Demodulation apparatus performing different frequency control functions using separately provided oscillators

• US 6,023,491 A
• Filed: 06/20/1995
• Issued: 02/08/2000
• Est. Priority Date: 06/21/1994
• Status: Expired due to Fees

• Current Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Inventors: Soga, Shigeru, Saka, Hiroshi, Urata, Kazunao
• Primary Examiner(s): Ghebretinsae, Temesghen
• Application Number:
  US08/492,488
• Time in Patent Office: 1,694 Days
• Field of Search: 375/326, 375/344, 375/281, 375/325, 375/332, 455/255, 455/257, 455/258, 455/263, 455/265, 455/314, 455/315, 455/179.1, 455/182.1, 455/182.2, 455/182.3, 455/183.1, 455/184.1, 455/192.1, 455/192.2 , 455/192.3, 455/196.1, 455/197.1, 455/207, 455/208, 455/209, 329/304, 329/306, 329/307, 329/345, 329/346
• US Class Current: 375/326
• CPC Class Codes: H04L 2027/0028 at passband only
• CPC Class Codes: H04L 2027/0057 quadrature phase
• CPC Class Codes: H04L 27/2273 associated with quadrature demodulation, e.g. Costas loop

Abstract

A QPSK modulation signal is processed with frequency conversion into an IF signal by a channel selection circuit, and then is converted into a baseband signal by a quadrature demodulator. The baseband signal has a wave thereof shaped by DTFs and then sent to a carrier recovery circuit. An offset frequency of the IF signal detected by an AFC circuit is compensated for by a baseband frequency converter including a complex multiplier and a DVCO. The frequency tracking circuit detects whether an operating frequency of another DVCO in the carrier recovery circuit goes beyond a range. By tracking an oscillation frequency of the DVCO in the baseband frequency converter so as not to go beyond the reference value, frequency deviation of the spectrum of signals which are input to the DTFs is controlled not to go beyond the reference value. By offsetting a frequency dividing ratio of a variable frequency divider of the channel selection circuit by an amount corresponding to the offset frequency of the IF signal detected by the frequency detector, the frequency deviation of the QPSK modulation signal can be compensated for whichever channel is selected.

Claims

1. A demodulation apparatus, comprising:

conversion means, which includes a local oscillator for generating a local oscillation signal having an oscillation frequency which changes in accordance with channel selection information, for selecting a signal corresponding to a prescribed channel from input modulation signals in accordance with the channel selection information and performing frequency conversion of the selected signal into an IF signal;

demodulation means for demodulating the IF signal into an I baseband signal and a Q baseband signal having a quadrature relationship to each other;

recovery means, which includes a recovered carrier generator for generating a recovered carrier based on the I baseband signal and the Q baseband signal having the quadrature relationship to each other, for outputting a recovered I output signal and a recovered Q output signal;

first control means for detecting a frequency error of the baseband signals based on the I baseband signal and the Q baseband signal, and feeding back the detected frequency error to a second voltage-controlled oscillator included in the first control means and having an output provided to the demodulation means so as to automatically remove the detected frequency error;

transmission means for detecting information on an offset frequency of the second voltage-controlled oscillator, which is a difference between a frequency of the second voltage-controlled oscillator and a prescribed frequency of the second voltage-controlled oscillator, and transmitting the detected information on the offset frequency to the conversion means; and

second control means included in the conversion means for controlling the oscillation frequency of the local oscillation signal generated by the local oscillator based on the information on the offset frequency to control a frequency of the IF signal so as to remove the offset frequency of the second voltage-controlled oscillator,

wherein a frequency control of the second voltage-controlled oscillator in the first control means and a frequency control of the local oscillator in the conversion means are performed using different oscillating means from each other.

2. A demodulation apparatus according to claim 1, wherein the conversion means includes:

a frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate the IF signal, and

a channel selection circuit, including the local oscillator having the oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the frequency converter,

wherein the demodulation means includes;

a quadrature demodulator for demodulating the IF signal to the I baseband signal and the Q baseband signal having a quadrature relationship to each other,

a local oscillator for generating an output to be sent to the quadrature demodulator,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals,

a first complex multiplier, connected to a first digital voltage-controlled oscillator serving as the second voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and an output from the first digital voltage-controlled oscillator, and

a low-pass wave-shaping filter for shaping a waveform of the output signal from the first complex multiplier,

wherein the recovery means includes;

a second complex multiplier for performing complex multiplication of the output signal from the low-pass wave-shaping filter and the recovered carrier, and

PLL means for smoothing a phase error of the output signal from the second complex multiplier and supplying the smoothed phase error to a second digital voltage-controlled oscillator to generate the recovered carrier,

wherein the transmission means include a frequency detector for detecting an oscillation frequency of the first digital voltage-controlled oscillator, and

wherein the channel selection circuit includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the local oscillator included in the channel selection circuit.

3. A demodulation apparatus according to claim 2, wherein the first control means includes at least one of:

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the first digital voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the second digital voltage-controlled oscillator goes beyond a range between positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the first digital voltage-controlled oscillator to allow the oscillation frequency of the second digital voltage-controlled oscillator within the range.

4. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

a frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate the IF signal, and

a channel selection circuit, including the local oscillator which comprises a first voltage-controlled oscillator, for supplying the local oscillation signal of the first voltage-controlled oscillator to the frequency converter,

wherein the demodulation means includes;

a quadrature demodulator for demodulating the IF signal to the I baseband signal and the Q baseband signal having the quadrature relationship to each other,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals, and

a low-pass wave-shaping filter for shaping a waveform of the output signals from the A/D converter,

wherein the recovered carrier generator in the recovery means includes;

a digital voltage-controlled oscillator for generating a recovered carrier,

wherein the recovery means further includes;

a complex multiplier, connected to the digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and filtered by the low-pass wave shaping filter with the output from the digital voltage-controlled oscillator, and

PLL means for smoothing a phase error of the output signal from the complex multiplier and supplying the smoothed phase error to the digital voltage-controlled oscillator to generate the recovered carrier,

wherein the transmission means include a frequency detector for detecting an offset frequency of the second voltage-controlled oscillator,

wherein the first control means includes automatic frequency control means for feeding back a frequency error signal detected from the baseband signals to the second voltage-controlled oscillator, and

wherein the second control means is fine-tune control means for receiving the offset frequency information obtained by the frequency detector included in the transmission means and fine-tuning the oscillation frequency of the first voltage-controlled oscillator included in the channel selection circuit.

5. A demodulation apparatus according to claim 4, wherein the first control means includes at least one of:

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the second voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the digital voltage-controlled oscillator goes beyond a range between a positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the second voltage-controlled oscillator to limit the oscillation frequency of the digital voltage-controlled oscillator within the range.

6. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

a first frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate a first IF signal,

a channel selection circuit, including the local oscillator having the oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the first frequency converter, and

a second frequency converter, connected to the second voltage-controlled oscillator as part of the demodulating means, for performing frequency conversion of the first IF signal to generate a second IF signal,

wherein the demodulation means includes;

a quadrature demodulator for demodulating the second IF signal to the I baseband signal and the Q baseband signal having a quadrature relationship to each other,

a local oscillator for generating an output to be sent to the quadrature demodulator,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals, and

a low-pass wave-shaping filter for shaping a waveform of the output signal from the A/D converter,

wherein the recovery means includes;

a complex multiplier, connected to a digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and the output from the digital voltage-controlled oscillator, and

PLL means for smoothing a phase error of the output signal from the complex multiplier and supplying the smoothed phase error to the digital voltage-controlled oscillator to generate the recovered carrier,

wherein the transmission means include a frequency detector for detecting an oscillation frequency of the second voltage-controlled oscillator, and

wherein the channel selection circuit includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the local oscillator included in the channel selection circuit.

7. A demodulation apparatus according to claim 6, wherein the first control means includes at least one of:

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the second voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the digital voltage-controlled oscillator goes beyond a range between positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the second voltage-controlled oscillator to allow the oscillation frequency of the digital voltage-controlled oscillator within the range.

8. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

a first frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate a first IF signal,

a channel selection circuit, including the local oscillator having an oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the first frequency converter, and

a second frequency converter, connected to the second voltage-controlled oscillator as part of the demodulating means, for performing frequency conversion of the first IF signal to generate a second IF signal,

wherein the demodulation means includes;

a band-pass wave-shaping filter for shaping a waveform of the second IF signal,

a quadrature demodulator for demodulating the second IF signal to the I baseband signal and the Q baseband signal having a quadrature relationship to each other,

a local oscillator having an oscillation frequency which is set to be equal to a center frequency of the band-pass wave-shaping filter, for generating an output to be sent to the quadrature demodulator, and

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals,

wherein the recovery means includes;

a complex multiplier, connected to a digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and the recovered carrier, and

PLL means for smoothing a phase error of the output signal from the complex multiplier and supplying the smoothed phase error to the digital voltage-controlled oscillator to generate the recovered carrier,

wherein the transmission means include a frequency detector for detecting an oscillation frequency of the second voltage-controlled oscillator, and

wherein the channel selection circuit includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the local oscillator included in the channel selection circuit.

9. A demodulation apparatus according to claim 8, wherein the first control means includes at least one of:

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the second voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the digital voltage-controlled oscillator goes beyond a range between positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the second voltage-controlled oscillator to allow the oscillation frequency of the digital voltage-controlled oscillator within the range.

10. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

a first frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate a first IF signal,

a channel selection circuit, including the local oscillator having an oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the first frequency converter, and

a second frequency converter, connected to the second voltage-controlled oscillator as part of the demodulating means, for performing frequency conversion of the first IF signal to generate a second IF signal,

wherein the demodulation means includes;

a band-pass wave-shaping filter for shaping a waveform of the second IF signal,

an A/D converter for converting the second IF signal into a digital signal,

a digital frequency converter for converting the digital second IF signal obtained by the A/D converter into a digital baseband signal, and

an I/Q separation circuit for dividing the digital baseband signal into an I baseband signal and a Q baseband signal having a quadrature relationship to each other, and

wherein the recovery means includes;

a complex multiplier, connected to a digital voltage-controlled oscillator, for performing complex multiplication of the I baseband signal and the Q baseband signal and the recovered carrier, and

PLL means for smoothing a phase error of the output signal from the complex multiplier and supplying the smoothed phase error to the digital voltage-controlled oscillator to generate the recovered carrier,

wherein the transmission means include a frequency detector for detecting an oscillation frequency of the second voltage-controlled oscillator, and

wherein the channel selection circuit includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the local oscillator included in the channel selection circuit.

11. A demodulation apparatus according to claim 10, wherein the first control means includes at least one of:

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the second voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the digital voltage-controlled oscillator goes beyond a range between positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the second voltage-controlled oscillator to allow the oscillation frequency of the digital voltage-controlled oscillator within the range.

12. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

a frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate the IF signal, and

a channel selection circuit, including the local oscillator having the oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the frequency converter,

wherein the demodulation means includes;

a quadrature demodulator for demodulating the IF signal to the I baseband signal and the Q baseband signal having a quadrature relationship to each other,

the second voltage-controlled oscillator for generating the output to be sent to the quadrature demodulator,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals,

a first complex multiplier, connected to a first digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and the output from the first digital voltage-controlled oscillator, and

a low-pass wave-shaping filter for shaping a waveform of the output signal from the first complex multiplier,

wherein the recovery means includes;

a second complex multiplier, connected to a second digital voltage-controlled oscillator, for performing complex multiplication of the output from the low-pass wave-shaping filter and the recovered carrier, and

PLL means for smoothing a phase error of the output signal from the second complex multiplier and supplying the smoothed phase error to the second digital voltage-controlled oscillator to generate the recovered carrier,

wherein the first control means includes;

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the second voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the second digital voltage-controlled oscillator goes beyond a range between a positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the first digital voltage-controlled oscillator to allow the oscillation frequency of the second digital voltage-controlled oscillator within the range,

wherein the transmission means include a frequency detector for detecting a frequency amount obtained by summing a frequency deviation of the oscillation frequency of the second voltage-controlled oscillator from a prescribed frequency and the oscillation frequency of the first digital voltage-controlled oscillator, and

wherein the channel selection circuit includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the local oscillator included in the channel selection circuit.

13. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

a first frequency converter for performing frequency conversion of the signal selected from the input modulation signals to generate a first IF signal,

a channel selection circuit, including the local oscillator having the oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the first frequency converter, and

a second frequency converter, connected to the second voltage-controlled oscillator as part of the demodulating means, for performing frequency conversion of the first IF signal to generate a second IF signal,

wherein the demodulation means includes;

a quadrature demodulator for demodulating the second IF signal to the I baseband signal and the Q baseband signal having a quadrature relationship to each other,

a local oscillator for generating an output to be sent to the quadrature demodulator,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals,

a first complex multiplier, connected to a first digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and the output from the first digital voltage-controlled oscillator, and

a low-pass wave-shaping filter for shaping a waveform of the output signal from the first complex multiplier,

wherein the recovery means includes;

a second complex multiplier, connected to a second digital voltage-controlled oscillator, for performing complex multiplication of the output from the low-pass wave-shaping filter and the recovered carrier, and

PLL means for smoothing a phase error of the output signal from the second complex multiplier and supplying the smoothed phase error to the second digital voltage-controlled oscillator to generate the recovered carrier,

wherein the first control means includes;

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the second voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the second digital voltage-controlled oscillator goes beyond a range between a positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the first digital voltage-controlled oscillator to allow the oscillation frequency of the second digital voltage-controlled oscillator within the range,

wherein the transmission means include a frequency detector for detecting a frequency amount obtained by summing a frequency deviation of the oscillation frequency of the second voltage-controlled oscillator from a prescribed frequency and the oscillation frequency of the first digital voltage-controlled oscillator, and

wherein the channel selection circuit includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the first voltage-controlled oscillator included in the channel selection circuit.

18. A demodulation apparatus according to claim 1,

wherein the conversion means includes:

frequency converter for performing frequency conversion of the signal selected from the input modulation signal to generate the IF signal, and

a channel selection circuit, including the local oscillator having the oscillation frequency which changes in accordance with the channel selection information, for supplying the local oscillation signal of the local oscillator to the frequency converter,

wherein the channel selection circuit includes fine-tune control means for receiving the offset frequency information of the demodulation means and fine-tuning the oscillation frequency of the local oscillator,

wherein the fine-tune control means controls the oscillation frequency of the local oscillator in a step-like manner discontinuously, and performs initial setting of the oscillation frequency of a prescribed oscillator which is different from the local oscillator in the state of being offset when a prescribed channel is selected so as to compensate for a remaining frequency error which cannot be compensated for by the discontinuous fine-tuning.

20. A demodulation apparatus according to claim 1, wherein the recovery means includes a complex multiplier, and detects a frequency error based on a change of a phase error of an output signal from the complex multiplier.

22. A demodulation apparatus according to claim 1, wherein the recovery means includes a complex multiplier, and detects a frequency error based on a change of a phase error of an input signal to the complex multiplier, using a differential detector and a frequency discriminator.

24. A demodulation apparatus according to claim 4,

wherein the fine-tune control means controls the oscillation frequency of the first voltage-controlled oscillator in a step-like manner discontinuously, and performs initial setting of the oscillation frequency of the second voltage-controlled oscillator in the state of being offset by a certain amount when a prescribed channel is selected so as to compensate for a remaining frequency error which cannot be compensated for by the discontinuous fine-tuning, the amount of the offset being set to correspond to the remaining frequency error, the compensation being performed by controlling the oscillation frequency of the second voltage-controlled oscillator.

25. A demodulation apparatus according to claim 4, wherein the automatic frequency control means detects the frequency error based on an input signal to the complex multiplier using a differential detector and a frequency discriminator.

14. A demodulation apparatus, comprising:

conversion means for performing frequency conversion of an input modulation signal into an IF signal;

demodulation means for demodulating the IF signal into an I baseband signal and a Q baseband signal having a quadrature relationship to each other;

local oscillation means, including a second voltage-controlled oscillator having an oscillation frequency which changes in accordance with frequency setting information, for supplying a local oscillation signal of the second voltage-controlled oscillator to the demodulation means;

recovery means, which includes a recovered carrier generator for generating a recovered carrier based on the I baseband signal and the Q baseband signal having the quadrature relationship to each other, for outputting a recovered I output signal and a recovered Q output signal;

first control means for detecting a frequency error of the baseband signals based on the I baseband signal and the Q baseband signal, and feeding back the detected frequency error to a first voltage-controlled oscillator included in the first control means and having an output provided to the conversion means so as to automatically remove the detected frequency error;

transmission means for detecting information on an offset frequency of the first voltage-controlled oscillator, which is a difference between a frequency of the first voltage-controlled oscillator and a prescribed frequency of the first voltage-controlled oscillator, and transmitting the detected information on the offset frequency to the local oscillation means; and

second control means included in the local oscillation means for controlling the oscillation frequency of the local oscillation means based on the information on the offset frequency to control a frequency of the I baseband signal and the Q baseband signal so as to remove the offset frequency of the first voltage-controlled oscillator,

wherein the first voltage-controlled oscillator in the first control means and the second voltage-controlled oscillator in the local oscillation means are constituted using different oscillating means from each other.

15. A demodulation apparatus according to claim 14,

wherein the conversion means includes:

a frequency converter, connected to the first voltage-controlled oscillator, for performing frequency conversion of the input modulation into an IF signal,

wherein the demodulation means includes;

a quadrature demodulator for demodulating the IF signal to the I baseband signal and the Q baseband signal having the quadrature relationship to each other,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals, and

a low-pass wave-shaping filter for shaping a waveform of the output signals from the A/D converter,

wherein the recovered carrier generator in the recovery means includes;

a digital voltage-controlled oscillator for generating a recovered carrier,

wherein the recovery means further includes;

a complex multiplier, connected to the digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and filtered by the low-pass wave-shaping filter with the output from the digital voltage-controlled oscillator, and

PLL means for smoothing a phase error of the output signal from the complex multiplier and supplying the smoothed phase error to the digital voltage-controlled oscillator to generate the recovered carrier,

wherein the transmission means include a frequency detector for detecting an offset frequency of the first voltage-controlled oscillator,

wherein the first control means includes automatic frequency control means for feeding back a frequency error signal detected from the baseband signals to the first voltage-controlled oscillator, and

wherein the second control means is fine-tune control means for receiving the offset frequency information obtained by the frequency detector included in the transmission means and fine-tuning the oscillation frequency of the second voltage-controlled oscillator included in the local oscillation means.

16. A demodulation apparatus according to claim 15, wherein the first control means includes at least one of:

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the first voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the digital voltage-controlled oscillator goes beyond a range between a positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the first voltage-controlled oscillator to limit the oscillation frequency of the digital voltage-controlled oscillator within the range.

17. A demodulation apparatus according to claim 14,

wherein the demodulation means includes:

a quadrature demodulator for demodulating the IF signal to the I baseband signal and the Q baseband signal having a quadrature relationship to each other,

an A/D converter for converting the I baseband signal and the Q baseband signal into digital signals,

a first complex multiplier, connected to a first digital voltage-controlled oscillator, for performing complex multiplication of the digital signals obtained by the A/D converter and the output from the first digital voltage-controlled oscillator, and

a low-pass wave-shaping filter for shaping a waveform of the output signal from the first complex multiplier,

wherein the second voltage-controlled oscillator included in the local oscillation means is connected to the quadrature demodulator,

wherein the recovery means includes;

a second complex multiplier, connected to a second digital voltage-controlled oscillator, for performing complex multiplication of the output from the low-pass wave-shaping filter and the recovered carrier, and

PLL means for smoothing a phase error of the output signal from the second complex multiplier and supplying the smoothed phase error to the second digital voltage-controlled oscillator to generate the recovered carrier,

wherein the first control means includes;

frequency control means for detecting an error of a frequency of the I baseband signal and the Q baseband signal from a prescribed frequency and smoothing the frequency error to control the oscillation frequency of the first voltage-controlled oscillator, and

a frequency tracking circuit for determining whether the oscillation frequency of the second digital voltage-controlled oscillator goes beyond a range between positive reference value and a negative reference value and, in the case when the oscillation frequency goes beyond one of the positive reference value and the negative reference value, tracking the oscillation frequency of the first digital voltage-controlled oscillator to allow the oscillation frequency of the second digital voltage-controlled oscillator within the range,

wherein the transmission means include a frequency detector for detecting a frequency amount obtained by summing a frequency deviation of the oscillation frequency of the first voltage-controlled oscillator from a prescribed frequency and the oscillation frequency of the first digital voltage-controlled oscillator, and

wherein the local oscillation means includes fine-tune control means for receiving the frequency information obtained by the frequency detector and fine-tuning the oscillation frequency of the second voltage-controlled oscillator included in the local oscillation means.

19. A demodulation apparatus according to claim 14,

wherein the channel selection circuit includes fine-tune control means for receiving the offset frequency information of the demodulation means and fine-tuning the oscillation frequency of the second voltage-controlled oscillator, and

wherein the fine-tune control means controls the oscillation frequency of the second voltage-controlled oscillator in a step-like manner discontinuously, and performs initial setting of the oscillation frequency of a prescribed oscillator which is different from the second voltage-controlled oscillator in the state of being offset when a prescribed channel is selected so as to compensate for a remaining frequency error which cannot be compensated for by the discontinuous fine-tuning.

21. A demodulation apparatus according to claim 14, wherein the recovery means includes a complex multiplier, and detects a frequency error based on a change of a phase error of an output signal from the complex multiplier.

23. A demodulation apparatus according to claim 14, wherein the recovery means includes a complex multiplier, and detects a frequency error based on a change of a phase error of an input signal to the complex multiplier, using a differential detector and a frequency discriminator.

26. A demodulation apparatus according to claim 16,

wherein the fine-tune control means controls the oscillation frequency of the second voltage-controlled oscillator in a step-like manner discontinuously, and performs initial setting of the oscillation frequency of the first voltage-controlled oscillator in the state of being offset by a certain amount when a prescribed channel is selected so as to compensate for a remaining frequency error which cannot be compensated for by the discontinuous fine-tuning, the amount of the offset being set to correspond to the remaining frequency error, the compensation being performed by controlling the oscillation frequency of the first voltage-controlled oscillator.

27. A demodulation apparatus according to claim 16, wherein the automatic frequency control means detects the frequency error based on an input signal to the complex multiplier using a differential detector and a frequency discriminator.