Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1998-06-29
2001-11-06
Vo, Nguyen (Department: 2745)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C333S139000, C455S317000
Reexamination Certificate
active
06314279
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to signal processing, and more specifically, to image rejection in signal down converters.
BACKGROUND OF THE INVENTION
Communication system receivers often employ image rejection to remove undesired information from received signals. The term “image rejection” is well known in the art to refer to the removal of certain undesirable information, such as a particular frequency band. In the context of wireless telephony, an example of undesired information is a conversation carried on another channel.
Image rejection in receivers has traditionally been provided by passive filters. More recently, image rejection is being implemented in receiver down converters to ease the rejection requirements of preceding passive filters. As is well known in the art, receiver down converters process a received signal and generate a down-converted signal having a lower frequency. A conventional receiver down converter is described with reference to
FIGS. 1
,
2
A and
2
B.
FIG. 1
is a block diagram illustrating a conventional image reject down converter arrangement
100
. A received signal s(t)
1
is processed by a passive filter
102
that removes undesirable information from signal s(t)
1
and provides a filtered signal s(t)
1
′. One example of a passive filter that is typically used for passive filter
102
is a surface acoustic wave (SAW) filter. Filtered signal s(t)
1
′ is provided to an automatic gain controller (AGC)
106
that adjusts the gain of signal s(t)′ to provide an output signal s(t)
1
″. The gain of signal s(t)
1
″ provided by AGC
106
is selected to be compatible with, i.e. not saturate, a band-pass (BP) filter
122
that is described in more detail hereinafter.
Signal s(t)
1
″ is provided to an image reject mixer
107
. Within image reject mixer
107
, signal s(t)
1
″ is provided to a mixer
108
along with a signal s
LO
generated by a local oscillator
110
and having a frequency of f
LO
. Mixer
108
generates a signal V
I
′. Signal s
LO
is also provided to a phase shifter
112
that generates a phase shifted signal s
LO
′ that is ninety (90) degrees out of phase with respect to signal s
LO
. Multiple phase shifters are sometimes employed to provide the signals s
LO
and s
LO
′ so long as the total phase shift between signals s
LO
and s
LO
′ is ninety (90) degrees. For example, two phase shifters providing phase shifts of plus forty five (+45) degrees and minus forty five (−45) degrees may be employed.
A mixer
114
combines signal s(t)
1
″ with phase shifted signal s
LO
′ and generates a signal V
Q
′. Signal V
I
′ is ninety (90) degrees out of phase with respect to signal V
Q
′. Signal V
I
′ is processed by a phase shifter
116
that generates a signal V
I
that is forty five (45) degrees out of phase with respect to signal V
I
′. A phase shifter
118
processes signal V
Q
′ and provides a signal V
Q
that is one hundred thirty-five (135) degrees out of phase with respect to signal V
Q
′. Thus, phase shifters
116
and
118
provide an additional ninety (90) degrees of phase shift of the unwanted image information contained in signals V
I
and V
Q
.
Signals V
I
and V
Q
are provided to a summer/subtractor
120
that selects either an upper or lower side band to be removed. Summer/subtractor
120
provides a down-mixed signal V
IF
having a center frequency of f
IF
. Signal V
IF
is processed by BP filter
122
centered at frequency f
IF
that provides a filtered signal V
IF
′ to a conventional demodulator
124
.
FIG. 2A
contains a chart
200
illustrating the spectrum of the conventional image reject down converter
100
of
FIG. 1
for a down-converted signal having a relatively high center frequency (f
IF
). The gain of passive filter
102
is represented by line
202
. Thus, the desired information in the form of a lower side band
204
, centered around the receiver frequency f
RF
, is not removed by passive filter
102
, since line
202
has a high cut-off at the lower f
3
and upper f
4
edges of lower side band
204
. However, the unwanted image in the form of an upper side band
206
, centered around an image frequency f
IM
, is removed by passive filter
102
, which has minimal gain (maximum attenuation) at the lower f
1
and upper f
2
edges of sideband
206
. The difference between the image frequency (f
IM
) and the local oscillator frequency (f
LO
) is the same as the difference in frequency between the local oscillator frequency (f
LO
) and the reference frequency (f
RF
). Summer/subtractor
120
is used to select either upper side band
206
or lower side band
204
to be removed from the received signal as follows:
V
IF
=V
I
+V
Q
(removes upper side band)
V
IF
=V
I
−V
Q
(removes lower side band)
A recent trend has been to implement communication system receivers as integrated devices, sometimes referred to as “on chip” receivers, to reduce their size and manufacturing costs. One approach for implementing receivers on integrated devices is to reduce the center frequency of the down-converted signal so that the demodulators can be integrated at a much smaller current. However, there are several drawbacks with using conventional down converter arrangement
100
for down-converted signals having lower center frequencies. Specifically, as the center frequency of down-converted signals is decreased, the upper and lower side bands
206
and
204
are closer to the local oscillator frequency (f
LO
). As the upper and lower side bands
206
and
204
move closer to the local oscillator frequency (f
LO
), the image rejection near the image frequencies to be removed can become insufficient to remove the entire image band due to roll-off in passive filter
102
.
FIG. 2B
contains a chart
210
that illustrates the spectrum of image reject down converter
100
for a down-converted signal having a relatively low center frequency (f
IF
), of about 4 times the signal bandwidth. As illustrated by chart
210
, upper side band
206
is not filtered in the same manner as in FIG.
2
(
a
). The upper side band is no longer sufficiently removed by passive filter
102
since upper side band
206
is situated in the roll-off region of filter
102
. Particularly, the upper edge f
2
and lower edge f
1
side band
206
are not equally filtered by passive filter
102
for down-converted signals having relatively low center frequencies (f
IF
). Thus, the upper side band
206
is not completely cut off as in FIG.
2
(
a
), and the partial attenuation which does occur is asymmetric, with the amplitude of the lower edge f
1
and upper edges f
2
being different.
FIG. 3
is a chart
300
illustrating image rejection provided by image reject mixer
107
(indicated by line
302
), passive filter
102
(indicated by line
304
), BP filter
122
(indicated by line
305
), and the total image rejection of image reject mixer
107
, passive filter
102
, and BP filter
122
(indicated by line
306
). The attenuation provided by image reject mixer
107
(line
302
) is centered at F
IF
, the location of which is represented by point
308
.
FIG. 3
does not necessarily completely and accurately illustrate the image rejection of image reject down converter
100
and filter
102
, and is provided to convey the limitations and asymmetric image rejection characteristics of image reject down converter
100
and filter
102
.
Where:
f
RF
is the receiver signal frequency,
f
IM
is the image frequency band to be removed,
f
IF
is the center frequency of the down-mixed signal,
f
1
is the lower edge of image band, and
f
2
is the upper edge of image band
then
f
LO
=f
RF
+f
IF
;
and
f
IM
=f
LO
+f
IF
=f
RF
+2
*f
IF
.
As is illustrated by chart
300
, image rejection at f
1
is different (asymmetric) from the image rejection at f
2
. In addition, the worst case image rejection is often unacceptable for certain applications. For example, the worst case image rejecti
Appiah Charles N.
Philips Electronics North America Corporation
Vo Nguyen
Wieghaus Brian J.
LandOfFree
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