Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2002-07-26
2003-04-01
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S322000, C327S323000
Reexamination Certificate
active
06542018
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an interface integrated circuit for data communication, and more particularly to a current mode step attenuation control circuit with digital technology.
BACKGROUND OF THE INVENTION
Voltage attenuation control has an important role in signal processing circuit with Digital Signal Process (DSP) as a kernel. At a signal input, when the signal amplitude is too large and modulation attenuation occurs, amplitude limit distortion will be lowered and accuracy of signal processing will be raised. At signal output, when signal amplitude is too large, attenuation will lower interference to adjacent channel and raise the whole system performance. Referring to
FIG. 1
, a conventional voltage attenuation circuit is consisted of one operational amplifier and two variable resistors, wherein V
in
is an input voltage signal, V
bias
is a DC bias voltage, V
out
is an output voltage signal and variable resistors R
1
and R
2
are used to adjust gain. Suppose signal V
in
has an AC component (VAC) and a DC component (VDC), and the VDC equals to a bias voltage of the operational amplifier, then the operational amplifier is an ideal amplifier and there is a formula
V
out
=
(
V
bias
-
V
in
)
R1
R2
=
(
V
DC
-
V
DC
-
V
AC
)
R1
R1
=
-
V
AC
R1
R2
It can be seen from the formula above that output voltage has a linear relationship with resistance ratio of variable resistors R
1
and R
2
, but the polarity is opposite to the input signal. By changing resistance of R
1
and R
2
, step voltage attenuation control can be implemented. In general, changing R
2
resistance is an easier way to attenuate by digital signal control, but when attenuation amplitude is large, connected larger resistance R
2
will cause larger noise. In this attenuation circuit which is consisted of resistors, some elements, such as MOS switch etc., work at nonlinear zone and there are conducting resistances, so the attenuation circuit must have larger resistance of R
2
, otherwise control accuracy is worse. As there are larger resistances, the attenuation circuit cannot be integrated by standard digital integrated circuit technology.
Another weakness of the attenuation circuit is that in a single power supply system, such as a single-positive power supply, the bias voltage setting is limited. In the formula V
bias
>abs(V
in
)+K, wherein function abs means taking a variable absolute value, K is the minimum setting value of DC bias voltage which is limited by output amplitude range of the operational amplifier. When two inputs of the operational amplifier are equal, output should be zero. Nevertheless, if output amplitude range is 0.5V . . . VCC−0.5(V), when two inputs are equal, according to attenuation setting the output should be 0.5V. Otherwise, in certain signal input range, output will have larger distortion, which comes from non-ideal working state of the operational amplifier.
The output of the attenuation circuit only have AC signal and partial DC component, so it cannot provide DC working point for successive circuit, i.e. it cannot be directly coupled with successive circuit. In order to provide ideal DC working point and to implement direct couple, a differential operational amplifier is used in general, which uses Common Mode Feed Back (CMFB) circuit to create DC working point for successive circuit.
FIG. 2
shows a step attenuation circuit controlled by 5 bits digital signal.
In
FIG. 2
, V
inP
and V
inN
are two complement input voltage signals, V
outP
and V
outN
are two complement output voltage signals too, b
0
~b
4
are five levels digital signal for step attenuation control. The 2 to 4 decoder decodes 2 Most Signification Bits (2MSB) of the digital signal. The outputs of 2 to 4 decoder are IN
0
~IN
3
which control cut-in or cut-off of full differential operational amplifier input switches: SW
1
P and SW
1
N, SW
2
P and SW
2
N, SW
3
P and SW
3
N, SW
4
P and SW
4
N, respectively. The 3 to 8 decoder decodes 3 Lowest Signification Bits (3LSB) of the digital signal. The outputs of 3 to 8 decoder are OUT
0
, OUT
1
, . . . OUT
7
which control cut-in and cut-off of full differential operational amplifier output switches: SWO
1
P and SWO
1
N, SWO
2
P and SWO
2
N, . . . SWO
7
P and SWO
7
N, SWO
8
P and SWO
8
N, respectively. The VDC is a needed DC component of output differential signal, CMFB
in
is the input of CMFB, and CMFB
out
is the output of CMFB.
The voltage attenuation control in circuit above is in segment, i.e. with 2MSB decoder, the control is divided into four segments and each 8 DB is a control segment. When a digital control signal input is b
4
b
3
b
2
b
1
b
0
=00000; in input part, switches SW
1
P and SW
1
N are cut off and others are cut in, so resistances R
1
P and R
1
N are cut in and other resistances are all shorted; in output part, all switches are cut off and all resistances are cut in; at this time voltage attenuation is 0 DB, the output is
V
outP
=
-
V
inN
*
∑
m
=
1
m
=
8
RO
(
m
)
P
R1N
V
outN
=
-
V
inP
*
∑
m
=
1
m
=
8
RO
(
m
)
N
R1P
In these two formulas, when all cuts (decreasing) in resistances in output equal to cutting in resistances in input, then attenuation is 0 DB, but signal polarity is opposite. When keeping 2MSB unchanged, but b
2
b
1
b
0
has been changed from 000 to 111, cutting in resistances, in input, have been kept unchanged and cutting in resistances, in output, have been changed from 8 items to 1 item. The attenuation is changed from 0 DB to −7 DB. When 2MSB has been changed from 00 to 11 one by one, in input, cutting in resistances have been changed from 1 item to 4 items, and four segments of control are sequentially performed.
The numerical expressions corresponding to a digital control signal is
m=
2*
b
4
+
b
3
n=
4*
b
2
+2*
b
1
+
b
0
For different inputs of a digital control signal, the attenuation is
V
outP
V
inN
=
-
∑
K
=
1
K
=
8
-
n
RO
(
K
)
P
∑
L
=
0
L
=
m
R
(
1
+
L
)
N
V
outN
V
inP
=
-
∑
K
=
1
K
=
8
-
n
RO
(
K
)
N
∑
L
=
0
L
=
m
R
(
1
+
L
)
P
Wherein m is a decimal number corresponding to 2MSB of a binary digital control signal, and n is a decimal number corresponding to 3LSB of a binary digital control signal. A calculated number n is used in attenuation calculation formulas to sum the corresponding cut in resistances in numerator and denominator.
It can be seen from the analysis above that a whole circuit voltage attenuation can be calculated as follow. First, calculate decimal number m and n with a digital control signal input. Then, sum resistances of cut in circuits to obtain input summed resistance and output summed resistance of a cut in circuit. Finally, with the attenuation calculating formulas mention above calculate voltage attenuation of corresponding digital control signal.
MOS switch has a conducting resistance, said above, when considering switch conducting resistance, control accuracy of voltage attenuation will be affected. Taking 0 DB attenuation as an example, after considering the conducting resistance effect of MOS switch, a real attenuation is
V
outP
V
inN
=
-
∑
m
=
1
m
=
8
RO
(
m
)
P
3
*
R
SW
(
ON
)
+
R1N
V
outN
V
inP
=
-
∑
m
=
1
m
=
8
RO
(
m
)
N
3
*
R
SW
(
ON
)
+
R1P
In the formulas above, numerator is a total output resistance of cut in circuits and denominator is a total input resistance of cut in circuits. The attenuation calculating formula is similar as above, the only difference is by considering the MOS switch conducting resistance effect in the acting on input circuit resistance. When an output switch is also cut in the circuit, switch conducting resistance effect is also considered in corresponding numerator i
Alston & Bird LLP
Huawei Technologies Co. Ltd.
Tran Toan
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