Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
1999-05-04
2001-01-30
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S553000, C327S552000
Reexamination Certificate
active
06181197
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of electronics and, more particularly, to a bandpass filter.
BACKGROUND OF THE INVENTION
Demodulation systems for the chrominance subcarrier of a video signal require use of bandpass filters accepting a signal with a relatively large dynamic range. The bandpass filters have quality coefficients defining the selectivity of these filters. These quality coefficients are relatively high, e.g., within the range 3 to 30.
In prior art demodulator circuits, these filters can be produced using integrated capacitors and transconductors. A transconductor is defined by its transconductance g
m
such that its output current i
s
=g
m
*&ugr;
e
. The variable &ugr;
e
is the voltage between the two input terminals. These transconductors must be able to be regulated to adjust the center frequency of the filter. The control of transconductance is provided by a servoloop using a filter structure analogous to that of the filter to be adjusted and a precise frequency reference.
A traditional example of a bandpass filter is illustrated in
FIG. 1
, and is described in an article titled “Active Filter Design Using Operational Transconductance Amplifiers: A Tutorial,” by Randall L. Geiger and Edgar Sanchez-Sinencio, published in IEEE Circuits and Devices Magazine, March 1985, pages 20-32. More particularly, a description of this bandpass filter can be found in this reference on the final two lines on page 25, and the first fifteen lines on page 27 viewed with FIG.
7
(
a
) therein. This reference discloses various structures for circuits (e.g., voltage controlled amplifiers, filters, etc.) based on transconductors for integration.
The filter illustrated in
FIG. 1
includes first and second transconductors
10
and
11
, respectively with transconductances g
m1
and g
m2
. The first input (+) of the first transconductor
10
is connected to ground. The second input (−input or reverse input) of the first transconductor
10
is connected to the second input (−input or reverse input) of the second transconductor
11
, and to the output S of the filter. The output of the first transconductor
10
is connected to the first input (+) of the second transconductor
11
, and to the input E of the filter through a first capacitor C
1
. The output of the second transconductor
11
is connected to ground through a second capacitor C
2
, and to a monitor amplifier
12
. The output of the monitor amplifier
12
is connected to the output S of the filter.
This filter can be formed, for monolithic applications, in MOS or bipolar technologies. The transfer function of this filter is such that:
v
s
v
e
=
C
1
g
m1
·
p
C
1
·
C
2
g
m1
·
g
m2
·
p
2
+
C
1
g
m1
·
p
+
1
The variable p is the Laplace variable with the resonant frequency defined as follows:
ω
0
=
g
m1
·
g
m2
C
1
·
C
2
The variable &ohgr;
0
can be adjusted as a result of regulating g
m1
and g
m2
.
The quality coefficient is such that:
Q
=
g
m1
·
C
2
g
m2
·
C
1
By choosing the transconductances g
m1
and g
m2
so that the ratio g
m1
/g
m2
remains constant, a quality coefficient value Q that is independent of &ohgr;
0
is obtained. To provide a precise adjustment of &ohgr;
0
, it is then necessary to vary the transconductances g
m1
and g
m2
in the same proportions, and to ensure the best possible matching between the transconductances g
m
of this filter and the transconductances g
m
of the servo-loop filter.
Firstly, this leads to a limiting of the ratio g
m1
/g
m2
to a value that does not exceed 4. The value of the quality coefficient can then be fixed by choosing the ratio C
2
/C
1
. In order to obtain a high quality coefficient, the ratio C
2
/C
1
must be large. This is incompatible with the constraints of circuit integration. In effect, poor matching of C
1 and C
2
is obtained if C
2
>>C
1
. The parasitic capacitances which are the same size as C
1
cause inaccuracy in the filter.
The output node
13
from the first transconductor
10
is particularly critical. In effect, it is at a high impedance. Small parasitic capacitances can cause changes in filter characteristics. Furthermore, a large overvoltage linked to the quality factor develops at this node
13
.
At resonance &ohgr;=&ohgr;
0
, the voltage &ugr;
c1
this node
13
is of the form:
v
cl
V
e
=
1
+
j
Q
&LeftBracketingBar;
v
cl
V
e
&RightBracketingBar;
=
1
+
Q
2
≈
Q
The input voltage dynamic range &ugr;
e
is then Q times less than the output dynamic range of the first transconductor
10
. Such a limitation means that a high penalty is paid in circuits with a low supply voltage.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a bandpass filter that allows one to avoid the above described disadvantages by permitting a significant improvement in its performance.
The present invention thus relates to a bandpass filter including first and second transconductors. The first input of the first transconductor is connected to ground. The second input of the first transconductor is connected to the second input of the second transconductor, and to the output of the filter. The output of the first transconductor is connected to the first input of the second transconductor, and to the input of the filter through a first capacitor. The output of the second transconductor is connected to ground through a second capacitor, and to a monitor amplifier. An output of the monitor amplifier is connected to the output of the filter. The bandpass filter further includes a third capacitor connected between the second input and the output of the first transconductor. A resistance bridge is also connected to the second input of the second transconductor, and to a common point between the second input of the first transconductor and the output of the filter. A second resistance is arranged between this second input of the second transconductor and ground.
Advantageously, the respective values of the first, second and third capacitors are such that:
C
1
=m−C
0
C
2
=C
0
C
3
=(1−m)C
0
Advantageously, the following relationships apply:
0.1
<m<
0.5 and 1.1(1
−m
)<
k<
0.9
with k=R
2
/(R
1
+R
2
), and R
1
and R
2
being the respective values of the first and second resistances.
This invention also relates to a television processing integrated circuit that includes at least one filter as specified above.
REFERENCES:
patent: 5001441 (1991-03-01), Gen-Kong
patent: 5317217 (1994-05-01), Rieger et al.
Urba{acute over (e)} et al.,IEEE Transactions on Consumer Electronics, “High Frequency Realization of C-OTA Second Order Active Filters”, May 10-12, 1982, P1106-1109, p. 1106, col. 1, line 9, figure 2.
Tanabe et al,IEEE Transactions on Consumer Electronics, “A Single Chip Y/C Signal Processing IC for VHS VCRS”, vol. 35, No. 4, pp. 741-747, p. 743, col. 1, line 7, p. 744, col. 1, line 23, figure 6.
Bret G{acute over (e)}rard
Debaty Pascal
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Galanthay Theodore E.
Le Dinh T.
STMicroelectronics S.A.
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