Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-12-11
2002-03-26
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Automatic frequency control
C375S332000
Reexamination Certificate
active
06363126
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a demodulator in a receiver, and more particularly to a demodulator of digital modulated signal.
Recently, as satellite broadcasting is being digitized, the demodulator by synchronous detection system of digital modulation method has come to be used in the receiver. Such demodulator is often provided with a function of automatic frequency control (AFC) using a voltage controlled oscillator (VCO).
FIG. 7
is an example of a schematic constitution of a conventional demodulator realizing an AFC circuit using VCO by digital signal processing.
In
FIG. 7
, a QPSK modulated signal and a local carrier issued from a local oscillator
22
are both put into a mixer
21
. In the mixer
21
,the QPSK modulated signal is converted in frequency to an intermediate frequency signal (IF signal), and is fed into an orthogonal detector
4
through a band pass filter (BPF)
3
. The orthogonal detector
4
converts the IF signal issued from the BPF
3
into orthogonal I, Q base band signals by the transmission output carrier of a VCO
14
. By A/D converters (ADC)
5
and
6
, analog I, Q base band signals are converted into digital I, Q base band signals. The digitized I, Q base band signals are shaped in waveform by low pass filters (LPF)
7
and
8
of waveform shaping type (digital transversal type) of low pass type, and transmitted to a carrier regenerator
9
.
The carrier regenerator
9
detects the phase difference of the phase of the input signal contained in the I, Q base band signals and the reference phase, and issues a reproduction carrier synchronized in phase with the I, Q base band signals from this phase difference signal. A frequency error detector
10
detects a frequency error between the center frequency of QPSK modulated signal and oscillation frequency of the VCO
14
, from the phase difference signal of the carrier regenerator
9
. This frequency error is smoothed by a frequency controller
11
, and converted into an analog signal by a D/A converter (DAC)
13
, then transmitted to the VCO
14
as control signal. The operating frequency of the VCO
14
is controlled by the control signal, and the frequency error is compensated.
The frequency error of the center frequency of the QPSK modulated signal and the oscillation frequency of the VCO
14
is always compensated by a feedback loop of AFC composed of the orthogonal detector
4
, carrier regenerator
9
, frequency error detector
10
, frequency controller
11
, DAC
13
, and VCO
14
. Accordingly, the output of the VCO
14
having the same frequency as the QPSK modulation frequency is transmitted to the orthogonal detector
4
, and the QPSK modulated signal is synchronously detected in the orthogonal detector
4
, and the I, Q base band signals low in error rate are always supplied to the carrier regenerator
9
.
In the satellite broadcast, incidentally, dielectric resonators are used in the local oscillator
22
and VCO
14
of the modulator of the BS converter. The dielectric resonators have ambient temperature dependence and time-course changes. Therefore, the frequency of the local oscillator
22
is often detuned from the reference frequency, and its detuning frequency may sometimes reach as much as several mega-Hertz. As for oscillation frequency of the local oscillator
22
, in addition to the average frequency detuning from the reference frequency, there is also a frequency drift due to changes of ambient temperature, and this frequency drift cannot be ignored, too.
Further, by using a dielectric resonator in the VCO
14
, the voltage-frequency conversion sensitivity characteristic of the VCO
14
has an offset amount, and this offset amount may cause a drift of several mega-Hertz due to time-course changes. Moreover, the inclination of the voltage-frequency conversion sensitivity characteristic may also change in the time course.
In the conventional demodulator described above, if the detuning frequency of the local oscillator
22
surpasses the passing frequency band of the BPF
3
, owing to the aforesaid cause, part of the QPSK modulated signal is cut off by the BPF
3
, and the error rate characteristic of the demodulator deteriorates significantly. To solve this problem, it has been proposed to detect the detuning frequency of the local oscillator
22
by the frequency error detector
10
, and control the local oscillator
22
so that part of the QPSK modulated signal may not be cut off by the BPF
3
, but since the detuning frequency value of the local oscillator
22
and the frequency offset value of the voltage-frequency conversion characteristic of the VCO
14
settle in a same AFC feedback loop, the detuning frequency value of the local oscillator
22
and the frequency offset value of the VCO
14
cannot be separated and detected from the frequency information of the AFC.
Besides, as the value of the frequency offset of the voltage-frequency conversion characteristic of the VCO
14
increases, the control frequency band that can be controlled by the AFC becomes narrower, and it may be out of the control frequency band of the AFC depending on the magnitude and direction of the detuning frequency of the local oscillator
22
, and control is not converged stably, and demodulation is disabled.
Still more, if the inclination of the voltage-frequency conversion characteristic changes in the time course, the control frequency band controllable by the AFC may be narrower, and the AFC is not converged stably, and demodulation is disabled.
These are the problems of the demodulator by the synchronous detection system of digital modulation system used in the conventional BS converter.
SUMMARY OF THE INVENTION
It is hence an object of the invention to solve the above problems, and to present a demodulator in which part of input signal is not cut off by the band pass filter if the detuning frequency of the local oscillator of the demodulator of BS converter surpasses the passing frequency band of the band pass filter, and the frequency offset of the voltage-frequency conversion sensitivity characteristic of the VCO is detected by separating from the detection of the detuning frequency of the local oscillator if the inclination of the voltage-frequency conversion sensitivity characteristic of the VCO changes in the time course, so that the AFC operates stably, thereby preventing deterioration of error rate characteristic or unstable demodulation.
To achieve the object, the invention presents a demodulator comprising a first mixer for converting an input modulated signal into an intermediate frequency signal by frequency conversion, a local oscillator for issuing a local oscillation signal to this mixer, a first band pass filter of band passing type for shaping the waveform of the intermediate frequency signal converted in frequency by the mixer, an orthogonal detector for demodulating the signal shaped in waveform by this filter into orthogonal signals of I, Q channels, a voltage controlled oscillator (VCO) for issuing an oscillation signal to this orthogonal detector, an A/D converter for converting the orthogonal signals of the I, Q channels into digital signals, a second low pass filter of low pass type for shaping the waveform of the output signal from this A/D converter, a carrier regenerator for detecting phase information relating to a carrier from the signal shaped in waveform by this low pass filter for obtaining a generated carrier, a frequency error detector for detecting the frequency error information between the intermediate frequency signal and oscillation frequency of the VCO on the basis of the phase information of this carrier regenerator, a frequency controller for controlling the oscillation frequency of the VCO by the frequency error information detected by this frequency error detector, a signal selector for selecting either the output of this frequency controller or the reference value, a D/A converter for converting the signal from this signal selector into an analog signal, and issuing to the VCO, a frequency divider for dividing the fr
Furukawa Hitonobu
Hayashi Sachiko
Mishima Akira
Ozeki Hiroaki
Takigawa Masami
Chin Stephen
Fan Chieh M.
Rossi & Associates
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