Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-05-13
2002-02-12
Bost, Dwayne (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S127500, C455S093000, C455S091000
Reexamination Certificate
active
06347221
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an orthogonal modulator for use in a digital mobile communication device, such as a portable telephone, and, more particularly, to an orthogonal modulator less susceptible to the influence of a carrier leak.
FIG. 1
is a schematic block diagram of a conventional orthogonal modulator
11
. The orthogonal modulator
11
includes a frequency multiplier
12
, a phase shifter
13
, modulation mixers
14
and
15
, and an adder
16
, all formed on a single semiconductor substrate.
The orthogonal modulator
11
is connected to a transformer
17
called a balloon to suppress the occurrence of a carrier leak. The transformer
17
receives a carrier signal LOin and supplies a carrier signal LO in phase with the carrier signal LOin and a carrier signal LOx of the opposite phase to that of the carrier signal LOin, to the orthogonal modulator
11
.
The frequency multiplier
12
multiplies the frequencies of the complementary carrier signals LO and LOx by two and supplies frequency-doubled carrier signals
2
LO and
2
LOx to the phase shifter
13
. The phase shifter
13
divides the frequencies of the frequency-doubled carrier signals
2
LO and
2
LOx by two to generate four carrier signals LO
0
, LO
90
, LO
180
and LO
270
whose phases are shifted from one another by 90 degrees. The carrier signals LO
0
and LO
180
are complementary to each other, and the carrier signals LO
90
and LO
270
complementary to each other.
The modulation mixer
14
multiplies a baseband signal I or Ix by the carrier signal LO
0
or LO
180
to produce a first modulation signal VI. The modulation mixer
15
multiplies a baseband signal Q or Qx by the carrier signal LO
90
or LO
270
to produce a second modulation signal V
2
. The adder
16
adds the first and second modulation signals V
1
and V
2
together and outputs an output signal RFout.
A double mode phenomenon, which is caused by the carrier leak, makes the modulating operation of the orthogonal modulator
11
to be unstable. The double mode phenomenon includes a good mode which indicates the spectrum of the output signal RFout as shown in
FIG. 3A and a
bad mode indicating the spectrum of the output signal RFout as shown in FIG.
3
B. The mode (i.e., the good mode or the bad mode) is determined by the timing of powering on a portable device and the carrier signals LO and LOx. Referring to
FIGS. 3A and 3B
, a component CL appears at the frequencies of the carrier signals LO and LOx, and a component Pout appears at positions shifted on the high-frequency side from the frequencies of the carrier signals LO and LOx by the frequencies of the baseband signals I to Qx. A component IR appears at positions shifted on the low-frequency side from the frequencies of the carrier signals LO and LOx by the frequencies of the baseband signals I to Qx.
It is believed that the double mode phenomenon may attributed to the following two factors. The first factor is phase differences between the carrier signal LO or LOx and the carrier signals LO
0
, LO
90
, LO
180
and LO
270
. The phase shifter
13
operates to make the rising edge of the frequency-doubled carrier signal
2
LO match with the rising edge of the carrier signal LO
0
. However, the rising edge of the carrier signal LO
0
coincides with the rising edge of the carrier signal LO in some cases as shown in FIG.
2
A and coincides with the falling edge of the carrier signal LO in other cases as shown in FIG.
2
B. That is, the phase shifter
13
generates a carrier signal LO
0
having a phase difference of 0 degrees to the carrier signal LO or a carrier signal LO
0
having a phase difference of 180 degrees to the carrier signal LO.
The second factor is the generation of a direct current (DC) component on the output signal RFout. As the carrier signal LO or the output signal RFout has a high frequency, it easily leaks in space. As this leaked carrier enters the modulation mixers
14
and
15
through their input terminals, the DC component appears.
More specifically, the output signals Vout (output signals V
1
and V
2
) of the modulation mixers
14
and
15
in an ideal state where the carrier signal LO does not leak are expressed by the following equation (1).
V
out
=
cos
(
2
π
f
LO
t
)
×
cos
(
2
π
f
BB
t
)
=
1
2
{
cos
2
π
(
f
LO
+
f
BB
)
t
+
cos
2
π
(
f
LO
-
f
BB
)
t
}
(
1
)
When the leaked carrier signal LO is input to the input terminal for the baseband signal I, Ix, Q or Qx, the output signals Vout of the modulation mixers
14
and
15
are given by the following equation (2).
V
out
=
cos
(
2
π
f
LO
t
+
φ
1
)
×
cos
(
2
π
f
LO
t
+
φ
2
)
=
1
2
{
cos
2
π
(
2
f
LO
+
φ
1
+
φ
2
)
t
+
cos
(
φ
1
-
φ
2
)
}
(
2
)
where &phgr;
1
is the input phase of the original carrier signal LO and &phgr;
2
is the leak phase (phase delay) of the leaked carrier signal LO, with the baseband signals ignored for the sake of convenience. The leak phase &phgr;
2
is nearly constant, and the input phase &phgr;
1
is 0 degree or 180 degrees (90 degrees or 270 degrees). Thus, the second term in the equation (2) that represents the DC component has two values. The two values for the second term causes the double mode phenomenon. Note that the first term in the equation (2) is hardly affected by the input phase &phgr;
1
and the leak phase &phgr;
2
because 2f
LO
is sufficiently larger than those phases.
Referring now to
FIG. 4
, an improved orthogonal modulator
21
suppresses the doube mode phenomenon is shown.
The improved orthogonal modulator
21
has two ½ frequency dividers
22
and
23
, each of which includes a flip-flop type phase shifter.
The first ½ frequency divider
22
frequency-divides the carrier signal LO or LOx by two to yield frequency-divided signals of phases different by 90 degrees from each other. The second ½ frequency divider
23
frequency-divides the carrier signal LO or LOx by two to yield frequency-divided signals. A first modulation mixer
24
multiplies the first baseband signal I by the frequency-divided signal from the first ½ frequency divider
22
. A second modulation mixer
25
multiplies the second baseband signal Q by the frequency-divided signal from the first ½ frequency divider
22
.
An adder
26
combines the output signals, Iout and Qout, from the first and second modulation mixers
24
and
25
, amplifies the resultant signal and outputs the amplified signal Sout. A frequency multiplier
27
multiplies the amplified signal Sout from the adder
26
by the output signal of the second ½ frequency divider
23
to yield an output signal RFout.
The output signals Iout and Qout of the first and second modulation mixers
24
and
25
, the output signal Sout of the adder
26
and the output signal Vout of the frequency multiplier
27
are given by the following equations (3) to (6)
I
out
=
cos
(
2
π
·
f
LO
/
2
·
t
)
×
cos
(
2
π
f
BB
t
)
=
1
2
{
cos
2
π
(
f
LO
/
2
+
f
BB
)
t
+
cos
2
π
(
f
LO
/
2
-
f
BB
)
t
}
(
3
)
Q
out
=
cos
(
2
π
·
f
LO
/
2
·
t
-
90
°
)
×
cos
(
2
π
f
BB
t
+
90
°
)
=
1
2
{
cos
2
π
(
f
LO
/
2
+
f
BB
)
t
-
cos
2
π
(
f
LO
/
2
-
f
BB
)
t
}
(
4
)
S
out
=
I
out
+
Q
out
=
cos
2
π
(
f
LO
/
2
+
f
BB
)
t
(
5
)
V
out
=
S
out
×
cos
(
2
π
·
f
LO
/
2
&
Takekawa Koji
Tsukahara Masahiro
Bost Dwayne
Trinh Sonny
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