Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
2000-05-23
2001-09-25
Grimm, Siegfried H. (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
C329S308000, C375S324000
Reexamination Certificate
active
06294952
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the calibration of a quadrature demodulator.
As an example of a synchronous detector, a quadrature demodulator synchronously detects two baseband signals, i.e., an I signal (in-phase component) and a Q signal (quadrature component) that are in quadrature with each other, such as signals modulated by QPSK (Quadrature PSK). A specific example of a quadrature demodulator
1
will be described with reference to FIG.
26
.
First, a frequency conversion section
10
accepts at its input a received signal. The section
10
converts the received signal into an intermediate frequency signal IF of a predetermined frequency and supplies this signal IF to an I signal conversion section
100
and a Q signal conversion section
200
. The I signal conversion section
100
receives a local signal of a frequency LF
1
from a reference oscillator
40
, mixes the local signal with the intermediate frequency signal IF, and generates an I signal (in-phase component). The Q signal conversion section
200
receives from the reference oscillator
40
a local signal of the same frequency LF
1
with its phase 90 degree shifted by a 90 degree phase shifter
42
, mixes such a local signal with the intermediate frequency signal IF, and generates a Q signal (quadrature component). In response to the I signal (in-phase component) and the Q signal (quadrature component), a succeeding circuit
90
performs a desired operation.
Here, factors such as temperature changes cause errors in the I signal conversion section
100
and the Q signal conversion section
200
. These errors are, e.g., amplitude, phase, and offset errors. These errors must be removed, or the sections
100
and
200
must be calibrated, since these errors adversely affect the operation of the succeeding circuit
90
.
The following method is available to calibrate the I signal conversion section
100
and the Q signal conversion section
200
. The user feeds a calibration signal of a predetermined frequency to the I and Q signal conversion sections
100
and
200
. At this point, the quadrature demodulator
1
must stop processing the received signal and adjusts attenuation values and the like of variable attenuators, variable delay units and the like in the I and Q signal conversion sections
100
and
200
to calibrate the sections
100
and
200
.
That is, during calibration, the user is not allowed to supply a signal to the quadrature demodulator
1
, or the quadrature demodulator
1
must stop its reception.
SUMMARY OF THE INVENTION
The present invention has been made to avoid the above inconvenience, and an object thereof is, therefore, to provide a quadrature demodulator, etc. capable of calibrating the I (Q) signal conversion section without stopping its receiving operation.
According to the present invention described in claim
1
, a quadrature demodulator includes: an addition unit for adding a user signal and a pseudo noise to generate a pseudo noise superimposed signal; a signal conversion unit for mixing the pseudo noise superimposed signal with a local signal of a predetermined frequency to generate a converted signal; a first multiplication unit for multiplying the converted signal with the pseudo noise to generate a correlated signal; and a first integration unit for integrating the correlated signal to generate an output.
According to the thus constructed quadrature demodulator, the pseudo noise superimposed signal containing a pseudo noise P(t) is converted into a converted signal by the signal conversion unit. The P(t) term in the converted signal is correlated by the first multiplication unit to become a P(t)
2
term in the correlated signal. When P(t)
2
is integrated in a sufficiently large interval, it becomes a constant not equal to 0. Hence, the p(t)
2
term in the correlated signal is integrated by the first integration unit, and is supplied as a function of the phase error or amplitude error of the signal conversion unit. If P(t) is integrated in a sufficiently large interval, it becomes 0. Therefore, the P(t) term in the correlated signal becomes 0 by the integration unit. Hence, the output of the first integration unit is a function of the phase error or amplitude error of the signal conversion unit. Therefore, the phase error of the signal conversion unit can be measured on the basis of the output of the first integration unit.
The pseudo noise includes but not restricted to M-sequence pseudo random patterns. Namely, assuming that the pseudo noise is P(t), the pseudo noise may be any noise which becomes a constant not equal to 0 when P(t)
2
is integrated in a sufficiently large interval and which becomes 0 when P(t) is integrated in a sufficiently large interval.
According to the present invention described in claim
2
, a quadrature demodulator according to claim
1
, further includes: a reference signal conversion unit for mixing the pseudo noise superimposed signal with the local signal of a predetermined local frequency without amplitude and phase errors to generate a reference converted signal; a second multiplication unit for multiplying the pseudo noise with the reference converted signal to generate a reference correlated signal; and a second integration unit for integrating the reference correlated signal to provide an output.
According to the thus constructed quadrature demodulator, the reference signal conversion unit has no amplitude and phase errors, and thus the output of the second integration unit contains no amplitude and phase errors. Hence, by comparing the output of the first integration unit containing an amplitude error with the output of the second integration unit containing no amplitude and phase errors, the amplitude error can be measured.
According to the present invention described in claim
3
, a quadrature demodulator according to claim
2
, further includes: an amplitude error correction unit for correcting an amplitude error of the signal conversion unit; and an average calculation unit for obtaining an average of the converted signals from the signal conversion unit.
A DC offset error can be calculated by the average calculation unit.
According to the present invention described in claim
4
, a quadrature demodulator according to claim
1
, further includes a phase error correction unit for adjusting the phase of the local signal to be supplied to the signal conversion unit so that the output of the first integration unit equals a predetermined value.
According to the present invention described in claim
5
, a quadrature demodulator according to claim
2
, further includes amplitude error measurement unit for obtaining an amplitude error on the basis of a value obtained by dividing the predetermined value, which is one of the outputs of the first integration unit, by the output of the second integration unit.
The present invention described in claim
6
, is a quadrature demodulator according to claim
4
or
5
, wherein the predetermined value is a maximum output value of the first integration unit.
The present invention described in claim
7
, is a quadrature demodulator according to claim
1
, wherein the pseudo noise is smaller than the user signal.
The present invention described in claim
8
, is a quadrature demodulator according to claim
7
, wherein the pseudo noise is substantially equal to a floor noise.
The present invention described in claim
9
, is a quadrature demodulator according to claim
1
, which further includes: a quadrature signal conversion unit for mixing a quadrature local signal with the pseudo noise superimposed signal to generate a quadrature converted signal, the quadrature local signal being obtained by shifting the phase of the local signal by 90 degree; a quadrature multiplication unit for multiplying the quadrature converted signal with the pseudo noise to generate a quadrature correlated signal; and a quadrature integration unit for integrating the quadrature correlated signal to provide an output.
The present invention described in claim
10
, is a quadrature demodulator according to
Advantest Corporation
Grimm Siegfried H.
Lowe Hauptman & Gilman & Berner LLP
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