JP2974296B2 - Radio selective call receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio selective call receiver having a frequency correction function.

[0002]

2. Description of the Related Art Conventionally, as a means for demodulating an FM-modulated multi-valued digital signal, as shown in FIG. 16, the phase characteristic of a discriminator D connected in parallel with a damping resistor R is used to obtain an FM signal. Convert the modulated signal to a voltage,
Further, there is one that performs demodulation. The multi-level signal converted into the voltage is subjected to multi-level determination at a fixed determination level (voltage value) at the next stage. However, since the phase characteristics of the individual discriminator D are affected by temperature changes and power supply voltage fluctuations, the characteristic (f−f) of the intermediate frequency-DC voltage value converted using the phase characteristics of the discriminator D is used. V characteristic or S curve characteristic). Also, if the transmitter is transmitted with an offset or the reference oscillator on the receiver is affected by the surrounding environment or misalignment, the intermediate frequency fluctuates on the output side of the mixer, and accurate multi-level It cannot be determined.

On the other hand, Japanese Patent Application Laid-Open No. 58-14618 discloses that a deviation between the average frequency of a frequency-converted intermediate frequency signal and the center frequency of the intermediate frequency is negatively fed back to a frequency control signal input terminal of a voltage controlled oscillator, and the intermediate There has been proposed a method of controlling the voltage control local oscillator so that the average frequency of the frequency signal matches the intermediate frequency center frequency. Further, as a method of correcting the temperature of the phase characteristic of the discriminator, a thermistor T is used for a damping resistor R connected in parallel with the discriminator D, as shown in FIG.
There is a method of performing correction using the B constant of the thermistor T.
In this case, it is necessary to match the B constant characteristic of the thermistor T with the temperature characteristic of the phase characteristic of the discriminator D.

As described above, when the frequency-phase characteristics (f-θ characteristics) fluctuate due to the individual difference of the discriminator D due to the temperature change or the power supply voltage fluctuation, the demodulation circuit f-
V characteristics also vary. In general, the receiver mixes the output of the local oscillator with the high-frequency received signal to create an intermediate frequency.At this time, the local oscillator is adjusted with a manual variable capacitor using a crystal oscillation circuit or a voltage controlled oscillator. There are many cases.

[0005]

However, in the conventional radio selective calling receiver, the fluctuation of the f-.theta. Characteristic of the discriminator D is not affected by the center frequency of the fV characteristic converted by the discriminator D. Fluctuations in the output voltage, transmission frequency fluctuations due to offset transmission on the transmitting side,
Due to changes in the local oscillator of the receiver due to environmental changes and adjustment deviations, the intermediate frequency signal of the mixer output deviates from the original center frequency, and the fV characteristics of the discriminator D fluctuate. However, there is a problem that it is not possible to secure a proper voltage amplitude, and it is not possible to make an accurate multi-value determination.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and fV characteristic fluctuation of a demodulation circuit due to temperature fluctuation of a frequency-phase characteristic of a discriminator and offset transmission on a transmitter side. The center frequency can be automatically corrected by following the fluctuation of the transmission frequency and the fluctuation of the intermediate frequency when the reference oscillation part on the receiver side is affected by the surrounding environment and adjustment deviation, etc. An object of the present invention is to provide a radio selective calling receiver capable of preventing erroneous determination of a digital signal.

[0007]

In order to achieve the above object, a radio selective call receiver according to the present invention detects a specific signal in an intermediate frequency signal obtained through a low-pass filter, and detects the specific signal. A signal detector for recognizing start and end, and a maximum value and a minimum value of the specific signal before recognizing the end of the specific signal detected by the signal detector. And calculating a center value voltage data by comparing the intermediate amplitude value and the reference voltage value obtained based on the center value voltage data based on the center value voltage data obtained by the calculation unit. The crystal oscillation unit controls the oscillation frequency to be input to the mixer for outputting the intermediate frequency signal so that the center voltage value of the output becomes the optimum value.

A radio selective call receiver according to a second aspect of the present invention detects a specific signal in an intermediate frequency signal obtained through a low-pass filter and recognizes the start and end of the specific signal. And comparing the reference voltage value with the intermediate amplitude voltage value obtained based on the maximum value and the minimum value of the specific signal until the end of the specific signal detected by the signal detection unit, An arithmetic unit for calculating data; and a smoothing unit for smoothing a voltage conversion value obtained by performing voltage conversion on the center value voltage data obtained by the arithmetic unit as needed. The crystal oscillation unit controls the oscillation frequency to be input to the mixer for outputting the intermediate frequency signal so that the center voltage value of the output of the band-pass filter becomes an optimum value.

In the radio selective calling receiver according to a third aspect of the present invention, the crystal oscillation section is provided with a voltage variable capacitance diode for adjusting an oscillation frequency based on the center value voltage data.

The radio selective calling receiver according to a fourth aspect of the present invention further comprises a battery saving off frequency counting section for counting the number of times of battery saving off at a predetermined timing, so that the number of times of battery saving becomes higher than a preset number of times. When the specific signal is not detected, the control unit outputs the initial center voltage value data stored in advance.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a radio selective calling receiver according to the present invention. In the figure, reference numeral 11 denotes an antenna, and a high-frequency signal received by the antenna 11 is represented by R
The signal is amplified by the F amplifier 12. Reference numeral 14 denotes an output of a voltage-controlled oscillation circuit (hereinafter, referred to as a VCO) section 18 that performs voltage-controlled oscillation with reference to a crystal oscillation section 20 as a reference oscillation section, and an output of the RF amplification section 12 that has passed through a filter 13. In the mixing mixer, the frequency conversion is performed here. The output signal of the mixer 14 is filtered by a filter 15 to remove noise outside the band. A mixer 16 mixes the output of the crystal oscillator 20 and the output of the filter 15, and converts the output into an intermediate frequency signal. This intermediate frequency signal is input to the phase detector 32 via the limiting amplifier 31 inside the FM demodulation circuit 30 via the filter 17. Also, 35
Is a discriminator, and this discriminator 35
Using the frequency-phase characteristic (f-θ characteristic), the phase detector 32 and the low-pass filter 33 convert the intermediate frequency signal into a frequency f-DC voltage V to obtain a desired S-curve characteristic. Have been.

Here, the low-pass filter 33 removes excess high-frequency noise, and then the A / D converter 34 converts the output data of the low-pass filter 33 into digital data.
Reference numeral 60 denotes a control unit which determines the digital signal as a multi-valued digital signal according to a certain criterion.
Is an A / D converter connected in parallel with the A / D converter 34, and the same signal is input to both. 42 is an operation unit,
The operation unit 42 stores the maximum value and the minimum value of the data digitally converted by the A / D conversion unit 41 in an internal register, calculates the center value of the maximum value data and the center value of the minimum value data, and sets the value in advance. The difference between the calculated data and the center value data serving as a reference is set as reference data, and the difference is sent to the D / A converter 43 in the next stage.

The D / A converter 43 converts the digital data of the comparison value calculated by the calculator 42 into analog data and inputs the analog data to the voltage variable capacitance diode of the crystal oscillator 20. Reference numeral 50 denotes a signal detection unit which sends a start signal for recognizing the specific signal to the arithmetic unit 42 when the specific signal is detected, and, when the specific signal ends, generates a specific signal end signal for notifying the end. , And the same is sent to the operation unit 42. Generally, a transmission signal from a base station includes its own address and message information. Since these signals are asynchronous signals, "1", "1",
There is a specific signal (preamble signal) that repeats “0”, “1”, “0”. The signal detection unit 50 detects the repetition signal of “1”, “2”, “1”, and “0” as the specific signal.

FIG. 2 is a functional block diagram showing a procedure for calculating the central value voltage data by the calculating section 42. The operation unit 42 samples the output signal digitally converted by the A / D conversion unit 41, and stores the maximum value and the minimum value of the amplitude value V as sampling data in an internal register. Further, the difference between the stored maximum value data and minimum value data is calculated, and half (１ ／) of the difference is calculated.
Is compared with a reference voltage VA that is preset center value data, and the center value voltage data of the difference between the two (reference voltage VA−addition output) is calculated in the next stage. It is sent to the D / A converter 43. This D
The / A converter 43 converts the center value voltage data into analog data, and inputs the analog data to the crystal oscillator 20.

The crystal oscillator 20 is configured as shown in FIG. 3, for example, and finely adjusts the oscillation frequency based on the capacitance values of the capacitors C1 and C2 using the crystal oscillator XTL1. In this circuit, the voltage variable capacitance diode X1 is connected in parallel with the capacitor C1, and the combined capacitance of the variable capacitance diode X1 and the capacitor C1 changes depending on the capacitance value of the variable capacitance diode X1. As shown in FIG. 11, the voltage variable capacitance diode X1 has a characteristic that the capacitance value changes depending on the applied voltage value. Therefore, the oscillation frequency of the crystal oscillation unit 20 becomes variable according to the voltage value calculated by the calculation unit 42. When the A characteristic of the fV characteristic shown in FIG. 10 is set to an optimum value, it matches the voltage value Va of the specific signal. The optimum case is obtained. The voltage obtained by converting the data by the D / A converter 43 is input to the crystal oscillator 20, and the oscillation frequency is controlled.

Next, the operation will be described in detail. FIG. 4 shows the relationship between the specific signal and the A / D converter 41. When the same specific signal (pre-andel signal) as that shown in FIG. 4 is detected by the signal detection unit 50, a start signal of recognition of the specific signal is input to the calculation unit 42. In the arithmetic section 42, the analog specific signal is converted into a digital signal by the A / D converter 41, and the digital signal is processed until the specific signal is completed (from the transmission of the recognition start signal to the input of the end signal). Among the data of (interval), the largest and smallest values of the voltage amplitude value are stored in an internal register.

Here, the following description is based on the assumption that the A / D converter 41 performs an output process using 8-bit data. Now, when the maximum value data = 1110000 and the minimum value data = 111000000 are stored in the register inside the operation unit 42, when the specific signal ends and the end signal is sent from the signal detection unit 50 to the operation unit 42, Arithmetic unit 42
Then, the minimum value data is added to the past two data (half value data and half of the difference between the minimum value data and the minimum value data) calculated and stored by the end signal (the central value between the maximum value and the minimum value). (Calculation output) Next, the calculation output and the reference voltage data VA (VA = 1101)
1000). As a result, "000000
00 "is obtained, and D / A
It is sent to the conversion unit 43.

That is, when this is represented by a symbolic expression, (11
110000-1100000) × (１ ／) + 11
000000 = 111011000 → calculated output data (VA), and 110101000-1101
1000 = 00000000 → operation output data (VA
A). Next, the D / A converter 43 converts the center value voltage data calculated by the calculator 42 into an analog signal, and inputs the obtained center value voltage data to the voltage variable capacitance diode X1 of the crystal oscillator 20. I do. However, as described above, in order to generally increase or decrease the capacitance of the voltage variable capacitance diode X1, a certain voltage value is required.
It is necessary to apply an offset corresponding to the offset setting voltage VSTB (V) to the conversion unit 43. Here, similarly, the A / D converter 41 is similarly offset by the offset setting voltage VSTB (V) to obtain the oscillation frequency of the crystal oscillation unit 20 such that the center voltage value of the output of the filter 33 matches Va at the optimum time. Of the capacitors C1 and C2 to be used.

The above describes a method in which the voltage amplitude data from the start to the end of the recognition of a specific signal is read and stored in the operation unit 42 to perform the operation, and the past data is used after the end signal is transmitted. However, as shown in FIG. 5, the data from the start signal to the end signal of the recognition of the specific signal is
At the same timing as above, the calculation is performed in the calculation unit 42, the data is converted into analog data at any time by the D / A conversion unit 43 in the next stage, and further input to the crystal oscillation unit 20 through the smoothing unit 45. Can also. FIG. 6 shows the specific signal output from the low-pass filter 33 in this case, the output data of the A / D conversion unit 41, the output data of the operation unit 42, and the D / A
7 shows waveforms of an output voltage of the conversion unit 43 and an output voltage of the smoothing unit 45 (input voltage of the voltage variable capacitance diode X1).
By outputting the specific signal as described above, the voltage value following the variation of the voltage amplitude in the specific signal is sent to the crystal oscillation unit 20 as needed to make the oscillation frequency variable,
The center voltage value of the output of the low-pass filter 33 is set to the optimum value V
a, it is possible to suppress an error in the multi-value determination due to the FM noise component.

The radio selective calling receiver has a battery saving function (BS) for turning on and off the power supply of the radio unit at a certain timing in order to extend the battery life.
Function). In the present invention, FIG.
As shown in (1), the battery saving off number counting section (BS counting section) 70 is controlled by the control section 60 by using the BS off of the BS function (power supply state, approximately 50 msec is performed about every 1 second). , And B from the previously detected specific signal (including the message signal)
The number of S-off times is counted, and a preset number (X
Times) If the next specific signal is not detected, the control unit 6
0 is the initial center voltage value data stored in advance in D /
The oscillation frequency of the crystal oscillator 20 can be controlled by transmitting the voltage to the A converter 43 and outputting the voltage.

FIG. 8 shows the relationship between the reception signal, the specific signal detection operation in the radio selective calling receiver, and the transmission frequency setting state of the crystal oscillator 20. FIG. 9 shows the relationship between the reception signal, the radio selective calling receiver in FIG. The relationship between the specific signal detection operation, the BS count operation, and the transmission frequency setting state of the crystal oscillation unit 20 is shown.

In FIG. 8, when the specific signal N (preamble signal) digitized by the A / D converter 34 is input to the control unit 60, the signal detection unit 50 repeats the signal "1" and "0". Is recognized as a specific signal (preamble signal) at the time of confirming n times, and at the same time, a start signal for recognition and an end signal at the end of the specific signal are sent to the arithmetic unit 42. The arithmetic unit 42 samples the output data while the specific signal is detected (while the end signal is output from the start signal), and calculates and stores the data of the maximum value and the minimum value of the voltage amplitude.

At the same time as the end of the specific signal, the past two data (maximum value data and minimum value data) stored in the register in the arithmetic section 42 are read out, and the difference between the two data is halved (1/2 times). ) And the difference between the reference voltage VA and the value obtained by adding the minimum value data (the center value of the difference) to the D / A conversion unit 43 as an output of the calculation unit 42, and controls the crystal oscillation unit 20. Therefore, for the message reception after the specific signal, the voltage set value applied to the voltage variable capacitance diode X1 of the crystal oscillation unit 20 calculated by the specific signal N is used.

On the other hand, in the radio selective calling receiver having the BS counting section 70 shown in FIG.
Is detected, as in the case described with reference to FIG.
The subsequent message reception is the setting of the crystal oscillation unit calculated using the specific signal N. However, if the BS-off is not detected X times or more from the end of the reception of the message signal until the next specific signal, The control unit 60 inputs amplitude data VAA = 0000000000 (optimal data) to the D / A conversion unit 43 and controls the voltage value applied to the voltage variable capacitance diode X1 of the crystal oscillation unit 20.

In general, a demodulated output (f-V) utilizing a discriminator 35 used as a demodulation element is used.
The characteristic (S curve characteristic) is usually as shown in FIG. 10, but the f-θ characteristic of the discriminator 35 fluctuates due to individual differences of the discriminator 21 and environmental conditions. As a result, as shown by the B and C characteristics in FIG.
V characteristics vary. Here, the operation of the FM demodulation circuit section 30 having the A characteristic in FIG. FIG. 12 shows the input (low-pass filter output) of the A / D converter 41, the output of the AD converter 41, the output data of the arithmetic unit 42, the waveform of the output of the D / A converter 43, and the crystal at the time of the A characteristic. 4 shows the relationship between the oscillation frequency deviations of the oscillation unit 20.

First, since the specific signal (preamble signal) is swung within the modulation frequency, the input waveform of the A / D converter 41 has an intermediate voltage of Va (V) and an amplitude of ΔVa (V).
When this is digitally converted by the A / D conversion unit 41, for example, the maximum value is “1110000” and the minimum value is “11000000”. With these data,
The calculation unit 42 calculates (maximum value−minimum value) × (１ ／) + minimum value to obtain data of the intermediate voltage value VA = 11011000. The arithmetic unit 42 compares the calculated center voltage value VA with the reference voltage value VA, and sends the difference, that is, the center value voltage data VAA = 000000000 to the D / A conversion unit 43.

For this reason, the D / A converter 43 converts the operation result data (VAA) into an offset setting unit (VST).
B) At 44, the voltage is converted into a DC voltage value offset by the offset setting voltage VSTB (V), and is applied to the voltage variable capacitance diode X1 of the crystal oscillation unit 20. This voltage variable capacitance diode X1 has a characteristic that the diode capacitance changes according to the applied voltage as shown in FIG. FIG.
As shown in FIG. 3, if the relationship between the oscillation frequency of the crystal oscillation unit 20 and the capacitance value of the voltage variable capacitance diode X1 is set, the voltage Va is input to the voltage variable capacitance diode X1 at the optimum time, The oscillation frequency deviation is Δ
fa = 0.

Next, the operation of the demodulation circuit section 30 having the characteristics shown in FIG. 10 at the time of fV conversion characteristics will be described.
In this case, as shown in FIG. 14, the A / D converter 41
The input waveform has a center voltage of Vb (V) (has a higher voltage value than Va) and an amplitude of ΔVb (V), which is digitally converted by the A / D converter 41.
The maximum value is “11111100” and the minimum value is “11001”
100 ". Based on this data, the calculation unit 42 calculates (maximum value−minimum value) × (１ ／) + minimum value, and data of the center voltage value VB = 11100100 is obtained. The arithmetic unit 42 calculates the center voltage value VB
Is compared with a reference voltage value VA, and the difference VBA = 0
0001100 is sent to the D / A converter 43.

For this reason, the D / A converter 43 converts the operation data (VBA) into a DC voltage direct value (VBA + VSTB) offset by the offset setting voltage VSTB (V). Diode X1
Is applied. As shown in FIGS. 11 and 13, the oscillation frequency deviation of the crystal oscillation unit 20 is more positive (Δf
b) results. Therefore, f shifted to the minus (−) side
Correction of the -V characteristic can be performed by shifting the oscillation frequency of the crystal oscillation unit 20, which is the reference oscillation unit, to the minus side, and accurate demodulation can be realized. As a result, the intermediate frequency of the output of the filter 17 mixed in the radio unit shifts to the minus side.

Finally, F having the C characteristic shown in FIG.
The operation of the M demodulation circuit unit 30 at the time of the fV conversion characteristic will be described. In this case, as shown in FIG. 15, the input waveform of the A / D converter 41 has an intermediate voltage of Vc (V) (a voltage value lower than Va) and an amplitude of ΔVc (V). Is digitally converted by the A / D converter 41.
For example, the maximum value is “11100100”, and the minimum value is “1”.
0110100 ". Based on this data, the arithmetic unit 4
In 2, the calculation of (maximum value−minimum value) × (１ ／) + minimum value is performed, and data of the center voltage value VC = 110001100 is obtained. In addition, the calculation unit 42 calculates a difference VCA between the calculated center voltage value VC and the reference voltage value VA = −0000011.
00 is sent to the D / A converter 43.

For this reason, the D / A converter 43 converts the operation data (VCA) into a DC voltage value (VCA + VSTB) offset by the VSTB (V), and the operation unit 42 calculates the center voltage value VC calculated. The difference between the reference voltage value VA and the reference voltage value VA is calculated and sent to the D / A converter 43. This D
The / A converter 43 converts the operation data into the DC voltage value Va-Vc
And the voltage variable capacitance diode X of the crystal oscillation unit 20
Apply to 1. From FIGS. 11 and 13, the oscillation frequency deviation of the crystal oscillation unit 20 has a deviation Δfc on the minus side as compared with the A characteristic, and the fV characteristic shifted to the plus (+) side.
The correction is performed by shifting the oscillation frequency of the crystal oscillation unit 20, which is the reference oscillation unit, to the plus side. As a result, the intermediate frequency of the output of the filter 17 mixed in the radio unit shifts to the plus side.

As described above, in the fV characteristic shown in FIG. 10, when the output voltage of the low-pass filter 33 becomes Va (V) at the intermediate frequency fc, the A / D converter 4
The output of 1 becomes VA, and this state becomes the optimum value. When the fv characteristic is the B characteristic, the output voltage value at the intermediate frequency fc becomes Vb (a value larger than Va), and it becomes difficult to demodulate the plus side (fc + y) in the FM demodulation circuit unit 30. Further, at the time of the C characteristic, the output voltage value at the intermediate frequency fc becomes Vc (a value smaller than Va), and it becomes difficult to demodulate the minus side (fc-y) in the FM demodulation circuit unit 30. fV characteristic is the B characteristic or C characteristic
Even when the characteristic fluctuates, the f-V
By correcting the center frequency deviation of the characteristic, it is possible to more accurately determine a multi-valued digital signal.

[0033]

As described above, according to the present invention, the difference between the intermediate amplitude value and the reference voltage value based on the maximum value and the minimum value of the specific signal until the end of the specific signal is recognized. The oscillation frequency of the crystal oscillation unit is automatically corrected by returning the obtained center voltage value data to the crystal oscillation unit which is the reference oscillation unit.
FV characteristic fluctuation of the FM demodulation circuit due to individual difference and temperature change of the discriminator constituting the FM demodulation circuit, fluctuation of the transmission frequency due to offset transmission on the transmitter side, and reference oscillation section on the reception side And the variation of the intermediate frequency due to the characteristic change due to adjustment deviation, etc.
The deviation of the center frequency of the characteristic can be corrected, whereby a reliable voltage amplitude can be ensured, and as a result, an effect that a multi-valued digital signal can be determined more accurately can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a radio selective calling receiver according to an embodiment of the present invention.

FIG. 2 is a functional block diagram showing a calculation processing procedure by a calculation unit in FIG. 1;

FIG. 3 is a circuit diagram showing details of a crystal oscillation unit in FIG. 1;

FIG. 4 is a time chart showing signals of respective parts of a block in FIG. 1;

FIG. 5 is a block diagram showing a radio selective calling receiver according to another embodiment of the present invention.

FIG. 6 is a time chart showing signals of respective parts of a block in FIG. 5;

FIG. 7 is a block diagram showing a radio selective calling receiver according to another embodiment of the present invention.

FIG. 8 is a time chart showing another signal of each section of the block in FIG. 1;

FIG. 9 is a time chart showing signals of respective parts of the block in FIG. 7;

FIG. 10 is a graph showing fV characteristics of the FM demodulation circuit according to the present invention.

FIG. 11 is a graph showing voltage-capacity characteristics of the voltage variable capacitance diode according to the present invention.

FIG. 12 is a time chart showing signals of respective blocks in the A characteristic of FIG. 10;

FIG. 13 is a graph showing a capacitance-oscillation frequency characteristic of the voltage variable capacitance diode according to the present invention.

FIG. 14 is a time chart showing signals of respective parts of a block in the B characteristic of FIG. 10;

FIG. 15 is a time chart showing signals of respective blocks in the C characteristic of FIG. 10;

FIG. 16 is a graph showing phase characteristics of a conventional discriminator.

FIG. 17 is a graph showing another phase characteristic of the conventional discriminator.

[Explanation of symbols]

16 Mixer 20 Crystal oscillator 33 Low-pass filter 42 Operation unit 45 Smoothing unit 50 Signal detection unit 70 Battery saving off frequency counting unit (BS counting unit) X1 Voltage variable capacitance diode

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03J 7/02-7/06 H04B 1/16 H04B 1/26 H04Q 7/14

Claims (4)

(57) [Claims] 1. A signal detecting section for detecting a specific signal in an intermediate frequency signal obtained through a low-pass filter, and recognizing the start and end of the timed signal, and recognizing the end of the specific signal detected by the signal detecting section. The maximum value and the minimum value of the specific signal are stored until the reference value is obtained, and the intermediate amplitude value calculated based on the maximum value and the minimum value is compared with the reference voltage value to calculate the center value voltage data. An arithmetic unit, based on the central value voltage data obtained by the arithmetic unit, such that the central voltage value of the output of the low-pass filter is set to an optimum value;
And a crystal oscillator for controlling an oscillation frequency to be input to the mixer for outputting the intermediate frequency signal. 2. A signal detector for detecting a specific signal in an intermediate frequency signal obtained through a low-pass filter, and recognizing the start and end of the specific signal, and recognizing the end of the specific signal detected by the signal detector. A calculation unit that calculates the center value voltage data by comparing the intermediate amplitude voltage value obtained based on the maximum value and the minimum value of the specific signal and the reference voltage value until A smoothing unit for smoothing a voltage conversion value obtained by voltage conversion of the center value voltage data as needed, based on an output of the smoothing unit, so that a center voltage value of an output of the low-pass filter is set to an optimum value. A radio selective calling receiver, comprising: a crystal oscillation unit that controls an oscillation frequency to be input to a mixer for outputting an intermediate frequency signal. 3. The radio selective call receiver according to claim 1, wherein the crystal oscillation unit has a voltage variable capacitance diode that adjusts an oscillation frequency based on the center value voltage data. . 4. A battery saving off number counting unit for counting the number of times of battery saving off at a certain timing is provided, and when the specific signal is not detected even when the number of times of battery saving exceeds a preset number of times, the control unit notifies the control unit in advance. 3. The radio selective calling receiver according to claim 1, wherein the stored initial center voltage value data is output.