JP2879016B2 - Variable frequency oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a variable frequency oscillator used in a wide band receiving apparatus and an OFDM receiving apparatus for receiving and demodulating an OFDM (Orthogonal Frequency Division Multiplex) signal.

[0002]

2. Description of the Related Art Recently, as a digital terrestrial broadcasting system, O.
The FDM system is being studied in Europe and Japan. This OFD
In the M system, since many orthogonal carriers are multiplexed in a band, the flatness of the phase characteristics and the amplitude characteristics of the band is important. Further, when phase noise is superimposed on the receiving device, orthogonality between the carriers is lost, and the error rate is deteriorated. Therefore, it is important to reduce the phase noise of the receiving device.

Similarly, when the center of the received signal deviates from the center frequency of the matched filter (Nyquist filter) of the receiving device, the carrier located at the end of the band is applied to the attenuation region of the filter, and the characteristics are deteriorated. Therefore, an AFC circuit that matches the center frequency of the received signal with the center frequency of the matched filter (Nyquist filter) of the receiver is also an important issue.

FIG. 11 shows a basic configuration of an OFDM receiver. In this OFDM receiver, a high-frequency reception signal is input from an input terminal 1, and a broadband reception OFDM is
A desired signal is selected from the signal and frequency-converted to an intermediate frequency or baseband. The analog signal from the tuner unit 2 is converted into a digital signal by an A / D (analog / digital) converter 7, and the digital signal is converted by a matching digital filter 8.
The OFDM signal is demodulated by extracting the multi-carrier band component of the FDM signal, reproducing the synchronizing signal by the synchronizing reproduction circuit 9, and performing FFT processing by the FFT (fast Fourier transform) circuit 10.

[0005] The receiving frequency band of the OFDM receiver is the same as the current VHF and UHF bands for terrestrial broadcasting, and is a wide band of about 50 to 900 MHz. In order to stably receive the broadband signal, the tuner unit 2 uses a double conversion method having excellent flatness of band characteristics.

That is, the received signal input to the tuner unit 2 is up-converted into a 1 GHz band by the mixer 3 and the first local oscillator 5, and a BPF (band-pass filter) 4
After the unnecessary wave is removed by the down converter (down con
v.) The frequency is converted in 2 and input to the A / D converter 7.

In this system, the first local oscillator 5 for tuning needs to have a frequency variable width of about 1.2 to 2.2 GHz in a wide band, so that the first local oscillator 5 has a wide band and low noise. is important.

Generally, the phase noise of an oscillation circuit is expressed by the following equation from a Leeson feedback model.

[0009]

(Equation 1)

Here, f0 is the oscillation frequency, Δf is the detuning frequency from the oscillation signal, fc is the low frequency and 1
The frequency at which the spectrum of the / f noise is the same as the white noise, QL is the load Q of the resonance circuit, PS is the oscillation output, and Pn
Indicates white noise density. From the above equation, if the 1 / f noise is small and the load Q of the resonance circuit is large, the phase noise of oscillation can be reduced.

As the first local oscillator 5, FIG.
A clap (modified Colpitts) oscillation circuit as shown in FIG. In this clap oscillation circuit, an LC series resonance circuit including an inductance element 16 and a capacitance element (variable capacitance diode) 17 is coupled to a base of a transistor 14, an emitter is grounded via a resistor 15, and a collector is connected to a capacitance element 13. Grounded through. Then, by applying a control voltage input from the terminal 19 via the inductance element 16 to the base of the transistor 14, an oscillation output is obtained from the terminal 12 to which the collector is connected.

That is, this oscillation circuit is of the LC series resonant common-collector or common-base type that can easily obtain a wide-band oscillation. However, in order to increase the bandwidth, it is necessary to use a variable capacitance diode having a small minimum capacitance value as the capacitance element 17. However, since a variable capacitance diode having a small minimum capacitance value has a large series resistance, the no-load Q of the resonance circuit tends to decrease as a whole.

On the other hand, as a wide-band variable frequency oscillator, an oscillation circuit having a transistor connected to a parallel resonance circuit as shown in FIG.
z varactor tuned transistor oscillators ”: Proc.
9th European Conf., Brighton, 1979.

This oscillation circuit includes an inductance element 53
A parallel resonance circuit 29 including an inductance element 31, a capacitance element (variable capacitance diode) 30, and an inductance element 31 is connected to an emitter of the transistor 22 whose base is grounded via an inductance element 52, and the collector thereof is output. Connect to terminal 28I. Then, the control voltage input from the terminal 32 is applied to the emitter of the transistor 22 via the inductance element 52, so that an oscillation output is obtained from the terminal.

That is, this oscillation circuit obtains oscillation in a wide band by coupling the emitter and the resonance circuit with the inductance 52, but the oscillation circuit of this configuration does not discuss the phase noise characteristics.

Also, the difference between the center frequency of the received signal and the center frequency of the matching filter (Nyquist filter) of the receiver is not sufficiently considered with respect to the frequency shift generated in the receiving tuner unit. AFC
Has not been considered.

FIG. 13 shows a state diagram of the carrier of the OFDM signal. If no frequency shift occurs in the receiving tuner, synchronous reproduction is performed on the center carrier fc of the OFDM reception signal as shown in FIG. 7A. However, if a frequency shift df occurs in the receiving tuner, the waveform shown in FIG. As described above, the synchronization pull-in point is shifted, and the carrier at the end of the OFDM signal is applied to the shoulder of the matched filter, so that information is lost and demodulation characteristics are deteriorated.

[0018]

As described above, the conventional O
In the FDM system, since a large number of mutually orthogonal carriers are multiplexed in a band, the flatness of the phase characteristics and amplitude characteristics of the band is important, and when the phase noise is superimposed on the receiver, the orthogonality between the carriers is reduced. Since the distortion and the error rate are degraded, it is important to reduce the phase noise of the receiver. In particular, it is necessary to reduce the phase noise of a wideband variable frequency oscillator.

Similarly, if the center frequency of the received signal is O
FDM receiver matched filter (Nyquist filter)
Deviates from the center frequency of the filter, the carrier located at the end of the band is applied to the attenuation region of the filter, and the characteristics are deteriorated. Therefore, it is also important to realize an AFC circuit that matches the center frequency of the received signal with the center frequency of the matched filter (Nyquist filter) of the receiver.

The present invention solves the above-mentioned problems, provides a wide-band variable frequency oscillator realizing low phase noise, and makes the center frequency of a received signal coincide with the center frequency of a matched filter to reduce characteristic deterioration. It is an object to provide a possible OFDM receiver.

[0021]

In order to solve the above problems, a variable frequency oscillator according to the present invention is configured as follows.

(1) A transistor element having a control electrode connected to a ground line, and a parallel connection formed by connecting a capacitor element and an inductive element in parallel between the transistor element and one of the controlled electrodes of the transistor element and the ground line. A resonance circuit, a first capacitor connected between one controlled electrode of the transistor element and the parallel resonance circuit, and a second capacitor connected between a control electrode of the transistor element and a ground line. By supplying a control voltage between one controlled electrode of the transistor element and the parallel resonance circuit, an oscillation signal is obtained from the other controlled electrode of the transistor element. did.

(2) In the configuration of (1), a third capacitor connected between one controlled electrode and the control electrode of the transistor element is further provided.

(3) The first control electrode is connected to a reference potential, one of the controlled electrodes is connected to a common power supply line, and the other is connected to a ground line.
And a second transistor element, and a parallel resonance circuit connected between the other controlled electrodes of the first and second transistor elements and connected in parallel with a capacitive element and an inductive element;
First and second time constant elements respectively connected between a control electrode of the first and second transistor elements and a ground line, and the first and second time constant elements are connected through the parallel resonance circuit.
By supplying a control voltage to the other controlled electrode of the second transistor element, an oscillation signal is obtained from at least one of the one controlled electrode of the first and second transistor elements.

(4) In the configuration of (3), a third and a fourth capacitor connected between one control electrode of each of the first and second transistor elements and a control voltage are further provided. I prepared for it.

That is, in the variable frequency oscillator according to the present invention, an emitter resonance type oscillation circuit advantageous for widening the band is used, and the connection between the emitter of the transistor and the resonator is capacitively coupled in order to increase the no-load Q of the resonator. . Further, in order to stabilize the power supply voltage fluctuation, a compensating capacitive element is inserted between the collector and the base of the transistor. Further, in order to suppress the influence on 1 / f noise, the base of the transistor has a ground impedance. Further, in order to reduce an increase in noise due to even-order harmonics, a differential oscillation circuit in which a parallel resonance circuit is inserted between the emitters of two transistors is used.

On the other hand, the OFDM receiver according to the present invention
It is configured as follows.

(5) OFDM (Orthogonal Freq) is obtained by mixing the oscillation output of the local oscillator and receiving the high frequency signal.
frequency conversion means for extracting a signal and converting it to an intermediate frequency or baseband OFDM signal, and OFD output from the frequency conversion means.
An analog / digital conversion circuit for converting the M signal into a digital signal; a variable digital filter for extracting a multicarrier band component of the OFDM signal from an output signal of the analog / digital conversion circuit; and a synchronization signal from an output of the variable digital filter. A synchronous reproducing circuit for reproducing, an FFT circuit for performing fast Fourier transform on an OFDM signal output from the synchronous reproducing circuit and demodulating the signal, a carrier frequency at the center of the OFDM signal input to the FFT circuit, and a center of the variable digital filter Frequency error detecting means for detecting a deviation from the frequency as a frequency error; and controlling the oscillation frequency of the local oscillator based on a frequency error signal obtained by the frequency error detecting means, thereby providing an output from the digital / analog conversion circuit. Intermediate frequency or baseband OFDM Frequency control means for matching the center frequency of the digital filter with the center frequency of the digital filter, and when the frequency error is detected by the frequency error detection means, the pass band of the variable digital filter is set wide to detect the frequency error. Band control means for setting the band of the variable digital filter to be narrow when the band has disappeared.

(6) In the configuration of (5), the frequency error detecting means includes a reference oscillation signal of the synchronous reproduction circuit and an O.
The frequency error is detected by detecting a deviation from the center carrier frequency of the FDM signal.

(7) In the configuration of (5), further, based on the frequency error signal obtained by the frequency error detecting means, the reference oscillation signal of the synchronous reproduction circuit is adjusted to the center of the OFDM signal input to the synchronous reproduction circuit. Phase control means for synchronizing the phase with the carrier.

(8) In the configuration of (5), the variable frequency oscillator according to any one of (1) and (2) is used as the local oscillator.

That is, in the OFDM receiver having the above configuration, the difference between the center frequency of the received signal and the center frequency of the matched filter (Nyquist filter) of the receiver is detected by the error detector and returned to the tuner. Use the configuration.

If the frequency shift is large, the OFDM carrier wave may fall on the shoulder of the matched filter, and information may be lost, and the demodulation characteristics may be degraded. By adjusting the bandwidth of the filter as a matched filter after correcting the frequency shift,
Even in an initial state having a frequency shift, relatively good reception characteristics can be obtained without losing information.

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to FIGS.

FIG. 1 shows a circuit configuration using a single transistor as a first embodiment of the variable frequency oscillator according to the present invention.

In FIG. 1, the base of the transistor 22 is grounded at a high frequency by a capacitive element 23, and the emitter of the transistor 22 is connected to a parallel resonance circuit 29 comprising an inductance element 31 and a capacitive element 30 via a capacitive element 24. Furthermore, it is connected to the control voltage input terminal 32 via the capacitive element 24.

A capacitor 25 is connected between the collector and the base of the transistor 22, and a matching circuit including an inductance element 26 and a capacitor 27 is connected to the collector. The collector of the transistor 22 is connected to an output terminal 28 so that an oscillation output can be obtained from the output terminal 28.

In the configuration of the oscillator according to the present embodiment, the base of the transistor 22 is grounded via the capacitance element 23 so that high-frequency components are released, so that transistor noise can be reduced.

Further, since the capacitive element 24 is interposed in the connection between the emitter of the transistor 22 and the parallel resonance circuit 29, the no-load Q of the parallel resonance circuit 29 can be improved. This is effective in reducing the phase noise of oscillation.

Furthermore, since the capacitance element 25 is connected between the collector and the base of the transistor 22, the variation of the parasitic capacitance between the collector and the base with respect to the power supply voltage can be suppressed to a small value. Therefore, the oscillation frequency can be stabilized and the phase noise can be reduced.

Here, if a variable capacitance diode is used as the capacitance element 30, the capacitance can be controlled by applying the control voltage Vt from the terminal 32, so that the oscillation frequency can be freely selected and the variable frequency oscillator can be used. Can be realized.

FIG. 2 shows the result of comparison of the no-load Q of the parallel resonance circuit 29 with a conventional oscillator. Solid line 5
5 is a no-load Q of the parallel resonance circuit 29 used in the oscillator of the present invention, and a broken line 56 is a no-load Q of the resonance circuit used in the common-collector oscillation circuit shown in FIG.
8 shows the no-load Q of the resonance circuit of the conventional oscillator in which the oscillation transistor and the resonator are coupled by an inductance element in the emitter resonance type oscillation circuit shown in FIG. As is clear from the figure, in the oscillator of the present invention, the no-load Q of the resonance circuit shows a high Q over the entire frequency band.

FIG. 3 shows the results obtained by measuring the measured values of the phase noise for various oscillation circuits. The solid line 60 is the case of the oscillation circuit according to the present invention, the broken line 59 is the case of the grounded collector oscillation circuit, the broken line 58 is the case of the emitter resonance type oscillation circuit,
9 shows phase noise characteristics of a conventional oscillation circuit in which an oscillation transistor and a resonance circuit are coupled by an inductance element. It can be seen that the phase noise characteristic of the oscillation circuit of the present invention is superior by 5 to 10 dB.

FIG. 4 shows the configuration of a second embodiment of the variable frequency oscillator according to the present invention. This oscillator
It is composed of a differential oscillation circuit using two transistors 33 and 34.

In FIG. 4, the bases of transistors 33 and 34 forming a differential pair are grounded at high frequencies by capacitive elements 43 and 42, respectively. Also, the transistor 33,
A parallel resonance circuit 44 is connected between the 34 emitters.

Further, the emitters of the transistors 33 and 34 are grounded via resistors 49 and 50, respectively. Capacitors 38 and 39 are connected between the collectors and bases of the transistors 33 and 34, respectively. Further, the collectors of the transistors 33 and 34 are connected to the load 35 and
It is connected to a power supply terminal 37 via. The collectors of the transistors 33 and 34 are connected to the output terminals 40 and 41, so that a differential oscillation output can be obtained from the output terminals 40 and 41.

Here, the parallel resonance circuit 44 includes an inductance element 4 between the emitters of the transistors 33 and 34.
5, and both ends of the inductance element 45 are connected to a control voltage input terminal 51 via capacitance elements (variable capacitance diodes) 46 and 47, respectively.

In this parallel resonance circuit 44, capacitors C1 and C2 are interposed between both ends of the inductance element 45 and each of the capacitors 46 and 47, respectively, so that the capacitors C1 and C2 are connected to each other. The space between them is grounded via resistors R1 and R2, respectively. In this way, even when the control voltage Vt becomes lower than the emitter voltage of the transistors 33 and 34, there is an effect that the emitter bias voltage is not affected.

In the present embodiment, since a differential oscillation circuit is used, even harmonics can be canceled. For this reason, generation of noise due to even-order harmonics can be suppressed, which is effective in reducing phase noise.

Further, since the bases of the transistors 33 and 34 are grounded by the capacitors 42 and 43, the noise of the transistors can be reduced. This is effective in reducing the phase noise of oscillation.

Since the capacitance elements 38 and 39 are connected between the collectors and bases of the transistors 33 and 34,
Variation of the parasitic capacitance between the collector and the base with respect to the power supply voltage can be reduced. Therefore, the oscillation frequency can be stabilized and the phase noise can be reduced.

As the capacitance elements 46 and 47 of the parallel resonance circuit 44, variable capacitance diodes are used. According to this, by applying the control voltage Vt input from the terminal 51 to each of the variable capacitance diodes 46 and 47, a variable frequency oscillation circuit can be configured.

FIG. 5 shows the configuration of the first embodiment of the OFDM receiver according to the present invention. Here, the same parts as those in FIG. 7 are denoted by the same reference numerals.

In this apparatus, the tuner 2 uses a double conversion method. That is, the multi-channel wideband OFDM signal input from the terminal 1 is converted into a first intermediate frequency signal by the mixer 3 and the wideband variable oscillation circuit 5, and is converted by the mixer 61 and the variable oscillation circuit 6 through the bandpass filter 4. It is converted into two intermediate frequency signals. The second intermediate frequency signal may be further converted into a third intermediate frequency signal or a baseband signal in the tuner unit 2 using another mixer and an oscillation circuit.

An output signal from the tuner unit 2 is converted from an analog signal to a digital signal by an A / D converter 7 and input to a synchronous reproduction circuit 9 via a digital filter 8 (matched filter). However, an adaptive filter having a variable bandwidth that can control the bandwidth by a control signal is used as the digital filter 8.

In the synchronous reproduction circuit 9, a number of OFDM carrier waves (f1 to f1) to which the internally generated reference oscillation signal is inputted are inputted.
The pull-in processing is performed so as to synchronize with the center carrier (fc) of the fn). The output of the synchronous reproduction circuit 9 is
And is OFDM demodulated.

The center frequency of the reference oscillation signal in the synchronous reproduction circuit 9 coincides with the center frequency of the digital filter 8, and the frequency and phase can be minutely controlled.

The output of the synchronous reproduction circuit 9 and the output of the FFT circuit 10 are both supplied to a frequency error detector 54. The frequency error detector 54 monitors the input / output relationship of the FFT circuit 10 and detects a frequency error based on the degree of deterioration of the FFT circuit 10. The detected frequency error signal is a microcomputer (hereinafter, referred to as a microcomputer) 63. Sent to

The microcomputer 63 includes buses 64, 66, 6
The frequency control for the local oscillators 5 and 6 of the tuner unit 2, the band control for the digital filter 8, the synchronization control for the synchronous reproduction circuit 9, and the control for taking in the frequency error for the frequency error detector 54 are performed through 7 and 68.

As described above, the synchronous reproduction circuit 9
Ideally, the internal reference oscillation signal is synchronized with the center carrier of the DM carrier. However, as shown in FIG. 6A, the OFDM carrier is shifted from the center carrier of the digital filter 8 by df due to a frequency error or the like in the tuner unit 2, so that synchronization is performed to the frequency (fc -1). . This is because, as described above, since the reference oscillation signal in the synchronous reproduction circuit 9 matches the center frequency of the digital filter 8, the center carrier of the OFDM carrier and the center frequency of the digital filter 8 are shifted by df. be equivalent to.

At this time, as shown in FIG. 13B of the conventional example, the carrier near one end fn is applied to the shoulder of the pass band of the digital filter 8, and the demodulation characteristics may be degraded. .

For this reason, in the present embodiment, the digital filter 8 is a variable bandwidth adaptive filter whose bandwidth can be controlled by a control signal 66 from a microcomputer (microcomputer) 63. In the initial state of channel selection, FIG.
(A) is set in a state where the bandwidth is widened as indicated by a broken line 73. If you set a wide bandwidth like this,
Although the S / N characteristic is deteriorated, the carrier wave near fn at one end does not fall on the shoulder of the pass band of the digital filter 8 and the demodulation characteristic is not deteriorated.

Next, the synchronous reproduction circuit 9 and the FFT circuit 10
Are input to a frequency error detector 54 to detect the frequency error df described above.
6, the OFDM carrier can be moved to the center of the band of the digital filter 8 as shown in FIG.

The above operation will be described again with reference to the flowchart of FIG.

First, channel selection is started. That is, a control signal from the microcomputer 63 is transmitted to the tuner unit 2 via the bus 64 to control the oscillation frequency of the local oscillators 5, 6, and the like. At this point, a frequency error df occurs in the tuner unit 2, and the center carrier of the OFDM signal and the center frequency of the digital filter 8 are shifted by df.

Next, the bandwidth of the filter 8 is
Is set to a wide bandwidth 73 by the control signal 66 from the controller, and the synchronization pull-in is completed by the synchronous reproduction circuit 9 while keeping the frequency error df. When the synchronization playback circuit 9 completes the synchronization pull-in, the microcomputer 63 sends a synchronization pull-in completion signal to the bus 67.
To send over.

At this time, the reference carrier wave in the synchronous reproduction circuit 9 is synchronized with a carrier wave shifted by df from the center carrier wave of the OFDM signal as shown in FIG. Here, the frequency error df is detected by the frequency error detector 54, and the detected df is transmitted to the microcomputer 63 via the bus 68.

The microcomputer 63 transmits an AFC signal to the tuner unit 2 via the bus 64 when the synchronization pull-in completion signal is received via the bus 67. The tuner unit 2 corrects a frequency error based on the AFC signal.

When the frequency error df becomes zero in the frequency error detector 54 due to the correction of the frequency error, the microcomputer 63 transmits a control signal 66 to the filter 8 to change the bandwidth of the filter 8 as shown in FIG. Narrow bandwidth 21 as shown in
Set to. Here, the narrow bandwidth 21 has, for example, the bandwidth of a matched filter (Nyquist bandwidth).

In the state where the channel selection is completed but the synchronization in the synchronous reproduction circuit 9 is not completed, the AFC
No control is applied.

According to the present embodiment, the outputs of the synchronous reproduction circuit 9 and the FFT circuit 10 are input to the frequency error detector 54 to detect the above-mentioned frequency error df, and this error signal is sent to the oscillation circuit of the tuner 2. Because I am going to return,
As shown in FIG. 6B, the OFDM carrier can be moved to the center of the band of the digital filter 8, whereby a good demodulation characteristic can be obtained.

Further, the bandwidth of the filter 8 is widened when there is a frequency error, and the bandwidth of the filter 8 is narrowed when the frequency error has disappeared.
There is an effect that a good demodulation characteristic can be obtained even if there is a frequency error.

FIG. 8 shows the configuration of a second embodiment of the OFDM receiver according to the present invention. Here, the same parts as those in the first embodiment shown in FIG.

In the embodiment shown in FIG. 5, the outputs of the synchronous reproduction circuit 9 and the FFT circuit 10 are input to a frequency error detector 54 to detect the above-mentioned frequency error df. And the OFDM carrier is moved to the center of the band of the digital filter 8 as shown in FIG.

However, only with this, the synchronization pull-in point remains at the carrier fc -1 as shown in FIG. 9A (that is, the synchronization pull-in point of the reference oscillation signal of the synchronous reproduction circuit 9 is the center carrier of the OFDM). Df). For this reason, in the present embodiment, the error signal df from the frequency error detector 54 is also fed back to the synchronous reproduction circuit 9, and
As shown in FIG. 9B, the synchronization pull-in point is changed to the carrier wave dc.

The above operation will be described again with reference to the flowchart of FIG.

Up to the point where the bandwidth of the digital filter 8 is reset to be narrow, it is the same as the embodiment of FIG. In the present embodiment, thereafter, based on the initial frequency error df stored in the microcomputer 63, a synchronization error signal is transmitted to the synchronization reproduction circuit 9 using the bus 72, and the synchronization error is corrected. Channel selection is completed.

According to the present embodiment, the outputs of the synchronous reproduction circuit 9 and the FFT circuit 10 are input to the frequency error detector 54 to detect the above-mentioned frequency error df, and this error signal is sent to the oscillation circuit of the tuner 2. Feedback to correct the frequency error,
Thereafter, the signal is further fed back to the synchronous reproduction circuit 9 to shift the OFDM carrier to the center of the band of the digital filter 8 as shown in FIG. Characteristics are obtained.

Note that the OFD of the first and second embodiments is
In the M receiving apparatus, if the variable frequency oscillator shown in FIG. 1 or FIG. 4 is used for the tuner units 5 and 6, low phase noise and stabilization of the oscillation frequency can be obtained, and it is needless to say that the present invention is more effective. Nor.

[0080]

As described above, according to the variable frequency oscillator of the present invention, transistor noise can be reduced by grounding the control electrode of the transistor element with the capacitive element, and the connection with the resonance circuit can be made by connecting the capacitive element. By doing through
Since the no-load Q of the resonance circuit can be improved, it is effective in reducing the phase noise of oscillation.

Further, by connecting a capacitance between the collector and the base of the transistor, it is possible to suppress the variation of the parasitic capacitance between the collector and the base with respect to the power supply voltage, thereby stabilizing the oscillation frequency and reducing the phase noise. Can be planned.

In addition, by employing a differential oscillation circuit configuration, even-order harmonics can be canceled.
The generation of noise due to even harmonics can be suppressed,
This is effective for lowering phase noise.

On the other hand, according to the OFDM receiving apparatus according to the present invention, for the AFC, the outputs of the synchronous reproduction circuit and the FFT circuit are input to the frequency error detector to detect the above-mentioned frequency error, and this error signal is output to the tuner. By returning the OFDM carrier to the center of the band of the digital filter by returning to the oscillation circuit of the section, good demodulation characteristics can be obtained.

Further, the configuration is such that the bandwidth of the filter is widened when there is a frequency error, and the bandwidth of the filter is narrowed when the frequency error has disappeared, so that good demodulation characteristics can be obtained even if there is a frequency error. effective.

Further, after correcting the frequency error, the synchronization error signal is fed back to the synchronization recovery circuit to move the OFDM carrier to the center of the band of the digital filter, and the synchronization pull-in point is also set to the center of the carrier to improve the frequency. The demodulation characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a variable frequency oscillator according to a first embodiment of the present invention.

FIG. 2 is a frequency characteristic diagram of no-load Q for explaining the effect of the oscillator according to the first embodiment.

FIG. 3 is a characteristic diagram showing a relationship between an offset frequency at 1.7 GHz and phase noise for explaining the effect of the oscillator according to the first embodiment.

FIG. 4 is a circuit diagram showing a configuration of a variable frequency oscillator according to a second embodiment of the present invention.

FIG. 5 is a block circuit diagram showing a configuration of a first embodiment of the OFDM receiver according to the present invention.

FIG. 6 is a frequency characteristic diagram for explaining the AFC operation of the OFDM receiver according to the first embodiment.

FIG. 7 is a flowchart for explaining the AFC operation of the OFDM receiving apparatus according to the first embodiment.

FIG. 8 is a block circuit diagram showing a configuration of a second embodiment of the OFDM receiver according to the present invention.

FIG. 9 is a frequency characteristic diagram for explaining the AFC operation of the OFDM receiver according to the second embodiment.

FIG. 10 shows the AFC of the OFDM receiver according to the second embodiment.
9 is a flowchart for explaining the operation.

FIG. 11 is a block circuit diagram showing a configuration of a conventional general OFDM receiver.

FIG. 12 is a circuit diagram showing a configuration of an oscillation circuit employed in a conventional variable frequency oscillator.

FIG. 13 is a frequency characteristic diagram for explaining a frequency error and a problem related to synchronous reproduction of the conventional OFDM receiver.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Signal input terminal 2 ... Tuner part 3,61 ... Mixer 5,6 ... Local oscillator 7 ... A / D converter 8 ... Digital filter 9 ... Synchronous reproduction circuit 10 ... FFT circuit 14,22,33,34 ... Transistor 17 , 31, 46, 47 ... Capacitance element (variable capacitance diode) 16, 31, 53, 26, 45 ... Inductance element 12, 28, 40, 41 ... Oscillation output terminal 19, 32, 51 ... Control voltage input terminal 13, 23 , 24, 25, 27, 42, 43, 38, 3
9 Capacitance element 49, 50 Resistor 54 Frequency error detector 11 OFDM demodulation signal output terminal 20 Carrier 21 Digital filter pass band 63 Microcomputer 64, 66, 67, 68, 69, 70 Bus

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

Claims (3)

(57) [Claims] 1. A transistor element having a base electrode connected to a ground line, a parallel resonance circuit connected between an emitter electrode of the transistor element and the ground line, and having a parallel connection of a variable capacitance element and an inductive element. A first capacitor connected between the emitter electrode of the transistor element and the parallel resonance circuit; a second capacitor connected between a base electrode of the transistor element and a ground line; A third capacitor connected between a base electrode and a collector electrode of the transistor element, wherein a frequency of a signal output from the collector of the transistor element by adjusting a level of a control voltage applied to the parallel resonance circuit; A variable frequency oscillator characterized in that the variable frequency oscillator is varied. 2. A first and a second transistor element each having a control electrode connected to a ground line, one controlled electrode both connected to a common power supply line, and the other controlled electrode connected to a reference potential. A parallel resonance circuit connected between the other controlled electrodes of the first and second transistor elements and having a variable capacitance element and an inductive element connected in parallel; and controlling the first and second transistor elements. A first and a second time constant element respectively connected between the electrode and a ground line; and a control voltage applied to the other controlled electrodes of the first and second transistor elements through the parallel resonance circuit. A variable frequency oscillator, wherein an oscillation signal is obtained from at least one of the controlled electrodes of the first and second transistor elements by supplying the oscillation signal. 3. The semiconductor device according to claim 1, further comprising third and fourth capacitors connected between one control electrode of each of said first and second transistor elements and a control voltage. 3. The variable frequency oscillator according to 2.