Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2003-04-22
2004-10-12
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S379000
Reexamination Certificate
active
06803813
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to active filters, and in particular, to calibration circuitry for tuning active filters.
2. Description of the Related Art
Increasing demands for wireless communications has accelerated developments of integrated circuits (ICs) in which all, or nearly all, necessary functions are integrated within one IC. Such single-chip implementations include filtering functions which have become increasingly critical and impose increasingly precise design requirements due to complex signal processing involved with high frequency digital signals.
Design and manufacturing of active continuous-time filters using IC technology poses many challenges due to the fact that integrated active filters require circuit components having values with high degrees of precision. Further, such precise component values must be maintained during the entire expected operating life of the circuitry. However, with normal variations in IC fabrication processes, normal operating temperature ranges and aging of components, such strict requirements are virtually impossible to maintain with little or no variations. Accordingly, it has become increasingly common to embed an automatic tuning mechanism within the IC as part of the overall circuitry.
Referring to
FIG. 1
, one conventional technique for tuning an active filter is based on indirect tuning where analog circuitry in the form of a phase-locked loop (PLL) provides the tuning mechanism. In accordance with well known conventional PLL techniques, the PLL
12
provides the control signal
13
to a master filter circuit
14
which, in turn, provides the feedback signal
15
to the PLL
12
. Together, the PLL
12
and master filter
14
form an oscillator from which the feedback signal
15
is generated and intended to be phase-locked to the input reference signal
11
r.
When the PLL
12
has achieved a phase locked condition, i.e., locked to the phase of the reference signal
11
r,
the master filter is tuned to its desired frequency characteristics. This tuning state of the master filter
14
is reflected in the control signal
13
during phase lock. Accordingly, with this same control signal
13
applied to the slave filter
16
, which is designed to match the master filter
14
, the slave filter
16
also now has the desired frequency characteristics for filtering the incoming signal
11
i
to produce the desired output signal
17
.
This tuning technique can be applied to a variety of integrated active filters, such as transconductance-capacitor (G
m
-C) filters or metal oxide semiconductor field effect transistor (MOSFET) resistor-capacitor (RC) filters. However, two problems associated with this technique involve the high degree of matching required between the master
14
and slave
16
filters, as well as the performance of the analog PLL circuit
12
. While problems associated with the matching of the master
14
and slave
16
filters can be minimized using direct tuning techniques, such as in-situ tuning, several forms of which are well known in the art, problems associated with the use of analog PLL circuitry
12
remain.
Referring to
FIG. 2
, one technique which has been used to overcome limitations of analog circuitry uses digital circuits for performing the tuning operation. Using digital circuitry in an automatic tuning mechanism can be advantageous in that the performance and complexity of the tuning mechanism can be scaled in a parallel manner with advances in very large scale integration (VLSI) technology. One such implementation
20
uses a phase comparison successive approximation tuning technique with which the complexity and limitations of analog tuning circuitry are significantly reduced. As shown, an input reference signal
21
(with signal frequency f
ref
) is filtered by a continuous time active filter
22
serving as the master filter. The filtered input signal
23
and the original input signal
21
are compared against respective threshold voltages V
1
, V
2
in voltage comparators
24
a,
24
b.
The resultant signal
25
a
based upon the filtered input signal
23
is latched in a flip-flop
26
in accordance with a clock signal
25
b
generated based upon the original input signal
21
. The latched signal
27
is used by a successive approximation register and control logic
28
to produce a digital approximation signal
29
which is converted to an analog control signal
31
by a digital-to-analog converter (DAC)
30
. This control signal
31
runes the master filter
22
, and also provides the control signal V
contrl
for the slave filter (not shown).
All of the components in this implementation
20
are well known in the art. For example, the successive approximation register
28
can be the 74/54LS502 manufactured by National Semiconductor Corporation of Santa Clara, Calif.
Notwithstanding the potential for improved performance, such an implementation
20
does have its own problems. For example, with the need for two comparators
24
a,
24
b,
any mismatches in the signal phases or other forms of incoherence between the input signals
23
,
21
to the comparators
24
a,
24
b
can result in erroneous data latching or phase jitter within the latched data signal
27
, thereby causing the approximation signal
29
, and therefore, the filter control signal
31
, to have errors.
SUMMARY OF THE INVENTION
In accordance with the presently claimed invention, a time constant-based calibration circuit is provided for tuning active filter circuitry. A time constant, e.g., corresponding to that of the active filter circuitry, within the calibration circuit is monitored and maintained at a desired value using successive approximation, with continuous calibration of the time constant performed using digital circuitry and a digital feedback signal to control the time constant.
In accordance with one embodiment of the presently claimed invention, time constant-based calibration circuitry for tuning active filter circuitry includes tunable signal generator circuitry, signal comparison circuitry and successive approximation circuitry. The tunable signal generator circuitry, having associated therewith a first controllable time constant, responds to reception of a digital tuning control signal and first and second reference signals having mutually proportional respective DC values by providing a DC signal and an AC signal, wherein the DC signal has a magnitude responsive to the digital tuning control signal and related to the first controllable time constant, and the AC signal has a predetermined waveshape with a temporally variant magnitude. The signal comparison circuitry, coupled to the tunable signal generator circuitry, compares the DC and AC signals and in response thereto provides a digital result signal indicative of a difference between the DC and AC signal magnitudes. The successive approximation circuitry, coupled to the signal comparison circuitry, responds to reception of the digital result signal by providing the digital tuning control signal indicative of an approximation corresponding to a plurality of successive values of the digital result signal.
In accordance with another embodiment of the presently claimed invention, time constant-based calibration circuitry for tuning active filter circuitry includes tunable signal generator means, signal comparison means and successive approximation means. The tunable signal generator means, having associated therewith a first controllable time constant, is for responding to reception of a digital tuning control signal and first and second reference signals having mutually proportional respective DC values by generating a DC signal and an AC signal, wherein the DC signal has a magnitude responsive to the digital tuning control signal and related to the first controllable time constant, and the AC signal has a predetermined waveshape with a temporally variant magnitude. The signal comparison means is for comparing the DC and AC signals and in response thereto generating a digital result signal indicative of
Callahan Timothy P.
Englund Terry L.
National Semiconductor Corporation
Vedder Price Kaufman & Kammholz P.C.
LandOfFree
Time constant-based calibration circuit for active filters does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Time constant-based calibration circuit for active filters, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Time constant-based calibration circuit for active filters will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3290937