&pgr;/2 phase shifter

Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Phase shift by less than period of input

Reexamination Certificate

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Details

C327S248000

Reexamination Certificate

active

06815993

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a &pgr;/2 phase shifter. More particularly, the present invention relates to a &pgr;/2 phase shifter capable of eliminating the effects of variation of elements in a circuit and parasitic capacitance and stably maintaining the phase difference of &pgr;/2 between output signals.
BACKGROUND ART
Recent development of digital mobile communication systems including a mobile phone system shows rapid progress. In the digital mobile communication systems, a roll-off &pgr;/4 shift QPSK (Quadrature Phase Shift Keying) method is generally used in order to improve the frequency utilization efficiency.
Generation of a roll-off &pgr;/4 shift QPSK signal requires a high-accuracy quadrature modulator. The quadrature modulator is a device for multiplying an input in-phase carrier and an input quadrature carrier having a phase difference of 90° (&pgr;/2) from the in-phase carrier by an in-phase-component and a quadrature component (both of which are a base band signal) of a complex envelope signal of a modulated wave, respectively, for output from a synthesizer. Accordingly, a &pgr;/2 phase shifter is required which divides an input carrier into two signals, i.e., in-phase (0°) and quadrature (90°=&pgr;/2) carrier signals.
Accuracy of the &pgr;/2 phase shifter in the quadrature modulator greatly affects modulation accuracy of the quadrature modulator. The reason for this is as follows: if the phase difference between two output signals of the &pgr;/2 phase shifter varies from &pgr;/2, that is, if quadrature phase error occurs, independency of the in-phase/quadrature signals after synthesis is degraded, causing degradation in modulation due to crosstalk between the in-phase/quadrature signals. In general, in the case of a QPSK signal, quadrature phase accuracy must be within ±3° in order to suppress this degradation to 0.1 dB or less as converted into a carrier-to-noise ratio (C/N).
As such a &pgr;/2 phase shifter for use in the quadrature modulator, the structure using a resistive element and a capacitive element is disclosed in, e.g., “Low Power Quadrature Modulator IC's for Digital Mobile Communications” (Yasushi YAMAO et al., IEICE Transactions, '93/11, Vol. J76-C-I, No. 11, pp. 453-455) and “Orthogonal Modulator” (Japanese Patent Laying-Open No. 6-252970).
Referring to
FIG. 14
, a conventional &pgr;/2 phase shifter
40
receives an input signal SIGIN and outputs signals SIGa, SIGb that are out of phase from each other by &pgr;/2.
The &pgr;/2 phase shifter
40
includes a capacitor
41
coupled between an input node Ni for receiving the input signal SIGIN and a node Nb for outputting one output signal SIGb, and a resistive element
42
coupled between the node Nb and a ground node
45
. The &pgr;/2 phase shifter
40
further includes a resistive element
43
coupled between the input node Ni and a node Na for outputting the other output signal SIGa, and a capacitor
44
coupled between the node Na and the ground node
45
.
The &pgr;/2 phase shifter
40
is designed so that the capacitors
41
,
44
have the same capacitance value C and the resistive elements
42
,
43
have the same resistance value R.
FIG. 15
shows vector representation of an ideal output state of the &pgr;/2 phase shifter
40
. Referring to
FIG. 15
, when the phase state of the signals is represented on the X-Y plane, the positive direction of the X axis corresponds to the phase &thgr;in (0°) of the input signal SIGIN as a reference.
The phase of the output signal SIGb is advanced by &thgr;b from that of the input signal SIGIN (&thgr;in) by the capacitor
41
and the resistive element
42
. In contrast, the phase of the other output signal SIGa is retarded by &thgr;a from that of the input signal SIGIN (&thgr;in) by the resistive element
43
and the capacitor
44
.
Ideally, by designing the resistance value R and the capacitance value C in
FIG. 14
according to the frequency of the input signal SIGIN so that &thgr;b=±45° (+&pgr;/4) and &thgr;a=−45° (−&pgr;/4), the phase difference between the output signals SIGb and SIGa, that is, (&thgr;b−&thgr;a), can be set to &pgr;/2.
In other words, phase accuracy of the conventional &pgr;/2 phase shifter
40
depends on accuracy of the resistance value of the resistive elements and the capacitance value of the capacitors.
However, when such a &pgr;/2 phase shifter is fabricated on LSI (Large Scale Integrated circuit), manufacturing variation in the resistance value and capacitance value of the resistive elements and capacitors formed on the integrated circuit becomes problematic. Moreover, when the &pgr;/2 phase shifter
40
having such a structure is used in, e.g., mobile phones using a high-frequency carrier of, e.g., 100 MHz level or higher, the effects of parasitic elements on these circuits also become problematic.
Referring to
FIG. 16
, in the conventional &pgr;/2 phase shifter
40
, the capacitor
41
coupled between the node Ni and the node Nb has Rtb
1
and Rtb
2
corresponding to the electrode resistance of the capacitor. The capacitance value Cb of the capacitor itself is not necessarily equal to the! design value C due to manufacturing variation.
The resistive element
42
coupled between the node Nb and the ground node
45
has a resistance value Rb varying from the design value R depending on the manufacturing variation, and an increased carrier frequency would cause parasitic capacitance Cpb.
Similarly, the resistive element
43
coupled between the node Ni and the node Na actually has a resistance value Ra that is not equal to the design value R due to the manufacturing variation, and operation in a high frequency region would cause parasitic capacitance Cpa. Moreover, the capacitor
44
coupled between the node Na and the ground node
45
actually has a capacitance value Ca that is not necessarily equal to the design value C due to the manufacturing variation, and has resistances Rta
1
and Rta
2
corresponding to the electrode resistance thereof.
FIG. 17
shows vector representation of the output state of the &pgr;/2 phase shifter
40
affected by these problems.
Referring to
FIG. 17
, as the resistance value and capacitance value of the capacitors
41
,
44
and the resistive elements
42
,
43
vary from the respective design values due to the manufacturing variation and the effects of the parasitic elements during high-frequency operation, the respective phase differences &thgr;b′, &thgr;a′ between the output signals SIGb, SIGa and the input signal SIGIN vary from the respective design values ±45° (+&pgr;/4), −45° (−&pgr;/4).
This hinders accurate setting of the phase difference between the output signals SIGb and SIGa of the &pgr;/2 phase shifter to &pgr;/2. when such a &pgr;/2 phase shifter is used in, e.g., a quadrature modulator, the phase difference between the output signals of the &pgr;/2 phase shifter varies from &pgr;/2, degrading independency of in-phase/quadrature signals after synthesis. This results in a degraded image suppression ratio upon demodulation, hindering desired modulation accuracy from being obtained by the quadrature modulator.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide the structure of a &pgr;/2 phase shifter for accurately adjusting the phase difference between output signals to &pgr;/2.
According to the present invention, a &pgr;/2 phase shifter includes a phase shifter circuit and a phase difference correction circuit. The phase shifter circuit receives an input signal and an inverted input signal thereof and produces first and second intermediate signals having a same amplitude and being out of phase from each other. The phase shifter circuit further produces first and second inverted intermediate signals respectively corresponding to the first and second intermediate signals with their respective phases inverted. The phase difference correction circuit receives the first and second intermediate signals produced by the phase shifter

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