Image-rejection receiver

Details Image-rejection receiver Image-rejection receiver

1999-09-22

2003-02-04

Cumming, William (Department: 2684)

Telecommunications

Receiver or analog modulated signal frequency converter

Noise or interference elimination

C455S285000

Reexamination Certificate

active

06516186

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an image-rejection receiver which is used, for instance, in digital radio communication.
FIG. 9
is a prior digital radio receiver which is a super heterodyne receiver, and is now popularly used. In
FIG. 9
, a first mixer
51
converts received radio frequency RF into intermediate frequency by using a first local frequency LOCAL
1
. The intermediate frequency passes a narrow band filter
52
for channel selection. The numeral
53
is a quadrature demodulator which converts the intermediate frequency signal into base band signal which has in-phase component BBI and quadrature-phase component BBQ.
The receiver of
FIG. 9
has the disadvantage because of the presence of the filter
52
. When we intend to mount all the components of a receiver in a single integrated circuit, it is difficult to implement a filter in an integrated circuit.
FIG. 10
is another prior image-rejection receiver which has no narrow band filter in an intermediate frequency stage, and is suitable for a single chip LSI (large scale integrated circuit).
In
FIG. 10
, the numerals
1
-
6
show a frequency mixer,
7
is a subtractor,
8
is an adder,
54
and
55
are a 90° phase shifter. The structure of
FIG. 10
can cancel an upconvert component generated in a second stage mixer (
3
-
6
) by using the phase relations of an upconvert component and a downconvert component. Therefore, no filter in an intermediate frequency stage is requested for rejecting an image frequency which would be the same frequency as the desired frequency on baseband. Therefore, the channel selection on baseband is possible in the structure of FIG.
10
.
The operation of
FIG. 10
is as follows.
It is assumed that receive frequency is expressed as cos(w
r
t), and first local signal in quadrature relations are cos(w
1
t) and sin(w
1
t), respectively.
Outputs I and Q of the first mixer (
1
,
2
) are;
I
=(1/2)[cos(
w
r
+w
1
)
t
+cos(
w
r
−w
1
)
t]
  (1)
Q
=(1/2)[sin(
w
r
+w
1
)
t
−sin(
w
r
−w
1
)
t]
  (2)
The frequency component (w
r
+w
1
) is very high frequency, and is eliminated by using a simple filter. Therefore, only the frequency component (w
r
−w
1
) is applied to the second mixers (
3
-
6
).
When we assume that second local signal LOCAL
2
applied to the second mixers
3
and
6
is cos(w
2
t), and second local signal applied to the second mixers
4
and
5
is sin(w
2
t), the outputs II, IQ, QQ and QI of the mixers
3
through
6
are expressed as follows.
II
=(1/4)[cos(
w
r
−w
1
+w
2
)
t
+cos(
w
r
−w
1
−w
2
)
t]
  (3)
 
IQ
=(1/4)[sin(
w
r
−w
1
+w
2
)
t
−sin(
w
r
−w
1
−w
2
)
t]
  (4)
QQ
=(1/4)[cos(
w
r
−w
1
+w
2
)
t
−cos(
w
r
−w
1
−w
2
)
t]
  (5)
QI
=−(1/4)[sin(
w
r
−w
1
+w
2
)
t
+sin(
w
r
−w
1
−w
2
)
t]
  (6)
Accordingly, the baseband outputs BBI and BBQ are as follows.
BBI
=(1/2)cos(
w
r
−w
1
−w
2
)
t
  (7)
BBQ
=−(1/2)sin(
w
r
−w
1
−w
2
)
t
  (8)
Therefore, it should be noted that an image frequency
(w
r
−w
1
+w
2
)  (9)
is eliminated in the baseband.
In
FIG. 10
, a pair of local frequencies (sine signal, and cosine signal) are generated by using 90° phase shifters
54
and
55
. When the phase difference obtained by the 90° phase shifter is not accurately 90°, an image signal is not completely eliminated, but is superposed on a desired signal on baseband. This image signal can not be eliminated by using a filter.
In particular, the first local frequency is very high frequency, and it is difficult to provide an ideal accurate 90° phase shifter. When the first local signal is not ideal, a pair of quadrature local frequencies are expressed by cos(w
1
t) and sin(w
1
t+Ø), where Ø is phase error.
When the phase error is not zero, the above equations (3) through (6) are modified as follows, respectively.
II
=(1/4)[cos(
w
r
−w
1
+w
2
)
t
+cos(
w
r
−w
1
−w
2
)
t]
  (10)
 
IQ
=(1/4)[sin(
w
r
−w
1
+w
2
)
t
−sin(
w
r
−w
1
−w
2
)
t]
  (11)
QQ
=(1/4)[cos((
w
r
−w
1
+w
2
)
t
−Ø)−cos((
w
r
−w
1
−w
2
)
t−Ø)]
  (12)
QI
=−(1/4)[sin((
w
r
−w
1
+w
2
)
t
−Ø)+sin((
w
r
−w
1
−w
2
)
t−Ø)]
  (13)
Accordingly, the equations (7) and (8) are modified as follows.
BBI
=(1/2)cos((
w
r
−w
1
−w
2
)
t−Ø/
2)cos(Ø/2)−(1/2)sin((
w
r
−w
1
−w
2
)
t−Ø/
2)sin(Ø/2)  (14)
BBQ
=(1/2)sin((
w
r
−w
1
−w
2
)
t−Ø/
2)cos(Ø/2)+(1/2)cos((
w
r
−w
1
−w
2
)
t−Ø/
2)sin(Ø/2)  (15)
Therefore, when an undesired radio frequency w
ri
of the frequency satisfying (w
ri
−w
1
+w
2
)=(w
r
−w
1
−w
2
) is superposed to a desired frequency w
r
emerging the baseband frequency (w
r
−w
1
−w
2
), it is impossible to separate the undesired radio frequency signal. This deteriorates receive sensitivity.
The above analysis considers an only phase error in the first local signal. Indeed, the second local signal may also have a phase error, and this causes further degradation of receive sensitivity.
Further, when the outputs II and QQ (or IQ and QI) of the second mixer have amplitude difference from each other, that difference affects to an output of the subtractor
7
and/or the adder
8
, and an image signal can not be eliminated. Assuming that an amplitude of QQ or QI is A when an amplitude of II or IQ is 1, the ratio IR of an image signal to a desired signal is expressed as follows.
 IR=10 log [(1
+A
2
−2
A
cos Ø)/(1
+A
2
+2
A
cos Ø)]
SUMMARY OF THE INVENTION
It is an object, therefore, of the present invention to provide a new and improved image-rejection receiver by overcoming the disadvantages and limitations of a prior image-rejection receiver.
It is also an object of the present invention to provide an image-rejection receiver which cancels an image signal completely.
The above and other objects are attained by an image-rejection receiver comprising; a first mixer receiving an input radio frequency signal and a first local frequency signal to provide sum and difference frequencies of those signals; a second mixer receiving said input radio frequency signal and said first local frequency signal through a first variable phase shifter which shifts phase of said first local frequency signal by 90° to provide sum and difference frequencies of those signals; a third mixer receiving an output of said first mixer and a second local frequency signal to provide sum and difference frequencies of those signals; a fourth mixer receiving said output of said first mixer and output of said second local frequency signal through a second variable phase shifter which shifts phase of said second local signal by 90° to provide sum and difference frequencies of those signals; a fifth mixer receiving an output of said second mixer and said output of said second variable phase shifter to provide sum and difference frequencies of those signals; a sixth mixer receiving said output of said second mixer and said second local frequency signal to provide sum and difference frequencies of those signals; a subtractor for providing difference between amplitude of an output of said third mixer and amplitude of an output of said fifth mixer as inphase component BBI; an adder for providing sum of amplitude of an output of said fourth mixer and amplitude of an output of said sixth mixer as quadrature-phase component BBQ; and a control circuit receiving at least three of outputs of said 3'rd thr

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