Demodulators – Phase shift keying or quadrature amplitude demodulator
Reexamination Certificate
1999-07-02
2001-02-20
Mis, David (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
C329S308000, C375S327000, C375S329000, C375S344000
Reexamination Certificate
active
06191649
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a digital demodulation device and method for demodulating a signal whose phase is digitally modulated.
BACKGROUND OF THE INVENTION
FIG. 7
shows the construction of a conventional quadrature demodulator. Conventional quadrature demodulators have been constructed to form a feedback loop arrangement. In-phase (I) component (a), and quadrature (Q) component (b) of the digital modulated signal are input to the quadrature demodulator via input terminals
101
and
103
.
These signals are inputted to A/D converters
105
and
107
and converted to digital values. The outputs of the A/Ds
105
and
107
, respectively, are inputted to a complex multiplier
109
. The digitized in-phase and quadrature components are demodulated at a complex multiplier
109
by using a local carrier which is introduced separately into the complex multiplier
109
. Then, a frequency error and a phase error are removed from the demodulated signals by controlling the local carrier (to be described later) in accordance with feedback operation.
The calculation of the complex multiplier is input to roll-off filters
111
and
113
, where they are shaped by being subjected to a filtering process. Then the shaped I and Q components (c) and (d) are output from the circuit through output terminals
137
and
139
.
The calculation of the complex multiplier is also supplied to a convolution circuit
19
. The values of signals (c) and (d) designate a coordinate on an I-Q orthogonal signal constellation. The designated coordinate will be referred to as a symbol. If a symbol of a subject signal matches a reference symbol, such a match means that no errors exist in the frequencies and phases of the subject signal. However, if the symbol deviates from the reference symbol, then, an error does exist. As a result of the error of a phase angle between the I-Q coordinate of the symbol and the reference symbol coordinate is detected by a detector
121
.
The convolution circuit
119
is provided before the phase detector
121
for rotating the symbols a sufficient number of rotations so that the symbols are positioned in the first quadrant of the I-Q plane. The phase detector
121
detects the phase angle between the symbol and the reference symbol.
A phase angle signal (e) detected at the phase detector
121
is supplied to an error detector
123
and a selector
127
. The differentiator
123
detects the error between the phases of sequential signals by calculating the difference between the input signals. Then, the differentiator
123
outputs a symbol to a sign discriminator
125
, which determines whether the sign of the symbol is positive and negative.
The selector
127
selects either the phase angle signal (e) of the phase detector
121
or the sign signal (f) obtained in the sign discriminator
125
. The signal selection at the selector
127
is organized by a selector controller
129
. When the quadrature demodulator executes a frequency synchronizing operation the selector
127
selects the sign signal (f) from the sign discriminator
125
. On the other hand, when the quadrature demodulator executes a phase synchronizing operation the selector
127
selects the phase angle signal (e) directly supplied from the phase detector
121
.
The gain of the signal selected at the selector
127
is properly adjusted in a gain adjuster
131
. Then, the signal is smoothed in a low pass filter (LPF)
133
to remove its high frequency. The smoothed signal of the LPF
133
is used as a control signal to control the local carrier generated at a numerically controlled oscillator (NCO)
135
. The local carrier is then converted into a cosine wave component and a sine wave component at a cosine converter
115
and a sine converter
117
, respectively. The cosine and the sine wave components are then multiplied with the in-phase and the quadrature components of the modulation signal at the complex multiplier
109
, in order to produce baseband signals, i.e., demodulated in-phase and quadrature components.
The feedback loop of the conventional quadrature demodulator, as shown in
FIG. 7
, disadvantageously has a loop delay. The loop delay represents the time that it takes for the signals input to travel through the feedback loop and return back to the input terminal. In order words, the time necessary for the input signal supplied to the complex multiplier
109
to leave and return back to the complex multiplier
109
via the feedback loop.
Since the quadrature demodulator for the digital demodulated signals digitally processes the signals, latches constructed by, for example, flip-flops circuits are added in many circuit elements for synchronizing signals among the circuit's elements.
Latches are indispensable components for all circuit elements. However, as the number of latches added to a circuit increases, the more likely the loop delay will increase. Further, the feedback loop includes additional circuits, which perform less important functions in the frequency or phase locked loops, such as the roll-off filters
111
and
113
. But these additional circuits also require latches. As a result, the loop delay of the feedback loop increases even further due to the increased number of latches.
If the loop delay of the feedback loop continues to increase, at some point, the circuit will begin to experience drawbacks, wherein the frequency pull-in range of the feedback loop will be reduced, and the phase jitter generated after the phase synchronization increases. By definition, the frequency pull-in range is the measure of the maximum reference frequency deviation from the nominal rate that can be overcome by a slave clock to pull itself into synchronization, and the phase jitters are short-time variations of the significant instants of a digital signal from its ideal position in time. Specifically, in the conventional circuit, the frequency pull-in range will begin to vary in accordance with the loop delay, which increases according to the number of latches. As a result, there is a need to decrease the number latches in order to improve the circuit's ability to efficiently demodulating the incoming signals.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention is to provide a quadrature demodulator and method which is able to decrease the number of latches. Thus, the invention is able to enlarge the frequency pull in range of a feedback loop and improve the circuit's ability to recover a carrier signal.
In order to achieve the above objective, a quadrature demodulator according to a first aspect of the present invention includes, a receiving circuit for receiving a quadrature modulated signal, a local oscillator for generating a local carrier, a complex multiplier for demodulating a quadrature modulation signal by complex multiplying the quadrature modulated signal with the local carrier generated in the local oscillator, a symbol error detector for detecting a symbol error between the carrier of the modulated signal and the local carrier from the signal demodulated at the complex-multiplier, a feedback loop for controlling the local carrier generated at the local oscillator by feeding back the symbol error detected at the symbol error detector to the local oscillator, a compensation signal generator for generating a compensation signal under the control of the symbol error branched from the feedback loop, and a compensator for compensating the local carrier to reduce the symbol error according to the compensation signal.
A quadrature demodulator according to a second aspect of the present invention includes, a first A/D converter for digitizing in-phase components of a quadrature modulation signal, a second A/D converter for digitizing quadrature components of the quadrature modulation'signal, a complex multiplier for complex multiplying the outputs from the first and the second A/D converters with a cosine wave component and a sine wave component to produce demodulated in-phase and quadrature components, a phase detector for detecting a phase
Nishikawa Masaki
Sugita Yasushi
Kabushiki Kaisha Toshiba
Mis David
Pillsbury Madison & Sutro LLP
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