U.S. patent application number 15/819684 was filed with the patent office on 2019-05-23 for analog to digital conversion using differential dither.
This patent application is currently assigned to Raytheon Company. The applicant listed for this patent is Raytheon Company. Invention is credited to John G. Heston, Ian S. Robinson, James Toplicar.
Application Number | 20190158110 15/819684 |
Document ID | / |
Family ID | 64650548 |
Filed Date | 2019-05-23 |
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United States Patent
Application |
20190158110 |
Kind Code |
A1 |
Robinson; Ian S. ; et
al. |
May 23, 2019 |
ANALOG TO DIGITAL CONVERSION USING DIFFERENTIAL DITHER
Abstract
An analog-to-digital conversion system includes two quantizers
having a least significant bit arranged in a parallel pair. An
input circuit coupled to the quantizers provides an analog input
signal to the quantizers. A dither generator coupled to the
quantizers provides an analog differential dither signal for
perturbing quantization of the analog input signal. A combiner
coupled to the quantizers adds respective outputs of the quantizers
to obtain a linearized digital representation of the analog input
signal.
Inventors: |
Robinson; Ian S.; (Waltham,
MA) ; Toplicar; James; (Waltham, MA) ; Heston;
John G.; (Waltham, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Raytheon Company |
Waltham |
MA |
US |
|
|
Assignee: |
Raytheon Company
Waltham
MA
|
Family ID: |
64650548 |
Appl. No.: |
15/819684 |
Filed: |
November 21, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03M 3/358 20130101;
H03M 1/36 20130101; H03M 3/328 20130101; H03M 1/0641 20130101; H03M
3/458 20130101; H03M 3/42 20130101 |
International
Class: |
H03M 3/00 20060101
H03M003/00 |
Claims
1. An analog-to-digital conversion system, comprising: a first
quantizer and a second quantizer arranged in a parallel pair, the
quantizers having a least significant bit (LSB) size; an input
circuit coupled to the quantizers and operational to provide an
analog input signal to the quantizers; a dither generator coupled
to the quantizers and operational to provide an analog differential
dither signal for perturbing quantization of the analog input
signal; and a combiner coupled to the quantizers and operational to
add respective outputs of the quantizers to obtain a linearized
digital representation of the analog input signal.
2. The system of claim 1 further comprising a voltage source
coupled to the second quantizer and operational to provide a DC
offset voltage to the second quantizer.
3. The system of claim 2 wherein the DC offset voltage is a DC
offset voltage of one half the LSB.
4. The system of claim 1 wherein: the dither generator is summed
with the analog input signal and transmitted to a positive voltage
reference terminal of the first quantizer; and the dither generator
is coupled to the second quantizer to provide the analog
differential dither signal at a negative voltage reference terminal
of the second quantizer.
5. The system of claim 1, wherein the analog differential dither
signal perturbs the analog input signal by an equal and opposite
amount within each quantizer.
6. The system of claim 1, wherein the analog differential dither
signal comprises uniform noise.
7. The system of claim 1, wherein each quantizer comprises a 3-bit
analog-to-digital converter.
8. The system of claim 1, further comprising a comparator coupled
to the input circuit and the combiner to measure a significant bit
of the analog input signal and extend the range of the
quantizers.
9. The system of claim 8, wherein the comparator is set at a most
significant bit (MSB).
10. The system of claim 8, wherein the comparator is set at a least
significant bit (LSB).
11. The system of claim 1, wherein the analog differential dither
signal is passed through a voltage inverter and summed with the
analog input signal to generate a sum, the sum being provided to
the second quantizer at a positive voltage reference terminal of
the second quantizer.
12. The system of claim 1, wherein the analog differential signal
is subtracted from the analog input signal to generate a
difference, the difference being provided to a positive voltage
reference terminal of the second quantizer.
13. A method of analog-to-digital conversion comprising:
configuring an input circuit to receive an analog input signal;
coupling the input circuit to a first quantizer and a second
quantizer, the quantizers being arranged in parallel and having a
least significant bit (LSB) size; coupling a dither generator and a
combiner to the quantizers; transmitting the analog input signal to
the quantizers; operating the dither generator to provide an analog
differential dither signal to the quantizers for perturbing
quantization of the analog input signal; and operating the combiner
to add respective outputs of the quantizers to obtain a linearized
digital representation of the analog input signal.
14. The method of claim 13 wherein, the analog input signal and the
analog differential dither signal are first summed before being
provided to the first quantizer via a positive voltage reference
terminal of the first quantizer.
15. The method of claim 13 further comprising subtracting the
analog differential dither signal from the analog input signal to
get a difference before providing the difference to the second
quantizer via a positive voltage reference terminal of the second
quantizer.
16. The method of claim 13 wherein, the analog differential dither
signal is provided to the second quantizer at a negative voltage
reference terminal of the second quantizer.
17. The method of claim 13 further comprising providing a DC offset
voltage to the second quantizer.
18. The method of claim 17 wherein the DC offset voltage is one
half of the LSB.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to the
analog-to-digital conversion, and more specifically, to
linearization through dithering.
BACKGROUND
[0002] High performance analog-to-digital converters (ADCs) are now
widely used in many applications, including RF receivers (e.g.,
radar) and electronic countermeasures, communication systems, test
instrumentation and others, that handle large dynamic ranges of
signal amplitudes of high data rate signals. Ideal ADCs have
equally spaced levels of voltage references against which the input
signal is compared. Ideal ADCs transfer energy from the frequencies
of the input signal or signals to other frequencies as a result of
the inherent non-linearity of their transfer function. The
transferred energy is often referred to as spurs, as they show up
as spikes in a spectrogram of the device output when the input is a
tone. Most ADCs suffer additional non-linearities. One particular
problem in such high performance ADCs is differential non-linearity
(DNL) errors. DNL error is generally defined as the difference
between an actual transfer function step width of an ADC and the
ideal "stair case" where each quantization level is spaced by
exactly 1 least significant bit (LSB). Errors are often due to
mismatches in the resistance ladder providing threshold reference
voltages and its comparator circuits. Spurs can cause a significant
degradation for some systems, especially where a large signal is
present and the system must reliably detect much smaller signals at
the same time. The spurs or distortion can cause false or missed
detections. The electronics industry is constantly striving to
improve the spurious free dynamic range (SFDR) of ADCs. A receiver
with excellent SFDR is able to detect small signals in the presence
of much larger ones. Non-linearities, for example DNL errors,
effectively decrease a receiver's SFDR rating.
[0003] A well-known technique called dithering is often required to
maximize SFDR. Dithering is the process of adding an uncorrelated
signal, such as pseudo random noise (PRN) or broadband noise, to a
desired analog signal prior to the analog input gate of the ADC. A
common approach to creating dither is to use a noise or thermal
diode whose output is summed with the wanted signal prior to
digitization. Although the injected dither does not eliminate the
errors, it whitens the resulting errors, spreading the spurs across
a wideband of frequencies with much less power at any frequency.
Without dither an input signal is repeatedly quantized at a
particular portion of the dynamic range with some given DNL errors
of the ADC, thereby repetitively providing the same error. The
repetition forces the spurious signals to be at a set of
frequencies and amplitudes for a given input. Adding dither to the
input results in the combined signal being converted at different
points in the dynamic range, across a wider set of reference
voltages interacting with different ones as the dither varies, even
when the wanted inputs signal has a constant waveform. Adding
dither improves the resolution and linearity of the conversion by
effectively smoothing the quantization errors of the ADC's transfer
function. However, while spurs are reduced, a commensurate increase
in the noise floor occurs as adding the dither is equivalent to
adding noise to the wanted signal. Many conventional systems simply
accept degradation of the noise floor to improve SFDR or
sub-optimize SFDR to avoid the additional noise.
[0004] FIG. 1A illustrates a prior art embodiment of a SFDR
maximization, wherein a digital PRN generator 10 generates a random
digital signal that is converted to an analog dither signal by a
high dynamic range digital-to-analog converter (DAC) 12 coupled to
a summer 14, which adds the analog dither signal to an analog input
signal before the dithered analog signal is digitized by an ADC 16.
The "known" random digital signal is subtracted from the converter
response at a digital subtractor 18. This is a more expensive
process for dither creation than a simple diode and will not be
100% random.
[0005] FIG. 1B shows another common technique for spur reduction,
wherein a wideband non-correlated signal is generated using a
thermal noise source 20. The signal from the thermal noise source
20 then passes through a low pass filter 22, which either
attenuates or passes the signal based on its frequency. The signal
is then added to the analog input signal within a summer 14.
Depending upon on how much noise must be injected, signal-to-noise
ratio (SNR) of an ADC 16 may be unduly sacrificed.
SUMMARY
[0006] There is a need therefore for a system of linearization
using low cost dither sources which reduces the size and number of
spurs without raising the noise floor.
[0007] In at least one aspect, the subject technology relates to an
analog to digital conversion system having a first quantizer and a
second quantizer arranged in a parallel pair. The quantizers have a
least significant bit (LSB) size. An input circuit is coupled to
the quantizers and operational to provide an analog input signal to
the quantizers. A dither generator is coupled to the quantizers and
operational to provide an analog differential dither signal for
perturbing quantization of the analog input signal. A combiner is
coupled to the quantizers and operational to add respective outputs
of the quantizers to obtain a linearized digital representation of
the analog input signal.
[0008] In at least some embodiments, a voltage source is coupled to
the second quantizer and operational to provide a voltage to the
second quantizer. The voltage can have a DC offset of 1/2 LSB
relative to the voltage reference of the first quantizer. In at
least some embodiments the dither generator is coupled to the first
quantizer to provide the analog differential dither signal at a
positive voltage reference terminal of the first quantizer and the
dither generator is coupled to the second quantizer to provide the
analog differential dither signal at a negative voltage reference
terminal of the second quantizer. The analog differential dither
signal perturbs the analog input signal by an equal and opposite
amount within each quantizer. The analog differential dither signal
can be a uniform noise.
[0009] In at least some embodiments a comparator is coupled to the
input circuit and the combiner to measure a significant bit of the
analog input signal and extend the range of the quantizers. The
comparator can be set at a most significant bit (MSB) or an LSB. In
some embodiments, the analog differential dither signal can be
passed through a voltage inverter and summed with the analog input
signal to generate a sum, the sum then being provided to the second
quantizer at a positive voltage reference terminal of the second
quantizer. The analog differential signal is subtracted from the
analog input signal to generate a difference, the difference being
provided to a positive voltage reference terminal of the second
quantizer.
[0010] In one aspect, the subject technology relates to a method of
analog-to-digital conversion. The method includes configuring an
input circuit to receive an analog input signal. The input circuit
is coupled to a first quantizer and a second quantizer, the
quantizers being arranged in parallel and having a least
significant bit (LSB) size. A dither generator and a combiner are
coupled to the quantizers. The analog input signal is transmitted
to the quantizers. The dither generator is operated to provide an
analog differential dither signal to the quantizers for perturbing
quantization of the analog input signal. The combiner is operated
to add respective outputs of the quantizers to obtain a linearized
digital representation of the analog input signal.
[0011] In some embodiments, the analog input signal and the digital
differential digital signal are first summed before being provided
to the first quantizer via a positive voltage reference terminal of
the first quantizer. In some embodiments, the differential dither
signal is subtracted from the analog input signal to get a
difference before providing the difference to the second quantizer
via a positive voltage reference terminal of the second quantizer.
The analog differential dither signal can also provided to the
second quantizer at a negative voltage reference terminal of the
second quantizer. In some embodiments a DC offset voltage is
provided to the second quantizer. The DC offset voltage can one
half of the LSB.
BRIEF DESCRIPTION OF THE FIGURES
[0012] The subject technology is further described in the detailed
description which follows, in reference to the noted plurality of
drawings by way of non-limiting examples of embodiments of the
present disclosure, in which like reference numerals represent
similar parts throughout the several views of the drawings.
[0013] FIGS. 1A and 1B are schematic circuit diagrams of example
prior art embodiments of dithered analog to digital conversion
systems.
[0014] FIG. 2 is a block circuit diagram of an exemplary analog to
digital conversion system in accordance with subject
technology.
[0015] FIG. 3 shows exemplary tables depicting the calculations
carried out within quantizers and a combiner in accordance with the
subject technology.
[0016] FIG. 4 is a block circuit diagram of an exemplary analog to
digital conversion system in accordance with subject
technology.
DETAILED DESCRIPTION
[0017] The details described and illustrated herein are by way of
example and for purposes of illustrative description of the
exemplary embodiments only and are presented in the case of
providing what is believed to be the most useful and readily
understood description of the principles and conceptual aspects of
the disclosure. In this regard, no attempt is made to show
structural details of the subject matter in more detail than is
necessary for the fundamental understanding of the disclosure, the
description taken with the drawings making apparent to those
skilled in that how the several forms of the present disclosure may
be embodied in practice with additional components or steps and/or
without all of the components or steps that are described. Further,
like reference numbers and designations in the various drawings
indicate like elements.
[0018] Referring now to FIG. 2, a block diagram of an analog to
digital conversion system 200 using differential dither in
accordance with the subject technology is shown. System 200 adds a
dither signal to an analog signal being digitized in order to
linearize (i.e., reduce the size and number of spurs of) the
quantization in a manner that removes noise from the final
digitized output more effectively than other known dither systems.
This implementation is less costly to achieve randomness and
decorrelation with other dither sources than conventional dithered
quantization. For example, an analog dither source, such as a
thermal diode or noise diode, can be used with the system 200 to
reduce cost. The addition of dither to the intended signal prior to
digitization, in essence, spreads the effect of the signal's
interaction with the quantization measurement system, reducing the
measurement system's imperfections.
[0019] Still referring to FIG. 2, the system 200 receives an analog
input signal from an outside source 202 along an input circuit 204.
The analog input signal can come from any analog signal source,
such as an antenna receiving an RF signal. The input circuit 204
includes optional front end circuitry 206 which can include any
necessary components for processing the analog input signal as are
typically used in the art. For example, the front end circuitry 204
can include an amplifier circuit, a frequency conversion circuit
having a local oscillator, an adder circuit, a gain control
circuit, or the like. The system 200 also includes a first
quantizer 208a and a second quantizer 208b (generally 208) which
form a parallel pair, each quantizer 208 having a shared least
significant bit (LSB) size. The analog input signal is passed from
the input circuit 204 into the quantizers 208 for conversion from
analog to digital (i.e. digitization). Each of the quantizers 208
are N-1 bit analog-to-digital converters which ultimately combine
to a single N-bit analog to digital converter ("N" being used
herein as a variable representing a numerical value), as discussed
further herein.
[0020] A dither generator 210 creates an analog noise signal which
serves as an analog differential dither signal within the system
200. In one embodiment, the dither generator 210 generates a
random, non-deterministic, analog noise signal such as through use
of a low-cost thermal noise diode (e.g. for analog Gaussian
dither). The quantum mechanical nature of electron-hole pairing
process within such devices produces a truly random noise signal
with very high bandwidth. In another, embodiment, the dither
generator 210 comprises a uniform analog noise generator to create
the analog noise signal. The analog noise signal is preferably
uncorrelated in time, uncorrelated with the desired analog input
signal, and has "white noise" properties. The probability
distribution of amplitudes should be as close to either Gaussian or
uniform as practical. The level of the analog noise signal should
be commensurate with the level of nonlinearity expected from the
quantizers 208. Since the noise power levels created by some dither
generators may be quite small, optional processing circuitry 212
can be coupled to the dither generator 210. The processing
circuitry 212 can include circuitry to control the gain of the
analog noise signal, low cost operational amplifiers, or the like,
as needed.
[0021] The dither generator 210 is coupled to the quantizers 208
and the analog noise signal is applied to the first and second
quantizers 208 as an analog differential dither signal for
perturbing the quantization of the analog input signal to remove
periodicity of quantization error and improve the dynamic range of
the quantizers 208. The dither signal is "differential" because
complimentary signals of opposite power are applied to each
quantizer 208. For example, the dither generator 210 might send a
signal having a power X to the quantizers 208. An analog
differential dither signal of +X would then be provided to the
first quantizer 208a while an analog differential dither signal of
-X would be applied to the second quantizer 208b. This can be
accomplished in several ways. In one embodiment, this is done by
summing the analog differential dither signal with the input
signals in summer 213 and providing or transmitting the sum to a
positive voltage reference terminal 214 of the first quantizer
208a. In this embodiment, the input signal alone is then provided
to the positive reference terminal 215 of the second quantizer 208b
and the dither signal is provided to a negative voltage reference
terminal 216 of the second quantizer 208b (the dither generator 210
being coupled to the respective reference terminals 214, 216),
where the quantizer 208b provides a digital representation of the
difference of the signals at its respective inputs 215, 216. In the
embodiment shown, the second quantizer 208b is also coupled to an
optional voltage source 218 which provides a DC offset to the
negative voltage reference terminal 216 of the second quantizer
208b. In some cases, the offset can be dependent on the LSB of the
quantizers 208. To that end, the voltage source 218 sometimes
provides a 1/2 LSB offset between the first quantizer 208a and the
second quantizer 208b.
[0022] Within each quantizer 208, the analog input signal is
combined with the respective (i.e. positive or negative) analog
differential dither signal. In other words, each quantizer 208
combines an opposite differential dither signal with the analog
input signal, perturbing the analog input signal in equal and
opposite amounts to form dithered analog signals. In the second
quantizer 208b, the analog input signal and analog differential
dither signal can also be combined with the optional offset voltage
from the voltage source 218 (e.g. a 1/2 LSB offset voltage). The
offset voltage can also be provided to the quantizer 208b as a sum
with the input signal or as a sum with the voltage references of
the quantizer 208b. In other embodiments, the dither signal can be
passed through a voltage inverter and then summed with the input
signal to generate a sum, the sum being provided to the positive
terminal of the second quantizer 208b. Alternatively, the dither
signal can be subtracted from the input analog signal to generate a
difference, the difference then being provided to the positive
terminal of a second quantizer. When N-1 bit quantizers are used to
replace an N-bit quantizer, the quantization noise will be
approximately the same as if an N-bit quantizer had been used.
[0023] The quantizers 208 include a plurality of comparators (not
distinctly shown) arranged in a parallel configuration for
comparing a corresponding plurality of threshold reference voltages
against the dithered analog signal. Each comparator can be coupled
to the input circuit 204 to receive the analog input signal, the
dither generator 210 to receive the analog differential dither
signal, and a combiner 220 at the comparator outputs. An N-bit
quantizer will include 2.sup.N-1 comparators. Therefore a 3-bit
quantizer has seven comparators. The quantizers 208 output digital
equivalent signals 222a, 222b (generally 222) representative of the
comparison of the dithered analog signals and the threshold
reference voltages carried out by the comparators. The output
digital equivalent signals 222 of each quantizer 208 are then added
within the combiner 220 and unwanted noise (e.g. quantization noise
and noise from the optional DC offset voltage) is removed to obtain
a linearized digital representation of the analog input signal
which is output as digital signal 224. Notably, since a
differential dither was applied to each quantizer 208, the
differential dither cancels out when the digital equivalent signals
222 are summed in the combiner 220 and no further steps need to be
taken to adjust for the inclusion of the analog differential dither
signals. This results in a digital signal 224 with a higher
linearity than if a single quantizer were used and does not suffer
from an increased noise floor as a result of dithering.
[0024] Referring now to FIG. 3, exemplary tables are shown
generally at 300 depicting a process of calculating and offsetting
significant bits within two quantizers and then summing the
significant bits. The underlying quantizers are 3-bit quantizers
and can functional similarly to the quantizers 208 shown in FIG. 2.
The first table 308a shows the incoming signal and significant bit
calculations of a first quantizer 208a. The "Example" column 330a
represents samples of the analog input signal provided to the first
quantizer 208a. Since the first quantizer 208a is 3-bit, the
samples of the analog input signal are digitized within the first
quantizer 208a by generating most significant bit (MSB), next most
significant bit (NMSB), and LSB values of either "0" or "1" for
each sample. The digitization within the second quantizer 208b is
shown in the second table 308b. The second quantizer 208b functions
similar to the first quantizer 208a, except a 1/2 LSB offset 318 is
added to the sample as shown in column 330b. Each quantizer 208
rounds down to the next whole number of the sample value. Therefore
in the rows highlighted by box 332, the offset 318 bumps the sample
value within the second quantizer 208b above the next highest whole
number, increasing the calculated significant bit value shown
within the second table 308b as compared to the first table 308a.
Further, in the top row of the second table 308b, the offset 318
has pushed the sample value above 8, which is above the range of
the 3-bit quantizer, as can be seen in block 334. Therefore another
comparator would need to be provided to gain full dynamic range for
the 3 bit quantizers (e.g. as seen in FIG. 4).
[0025] Still referring to FIG. 3, after the analog input signals
are digitized within each quantizer 208, they are then sent to a
combiner to be summed (e.g. the combiner 220 of FIG. 2). The
combination of the significant bit calculations of the quantizers
208 shown in tables 308a, 308b is shown in summing table 320. The
data shown in table 320 can then be relied upon to obtain a
linearized digital representation of the analog input signal. It is
notable that since the sample 332 was outside the range of the
second quantizer 208b, the resulting error in significant bits was
carried over in the summing table 320 and can be seen in the final
block 336 of the first row of the summing table 320.
[0026] Referring now to FIG. 4 a block diagram of an analog to
digital conversion system 200 using differential dither in
accordance with the subject technology is shown generally at 400.
The system 400 is similar to the system 200 of FIG. 2, except that
a separate comparator 440 is included. As noted above, the two
quantizers 208 are typically N-1 bit quantizers meant to replace a
single N-bit quantizer. An N-bit quantizer has 2.sup.N-1
comparators, so a 4-bit quantizer would have fifteen comparators.
Two N-1 bit quantizers replacing a single N-bit quantizer would
therefore have one less total comparator (e.g. fourteen
comparators). Therefore to gain full dynamic range of an N-bit
quantizer, an additional comparator 440 is included. The comparator
440 can be set at an MSB or an LSB of the quantizers 208 and can be
calibrated to account for samples outside the range of the
quantizers 208 (e.g. sample 332 of FIG. 3). While two 3-bit
quantizers 208 are given as an example, two N-1 bit quantizers
replacing a single N-bit quantizer will always have one less
comparator, so including an additional comparator 440 is beneficial
when two quantizers of any number of bits are used to replace a
single quantizer.
[0027] Various embodiments of the above-described systems and
methods may be implemented in digital electronic circuitry, in
computer hardware, firmware, and/or software. The implementation
can be as a computer program product (i.e., a computer program
tangibly embodied in an information carrier). The implementation
can, for example, be in a machine-readable storage device and/or in
a propagated signal, for execution by, or to control the operation
of, data processing apparatus. The implementation can, for example,
be a programmable processor, a computer, and/or multiple computers.
Processors suitable for the execution of a computer program
include, by way of example, both general and special purpose
microprocessors, and any one or more processors of any kind of
digital computer. Generally, a processor receives instructions and
data from a read-only memory or a random access memory or both. The
essential elements of a computer are a processor for executing
instructions and one or more memory devices for storing
instructions and data. Generally, a computer can be operatively
coupled to receive data from and/or transfer data to one or more
mass storage devices for storing data, e.g., magnetic,
magneto-optical disks, or optical disks.
[0028] A computer program can be written in any form of programming
language, including compiled and/or interpreted languages, and the
computer program can be deployed in any form, including as a
stand-alone program or as a subroutine, element, and/or other unit
suitable for use in a computing environment. A computer program can
be deployed to be executed on one computer or on multiple computers
at one site.
[0029] The circuitry can, for example, be a FPGA (field
programmable gate array) and/or an ASIC (application-specific
integrated circuit). Modules, subroutines, and software agents can
refer to portions of the computer program, the processor, the
special circuitry, software, and/or hardware, e.g., a controller
such as a microcontroller, which implements that functionality.
[0030] It will be appreciated by those of ordinary skill in the
pertinent art that the functions of several elements may, in
alternative embodiments, be carried out by fewer elements or a
single element. Similarly, in some embodiments, any functional
element may perform fewer, or different, operations than those
described with respect to the illustrated embodiment. Also,
functional elements (e.g., electronics, circuitry, and the like)
shown as distinct for purposes of illustration may be incorporated
within other functional elements in a particular
implementation.
[0031] While the subject technology has been described with respect
to preferred embodiments, those skilled in the art will readily
appreciate that various changes and/or modifications can be made to
the subject technology without departing from the spirit or scope
of the subject technology. For example, each claim may depend from
any or all claims in a multiple dependent manner even though such
has not been originally claimed. It is noted that the foregoing
examples have been provided merely for the purpose of explanation
and are in no way to be construed as limiting of the present
disclosure.
* * * * *