Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2002-12-20
2004-09-14
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S555000
Reexamination Certificate
active
06791400
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the frequency tuning circuit, more particularly to a frequency-tuning loop in the Transconductor-Capacitor filter.
2. Description of the Prior Art
A filter is a common but important unit in a general signal processing system, the function of which is to eliminate the unnecessary band and to preserve or amplify the necessary ones. In the production process of advanced integrated circuits, it is a general and common trend to integrate filters into the design of the chip.
The cut-off frequency and the reciprocal of the time constant of the filter are in direct ratio. The time constant in the active RC filter is symbolized by the R.C value and that in the transconductor-capacitor filter is symbolized by the C/Gm value. However, due to the uncertainty of the integrated-circuit-fabrication process, the resistance value, the capacitance value, and even the value of resistance value multiplying capacitance value produced, the most important in the filter, range greatly, and therefore the frequency precision of the filters produced is below standard or unstable.
Therefore, a frequency-tuning loop is needed to set the filter, which can focus on a signal-inputting resource (such as a constant clock signal), to measure the time constant, and to alter or adjust the cut-off frequency of the filter.
A Transconductor-Capacitor filter is a common filtering technique that has a high-speed feature. The most important thing in designing a Transconductor-Capacitor filter is to make sure that the cut-off frequency is controlled in a designed range that does not change with the production process, temperature, or time. There are two common controlling methods, one is to make the calibration to the cut-off frequency of the filter according to the inputting clock, which can repair all the differences in the production process. Having the advantage, that once it is calibrated it will not affect the filter anymore. The disadvantage being, that precision in the calibration is limited—incapable of doing repairs to the variables such as temperature changing, time aging, etc. At the same time; the other method uses a continuous tuning to the cut-off frequency of the filter according to the consistency of the inputting clock signal, having the advantage that it can do repairs to the variables such as temperature changing, time aging, etc. But also the disadvantage is that the offset current produced by the Transconductor will affect the adjusted results and therefore lower the degree of precision needed.
As shown in
FIG. 1A
, a block diagram of a prior Transconductor-Capacitor filter with a frequency-tuning loop, after the inputting signal goes through the Transconductor-Capacitor filter
110
, there is a filtered outputting signal, and this Transconductor-Capacitor filter
110
has a frequency-tuning Loop
112
that receives a fixed clock and generates tuning-voltage for the Transconductor-Capacitor filter
110
to adjust the outputting signal according to the calibration of this clock.
FIG. 1B
is a model diagram of the interior circuit of the frequency-tuning loop
112
in
FIG. 1A
, a tuning circuit designed according to “a novel approach for the automatic tuning of continuous-time filters” in IEEE proc. ISCAS-91, the idea of which is to use the charge-transforming negative-feedback loop. The transconductor
120
itself to form the resistance
R
=
1
Gm
of equal effect with the negative-feedback while the voltage produced by the current supply I
r
from the positive end of the Transconductor
120
going through Gm is
V
=
I
r
Gm
.
The on and off of the first switch
122
and the second switch
123
are respectively controlled by the clock and their on-off conditions are different. When the check controlling first switch
122
is high-level then conductive, the capacitance C
1p
is filled with charge Q
p
, and
Q
p
=
C
1
p
*
V
=
C
1
p
*
I
r
Gm
,
and at this time the second switch
123
is off and in open-circuit condition, which makes the capacitance C
1p
and the later circuit unable to affect each other. And when the high-level of the clock turns to low-level, the first switch
122
is switched off and becomes an open-circuit, and the second switch
123
conductive, and at this time the charge Q
p
deposited in the capacitance C
1p
is transformed to outputting voltage
Δ
V
cp
,
Δ
V
cp
=
-
Q
p
C
2
p
=
-
C
1
p
C
2
p
*
I
r
Gm
,
on the integrator circuit constituted by integrator
124
, the capacitance C
2p
, and the capacitance C
2n
.
Moreover, since there is another current supply
125
on the inputting end of the integrator
124
, the current value of which is N*I
r
, therefore the outputting of the integrator increases constantly, and in a unit of clock period T=1/f, the increased voltage of the integrator
124
due to the current resource
125
is
Δ
V
1
=
I
C
2
p
*
T
=
N
*
I
r
C
2
p
*
1
f
.
And when the negative-feedback loop reaches balance in the end, &Dgr;V
cp
+&Dgr;V
1
=0, the Equation 1 can also be reached:
-
C
1
p
C
2
p
*
I
r
Gm
+
N
*
I
r
C
2
p
*
1
f
=
0
∴
⇒
Gm
C
1
p
=
f
N
(Equation 1)
Therefore, we can see that the tuning-frequency of the frequency-tuning loop
112
can be controlled with its f
value, thus the cut-off frequency of the Transconductor-Capacitor filter can be tuned to the designed value with this frequency-tuning loop
112
.
Besides, the units connected to the other inputting end of the transconductor
120
, such as the third switch
126
, the fourth switch
127
, and the capacitance C
1n
, C
2n
, have similar way of connecting among each unit itself, the principle of action, and other units as the way described above, and only that it is connected to the other end so the outputting voltage &Dgr;V
cn
is different from &Dgr;V
cp
by a negative sign. Then &Dgr;V
cn
and &Dgr;V
cp
outputted after going through a differential to single converter
128
and a low-pass filter
129
is the needed tuning-voltage.
However, the circuit mentioned above is in an ideal situation, and in a real situation, unavoidably, there will be an offset current.
FIG. 1C
is a practical equal-effect model of
FIG. 1B
to illustrate the real situation, in which offset current
130
indicates the offset current produced by the transconductor
120
in the real situation, therefore a real transconductor can be equaled to an ideal transconductor
120
plus an offset current
130
, and the current produced by this offset current
130
is marked I
offset
. Thereupon the current going through this differential to single converting transconductor
120
here becomes I
r
+I
offset
. And making use of the said analyzing method, Equation 2 can be reached:
-
C
1
p
C
2
p
*
(
I
r
+
I
offset
)
Gm
+
N
*
I
r
C
2
p
*
1
f
=
0
∴
⇒
Gm
C
1
p
=
f
(
1
+
I
offset
I
r
)
N
(Equation 2)
It can be seen that the offset current produced by the Transconductor
120
will affect the value of the tuning frequency, and the error amount and the value are in direct ratio. And since the offset current changes because of the producing process of the Transconductor itself, the temperature of the environment, and the time factor, a frequency-tuning loop that is not affected by the offset current produced by the Transconductor is needed to upgrade the precision of the frequency-tuning loop.
SUMMARY OF THE INVENTION
Due to the several disadvantages in controlling the cut-off frequency of the traditional Transconductor-Capacitor filter in the background of invention described above, the invention provides a frequency-tuning loop used in the Transconductor-Capacitor filter in order to overcome the traditional problems.
The main purpose of the invention is to provide a frequency-tuning loop that is not affected by the offset current produced by the referential Transconductor in order to upgrade the precision of frequency of the Transcondu
Cunningham Terry D.
Industrial Technology Research Institute
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