Apparatus for detecting when a dynamic signal is stable

An apparatus for detecting when a dynamic signal is stable. The apparatus is couplable to a first charge pump connected to receive the dynamic signal and generate a first signal as a function of the dynamic signal. The first charge pump has a predetermined capacitance and current. In one embodiment, the apparatus includes: (1) a second charge pump connected to receive the dynamic signal and in parallel with the first charge pump, the second charge pump having a smaller capacitance and discharge current than the first charge pump and configured to produce a second signal as a function of the dynamic signal, (2) a third charge pump connected to receive the dynamic signal and in parallel with the first charge pump and the second charge pump, the third charge pump having a smaller capacitance and larger discharge current than the first charge pump and configured to produce a third signal as a function of the dynamic signal and (3) control logic configured to receive the second and third signals and produce an active signal when the second and third signals are substantially opposite to each other, logically, indicating that the dynamic signal is stable.

TECHNICAL FIELD OF THE INVENTION 
The present invention is directed, in general, to an apparatus and method 
for detecting stability of a dynamic signal, and more specifically, to an 
apparatus that employs charge pumps to detect when a dynamic signal is 
stable. 
BACKGROUND OF THE INVENTION 
It is desirable, in many applications, to adjust the gain levels of a 
signal. For example, when a signal is transmitted over a line, it may be 
necessary to increase the gain of the signal to account for degradation in 
signal clarity and strength. Signal gain may require adjustment (up or 
down) depending on, for instance, the distance a signal travels from 
point-to-point. One common device used to adjust signal gain is an 
automatic gain control (AGC) loop. An AGC loop automatically adjusts the 
output gain of a signal to a specific level as a function of its input. 
In larger systems, it may be necessary to employ several AGC loops 
connected together to control signal gain. A requirement associated with 
nesting interconnecting AGC loops is that when one loop is active, the 
other loop(s) should remain stable to avoid transmitting an unstable 
signal (garbage data) to the active loop thereby ensuring that the system 
converges. To verify the stability of an AGC loop, many devices monitor 
the voltage produced by a charge pump of an AGC loop to determine when the 
AGC loop is stable. The charge pumps are typically used in the AGC loop to 
boost or reduce the voltage of a signal to a particular level, then freeze 
the loop at that level. By monitoring the output voltage (a signal) of the 
charge pump, it is possible, in principal, to ascertain when an AGC loop 
is stable. 
Determining when the resulting output signal of the AGC loop is stable 
based on the output voltage of the charge pump, however, can be arduous or 
impractical. First, it is difficult to detect the stability of the loop by 
monitoring the output voltage of the charge pump because the final values 
of the output voltage are often unknown. Second, it is also difficult to 
detect the stability of the loop by comparing the output voltage of the 
charge pump because the values of the output voltage are dynamic having 
differing values even when the loop is stable. Third, to reduce the 
variations in the output voltage of the charge pump when the loop becomes 
stable requires a large capacitor (resulting in a large time constant) in 
the charge pump. As a result, the variations in the output voltage are 
dampened during both the active and stable periods. Thus, to detect the 
stability of the loop by monitoring the variations (as the variations are 
being tempered) in the output voltage with the constraints on the 
allowable time interval is difficult, if not impossible. 
Presently, most detection circuits are design specific. The detection 
circuits only function with specific AGC loops having predetermined charge 
pump voltages and time intervals. Other detection circuits are complicated 
and expensive. Thus, it is difficult and often expensive to design 
stability detection circuits to operate in a multiple interconnected AGC 
loop environment, since each AGC loop may have a unique stability 
characteristic. 
Accordingly, what is needed in the art is an improved apparatus that 
automatically detects when a dynamic signal is stable, without knowing, 
beforehand, the level and time that is takes a particular dynamic signal 
to reach stability. Additionally, such an apparatus should employ a 
simple, low-cost and substantially standardized design (for easy 
interchangeability in different systems). 
SUMMARY OF THE INVENTION 
To overcome the deficiencies in the prior art, the present invention 
provides an apparatus for detecting when a dynamic signal is stable. The 
apparatus is couplable to a first charge pump connected to receive the 
dynamic signal and generate a first signal as a function of the dynamic 
signal. The first charge pump has a predetermined capacitance and current. 
In one embodiment, the apparatus includes: (1) a second charge pump 
connected to receive the dynamic signal and in parallel with the first 
charge pump, the second charge pump having a smaller capacitance and 
discharge current than the first charge pump and configured to produce a 
second signal as a function of the dynamic signal, (2) a third charge pump 
connected to receive the dynamic signal and in parallel with the first 
charge pump and the second charge pump, the third charge pump having a 
smaller capacitance and larger discharge current than the first charge 
pump and configured to produce a third signal as a function of the dynamic 
signal and (3) control logic configured to receive the second and third 
signals and produce an active signal when the second and third signals are 
substantially opposite to each other, logically, indicating that the 
dynamic signal is stable. 
In another embodiment, the apparatus, includes (1) a second charge pump 
connected to receive the dynamic signal and in parallel with the first 
charge pump, the second charge pump having a smaller capacitance and 
larger charge current than the first charge pump and configured to produce 
a second signal as a function of the dynamic signal, (2) a third charge 
pump connected to receive the dynamic signal and in parallel with the 
first charge pump and the second charge pump, the third charge pump having 
a smaller capacitance and smaller charge current than the first charge 
pump and configured to produce a third signal as a function of the dynamic 
signal and (3) control logic configured to receive the second and third 
signals and produce an active signal when the second and third signals are 
inverse to each other, logically, indicating that the dynamic signal is 
stable. 
Thus, the present invention is able to automatically detect when a dynamic 
signal is stable, without knowing beforehand, when a dynamic signal level 
has reached stability and how long it takes to do so. Additionally, the 
present invention employs a simple, low-cost and substantially 
standardized design; making the present invention a desirable detector. 
The foregoing has outlined, rather broadly, features of the present 
invention so that those skilled in the art may better understand the 
detailed description of the invention that follows. Additional features of 
the invention will be described hereinafter that form the subject of the 
claims of the invention. Those skilled in the art should appreciate that 
they can readily use the disclosed conception and specific embodiment as a 
basis for designing or modifying other structures for carrying out the 
same purposes of the present invention. Those skilled in the art should 
also realize that such equivalent constructions do not depart from the 
spirit and scope of the invention in its broadest form.

DETAILED DESCRIPTION 
Referring initially to FIG. 1, illustrated is a block diagram of an 
exemplary environment for the present invention. The environment is a 
general telecommunication system 100 including: a line 102, an equalizer 
104, a slicer 106 and a data recoveror 108. 
FIG. 2 illustrates a block diagram of the equalizer 104 of FIG. 1. The 
equalizer 104 adjusts the gain of an input signal in different frequencies 
based on its output. As shown in the exemplary embodiment, the equalizer 
104 includes a programmable gain amplifier 201 with its gain controlled by 
a first automatic gain controller 204 and a pulse shaping filter 200 with 
its gain controlled by a second automatic gain controller 202. The total 
gain of an output voltage Vout versus an input voltage Vin is the product 
of the gain of the programmable gain amplifier 201 and the gain of the 
pulse shaping filter 200. 
When a first automatic gain control loop, including the programmable gain 
amplifier 201 and the first automatic gain controller 204, is active, the 
second automatic gain control loop, including the pulse shaping filter 200 
and the second automatic gain controller 202, should be stable. Otherwise, 
a change in the output voltage Vout contains information about changes of 
the first and second automatic gain control loops. This information, 
however, cannot be used to modify the programmable gain amplifier 201 
through the first automatic gain controller 204 because the first 
automatic gain control loop will be confused by the variations from the 
second automatic gain control loop. Moreover, the change in the second 
automatic gain control loop may change the output voltage Vout in a 
direction that precludes the first and second automatic control loops from 
converging. A similar situation arises when the second automatic control 
loop is active. 
Therefore, to make sure only one loop is operating at a time, an apparatus 
or detector is used to detect when an operating loop becomes stable. When 
the operating loop becomes stable, it is time to start another loop and 
monitor the stability of the new operating loop. Since it is preferable to 
include a detector per automatic gain control loop, the detector should be 
designed to meet the specifications of the particular loop it is serving. 
The telecommunications system 100 including the equalizer is, obviously, 
only one application where the present invention may be employed; other 
environments and applications will become apparent from the foregoing 
description. 
FIG. 3 illustrates a block diagram of an embodiment of an apparatus or 
detector 300 constructed according to the principles of the present 
invention. The detector 300 is parallel-coupled to a first charge pump 302 
and includes parallel-coupled second and third charge pumps 304, 306. The 
charge pumps 302, 304, 306 receive first and second dynamic signals Vup, 
Vdown with an unknown stability level or amount of time to reach 
stability. In a preferred embodiment, the first charge pump 302 is 
employed in an automatic gain control loop as described above. Of course, 
other applications are well within the broad scope of the present 
invention. Additionally, while the illustrated embodiment introduces first 
and second dynamic input signals Vup, Vdown, it is possible that only one 
signal (with either the first or second signal Vup, Vdown being held 
constant) or many signals be employed and still be within the broad scope 
of the present invention. 
The first charge pump 302 has a predetermined capacitance and charge and 
discharge currents and generates an output signal VA as a function of the 
dynamic input signals Vup, Vdown. Typically, the output signal VA varies 
up or down until the dynamic input signals Vup, Vdown reach stability. At 
stability, the output signal VA varies within a relatively small voltage 
range. 
The second and third charge pumps 304, 306 are connected in parallel with 
the first charge pump 302 and are generally used to mirror the main charge 
pump (in this case, the first charge pump 302). As will be described, the 
circuitry of the second and third charge pumps 304, 306 are substantially 
similar to that of the first charge pump 302, except that different-sized 
transistors and different-valued capacitors are employed. For example, in 
one exemplary embodiment, the second charge pump 304 has a smaller 
capacitance and discharge current than the first charge pump 302 and the 
third charge pump 306 has a smaller capacitance, but larger discharge 
current than the first charge pump 302. The second charge pump 304 
generates a high output signal VAH and the third charge pump 306 generates 
a low output signal VAL, both as a function of the dynamic input signals 
Vup, Vdown. 
A control logic: 308 (in this exemplary embodiment, including an inverter 
310 and an AND gate 312) receive the high output signal VAH and low output 
signal VAL and produce an enable signal when the high and low output 
signals VAH, VAL are substantially opposite to each other, logically 
(e.g., logic high and logic low), indicating that the dynamic signal is 
stable. 
FIG. 4 illustrates a schematic diagram of an exemplary charge pump 
structure used in accordance with the present invention. The charge pump 
is coupled to first and second dynamic input signals Vup, Vdown and 
produces an output signal VA. The charge pump includes first, second, 
third and fourth transistors M1-M4 and a capacitor C1. The schematic of 
the charge pump is submitted for exemplary purposes only and other charge 
pumps can be employed in accordance with the present invention. For 
instance, a single dynamic input signal could be used to replace one of 
the first and second dynamic input signals Vup, Vdown, depending on the 
application. For such an application, the third transistor M3 would be 
eliminated as should be understood by those skilled in the art. Also, 
other devices can be employed in lieu of the transistors M1-M4, such as, 
without limitation, bipolar, complementary metal-oxide semiconductor or 
gallium arsenide devices. 
As explained above, the second and third charge pumps 304, 306 of FIG. 3 
are analogous to the charge pump illustrated in FIG. 4, except that the 
second and third charge pumps 304, 306 employ a different sized second 
transistor M2 and different values for the capacitor C1. The size of the 
second transistor M2 employed in the second charge pump 304, is designed, 
in purpose, to be smaller than that in first charge pump 302. The size of 
second transistor M2 employed in third charge pump 306, is designed to be 
larger than that of first charge pump 302. The capacitors C1 used in both 
the second and third charge pumps 304, 306 are designed to be smaller than 
that in first charge pump 302. 
In the charge pump illustrated in FIG. 4, the output signal VA varies due 
to the fact that the charge current in the first transistor M1 is, 
typically, not equal to the discharge current in the second transistor M2. 
When the dynamic input signals Vup, Vdown are received (before the dynamic 
input signals Vup, Vdown are stable), the average charge current in the 
first transistor M1 is different than the average discharge current of the 
second transistor M2. During equilibrium, the average charge current 
through the first transistor M1 is equal to the average discharge current 
in the second transistor M2 and the output signal VA is stable. 
With continuing reference to FIG. 3 and assuming that the first, second and 
third charge pumps 302, 304, 306 include substantially similar components, 
the capacitors C1 of the second and third charge pumps 304, 306 are 
smaller than that of the first charge pump 302. A purpose of employing a 
smaller capacitor C1 is that the high and low output signals VAH, VAL can 
vary corresponding to: 
EQU dVAH/dt=(Ic-Id)/C 
where Ic is the charge current of the first transistor M1, Id is the 
discharge current of the second transistor M2, C represents the value of 
the capacitor C1 and dVAH/dt represents the change of the high output 
signal VAH over time. Obviously, the smaller the value of the capacitor 
C1, the faster the high output signal VAH varies over time. The high and 
low output signals VAH, VAL can, therefore, increase or decrease 
relatively quickly to an upper (e.g., supply voltage Vdd) and lower (e.g., 
the ground) limits, respectively. 
Since the size of the first transistor M1 in the second charge pump 304 is 
the same as the first transistor M1 in the first charge pump 302 (thereby 
providing equivalent charge current) and the size of the second transistor 
M2 in the second charge pump 304 is smaller than the second transistor M2 
in the first charge pump 302 (thereby providing smaller discharge 
current), the average current through the first transistor M1 in the 
second charge pump 304 is larger than that of the second transistor M2 
(during a period when is the first charge pump 302 is stable). Thus, the 
high output signal VAH increases. Since a smaller capacitor C1 is employed 
in the second charge pump 304, the high output signal VAH substantially 
reaches the supply voltage Vdd. Similarly, since the size of the first 
transitor M1 in the third charge pump 306 is the same as the first 
transistor M1 in the first charge pump 302 and the size of the second 
transistor M2 in the third charge pump 306 is larger than the second 
transistor M2 in the first charge pump 302, the average current through 
the first transistor M1 in the third charge pump 306 is smaller than that 
of the second transistor M2. Thus, the low output signal VAL decreases. 
Since a smaller capacitor C1 is employed in the third charge pump 306, the 
low output signal VAL substantially reaches the ground. Finally, the 
control logic 308 generates an active enable signal indicating when the 
first control pump 302 is stable. 
The principles of the present invention can more clearly be described with 
reference to FIGS. 5 and 6. FIG. 5 illustrates a dynamic output signal VA 
of the first charge pump 302 of FIG. 3 as the output signal VA varies from 
a lower to a higher voltage and, then, becomes stable. FIG. 6, on the 
other hand, illustrates a dynamic output signal VA of the first charge 
pump 302 of FIG. 3 as the output signal VA varies from a higher to a lower 
voltage and, then, becomes stable. As illustrated in FIGS. 5 and 6, the 
final value of the output signal VA is a function of the starting point. 
In FIG. 5, as the output signal VA goes high (meaning that the charge 
current of the first transitor M1 of the first charge pump 302 is larger 
than the discharge current of the second transitor M2 of the first charge 
pump 302), the high and low output signals VAH, VAL of the second and 
third charge pumps 304, 306, respectively, go high. After the output 
signal Va is stable, the high output signal VAH is maintained about the 
supply voltage Vdd and the low output signal VAL drops to ground resulting 
in an active enable signal from the control logic 308. In FIG. 6, 
conversely, the high and low output signals VAH, VAL begin at the ground 
and after the output signal VA is stable, the high output signal VAH 
migrates to the supply voltage resulting in an active enable signal from 
the control logic 308. 
It should be noted that the second and third charge pumps 304, 306 can be 
configured to operate in other ways. For instance, it is possible to 
modify the second and third charge pumps 304, 306 to have larger or 
smaller charge currents (instead of discharge currents) than the first 
charge pump 302, as should be apparent to those skilled in the art. 
Additionally, the roles of the second and third charge pumps 304, 306 may 
be reversed such that the high output signal VAH comes from the third 
charge pump 306 and the low output signal VAL comes from the second charge 
pump 304. 
FIG. 7 illustrates a schematic diagram of an exemplary implementation for 
interconnecting the charge pumps to form an embodiment of an apparatus 
constructed according to the principles of the present invention. As an 
example, first, second and third charge pumps 702, 704, 706 are 
interconnected to form an embodiment of a detector 700. In this exemplary 
embodiment, it is advantageous for the charge pumps 702, 704, 706 to share 
a current source I21 and transistors M31, M41, thereby simplifying the 
circuit. As a result, the second and third charge pumps 704, 706 contain 
only first transistors M12, M13 and second transistors M22, M23, current 
sources I12, I13 and capacitors C12, C13, respectively, thereby providing 
a less complex and smaller detector 700 (see FIGS. 3 and 4 for 
comparison). While the control logic 708, in the illustrated embodiment, 
includes an inverter El and NOR gate E2, those skilled in the art 
understand that other embodiments capable of performing analogous 
functions are well within the broad scope of the present invention. 
The sizes for the second transistor M2i and the values of the capacitors 
Cli should be chosen in a way to enable the detector 300 to operate well 
within its maximum time for arriving at a detection measurement. 
Exemplary embodiments of the present invention have been illustrated above 
with reference to specific electronic components. Those skilled in the art 
are aware, however, that components may be substituted (not necessarily 
with components of the same type) to create desired conditions or 
accomplish desired results. For instance, multiple components may be 
substituted for a single component and vice-versa. 
Although the present invention has been described in detail, those skilled 
in the art should understand that they can make various changes, 
substitutions and alterations herein without departing from the spirit and 
scope of the invention in its broadest form.