Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2002-08-09
2003-07-15
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
Reexamination Certificate
active
06593803
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to Japanese Patent Application No. JP 2001-245866, filed on Aug. 14, 2001, the disclosure of such application being herein incorporated by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active filter circuit to be utilized in a variety of electronic devices, for example for deriving a signal under a designated frequency.
2. Description of Related Art
In a variety of electronic devices, various active filters are used in order to isolate and extract a specific signal or to remove noises from signals of a wide range of frequencies. As an example of such active filters, a MOSFET-C filter (a low pass filter)
100
as shown in
FIG. 7
is known as a configuration of a monolithic active filter (an active filter formed as a semiconductor integrated circuit).
A MOSFET-C filter operates in a non-saturated area of a MOSFET (Metal Oxide Semiconductor Field-effect Transistor: a field-effect transistor having a metal oxide film semiconductor structure). In other words, the MOSFET-C filter utilizes a linear operating area as a variable resistance, and it is known as a filter (a triode filter) which utilizes triode action as a variable resistance.
In other words, a MOSFET-C filter
100
as illustrated in
FIG. 7
is configured such that circuit sections each having a plurality of MOSFET used as variable resistors, capacitance elements and an operational amplifier are connected to each other in multi-stage and a plurality of capacitance elements are used.
As described in an existing document (M. Banu and Y. Tsividis, “An Elliptic Continuous-Time CMOS Filter with On-Chip Automatic Tuning,” IEEE Journal of Solid-State Circuits, vol. 20, no.6, pp1114-1121, December 1985), a MOSFET-C filter IC (integrated circuit) operable with a ±5 V power supply has been realized by Banu et al.
In recent years, however, voltage of a power supply for an IC has been reduced because of requirements for a more efficient use of a battery taking in consideration installation of the battery in a so-called a mobile terminal such as a mobile communications terminal. Also, because of miniaturization of the semiconductor fabricating process, there is a strong need for an IC to be operable with a single power supply of 2.7 V or less.
Generally, a MOSFET-C filter needs a power supply of relatively high voltage as compared, for example, with a Gm-C filter (temporally continuous filter) which is another method for designing a monolithic filter. This is because, in order to let a MOSFET used as a variable resistor perform a good linear action (a triode action), sufficiently high gate voltage (VG) must be applied to the gate terminal of the MOSFET.
The MOSFET-C filter, therefore, has a serious drawback with respect to lowering the voltage of the power supply in terms of circuit design. Even in a conventional power supply of 2.7 V, it is difficult to obtain a MOSFET-C filter of a higher dynamic range.
In order to solve the problem, as shown in
FIG. 8
, for example, a charge pump section (a charge pump circuit)
204
is provided within a frequency adjusting section
200
which supplies a gate voltage (VG) to a gate terminal of each MOSFET of the MOSFET-C filter. The MOSFET-C filter
100
may be driven and controlled by the gate voltage (VG) which is increased to a higher voltage than that of the power supply in the charge pump section
204
.
As shown in
FIG. 7
, for example, since a terminal used for frequency adjusting in the MOSFET-C filter
100
is the gate terminal of the MOSFET, a simple charge pump section may easily provide a gate voltage higher than the power supply voltage, thus the MOSFET filter
100
may be applied to a filter circuit having a high dynamic range which is required in a receiving circuit of a mobile communications terminal.
In other words, as shown in
FIG. 9
, the MOSFET-C filter
100
and the frequency adjusting section
200
supplying the gate voltage (VG) thereto make it possible to configure an active filter circuit having a high dynamic range.
It is to be noted that the frequency adjusting section
200
as shown in
FIG. 8
is a design of so-called DLL (Delay Locked Loop) which is configured to be locked when a predetermined phase difference, 90 degrees, for example, is caused between a phase of an output signal from a filter (a MOSFET-C filter for frequency adjusting)
201
having a function of a delay circuit and that of an input signal (reference clock signal CLK) to the frequency adjusting section
200
.
In this case, the output signal from the filter
201
and an input signal to the frequency adjusting section
200
are multiplied by a multiplier
202
. The multiplier
202
outputs a signal comprised of a doubled signal component and a DC component which are supplied to a loop filter
203
. The loop filter
203
extracts only the DC component, which is then supplied to a charge pump section
204
as a control signal.
A voltage charging action (voltage increasing action) at the charge pump section
204
is carried out until the phase difference between the output signal from the filter
201
and the input signal to the frequency adjusting section
200
is a predetermined value (90 degrees, for example) so as to obtain a gate voltage of a target level.
When the phase difference between the output signal from the filter
201
and the input signal to the frequency adjusting section
200
reaches the predetermined value, the charge pump
204
is locked to supply the gate voltage (VG) of the target level to each gate terminal of the filter
201
of the frequency adjusting section
200
and the MOSFET-C filter
100
, whereby the MOSFET-C filter is driven and controlled.
It is to be noted that, the following documents (1) and (2) describe how to increase a gate voltage of an active filter by means of a charge pump section:
(1). G. L. E. Monna, J. C. Sandee, C. J. M. Verhoeven, G. Groenewold, and A. H. M. van Roermund, “Charge Pump for Optimal Dynamic Range Filters,” Proceedings,1994 IEEE International Symposium on Circuits and System, vol.5,pp747-750,1994;
(2). A. Yoshizawa and Y. Tsividis, “An Anti-Blocker Structure MOSFET-C Filter For a Direct Conversion Receiver,” Proceeding,2001 IEEE Custom Integrated Circuit Conference.
However, such a configuration in which the frequency adjusting section
200
having the charge pump section
204
drives a cut-off frequency control terminal (a gate terminal) of the MOSFET-C filter
100
may cause a clock signal for driving the charge pump section
204
to leak into the MOSFET-C filter
100
and present a problem in that a dynamic range of the MOSFET-C filter
100
is limited.
For example, noise from the frequency adjusting section
200
may be mixed into a gate terminal of the MOSFET-C filter
100
shown in
FIG. 7
, increasing the output signal from the MOSFET-C filter
100
to an excessively high level. If the signal coincides with timing of a clock signal and happens to be picked up, the dynamic range of the MOSFET-C filter, resulting in a worse characteristic of the MOSFET-C filter.
Further, in a receiving circuit of a mobile communications terminal, for example, less power consumption in the circuit is preferable in an effort to extend stand-by time for receiving calls. However, in order to control the cut-off frequency of an active filter, it is necessary for an analogue control to keep a dedicated circuit such as the frequency adjusting section
200
as shown in
FIG. 8
, for example, constantly active, thus resulting in a burden for reduction of power consumption.
Still further, when a digital control is used in order to control a cut-off frequency of an active filter, a D/A (digital to analogue) converter is required. As a result, the size of the circuit is increased and also its power consumption is increased.
SUMMARY OF THE INVENTION
In view of the above description of the existing problems related to the conventional art, the present invention provides an active filter circuit capable
Le Dinh T.
Maioli Jay H.
Sony Corpration
LandOfFree
Active filter circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Active filter circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Active filter circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3031394