIG. 5 is a block diagram showing the configuration of a third embodiment of the direct conversion receiver according to the present invention. 5, the same components as those in FIG. 4 are given the same numbers. In FIG. 5, 19 is a variable attenuator,
20 is a delay line, 21 is a subtractor, and 22 is an output terminal. The difference from the second embodiment is that the signal from the signal source 13 is used to remove the effect of spreading the spectrum. The signal from the signal source 13 is input to the subtractor 21 through the variable attenuator 19 and the delay line 20. On the other hand, the output of the demodulator 15 is also input to the subtractor 21. The difference between the voltages of these two signals is output from the subtractor 21, and an output signal can be obtained at the output terminal 22. At this time, the variable attenuator 19 and the delay line 20 are adjusted so that the component of the signal source 13 is canceled from the output of the demodulator 15. This embodiment has the following advantages. That is, since the spectrum is spread and its effect is removed based on the signal source 13, a complicated removing means is not required and the configuration can be simplified. Further, the removal can be performed with high accuracy. Furthermore, even if the signal source is arbitrarily changed, adjustment of the removing means is unnecessary.
For example, when the frequency or amplitude of the signal source 13 is changed, the signal input to the subtractor 21 also changes, so that adjustment of the subtractor 21 is unnecessary. The subtractor 21 is
It can be easily configured with a differential amplifier such as an operational amplifier. More simply, it is possible to combine several resistors.

(Fourth Embodiment) FIG. 6 is a block diagram showing the configuration of a fourth embodiment of the direct conversion receiver according to the present invention. 6, the same components as those in FIG. 4 are given the same numbers. In FIG. 6, 30 is a low pass filter. The difference from the second embodiment is that the low-pass filter 30 is used to remove the effect of spreading the spectrum. By setting the cutoff frequency of the low-pass filter 30 lower than the frequency of the signal source 13, the component of the signal source 13 can be removed from the output of the demodulator 15. In this embodiment, there is an advantage that the removing means can be composed of only a filter. However, in order to selectively remove the component of the signal source 13 from the output of the demodulator 15, the modulation frequency of the received signal and the frequency of the signal source 13 need to have different values. When the low pass filter is used as described above, the frequency of the signal source needs to be higher than the modulation frequency of the received signal. Low pass filter is LC filter, R
A C filter, an active filter, or the like can be adopted, and realization is easy. A band stop filter may be used instead of the low pass filter.

(Fifth Embodiment) FIG. 7 is a block diagram showing the structure of a fifth embodiment of the direct conversion receiver according to the present invention. In FIG. 7, the same components as those in FIG. 3 are given the same numbers. In FIG. 7, 40 is a first local oscillator, 41 is a second local oscillator, and 42 is a mixer. The feature of this embodiment resides in a method of removing the effect of spreading the spectrum of the received signal by the signal from the signal source 13. That is, the output of the mixer 11 and the output of the second local oscillator that has been frequency-modulated by the signal from the signal source 13 are input to the mixer 11 for removal. Here, the first local oscillator 40 and the second local oscillator 41 output the same frequency-modulated signals by the signal from the signal source 13. However, the center frequency of the first local oscillator 40 is set to be substantially the same as the center frequency of the received signal, but the center frequency of the second local oscillator 42 can be set arbitrarily. However, considering the demodulation operation in the demodulator 15,
It is appropriate to set the frequency to several tens kHz to several hundreds kHz. As described above, since the first local oscillator 40 and the second local oscillator 41 are frequency-modulated by the same signal source 13, there is an advantage that the component of the signal source 13 can be surely removed by the mixer 42. Further, there is an advantage that the frequency of the second local oscillator 42 can be set to a frequency convenient for demodulation operation in the demodulator 15.

(Sixth Embodiment) FIG. 8 is a block diagram showing the configuration of a sixth embodiment of the direct conversion receiver according to the present invention. 8, the same components as those in FIG. 7 are denoted by the same reference numerals. In FIG. 8, 50 is a mixer. The difference from the fifth embodiment lies in the method of generating the frequency modulation signal input to the mixer 11. That is, the output of the first local oscillator 51, which is frequency-modulated by the signal of the signal source 13, is mixed with the output of the second local oscillator 52 by the mixer 50 to be converted into the same center frequency as the received signal. It is input to the mixer 11. In addition, the first local oscillator 4
The output of 0 is also input to the mixer 42. In this embodiment, only the first local oscillator 40 is frequency-modulated. Therefore, the signal input to the mixer 11 and the signal input to the mixer 42 are subjected to exactly the same modulation. That is, there is an advantage that the operation of removing the component of the signal source 13 performed by the mixer 42 becomes more accurate. When the reception frequency of the receiver is changed to several channels and used, the second local oscillator 41 is used for channel switching, and the first local oscillator 40 is used for modulation signal generation. There is an advantage that functions can be shared.

(Embodiment 7) FIG. 9 is a block diagram showing the configuration of a seventh embodiment of the direct conversion receiver according to the present invention. In FIG. 9, the same components as those in FIG. 3 are given the same numbers. In FIG. 9, reference numeral 60 is a control circuit.
The difference from the first embodiment is that the output signal of the demodulator 15 is input to the control circuit 60 and the setting of the signal source 13 is changed by the control signal from the control circuit 60. The purpose of this embodiment is to suppress the loss of some information of the received signal by the capacitor 16 for removing the DC component, but the amount of information lost is actually different depending on the type or state of the received signal. ing. Particularly when the frequency displacement is subject to a small modulation, the amount of information lost due to the influence of the capacitor increases. There are also differences depending on the type of modulation. According to this, the frequency and amplitude of the signal source 13 are optimally set. The type and state of the received signal is identified by the control circuit 60 using the output from the demodulator 15, and the frequency and amplitude of the signal source 13 are determined. This allows stable demodulation even when the type and state of the received signal changes.

(Embodiment 8) FIG. 10 is a block diagram showing the configuration of an eighth embodiment of the direct conversion receiver according to the present invention. In FIG. 10, the same components as those in FIG. 9 are given the same numbers. In FIG. 10, 70 is a switch. The difference from the seventh embodiment is that the switch 70 is operated by the control signal from the control circuit 60.
When the switch 70 is on, the signal from the signal source 13 causes the local oscillator 12 to undergo frequency modulation. For example, 2
With the value FSK signal, as shown in FIG. 2A, when the frequency of the local oscillator 12 is near the center frequency of the received signal, demodulation is possible without adding modulation to the local oscillator 12. If the deviation occurs, it becomes difficult to demodulate rapidly. Therefore, the control circuit 60 identifies that the frequency of the local oscillator 13 is deviated by using the signal from the demodulator 15, and the control circuit 60 modulates the local oscillator 12 by turning on the switch 70. This allows demodulation. In other words, operate only when demodulation is difficult,
Demodulation can be enabled. However, depending on the configuration of the demodulator 15, there is one that does not output when the frequency of the local oscillator 12 is deviated. You can also do

[0037]

As is apparent from the above description, the following effects can be obtained by the direct conversion receiver of the present invention. A signal source, a local oscillator that is frequency-modulated by a signal from the signal source, a mixer that inputs the output of the local oscillator and an input signal from the antenna, and a removing unit that removes the frequency component of the signal source from the output of the mixer. By including the demodulating means for inputting the output of the removing means, stable demodulation can be performed even when the modulation index of the received signal is small or the frequency of the local oscillator is deviated.

Further, a signal source, a local oscillator frequency-modulated by a signal from the signal source, a mixer for inputting the output of the local oscillator and an input signal from the antenna, demodulation means for inputting the output of the mixer, and demodulation The circuit of the removing means can be easily configured by providing the removing means for removing the voltage change component corresponding to the voltage change of the signal source from the output of the means.

Further, by adjusting the phase and amplitude of the output from the demodulation means and the signal from the signal source to obtain the difference,
By canceling the voltage change component of the signal source from the output of the demodulation means, the removal operation can be performed with high accuracy in addition to the above effects. Further, even if the signal source is arbitrarily changed, the adjustment of the removing means becomes unnecessary.

Further, by providing the removing means with a low-pass filter or a band-stop filter for attenuating the frequency component of the signal source, the removing means can be constituted by only the filter.

A signal source, a first local oscillator frequency-modulated by a signal from the signal source, a first mixer for inputting an output of the first local oscillator and an input signal from the antenna, and a signal source Second frequency-modulated by the signal from
Of the local mixer, the second mixer for inputting the output of the first mixer and the output of the second local oscillator, and the demodulation means for inputting the output of the second mixer, the removal operation is further ensured. The frequency of the second local oscillator,
The frequency can be set to a frequency convenient for demodulation operation in the demodulator.

Further, the signal source, the first local oscillator frequency-modulated by the signal from the signal source, the second local oscillator, the output of the first local oscillator and the output of the second local oscillator are input. A first mixer, a second mixer for inputting the output of the first mixer and an input signal from the antenna, and a second mixer
In addition to the above effects, by providing a third mixer that inputs the output of the mixer and the output of the first local oscillator, and a demodulation unit that inputs the output of the third mixer, The function can be shared by the second local oscillator.

Further, the reception state or the type of the input signal is discriminated using the input signal from the antenna or the signal from the demodulation means, and the frequency or amplitude of the signal source is changed according to the discrimination result. Even when the type and state change, stable demodulation can be performed.

Further, the reception state or the type of the input signal is identified using the input signal from the antenna or the signal from the demodulation means, and whether the local oscillator is frequency-modulated or not is selected according to the identification result. The demodulation can be enabled by operating only when it is difficult to demodulate.

[Brief description of drawings]

FIG. 1 is a block diagram showing the principle of a direct conversion receiver of the present invention.

FIG. 2A shows a spectrum of a binary FSK signal without FM modulation. FIG. 2B shows a spectrum of a binary FSK signal with FM modulation.

FIG. 3 is a block diagram showing a configuration of a first embodiment of a direct conversion receiver according to the present invention.

FIG. 4 is a block diagram showing the configuration of a second embodiment of the direct conversion receiver according to the present invention.

FIG. 5 is a block diagram showing the configuration of a third embodiment of the direct conversion receiver according to the present invention.

FIG. 6 is a block diagram showing the configuration of a fourth embodiment of a direct conversion receiver according to the present invention.

FIG. 7 is a block diagram showing the configuration of a fifth embodiment of the direct conversion receiver according to the present invention.

FIG. 8 is a block diagram showing the configuration of a sixth embodiment of the direct conversion receiver according to the present invention.

FIG. 9 is a block diagram showing the configuration of a seventh embodiment of the direct conversion receiver according to the present invention.

FIG. 10 is a block diagram showing the configuration of an eighth embodiment of the direct conversion receiver according to the present invention.

FIG. 11 is a block diagram showing the configuration of a conventional direct conversion receiver.

[Explanation of symbols]

 10 antenna 11 mixer 12 local oscillator 13 signal source 14 removing means 15 demodulator 16 capacitor 17 low pass filter 18 90 degree phase shifter 19 variable attenuator 20 delay line 21 subtractor 30 low pass filter 40 first local oscillator 41 second Local oscillator 42 Mixer 50 Mixer 60 Control circuit 70 Switch

Claims (8)

[Claims] 1. A signal source, a local oscillator frequency-modulated by a signal from the signal source, a mixer for inputting an output of the local oscillator and an input signal from an antenna, and an output of the mixer for converting the signal source to A direct conversion receiver comprising a removing means for removing a frequency component and a demodulating means for receiving an output of the removing means. 2. A signal source, a local oscillator frequency-modulated by a signal from the signal source, a mixer for inputting the output of the local oscillator and an input signal from an antenna, and demodulation means for inputting the output of the mixer. And a removing means for removing the voltage change component corresponding to the voltage change of the signal source from the output of the demodulating means. 3. The voltage change component of the signal source is canceled from the output of the demodulating means by adjusting the phase and amplitude of the output from the demodulating means and the signal from the signal source to obtain a difference. Direct conversion receiver. 4. The direct conversion receiver according to claim 2, wherein the removing means comprises a low pass filter or a band stop filter for attenuating the frequency component of the signal source. 5. A signal source, a first local oscillator that is frequency-modulated by a signal from the signal source, and a first mixer that receives an output of the first local oscillator and an input signal from an antenna. A second local oscillator frequency-modulated by a signal from the signal source, a second mixer for inputting the output of the first mixer and the output of the second local oscillator, and an output of the second mixer A direct conversion receiver equipped with demodulation means for inputting. 6. A signal source, a first local oscillator frequency-modulated by a signal from the signal source, a second local oscillator, an output of the first local oscillator, and a second local oscillator. A first mixer for inputting an output, a second mixer for inputting an output of the first mixer and an input signal from an antenna, an output of the second mixer and an output of the first local oscillator are input. A direct conversion receiver including a third mixer that performs the above, and demodulation means that inputs the output of the third mixer. 7. The reception state or the type of the input signal is discriminated using the input signal from the antenna or the signal from the demodulation means, and the frequency or amplitude of the signal source is changed according to the discrimination result. The direct conversion receiver according to claim 1. 8. A receiving state or a type of an input signal is identified using an input signal from an antenna or a signal from a demodulation means, and whether to perform frequency modulation on a local oscillator is selected according to the identification result. The direct conversion receiver according to any one of 1 to 6.