Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Phase shift by less than period of input
Reexamination Certificate
1999-06-18
2001-01-09
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Phase shift by less than period of input
C327S258000, C327S306000, C327S558000, C327S559000
Reexamination Certificate
active
06172543
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a 90° phase shift circuit and, more particularly, to a 90° phase circuit for generating I-signal and Q-signal having a phase difference of 90° therebetween and suitable for use in a radio communication system.
(b) Description of the Related Art
90° phase shift circuits are generally used in radio communication systems. Referring to
FIG. 1
, a conventional 90° phase shift circuit includes a high-pass filter (HPF)
12
having therein a serial capacitor (C) an a parallel resistor (R), a low-pass filter
13
having therein a serial resistor (R) and a parallel capacitor (C), a first limiter amplifier
14
for receiving an output from the HPF
12
to deliver a Q-signal, a second limiter amplifier
15
for receiving an output from the LPF
13
to deliver an I-signal.
An input signal having an angular frequency of &ohgr; and supplied through the input terminal
16
common to HPF
12
and LPF
13
leads in phase after passing the HPF
12
, and lags in phase after passing the LPF
13
. The ratio of an output voltage V
1
from the HPF
12
to an output voltage V
2
from the LPF
13
is expressed by:
V
1
/
V
2
=
j&ohgr;·C·R.
As understood from the above equation, the phase difference between the output voltages V
1
and V
2
is 90 degrees irrespective of the angular frequency &ohgr;.
The practical phase error of the output signals from the HPF
12
and the LPF
13
depends on the relative accuracy between the capacitances (C) and between the resistors (R). In general, if the fabrication techniques used for current semiconductor integrated circuits are applied to the conventional 90° phase shift circuit, a high relative accuracy can be obtained between the resistors and between the capacitors, and accordingly, the phase error can be suppressed to a suitable level.
On the other hand, the amplitudes of the output voltages V
1
and V
2
coincide with each other at an angular frequency &ohgr; wherein the following equation holds:
&ohgr;·
C·R=
1.
Accordingly, the amplitude error depends on the absolute accuracy of the resistors and the capacitors. Thus, in the conventional 90° phase shift circuit, limiter amplifiers are provided at the outputs of the HPF
12
and the LPF
13
for controlling the amplitude difference between the output voltages V
1
and V
2
to compensate the absolute errors of the resistors and the capacitors.
However, in the conventional 90° phase shift circuit as described above, since the phases of the outputs from the limiter amplifiers
14
and
15
depend on the respective input amplitudes of the limiter amplifiers
14
and
15
, a phase error is generated by the fact that the amplitude error is suppressed by using the limiter amplifiers. That is, the presence of the limiter amplifiers
14
and
15
itself generates a new phase error.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a 90° phase shift circuit having a higher level of accuracy in phase and amplitude by suppressing the amplitude error without using limiter amplifiers.
The present invention provides a 90° phase shift circuit including: a high-pass filter including a first capacitor connected between an input terminal and a first output terminal and a first resistor connected between the first output terminal and a reference line, the high-pass filter outputting a first signal having a first amplitude and a first phase; a low-pass filter including a second resistor connected between the input terminal and a second output terminal and a second capacitor connected between the second output terminal and the reference line, the low-pass filter outputting a second signal having a second amplitude and a second phase which is 90° behind with respect to the first phase; and a level comparator for comparing the first amplitude against the second amplitude to feed-back a control signal to the high-pass filter and the low-pass filter, the control signal controlling cut-off frequencies of the high-pass filter and the low-pass filter based on a difference between the first amplitude and the second amplitude.
In accordance with the 90° phase shift circuit of the present invention, since the amplitude error can be removed by a feed-back control using a level comparator, the 90° phase shift circuit is free from the new phase error as encountered in the conventional circuit.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.
REFERENCES:
patent: 5051972 (1991-09-01), Yamamuro
patent: 5608796 (1997-03-01), Banu et al.
patent: 3-121610 (1991-05-01), None
Cox Cassandra
McGuireWoods LLP
NEC Corporation
Wells Kenneth B.
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