JPH0964924A - Fsk demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an FSK demodulator with a high dynamic range in which the presence of a signal is detected even under a lower reception electric field strength. SOLUTION: The demodulator is provided with a 1st filter 21 to pass through only a prescribed frequency component from a radio signal to extract a data signal, a 2nd filter 22 to pass through only a prescribed frequency component from the radio signal to extract a preamble signal, a signal presence detection means 50 to detect the preamble signal included in the radio signal to detect the presence of the data signal, a data extract means 40 to extract data from the radio signal, and changeover means 30, 31 receiving a switching control signal from the signal presence detection means 50 to select the 1st and 2nd filters 21, 22.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an FSK demodulator in a mobile terminal system. In a mobile terminal system, for example, in communication between a mobile terminal and a radio base station, FS
It is exchanged by a signal such as a K modulation method. In the FSK modulation method, as shown in FIG. 13, numerical values "0" and "1" are assigned to different frequencies f1 and f2, respectively. Therefore, in order to restore the data from the signal transmitted by such a modulation method, there is a demodulation circuit that demodulates the frequency into numerical values "0" and "1".

FIG. 14 is a block diagram showing a conventional configuration example of an FSK demodulator. In the figure, 1 is the first antenna,
2 is a second antenna. Reference numeral 3 is a switch for selecting one of these antenna outputs. The signal received from the antenna has its noise removed by the subsequent filter 4, is amplified by the low noise amplifier 5, and enters the mixer 6. Reference numeral 7 is a local oscillator that enters the other input of the mixer 6. The frequency of the signal received by the antenna is, for example, 2.4 GHz. Therefore, when a frequency of 2.1 GHz is applied to the mixer 6 from the local oscillator 7, the mixer 6 mixes these two signals.
Then, from the mixer 6, 2.4 GHz and 2.1 GH
The frequency difference of 300 MHz is output. In this way, the signal lower than the frequency of the received signal is I
It is called an F signal (intermediate frequency signal).

This IF signal enters the subsequent bandpass filter 8 and the high frequency component generated by the mixer 6 is removed. The output of the filter 8 enters a limiter amplifier 9. The limiter amplifier 9 is formed by connecting a plurality of amplifiers in series, and makes it constant (amplitude limiting) that the amplitude increases or decreases depending on the received power. The output of the limiter amplifier 9 enters a discriminator (frequency discriminator) 10. The discriminator 10 converts an input frequency signal into a voltage signal corresponding to the frequency.

The output voltage of the discriminator 10 enters a comparator 11 and is compared with a reference value to obtain "0",
It is converted into digital data of "1". The output of the comparator 11 is the subsequent BTR (Bit Timing).
After being subjected to bit synchronization by the Recovery circuit, it is sent to the circuit in the subsequent stage.

On the other hand, the saturation current of the limiter amplifier 9 is converted into a voltage signal (RS) by a current / voltage conversion resistor (not shown).
SI signal: Received Signal Str
length Indication). This RSSI signal is converted into a digital signal by the subsequent A / D converter 13. This digital signal is compared with a predetermined threshold value set in the threshold value setting unit 16, and if it is larger than the threshold value, it is determined that there is a transmission signal, and the circuit starts receiving the radio signal. Here, the control unit 14 compares the RSSI value when it is received by the antenna 1 with the RSSI value when it is received by the antenna 2, and sends a switching signal to the switch 3 to select the antenna with the larger value. give. By performing such diversity control, the bit error rate (bit error rate) is improved.

FIG. 15 is a diagram showing an example of RSSI characteristics.
The vertical axis represents the RSSI voltage [V], and the horizontal axis represents the incoming power of the wireless signal [dBm]. The area A in the drawing is the dynamic range, and this dynamic range affects the performance of the circuit, but this dynamic range depends exclusively on the limiter amplifier 9.

[0007]

By the way, the problem with a radio receiver is that it can communicate even at low reception field strengths.
Therefore, the low noise amplifier (the low noise amplifier 5 described above)
And the performance of filters are being studied. However, the detection of the presence or absence of a signal must be reliably performed even with a low received electric field strength. This largely depends on the dynamic range of the limiter amplifier 9. Therefore, it is necessary to expand the dynamic range of the limiter amplifier 9.

That is, the dynamic range cannot be expanded without improving the performance of the limiter amplifier 9. That is, it is unlikely that the reception performance will be improved at low reception electric field strength. here,
Considering the limiter amplifier 9, the limiter amplifier performs limiting (amplitude limitation) by connecting several amplifiers in series and using them in a saturation region. That is, at low received electric field strength, noise is also amplified at the same time. Therefore, it is difficult to expand the dynamic range because the operation at lower received electric field strength is hindered. The noise is mainly caused by the low noise amplifier 5.

The present invention has been made in view of the above problems, and has a wide dynamic range capable of detecting the presence or absence of a signal even with a lower received electric field strength.
The purpose is to provide a demodulator.

[0010]

FIG. 1 is a block diagram showing the principle of the present invention. The same components as those in FIG. 14 are denoted by the same reference numerals. In the figure, 1 is an antenna, 20 is a radio signal reception processing unit that extracts and amplifies only a signal component from a signal received by the antenna 1, and 6 is an input of one of the outputs of the radio signal reception processing unit 20. The mixer 7 is a local oscillator that gives a local frequency to the mixer 6.
Reference numeral 30 denotes a first switcher for receiving the output of the mixer 6, 21
Is a first filter connected to one side of the first switching unit 30, and 22 is a second filter also connected to the other side of the first switching unit 30. Reference numeral 31 is a second switcher which is connected to these filters and selects one of them.

The first switching device 30 and the second switching device 31 constitute a switching means for switching the first and second filters.

The first filter 21 is for passing only a predetermined frequency component from the radio signal to extract the data signal, and the second filter 22 is for passing only the predetermined frequency component from the radio signal. It is for extracting the preamble signal.

Reference numeral 9 is a limiter amplifier for receiving the output of the second switching device 31, 40 is a data extracting means for receiving the output of the limiter amplifier 9 and extracting data from a radio signal,
Reference numeral 50 is a signal presence / absence detecting means for detecting the presence / absence of a data signal by detecting the preamble signal included in the radio signal. From the signal presence / absence detection means 50, the first and second
A switching control signal is applied to the switching devices 30 and 31.

The structure of the present invention is characterized in that the second filter 22 for accurately extracting only the preamble signal is provided. Then, the second filter 22 allows only the preamble signal component to pass therethrough, and the signal presence / absence detection means 50 can accurately detect the presence / absence of data from the output of the second filter 22. Therefore, it is possible to provide an FSK demodulator with a wide dynamic range capable of detecting the presence or absence of a signal even with a lower received electric field strength.

In this case, diversity control is performed from the output of the second filter. According to the configuration of the present invention, the signal presence / absence detecting means is R
Since the switching means is controlled so as to select the antenna with the higher SSI, it is possible to reliably detect the presence or absence of a signal even with a lower received electric field strength.

Further, two switching means are provided after the first filter, and a second filter for detecting a preamble is connected in parallel to these switching means, and when the preamble signal is received, the switching means is provided with the first switching means. The output of the filter is made to pass through the second filter, and the two changeover switch means pass the output of the first filter 1 as it is when receiving data.

According to the structure of the present invention, by operating the second filter only when detecting the presence / absence of a data signal, the presence / absence of a signal can be reliably detected even at a lower received electric field strength. You can

Further, a mixer is added before and after the second filter to convert the frequency to a predetermined frequency, a comb filter for preamble is applied, and then the original frequency is restored.

According to the structure of the present invention, the reception frequency can be lowered by the mixer to facilitate the production of the comb filter for the preamble.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 2 is a block diagram showing the essential parts of the first embodiment of the present invention. 1 and 1
The same parts as 4 are designated by the same reference numerals. In the figure, 6 is a mixer for mixing the received signal and the signal of the local oscillator 7. Although not shown, the antenna, the filter 4, and the low noise amplifier 5 configure the radio signal reception processing unit 20 of FIG. 1.

Reference numeral 30 is a first switching device (hereinafter abbreviated as switch SW1) for switching the filter, and 31 is a second switching device (hereinafter abbreviated as switch SW2) for similarly switching the filter. These switching devices 3
As 0 and 31, for example, analog switches that operate in the video band are used. These switches SW1 and SW
A first filter 21 (hereinafter abbreviated as filter 1) and a second filter 22 (hereinafter abbreviated as filter 2) are connected between the two.

The filter 1 is a filter used when receiving signal data (hereinafter simply referred to as data), and the filter 2 is a filter used when detecting the presence or absence of a signal from the preamble. Reference numeral 9 is a limiter amplifier which receives the output of the switch SW2, and 10 is a discriminator which receives the output of the limiter amplifier 9 and converts a frequency signal into a voltage signal. 13 is the RSS of the limiter amplifier 9 as well.
An A / D converter that receives an I signal and converts it into digital data. Reference numeral 15 is a comparator for comparing the output of the A / D converter 13 with a predetermined threshold value of the threshold value setting section 16.
The switch SW receives the output of the comparator 15
This is a control unit that controls switching between 1 and SW2. A /
The D converter 13, the comparator 15, and the control unit 17 constitute the signal presence / absence detection means 50 of FIG. 1, and the circuits after the discriminator 10 constitute the data extraction means 40 of FIG. The operation of the circuit thus configured will be described as follows.

A burst signal is transmitted from a radio base station (not shown). The frame of the burst signal has the format shown in FIG. That is, it is composed of a preamble portion D1 composed of alternating signals of 0 and 1, and a data portion D2. FIG. 4 is a diagram showing the transmission spectrum of the data part and the preamble part. The vertical axis represents intensity and the horizontal axis represents frequency. In the data part, since 0 and 1 data appear at random, this spectrum is as shown in (a). On the other hand, in the preamble part, since the data of 0 and 1 alternately appear, the spectrum thereof is as shown in (b).

In order to pass only the data and preamble component having such respective characteristics, the filter 1
Has a characteristic as shown in FIG. 5, and the filter 2 has a characteristic as shown in FIG. In each case, the vertical axis is intensity and the horizontal axis is frequency. Conventionally, since both the preamble part and the data part have passed through the filter 1, the optimum frequency component filtering process is performed on the data, but there is a problem in the filtering process on the preamble.

FIG. 6 is an explanatory diagram for comparing the filter characteristic and the spectrum of the preamble. The characteristic of the filter 1 is fi
11 and the spectrum of the preamble for such a filter characteristic is as shown by fil2. That is, in the conventional filter, the frequency of the preamble is passed, but at the same time, the noise component around it (noise between components appearing at frequency intervals of half the bit rate) is also passed. Therefore, the remaining noise is amplified by the limiter amplifier 9, which hinders the determination of the presence or absence of a signal and the operation of the BTR 12 (see FIG. 14).

Therefore, in the present invention, the preamble part is made to pass the filter 2, which passes only the frequency component shown in FIG. 4B, instead of the filter 1. FIG. 7 is a diagram showing a characteristic example of the filter 2. FIG.
In order to allow only the frequency component shown in (b) of FIG. 4 to pass, it has a plurality of peaks with a narrow pass band. For this reason,
It is called a comb filter.

At the time of waiting for a signal, the control section 17 gives a control signal to the switches SW1 and SW2 to select the filter 2 side. The radio signal dropped to the IF signal of, for example, 300 MHz by the mixer 6 is passed by the filter 2 only the frequency component of the preamble. The characteristic of the filter 2 is a characteristic of passing only the preamble component as shown in FIG. This gives the signal-to-noise ratio (SNR)
Is much improved. The spectrum of the preamble that has passed through the filter 2 does not include a noise component, as shown in FIG. This wireless signal is amplified by the limiter amplifier 9 and outputs an RSSI voltage signal.

Since the noise is reduced, the dynamic range of the limiter amplifier 9 is expanded and the output noise is also reduced. This RSSI signal is supplied to the A / D converter 13
Are converted into digital data by. Since the noise superimposed on the RSSI is reduced, the signal fluctuation of the A / D converter 13 is reduced. The output data of the A / D converter 13 is compared with a threshold value by the subsequent comparator 15. Since the output data of the A / D converter 13 at this time does not include a noise component, the comparator 15 stabilizes the comparison with the threshold value, and the signal detection probability is improved. In this way, the comparator 15 can accurately detect the presence or absence of the preamble. The comparator 15 outputs "1" when RSSI is larger than the threshold value.
Output level signal. This "1" level indicates that there is a preamble. That is, it indicates that a wireless signal is coming. When the level is "0", it indicates that there is no preamble. That is, it indicates that no radio signal is coming.

"1" indicating that the RSSI of the limiter amplifier 9 is larger than the threshold value and the reception signal of the comparator 15 is present.
When the signal is recognized, the control unit 17 waits for a certain time and then switches the switches SW1 and SW2 to the filter 1 side.
The circuit shown in the figure then uses the filter 1 to receive the normal data portion.

As described above, according to the present invention, by using the filter 2 that passes only the preamble signal component, it is possible to accurately detect the presence or absence of the incoming of the radio signal even with a lower received electric field strength. Therefore, it is possible to provide an FSK demodulator with a wide dynamic range that can detect the presence or absence of a signal even with a lower received electric field strength.

FIG. 9 is a block diagram showing the essential parts of a second embodiment of the present invention. The same parts as those in FIGS. 2 and 14 are designated by the same reference numerals. In this embodiment, two antennas 1 and 2 are provided, and the one with higher sensitivity is selected. The antennas 1 and 2 are input to a switch 3, and the switch 3 supplies one of these antenna outputs to a filter 4. The output of the switch 3 enters the filter 4, the noise component is removed, and then it is amplified by the low noise amplifier 5 and given to the mixer 6. The output frequency of the low noise amplifier 5 is 2.4.
GHz. This frequency signal is input to one input of the mixer 6. On the other hand, 2.1 GHz from the local oscillator 7
Signal enters the other input of the mixer 6. These signals are mixed by the mixer 6 and become an intermediate frequency signal of 300 MHz.

The output of mixer 6 enters switch SW1.
The output of the switch SW1 is connected to either the filter 1 or the filter 2, and the output of the filter enters the limiter amplifier 9 via the switch SW2. The RSSI output of the limiter amplifier 9 is converted into digital data by the A / D converter 13. The output of the A / D converter 13 enters the control unit 17. Then, the control unit 17 causes the switching unit 3 and the switch SW1.
Switching control of SW2 and SW2 is performed. The operation of the circuit thus configured will be described as follows.

First, the control unit 17 fixes the antenna to one of the antennas (for example, the antenna 1) and sets the filter to the filter 2.
Fixed to. In this state, the RSSI of the limiter amplifier 9
The output is measured by the A / D converter 13. This measured value is stored in the control unit 17. Next, the control unit 17 fixes the antenna on the other side (for example, the antenna 2) and fixes the filter on the filter 2. In this state, the RSS of the limiter amplifier 9
The I output is measured by the A / D converter 13. This measured value is stored in the control unit 17. The control unit 17 selects the antenna with the higher one of the two stored RSSI values.

In this way, the antenna is fixed and the presence or absence of a received signal is detected. According to this embodiment, since the SNR at the time of receiving the preamble is improved, the noise superimposed on the RSSI is reduced. As a result, the signal fluctuation of the data converted into digital data by the A / D converter 13 is reduced. Therefore, in diversity control in which the larger RSSI is selected, the accuracy of antenna selection is improved. As a result, the error rate is improved even in normal reception, and the error rate at low reception field strength is improved. According to this embodiment, since the control unit 17 selects the switcher 3 so as to select the antenna with the higher RSSI, the presence or absence of a signal can be reliably detected even with a lower received electric field strength.

When receiving normal data, the control unit 1
Reference numeral 7 gives a switching signal to the switches SW1 and SW2, selects the filter 2 and receives normal data. FIG.
Reference numeral 0 is a block diagram showing an essential part of the third embodiment of the present invention. The same components as those in FIG. 2 are denoted by the same reference numerals. In this embodiment, a filter 1 is directly connected after a mixer 6, and a switching circuit composed of switches SW1 and SW2 and a filter 3 is provided after the filter 1. One contact of the switches SW1 and SW2 is directly connected, and the other contact is connected through the filter 3. The control unit 17 receives the output of the comparator 15 and controls the switching of the switches SW1 and SW2. The operation of the circuit thus configured will be described as follows.

The control section 17 gives a switching signal to the switches SW1 and SW2 to select the filter 2. The radio signal received in this state becomes an intermediate frequency signal by the mixer 6 and enters the filter 1. Since the filter 1 has the characteristics shown in FIG. 5, only the signal containing the signal component is passed. The signal that has passed through filter 1 then enters filter 2. The filter 3 has the characteristics as shown in FIG. 11, and only the comb-shaped peak in the central portion is passed. This filter has a characteristic of passing both ends of the comb-shaped portion in the figure, but since the portions corresponding to both ends are already removed by the filter 1, the characteristic of this filter becomes substantially the same as the characteristic shown in FIG. . As a result, filter 3
From which noise is removed and only the preamble component is output.

This radio signal enters the limiter amplifier 9 that follows and is amplified. The limiter amplifier 9 outputs an RSSI voltage signal. In this case, since the noise is reduced by the filter 3, the dynamic range of the limiter amplifier 9 is expanded and the output noise is also reduced. This RSSI signal is converted into digital data by the A / D converter 13. Since the noise superimposed on the RSSI is reduced, the signal fluctuation of the A / D converter 13 is reduced. The output data of the A / D converter 13 is compared with a threshold value by the subsequent comparator 15. Since the output data of the A / D converter 13 at this time does not include a noise component, the comparator 15 stabilizes the comparison with the threshold value, and the signal detection probability is improved. In this way, the comparator 15 can accurately detect the presence or absence of the preamble. The comparator 15 outputs a "1" level signal when it is larger than the threshold value. This "1" level indicates that there is a preamble. That is, it indicates that a wireless signal is coming. When the level is "0", it indicates that there is no preamble. That is, it indicates that no radio signal is coming.

"1" indicating that the RSSI of the limiter amplifier 9 is larger than the threshold value and the reception signal of the comparator 15 is present.
When the signal is recognized, the control unit 17 waits for a certain time and then switches the switches SW1 and SW2 to the filter 1 side.
The circuit shown in the figure then uses the filter 1 to receive the normal data portion.

According to this embodiment, by operating the second filter (filter 2) only when detecting the presence / absence of a data signal, the presence / absence of a signal can be reliably detected even with a lower received electric field strength. it can.

FIG. 12 is a block diagram showing the essential parts of a fourth embodiment of the present invention. The same components as those in FIG. 10 are denoted by the same reference numerals. In this embodiment, a comb filter is easily made. When a comb filter is used as the filter 2, the width Tw (see FIG. 7) of each peak is extremely small. Comb filters with small width peaks are generally difficult to make. Therefore, in this embodiment, after the frequency of the received radio signal is lowered to a lower frequency by the mixer, a comb filter is applied to this low frequency, and the radio signal passed through the comb filter is returned to the original frequency. It was done like this.

In the figure, 23 has a switch S at one end.
A filter connected to W1 (hereinafter abbreviated as filter 4), 34 is a mixer for receiving the output of the filter 5 at one input, 32 is a local oscillator, and 33 is an output of the local oscillator 32 in two directions. It is a hybrid. One output of the hybrid 33 is the mixer 3
It is in the other input of 4. 24 is a comb filter (hereinafter abbreviated as filter 5) that receives the output of the mixer 34,
The output of the filter 5 to one input and the hybrid 3
3 is a mixer that receives the output of 3 at the other input. 25 is
A filter for receiving the output of the mixer 35 (hereinafter referred to as filter 6
Abbreviated). The operation of the circuit thus configured will be described as follows.

When the preamble is detected, the control unit 17
A switching signal is given to the switches SW1 and SW2 to set the contact to the lower side. As a result, the radio signal converted into the intermediate frequency by the mixer 6 enters the filter 4 via the switch SW1. The filter 4 removes harmonics generated by the mixer 6. Here, the output frequency of the filter 4 is set to fa. The output of the filter 4 enters the mixer 34,
The frequency given by the hybrid 33 is converted into a signal of frequency fb.

As a result, the comb filter (filter 5)
Is easier to make because the Q value of the filter is smaller than that in the case of fa. The radio signal converted to the frequency fb is allowed to pass only a predetermined frequency component by the filter 5. The output of the filter 5 enters the mixer 35, and the local frequency (fb-fa) is input to the mixer 35 from the hybrid 33, and the mixed signal is mixed back to the original frequency fa signal. The output of the mixer 35 enters the filter 4, and the harmonics generated by the mixer 35 are removed. The radio signal of frequency fa from which the harmonics have been removed is noise-free.

This radio signal enters the limiter amplifier 9 via the switch SW2 and is amplified. The limiter amplifier 9 outputs an RSSI voltage signal. In this case,
Since the noise is reduced by the filter 2, the dynamic range of the limiter amplifier 9 is widened, and the output noise is also reduced. This RSSI signal is sent to the A / D converter 1
It is converted by 3 into digital data. RSSI
By reducing the noise superimposed on, the signal fluctuation of the A / D converter 13 is reduced. The output data of the A / D converter 13 is compared with a threshold value by the subsequent comparator 15. Since the output data of the A / D converter 13 at this time does not include a noise component, the comparator 1
According to 5, the comparison with the threshold value becomes stable and the signal detection probability is improved. In this way, the comparator 15 can accurately detect the presence or absence of the preamble. The comparator 15 outputs a "1" level signal when it is larger than the threshold value. This "1" level indicates that there is a preamble. That is, it indicates that a wireless signal is coming. When the level is "0", it indicates that there is no preamble. That is, it indicates that no radio signal is coming.

"1" indicating that the RSSI of the limiter amplifier 9 is larger than the threshold value and the reception signal of the comparator 15 is present.
When the signal is recognized, the control unit 17 waits for a certain time and then switches the switches SW1 and SW2 to the filter 1 side.
The circuit shown in the figure then uses the filter 1 to receive the normal data portion. The filters 4 and 5 shown in the figure may be bandpass filters (BPF) or lowpass filters (LPF) as long as they can remove harmonics.

According to this embodiment, the reception frequency can be lowered by the mixer to facilitate the production of the comb filter for the preamble. Similarly, in diversity control, the bit error rate is improved because the RSSI variation is reduced and the antenna selection error is reduced. Also, the operation of the discriminator 10 is stable, and the SN of the output signal is
R is also improved. That is, the operation of the BTR is improved.

In the above embodiment, the switches SW1 and SW
Although the case where two switches are provided independently has been shown, these switches may have a hybrid configuration. As described above, according to the present invention, noise entering the limiter amplifier is reduced, so that the dynamic range of RSSI is widened and the signal detection accuracy is improved. Further, since the fluctuation of RSSI due to noise is reduced, diversity control is stabilized. This improves the bit error rate. Further, since the SNR of the preamble is improved, the synchronization establishment of BTR or the like becomes fast and accurate. As described above, excellent communication (for example, a bit error rate of 10 ⁻⁵ or less) can be performed even with a low reception electric field strength.

[0048]

As described above in detail, according to the present invention, the first filter for passing only the predetermined frequency component from the radio signal to extract the data signal and the predetermined filter from the radio signal. A second filter for extracting a preamble signal by passing only a frequency component, a signal presence / absence detecting unit for detecting a preamble signal included in a radio signal to detect the presence / absence of a data signal, and data from the radio signal A lower received electric field strength is provided by including a data extraction unit for extracting and a switching unit for receiving one of the switching control signals from the signal presence / absence detection unit and selecting one of the first and second filters. However, FSK with a wide dynamic range that can detect the presence or absence of a signal
A demodulator can be provided.

In this case, the signal presence / absence detecting means R
Since the switching means is controlled so as to select the antenna with the higher SSI, it is possible to reliably detect the presence or absence of a signal even with a lower received electric field strength.

By operating the second filter only when detecting the presence / absence of a data signal, it is possible to reliably detect the presence / absence of a signal even with a lower received electric field strength.

Further, the reception frequency can be lowered by the mixer to facilitate the production of the comb filter for the preamble. As described above, according to the present invention, it is possible to provide an FSK demodulator with a high dynamic range that can detect the presence or absence of a signal even with a lower received electric field strength, and the practical effect is great.

[Brief description of drawings]

FIG. 1 is a principle block diagram of the present invention.

FIG. 2 is a block diagram showing a main part of a first embodiment example of the present invention.

FIG. 3 is a diagram showing a format example of a burst signal.

FIG. 4 is a diagram showing transmission spectra of a data part and a preamble part.

FIG. 5 is a diagram showing a characteristic example of the filter 1.

FIG. 6 is an explanatory diagram comparing a filter characteristic and a spectrum of a preamble.

FIG. 7 is a diagram showing a characteristic example of the filter 2.

FIG. 8 is a diagram showing a spectrum of a preamble after passing through a filter.

FIG. 9 is a block diagram showing a main part of a second embodiment example of the present invention.

FIG. 10 is a block diagram showing a main part of a third embodiment of the present invention.

FIG. 11 is a diagram showing an example of frequency characteristics of a comb filter.

FIG. 12 is a block diagram showing a main part of a fourth embodiment of the present invention.

FIG. 13 is an explanatory diagram of an FSK modulation method.

FIG. 14 is a block diagram showing a conventional configuration example of an FSK demodulator.

FIG. 15 is a diagram showing an example of RSSI characteristics.

[Explanation of symbols]

 1 Antenna 6 Mixer 7 Local Oscillator 9 Limiter Amplifier 20 Radio Signal Receiving Processor 21 Filter 22 Filter 30 Switcher 31 Switcher 40 Data Extracting Means 50 Signal Presence / Absence Detecting Means

Claims (4)

[Claims] 1. A first filter for passing only a predetermined frequency component from a radio signal to extract a data signal, and a first filter for passing only a predetermined frequency component from a radio signal to extract a preamble signal. No. 2 filter, a signal presence / absence detecting means for detecting the presence / absence of a data signal by detecting a preamble signal included in a wireless signal, a data extracting means for extracting data from a wireless signal, and a switching from the signal presence / absence detecting means. An FSK demodulator, comprising: a switching unit that receives a control signal and selects one of the first and second filters. 2. The F according to claim 1, wherein diversity control is performed based on an output of the second filter.
SK demodulator. 3. Two switching means are provided after the first filter, and a second preamble detection second circuit is provided in parallel with the switching means.
When the preamble signal is received, the switching means is connected to the first filter.
2. The FSK demodulator according to claim 1, wherein the output of the filter is passed through the second filter, and when the data is received, the switching means passes the output of the first filter 1 as it is. 4. A mixer is added before and after the second filter to convert the frequency to a predetermined frequency, apply a comb filter for preamble, and then return to the original frequency. 1. The FSK demodulator according to 1.