JPH0654013A - Frequency conversion circuit for receiver - Google Patents

Abstract

PURPOSE:To allow the conversion circuit to be applied to, e.g. a reception circuit of a mobile equipment in a mobile communication system to enhance the frequency stability of an intermediate frequency signal subject to frequency conversion. CONSTITUTION:The conversion circuit is provided with a local oscillation signal generating circuit 101 generating a 1st local oscillation signal and a 2nd local oscillation signal, a 1st mixer 102 frequency-converting the modulated reception signal into a 1st intermediate frequency signal by mixing the signal with the said 1st local oscillation signal, a 2nd mixer 103 frequency-converting the 1st intermediate frequency signal into a 2nd intermediate frequency signal by mixing the 1st intermediate frequency signal with the 2nd local oscillation signal, a modulation wave elimination circuit 104 eliminating a modulation wave from the 2nd intermediate frequency signal to extract the carrier, and a deviation detection circuit 105 detects the frequency deviation of the carrier extracted by the modulation wave elimination circuit 104 from its true frequency, and the local oscillation signal generating circuit 101 controls the frequency of the 1st local oscillation signal and the 2nd local oscillation signal generated from itself in a decreasing direction of the frequency deviation detected by the deviation detection circuit 105.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency conversion circuit used in a receiving circuit of a mobile device in a mobile communication system, for example.

Generally, in a mobile communication system such as an automobile telephone system, a reference oscillator is provided in a radio communication device, and a local oscillator output for frequency conversion or a carrier wave for modulating a transmission signal is obtained based on the reference oscillator output. I have to.

In such a mobile communication system,
With the increasing demand in recent years, effective use of the frequency spectrum by narrowing the channel spacing and interleaving the frequency arrangement is desired. In order to meet this demand, high frequency stability of a radio frequency signal (hereinafter referred to as RF signal) is required, and for that purpose, a highly accurate reference oscillator having high frequency stability of the oscillation frequency is required.

[0004]

2. Description of the Related Art Generally, a base station can be provided with a highly accurate oscillator having a frequency stability of 0.1 ppm or less due to its nature. On the other hand, the mobile station has severe operating conditions, so the frequency stability of the reference oscillator is 3
The limit is about ppm.

Therefore, in the conventional mobile unit, the oscillation frequency of the reference oscillator on the mobile unit side is controlled based on the RF signal sent from the base station. With such a configuration, the RF signal transmitted from the base station has high frequency stability, and thus the signal stability of the RF signal transmitted from the mobile device can also be increased.

FIG. 8 shows an example of a conventional mobile device having such a configuration. In FIG. 8, 1 is a receiving antenna, 2 is a bandpass filter, 3 is a mixer that frequency-converts the received signal into a first intermediate frequency signal IF1, and 21 is a mixer 3 for the first local oscillation signal.
Frequency synthesizer 5 is supplied to a bandpass filter, 6
Is a mixer that frequency-converts the first intermediate frequency signal IF1 into a second intermediate frequency signal IF2, and 22 is a second local oscillator that supplies the second local oscillation signal to the mixer 6, and is composed of a crystal oscillator or the like having high frequency stability. , 9 is a bandpass filter, 10 is an amplitude limiting amplifier, and 11 is a demodulation circuit.

Here, the frequency of the first intermediate frequency signal is f
_IF1 + Δf _IF1 (where f _IF1 is the true value, Δf _IF1 is the frequency deviation from the true value), and the frequency of the second intermediate frequency signal is f
_IF2 + Δf _IF2 (where f _IF2 is the true value, Δf _IF2 is the frequency deviation from the true value), and the frequency of the first local oscillation signal is f _L1
+ Δf _L1 (where f _L1 is the true value, Δf _L1 is the frequency deviation from the true value), and the frequency of the second local oscillation signal is f _L2 + Δf
_L2 (where f _L2 is a true value and Δf _L2 is a frequency deviation from the true value).

Reference numeral 23 is a bandpass filter, 24 is a frequency counter for counting the frequency of the second intermediate frequency signal, and 2 is a frequency counter.
_{Reference numeral} 5 is a table for storing and holding the true value f _IF2 of the frequency of the second intermediate frequency signal, and 26 is a frequency deviation (Δf _{IF2 ==)} between the measured value of the second intermediate frequency from the frequency counter 24 and the true value of the table 25. A subtractor that calculates Δf _L1 + Δf _L2 ),
27 is the output frequency of the second local oscillator 22 (f _L2 + Δ
f _L2 ) is counted by a frequency counter, 28 is a table for storing the true value f _L2 of the output frequency of the second local oscillator, and 29 is a measured value (f _L2 + Δf) of the second local oscillation frequency from the frequency counter 27. _L2 ) and the true value f of table 18
Subtractor for calculating a frequency deviation Delta] f _L2 and _L2, 30 is a subtractor from the frequency deviation of the subtractor _{26 (Δf L1 + Δf L2)} 29
The arithmetic unit for extracting a frequency deviation Delta] f _L1 minus the frequency deviation Delta] f _L2, 31 is a D / A converter for digital / analog conversion frequency deviation Delta] f _L1 of the arithmetic unit 30 to an analog voltage, 32 a D / A converter The reference oscillator is a voltage controlled oscillator having an output voltage of 32 as a control voltage, and a reference oscillation signal of the reference oscillator 32 is supplied to the frequency synthesizer 21 as a signal for providing a frequency reference for its operation.

The reference oscillation signal of the reference oscillator 32 is used to generate a carrier wave for modulation in the transmission side circuit. That is, 33 is a frequency synthesizer that generates a carrier wave for transmission based on the reference oscillation signal, and 34 is a mixer that multiplies the carrier wave by the carrier wave and converts the frequency into an RF signal.
Reference numeral 35 is a bandpass filter, and 36 is a transmission antenna.

In this circuit, the reference oscillator 3 is set so that the frequency deviation Δf _L1 which is the output of the arithmetic unit 30 becomes zero.
Frequency synthesizer 2 by controlling the oscillation frequency of 2
The frequency deviation Δf _L1 at the output of 1 is set to zero. With this AFC circuit, the frequency stability of the reference oscillator 32 can be made to conform to the frequency stability of the transmission signal on the base station side. The frequency stability can be made highly stable.

In the second intermediate frequency signal input to the demodulation circuit 11, Δf _L1 is suppressed as its frequency deviation, but Δf _L2 still remains. Therefore, a temperature-compensated crystal oscillator having a relatively high frequency stability is used as the second local oscillator 22 so that the frequency deviation Δf _L2 thereof is minimized.

[0012]

The above-mentioned conventional circuit has the following problems. (1) In the conventional mobile communication system, the signal processing of the voice, which is the main signal, is analogized, and the frequency modulation method is adopted as the modulation method.
A high frequency stability was not required for the intermediate frequency signal input to the demodulation circuit that frequency-converted the F signal.

On the other hand, in recent years, in mobile communication systems, digitization of signal processing such as voice has been considered. In this digital mobile communication system, generally, π / 4 shift QPSK and QPSK are used as a modulation method.
Digital phase modulation methods such as K and QAM are adopted.

For example, when the π / 4 shift QPSK modulation method is adopted, the frequency of the second intermediate frequency signal IF2, which is the input signal to the demodulation circuit, deviates from the true value due to temperature fluctuations or power supply voltage fluctuations. Occurs, the frequency deviation is converted into a phase error, which causes deterioration of the error rate. Therefore, in the digital mobile communication system, it is necessary to stabilize the second intermediate frequency input to the demodulation circuit.

(2) In the case of a digital modulation method such as π / 4 shift QPSK modulation, the code sequence of the modulated signal may be a sequence of "1" or "0". When such a code deviation occurs, the second side of the receiving side
In the intermediate frequency conversion stage, the frequency spectrum of the modulated signal (second intermediate frequency signal) is the carrier frequency f ₀ (= f
_R− f _L1 −f _L2 ) as the center, biased to the positive or negative side. For example, a maximum deviation of about ± 2.625 kHz occurs with respect to the second intermediate frequency f _IF2 = 455 kHz.

Therefore, the frequency of the second intermediate frequency signal IF2 (that is, the frequency of the output signal of the amplitude limiting amplifier 10).
When the frequency counter is directly counted by the frequency counter, the measured value includes an error due to the sign deviation in addition to the frequency deviation (Δf _L1 + Δf _L2 ) described above. Therefore, the reference oscillator is based on the measured value including this error. With the control, the frequency deviation of the second intermediate frequency signal IF2 cannot be sufficiently suppressed.

This code deviation can be usually prevented by scrambling the modulated signal in the base station so that "1" or "0" does not continue. However, since the degree of code bias prevention effect due to scrambling is proportional to the period of the PN pattern (pseudo noise pattern), in order to obtain the desired code bias prevention effect, PN
It is necessary to lengthen the cycle of the pattern, and specifically, for example, a cycle of about 1.5 sec is required.

On the other hand, the count time of the frequency counter 24 cannot be made too long because it is necessary to shorten the time from the start of reception until the AFC operation converges. Specifically, for example, only a time of about 100 msec can be set. Therefore, even if the base station side scrambles the modulated signal, since the count time of the frequency counter 24 is short, in reality, it is almost impossible to prevent the count error of the frequency counter 24 due to the code deviation.

(3) Further, in the conventional circuit, in order to obtain the frequency deviation Δf _L2 of the oscillation frequency of the second local oscillator 22,
The oscillation frequency (f _L2 + Δf _L2 ) of the second local oscillator 22 is directly counted by the frequency counter 27, and its true value f _L2
And the difference Δf _L2 from

However, in this case, the second local oscillation frequency is so high that it cannot normally be counted by a frequency counter composed of CMOS transistors. Therefore, it is necessary to divide the second local oscillation frequency to lower the frequency. However, when such frequency division processing is performed, it is necessary to lengthen the count time in order to obtain the same counting accuracy as when the frequency division processing is not performed, but this is to increase the convergence time of the AFC operation. Become.

(4) Since the second local oscillator is required to have relatively high frequency stability, a temperature compensation type crystal oscillator or the like is used, but these oscillators are expensive.

The present invention has been made in view of the above problems, and an object thereof is to improve the frequency stability of an intermediate frequency signal frequency-converted in a frequency conversion circuit of a receiver using a digital modulation method or the like. To raise.

[0023]

FIG. 1 is a diagram illustrating the principle of the present invention. The frequency conversion circuit of the receiver of the present invention is
As one form, a local oscillation signal generation circuit 101 for generating a first local oscillation signal and a second local oscillation signal and a first intermediate frequency signal by mixing a modulated reception signal with the first local oscillation signal. First mixer 10 for frequency conversion
2, a second mixer 103 for frequency-converting the first intermediate frequency signal with the second local oscillation signal into a second intermediate frequency signal, and a modulated wave from the second intermediate frequency signal of the second mixer 103. Wave removing circuit 10 for removing the carrier wave to extract the carrier wave
4 and a deviation detection circuit 105 for detecting a frequency deviation of the carrier wave extracted by the modulated wave removal circuit 104 from its true value. The local oscillation signal generation circuit 101 includes a frequency deviation detected by the deviation detection circuit 105. Is controlled to decrease the frequencies of the first local oscillation signal and the second local oscillation signal generated.

The local oscillation signal generation circuit 101 described above is a variable frequency control type reference oscillator 1 for generating a reference oscillation signal.
06, a first local oscillator 107 that generates a first local oscillation signal based on the reference oscillation signal, and a second local oscillator 108 that generates a second local oscillation signal based on the reference oscillation signal. The reference oscillator 106 can be configured to control the frequency of the reference oscillation signal generated in the direction in which the frequency deviation detected by the deviation detection circuit 105 decreases.

The first local oscillator 107 may be composed of a frequency synthesizer using a phase-locked loop circuit, and the second local oscillator 108 may be composed of a phase-locked loop circuit.

The frequency conversion circuit of the receiver of the present invention is
As another mode, a local oscillation signal generation circuit that generates a local oscillation signal, a mixer that mixes the modulated signal with the local oscillation signal to perform frequency conversion into an intermediate frequency signal that is input to a demodulation circuit, and a mixing Equipped with a modulation wave removing circuit for removing the modulation wave from the intermediate frequency signal of the converter to extract the carrier wave, and a deviation detecting circuit for detecting the frequency deviation of the carrier wave extracted by the modulation wave removing circuit from its true value, The oscillation signal generation circuit is configured to control the frequency of the local oscillation signal generated in the direction in which the frequency deviation detected by the deviation detection circuit decreases.

This local oscillation signal generation circuit uses a variable frequency control type reference oscillator for generating a reference oscillation signal and a phase for generating a local oscillation signal phase-locked to the reference oscillation signal of the reference oscillator. It can be composed of a local oscillator composed of a synchronous circuit.

Further, the above-mentioned modulated wave removing circuit has a modulated signal component separating means for separating the modulated signal component by multiplying the inputted intermediate frequency signal, and the modulated signal separated by the modulated signal component separating means. A multiplied carrier component for multiplying the component and the reproduced clock generated by the demodulator to extract a multiplied carrier component from the modulated signal component, and a multiplied carrier component of the multiplied carrier extracting unit for dividing the original carrier component It can be configured by including a frequency dividing means for changing the frequency carrier.

The receiver using each frequency conversion circuit described above is suitable for application to a communication system using a digital phase modulation method.

[0030]

In the frequency conversion circuit of the receiver of the first embodiment, the modulated wave removing circuit 104 removes the modulated wave from the second intermediate frequency signal of the second mixer 103 and extracts the carrier wave. The deviation detecting circuit 105 uses the modulated wave removing circuit 104.
The frequency of the carrier wave extracted in step 1 is compared with its true value, and the frequency deviation from the true value is detected. The local oscillation signal generation circuit 101 controls the frequencies of the first local oscillation signal and the second local oscillation signal generated in the direction in which the frequency deviation detected by the deviation detection circuit 105 decreases. By doing so, the frequency deviation of the second intermediate frequency signal input to the demodulation circuit can be eliminated including the error due to the code deviation.

Further, the local oscillation signal generating circuit 101 includes a reference oscillator 106, a first local oscillator 107 and a second local oscillator 1.
08, the frequency of the reference oscillation signal generated by the reference oscillator 106 is controlled so that the frequency deviation detected by the deviation detection circuit 105 decreases.

In the frequency conversion circuit of the receiver of the second embodiment, the modulated wave removing circuit removes the modulated wave from the intermediate frequency signal of the mixer to extract the carrier wave. The deviation detection circuit compares the frequency of the extracted carrier wave with its true value and detects the frequency deviation from the true value. The local oscillation signal generation circuit controls the frequency of the local oscillation signal generated in the direction in which the frequency deviation detected by the deviation detection circuit decreases. By doing so, the frequency deviation of the intermediate frequency signal input to the demodulation circuit can be eliminated including the error due to the code deviation.

Further, in the above-mentioned modulated wave removing circuit, the modulated signal component separating means separates the modulated signal component by multiplying the input intermediate frequency signal, and the multiplied carrier wave extracting means separates the modulated signal component. The modulated signal component separated by the separating means is multiplied by the reproduction clock generated by the demodulator to extract the multiplied carrier component from the modulated signal component, and the dividing means extracts the multiplied carrier component of the multiplied carrier extracting means. Is divided into the carrier wave of the original carrier wave frequency. This makes it possible to extract the carrier wave component from which the modulated wave has been removed from the intermediate frequency signal and suppress the error due to the code bias.

[0034]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows a receiver circuit using a frequency conversion circuit as an embodiment of the present invention. The receiving circuit of this embodiment is mounted on a mobile device of a digital mobile communication system using a π / 4 shift QPSK modulation system as a modulation system. This receiving circuit uses a double superheterodyne system as a receiving system, an RF signal frequency f _R of about 820 MHz to 900 MHz is used, and a first intermediate frequency f _IF1 of the receiving circuit is 130 MHz and a second intermediate frequency f. 455 kHz is used as _IF2 .

In FIG. 2, the RF signal of the frequency f _R received by the antenna 1 passes through the bandpass filter 2 and is input to the mixer 3, where it is multiplied by the first local oscillator output from the first local oscillator 4. It is combined and frequency-converted into the first intermediate frequency signal IF1 having the first intermediate frequency f _IF1 .

The first intermediate frequency signal IF1 further passes through the bandpass filter 5 and is input to the mixer 6, where it is multiplied by the second local oscillator output from the second local oscillator 7 to obtain the second intermediate frequency f _IF2 . The frequency is converted into two intermediate frequency signals IF2. The second intermediate frequency signal IF2 passes through the bandpass filter 9 and the amplitude limiting amplifier 10 and is input to the modulated wave removing circuit 8 and the demodulating circuit 11.

The demodulation circuit 11 demodulates the baseband signal from the second intermediate signal IF2. The demodulation circuit 11 includes a clock reproduction circuit, reproduces a clock having a modulation frequency (symbol frequency) f _S on the transmission side, and further divides this clock by 4 to modulate a reproduction clock having a frequency f _S / 4. It is supplied to the wave removing circuit 8.

The modulated wave removing circuit 8 removes the modulated wave from the second intermediate frequency signal IF2 output from the amplitude limiting amplifier 10 to obtain an unmodulated carrier (f ₀ = 455k).
This is a circuit for extracting a carrier wave of Hz), and is for removing the influence of code bias in the frequency measurement of the second intermediate frequency signal. This unmodulated carrier is frequency counter 1
Entered in 2.

The frequency counter 12 is a circuit for measuring the frequency f _{0 of} the carrier wave from which the modulation is removed as an actual measurement value, and the actual measurement value is input to one input terminal of the subtractor 14. The data from the table 13 is input to the other input terminal of the subtractor 14. This table 13
_Stores the true value of the second intermediate frequency f _IF2 (that is, 455 kHz without frequency deviation) as data. Therefore, the subtractor 14 calculates and outputs the frequency deviation Δf _IF2 between the measured value and the true value of the second intermediate frequency f _IF2 .

Frequency deviation Δf output from the subtractor 14
_IF2 is converted into an analog voltage by the D / A converter 15 and then supplied to the reference oscillator 16 as a control voltage. The reference oscillator 16 is composed of a voltage controlled oscillator (VCO). The reference oscillator 16 does not have a high frequency stability as a single unit, and a frequency oscillator of about 3 ppm is used. The output of the reference oscillator 16 is supplied as a reference signal which gives the first local oscillator 4 and the second local oscillator 7 a frequency reference for their operation.

Here, the modulated wave removing circuit 8, the frequency counter 12, the subtractor 14, the D / A converter 15 and the like constitute an automatic frequency control (AFC) circuit for automatically controlling the oscillation frequency of the reference oscillator 16. It is a thing.

The timing generation circuit 17 generates a time base signal and a gate signal for controlling the operation of the frequency counter 12 and the like based on the output of the reference oscillator 16.

FIG. 3 shows a detailed configuration example of the first local oscillator 4. The first local oscillator 4 has a known configuration including a synthesizer using a PLL (phase locked loop), and has a voltage controlled oscillator 41, a variable frequency divider 42, a phase comparator 43, a frequency divider 44, and an integrator. It consists of 45 mag. The first local oscillator 4 outputs different RF signal frequencies f _R for the respective receiving channels by the mixer 3 so that the first intermediate frequency f _R is the same.
First to supply to mixer 3 for frequency conversion to _IF1
The oscillation frequency f _{L1 of the} output of the local oscillator is changed according to the frequency f _R of the received RF signal, and therefore the variable frequency divider 42 is used.

To explain the operation, the first local oscillator output from the voltage controlled oscillator 41 is frequency-divided by the variable frequency divider 42 (for example, 25 kHz) and input to one input terminal of the phase comparator 43. It The reference oscillator output from the reference oscillator 16 is applied to the other input terminal of the phase comparator 43 by the frequency divider 4
It is divided by 4 (divided into 25 kHz) and input. The phase comparator 43 detects the phase error between the both input signals and supplies the phase error as a control voltage to the voltage controlled oscillator 41 through the integrator 45. This allows the voltage controlled oscillator 4
The first local oscillator output from 1 is phase-locked to the reference oscillator output. Note that the frequency division ratio of the variable frequency divider 42 is controlled so that the first intermediate frequency f _IF1 is kept constant based on the channel selection data even when the reception channel is switched.

FIG. 4 shows a detailed configuration example of the second local oscillator 7. The second local oscillator 7 also has a known configuration including a PLL circuit, and includes a voltage controlled oscillator 71, a frequency divider 72, a phase comparator 73, a frequency divider 74, an integrator 75, and the like. The second local oscillator 7 is provided to the mixer 6 so that the first intermediate frequency signal IF1 output from the mixer 3 is further converted into the second intermediate frequency signal IF2 having the second intermediate frequency f _IF2 by the mixer 6. It is a circuit that supplies the second local oscillator output at the oscillation frequency.
_{Since IF1} is a constant frequency, the second local oscillation frequency f _L2
Need not have a variable frequency, and therefore, a synthesizer configuration such as the first local oscillator 4 is not provided.

To explain the operation, the output of the second local oscillator of the voltage controlled oscillator 71 is divided by the frequency divider 72 (for example, 5 kHz).
And is input to one of the input terminals of the phase comparator 73. The output from the reference oscillator 16 is frequency-divided by the frequency divider 74 (divided into 5 kHz) and input to the other input terminal of the phase comparator 73. The phase comparator 73 detects the phase error between the both input signals and detects the phase error from the integrator 7.
The voltage is supplied as a control voltage to the voltage controlled oscillator 71 through 5. As a result, the second voltage output from the voltage controlled oscillator 71
The local oscillator output is phase locked to the reference oscillator output.

FIG. 5 shows a detailed configuration example of the modulated wave removing circuit 8. In FIG. 5, 81 is the amplitude limiting amplifier 10.
The second intermediate frequency signal IF2 output from
It is a multiplier. The multiplied output of the quadruple multiplier 81 is supplied to one input terminal of the multiplier 83 through the bandpass filter 82. The center frequency of the bandpass filter 82 is four times the carrier frequency (455 kHz) of the second intermediate frequency signal IF2. The demodulation circuit 11 is connected to the other input terminal of the multiplier 83.
The recovered clock of frequency f _S / 4 is input from
The modulated wave is removed from the second intermediate frequency signal IF2 by multiplying this by the signal obtained by multiplying the above-mentioned second intermediate frequency signal IF2 by 4 through the bandpass filter 82,
A quadruple carrier component (however, the frequency is 4f ₀ ) is extracted. This extracted carrier wave component is divided by 4 by the divide-by-four frequency divider 84, whereby the frequency of the extracted carrier wave component is reduced to 1/4 and is returned to the original frequency f ₀ .

The operation of the apparatus of the embodiment will be described below. First, the outline of the AFC operation will be described. Generally, the RF signal transmitted by the base station has an accuracy of 0.1 pp as described above.
It is generated using a reference oscillator of m or less, and its frequency stability is extremely high. Therefore, if the true value of the frequency of the RF signal is f _R , the actual frequency of the RF signal received by the antenna 1 on the mobile device side and passed through the bandpass filter 2 can also be substantially regarded as f _R.

On the other hand, the output of the first local oscillator 4 is generated based on the oscillator output of the reference oscillator 16 having an accuracy of about 3 ppm. Therefore, this first local oscillator output has a frequency deviation corresponding to the frequency deviation of the reference oscillator 16. If the frequency deviation of the first local oscillator output is Δf _L1 , the actual frequency of the first local oscillator output is f _L1 + Δf _L1 . Here, f _L1 is the true value of the frequency of the output of the first local oscillator.

[0050] Thus, the first frequency of the intermediate frequency signal IF1, i.e. the first intermediate frequency f _IF1 outputted from the pre-mixer 3, f _IF1 = f _R - a (f _L1 + Δf _L1).

Similarly, the output of the second local oscillator 7 also has a frequency deviation corresponding to the frequency deviation of the reference oscillator 16. If this frequency deviation is Δf _L2 , the actual frequency of the output of the second local oscillator is f _L2 + Δf _L2 . Here, f _L2 is the true value of the frequency of the second local oscillator output.

[0052] Thus, the frequency of the second intermediate frequency signal IF2 output from the subsequent stage of the mixer 6, i.e. the second intermediate frequency f _{IF2 is, f IF2 = f R - (} f L1 + Δf L1) - (f L2 + Δf L2 ). Therefore, the second intermediate frequency f _IF2 is caused by the frequency deviation of the output of the reference oscillator 16, and the frequency deviation of + (Δf _L1 + Δf _L2 ) with respect to its true value (f _R −f _L1 −f _L2 ). Will have.

The modulated wave removing circuit 8 removes the modulated wave from the second intermediate frequency signal IF2 and extracts the carrier wave.
The true _value of the frequency f ₀ of the carrier wave is f ₀ = f _R −f _L1 −f _L2 if there is no influence of the code bias, but the actual value is the frequency deviation (Δf _L1 + Δf _L2 ). _Including , f ₀ = f _IF2 = f _R − (f _L1 + Δf _L1 ) − (f _L2 + Δf _L2 ).

The frequency component of the carrier wave component extracted by the modulated wave removing circuit 8 is counted by the frequency counter 12 as an actually measured value, and then the table 13 is calculated by the subtracter 14.
True value (f _R − of the second intermediate frequency f _IF2 stored in
f _L1 −f _L2 ) and thereby the frequency deviation (Δf _L1 + Δf _L2 ) from the true value of the second intermediate frequency signal IF 2 is extracted.

This frequency deviation is converted into an analog value by the D / A converter 15 and input to the reference oscillator 16 as a control voltage. As a result, the reference oscillator 16 causes the first local oscillator 4 and the second local oscillator 7 to move in the direction in which this frequency deviation (Δf _L1 + Δf _L2 ) is removed from the second intermediate frequency signal IF2.
Controls the frequency of the reference oscillator output supplied to.

Therefore, even if the reference oscillator 16 has a low frequency stability, the frequency stability of the carrier wave of the second intermediate frequency signal IF2 input to the demodulation circuit 11 is equal to the above-mentioned AF.
Due to the C operation, the frequency becomes extremely high in accordance with the frequency stability of the reference oscillator on the base station side, whereby the demodulation circuit 11
It is possible to prevent the error rate from being deteriorated.

Next, the operation of removing the modulated wave from the second intermediate frequency signal IF2 by the modulated wave removing circuit 8 will be described. That is, the carrier wave component is extracted from the second intermediate frequency signal IF2 output from the amplitude limiting amplifier 10 by removing the modulation wave that causes the deviation due to the sign deviation, and the frequency of the carrier wave component is counted by the frequency counter 12. By doing so, the influence of the code bias is removed.

The operation of the modulated wave removing circuit 8 will be described in more detail. In the case of the conventional analog frequency modulation method, as shown in FIG. 9, the frequency spectrum of the modulated signal is modulated by the value of the ratio of the modulation frequency f _S to the frequency displacement f _D centering on the carrier frequency f _0. lined accordance Bessel function B (f _D / f _S) at an interval of _S.

On the other hand, in the case of a digital modulation method such as π / 4 shift QPSK modulation in digital mobile communication, the frequency spectrum of the modulated signal is as shown in FIG.
As shown in (a), the transmitter side is lined up in a clogged state because the digital modulation signal has a frequency band similar to that of a noise signal.

The frequency of the amplified output of the amplitude limiting amplifier 10 is first multiplied by 4 by the quadrupler 81 of FIG. Part 4
By further passing the multiplied output through the band-pass filter 82, the frequency becomes (4f ₀ −
The components S1 and S2 of the modulated signal of f _s / 4) and (4f ₀ + f _s / 4) are obtained.

By multiplying the quadrupled output and the regenerated clock of the frequency f _S / 4 regenerated by the demodulation circuit 11 by the multiplier 83, the quadrupled carrier component S of the modulated signal is obtained.
3 (having a frequency of 4f ₀ ) is extracted. This quadrupled carrier component S3 is divided by 4 by the quadrature divider 84 to set the frequency to 1
When set to / 4, the frequency of the quadruple carrier component S3 is converted to the original carrier frequency f ₀ .

Various modifications are possible in carrying out the present invention. For example, in the embodiment described above, the present invention
The case where the present invention is applied to the communication device of the 4-shift QPSK modulation method has been described, but the present invention is not limited to this, and can be applied to a digital phase modulation method such as QPSK or QAM.

For example, when the present invention is applied to the QPSK modulation system, the structure of the modulated wave removing circuit shown in FIG. 5 may be used, but it may be changed as follows. That is, in FIG. 5, instead of the 4 × multiplier 81, a 2 × multiplier is used.
2 instead of the bandpass filter 82 having the center frequency at 4f ₀
A bandpass filter having a center frequency at f ₀ is used, and a 2 frequency divider is used instead of the 4 frequency divider 83. The frequency spectrum in this case is as shown in FIG. 7, and the frequency of the carrier component in the signal output from the bandpass filter is 2
It becomes f ₀ .

In the above embodiment, the present invention is applied to the receiver of the double superheterodyne system for converting the received signal into the first and second intermediate frequency signals, but the present invention is not limited to this. The present invention is not limited to this, and can be applied to a single super-heterodyne receiver that performs frequency conversion into a single intermediate frequency signal. Stability can be lost.

In the above-described embodiment, the first local oscillation signal and the second local oscillation signal are generated by the circuit composed of the first local oscillator of the frequency synthesizer configuration, the second local oscillator of the phase locked loop configuration, and the reference oscillator. However, this is because the output of the reference oscillator is also used for carrier generation on the transmission side, and the present invention is not limited to this configuration. For example, the first local oscillator and the second local oscillator may be configured by voltage-controlled oscillators without providing the reference oscillator, and their oscillation frequencies may be controlled so that the frequency deviation of the second intermediate frequency signal is eliminated. .

The present invention can be applied not only to a digital modulation type receiver, but also to a conventional analog modulation type receiver.

[0067]

As described above, the present invention has the following effects. (1) First, since the first and second local oscillation frequencies are controlled by the AFC circuit, even if the frequency stability of the reference oscillator 16 is low, the frequency stability of the carrier of the second intermediate frequency signal IF2 is Therefore, the error rate in the demodulation circuit 11 can be reduced.

(2) In particular, in order to measure the frequency deviation (Δf _L1 + Δf _L2 ) of the second intermediate frequency f _IF2 , the modulated wave removing circuit 8 removes the modulated wave from the second intermediate frequency signal IF 2 and the carrier component S 3 Is extracted and the frequency of the carrier component is counted.
Even if the intermediate frequency signal IF2 shifts, the frequency deviation (Δf _L1 + Δf _L2 ) in the second intermediate frequency signal IF2 can be accurately measured without being affected by the shift.

(3) Further, since the frequency f _IF2 of the second intermediate frequency signal IF2 is counted by the frequency counter in order to obtain the frequency deviation of the second intermediate frequency signal IF2, the convergence time of the AFC operation is shortened. be able to.

That is, in order to obtain the frequency deviation (Δf _L1 + Δf _L2 ) of the second intermediate frequency signal IF 2, the oscillation frequency (f _L1 + Δf _L1 ) of the first local oscillator 4 and the oscillation frequency (f _{A method is possible in which L2} + Δf _L2 ) is directly counted by a frequency counter to obtain the difference between the true values f _L1 and f _L2 , but in that case, the first and second local oscillation frequencies are very Since it is a high frequency, C as a counter
Since the MOS transistor configuration cannot be used, there arises a problem that the convergence time of the AFC operation as described above becomes long.

On the other hand, in the present invention, as shown in the embodiment, the carrier frequency f _{0 of} the second intermediate frequency signal IF2 (=
Since the frequency counter has a CMOS transistor configuration, the counting time can be short.
The convergence time of the AFC operation can be shortened.

(4) Further, as described above, the count time of the frequency counter can be shortened, so that
Since it is possible to repeat the counting operation a plurality of times within a preset time for the AFC operation of the FC circuit, the average value of the plurality of count values can be used as the final count value. The counting accuracy can be improved. (5) Further, it is not necessary to use a highly accurate and expensive oscillator such as a temperature-compensated crystal oscillator for the second local oscillator.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a receiving circuit using a frequency conversion circuit as an embodiment of the present invention.

FIG. 3 is a diagram showing a configuration example of a first local oscillator in the embodiment circuit.

FIG. 4 is a diagram showing a configuration example of a second local oscillator in the embodiment circuit.

FIG. 5 is a diagram showing a configuration example of a modulated wave removing circuit in the embodiment circuit.

FIG. 6 is a diagram showing a frequency spectrum at each processing stage in the modulated wave removing circuit in the embodiment circuit.

FIG. 7 is a diagram showing a frequency spectrum at each processing stage in a modulated wave removing circuit according to another embodiment.

FIG. 8 is a diagram showing a conventional frequency conversion circuit.

FIG. 9 is a diagram showing a frequency spectrum of a modulated signal in the case of a conventional analog frequency modulation method.

[Explanation of symbols]

 1 Antenna 2, 5, 9, 82 Bandpass Filter 3, 6 Mixer 4 1st Local Oscillator 7 2nd Local Oscillator 8 Modulated Wave Removal Circuit 10 Amplitude Limiting Amplifier 11 Demodulation Circuit 12 Frequency Counter 13 Table 14 Subtractor 15 D / A Conversion 16 Reference Oscillator 17 Timing Generator 41 71 Voltage Controlled Oscillator 42 Variable Divider 43, 73 Phase Comparator 44, 72, 74 Divider 45, 74 Integrator 81 4 Multiplier 83 Multiplier 84 4 Divider

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshifumi Toda 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Limited

Claims (7)

[Claims] 1. A local oscillation signal generation circuit (101) for generating a first local oscillation signal and a second local oscillation signal, and a first intermediate frequency by mixing a modulated reception signal with the first local oscillation signal. A first mixer (102) for frequency converting into a signal, and a second mixer (103) for frequency converting into a second intermediate frequency signal by mixing the first intermediate frequency signal with the second local oscillation signal, A modulation wave removing circuit (104) for removing a modulation wave from the second intermediate frequency signal of the second mixer to extract a carrier wave, and a frequency deviation of the carrier wave extracted by the modulation wave removing circuit from its true value. And a deviation detection circuit (105) for detecting, wherein the local oscillation signal generation circuit is configured to detect the first local oscillation signal and the second local oscillation signal generated in a direction in which the frequency deviation detected by the deviation detection circuit decreases. Control the frequency Frequency conversion circuit of a receiver configured to. 2. The local oscillation signal generating circuit (101) comprises a frequency-variable control type reference oscillator (106) for generating a reference oscillation signal, and a first local oscillation signal for generating a first local oscillation signal based on the reference oscillation signal. A local oscillator (107) and a second local oscillator (108) for generating a second local oscillation signal based on the reference oscillation signal, the reference oscillator being the frequency detected by the deviation detection circuit. The frequency conversion circuit of the receiver according to claim 1, wherein the frequency conversion circuit is configured to control the frequency of the generated reference oscillation signal in the direction in which the deviation decreases. 3. The frequency conversion circuit for a receiver according to claim 2, wherein the first local oscillator is composed of a frequency synthesizer using a phase locked loop, and the second local oscillator is composed of a phase locked loop. 4. A local oscillation signal generation circuit for generating a local oscillation signal; a mixer for frequency-converting the modulated signal with the local oscillation signal into an intermediate frequency signal input to a demodulation circuit; A modulation wave removing circuit that removes the modulation wave from the intermediate frequency signal of the mixer to extract the carrier wave, and a deviation detection circuit that detects the frequency deviation of the carrier wave extracted by the modulation wave removing circuit from its true value The frequency conversion circuit of the receiver configured to control the frequency of the local oscillation signal generated by the local oscillation signal generation circuit in a direction in which the frequency deviation detected by the deviation detection circuit decreases. 5. The local oscillation signal generation circuit includes a frequency variable control type reference oscillator for generating a reference oscillation signal, and a local oscillation signal phase-locked to the reference oscillation signal using the reference oscillation signal of the reference oscillator. 5. The frequency conversion circuit for a receiver according to claim 4, further comprising a local oscillator composed of a phase-locked loop circuit for generating. 6. The modulated wave removing circuit includes a modulated signal component separating means for separating a modulated signal component by multiplying an input intermediate frequency signal, and a modulated signal separated by the modulated signal component separating means. Multiplying carrier wave extracting means for multiplying the signal component and the reproduction clock generated by the demodulator to extract a multiplied carrier wave component from the components of the modulated signal, and dividing the multiplying carrier wave component of the multiplying carrier wave extracting means. 6. The frequency conversion circuit of the receiver according to claim 1, further comprising: a frequency dividing unit that converts the carrier wave to the original carrier frequency. 7. The frequency conversion circuit for a receiver according to claim 1, wherein the received signal is digitally phase-modulated.