U.S. patent application number 12/417743 was filed with the patent office on 2009-07-30 for high speed arbitrary waveform generator.
This patent application is currently assigned to LeCroy Corporation. Invention is credited to Peter James Pupalaikis.
Application Number | 20090189651 12/417743 |
Document ID | / |
Family ID | 40252659 |
Filed Date | 2009-07-30 |
United States Patent
Application |
20090189651 |
Kind Code |
A1 |
Pupalaikis; Peter James |
July 30, 2009 |
High Speed Arbitrary Waveform Generator
Abstract
A high-speed arbitrary waveform generator (AWG) that utilizes
multiple digital-to-analog converters (D/A converters) and
overcomes bandwidth limitations of individual D/A converters to
produce high-speed waveforms.
Inventors: |
Pupalaikis; Peter James;
(Ramsey, NJ) |
Correspondence
Address: |
LECROY CORPORATION
700 CHESTNUT RIDGE ROAD
CHESTNUT RIDGE
NY
10977
US
|
Assignee: |
LeCroy Corporation
Chestnut Ridge
NY
|
Family ID: |
40252659 |
Appl. No.: |
12/417743 |
Filed: |
April 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11827082 |
Jul 10, 2007 |
7535394 |
|
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12417743 |
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Current U.S.
Class: |
327/105 |
Current CPC
Class: |
G06G 7/26 20130101 |
Class at
Publication: |
327/105 |
International
Class: |
H03B 21/00 20060101
H03B021/00 |
Claims
1. An apparatus for generating a signal with contiguous content,
comprising; a plurality of digital-to-analog converters, each
having a bandwidth and each producing a D/A output signal; at least
one upconverter, the at least one upconverter receiving a D/A
output signal and producing an upconverted signal; and a combining
element that receives and combines at least one D/A output signal
and at least one upconverted signal and produces a final output
signal; whereby the final output signal includes spectral content
occupying a substantially contiguous frequency band whose width is
greater than the bandwidth of any one of the plurality of
digital-to-analog converters.
2. The apparatus of claim 1, wherein the final signal output is a
substantially accurate analog representation of a desired digital
signal.
3. The apparatus of claim 2, wherein the final output signal spans
a frequency range substantially similar to that of the desired
digital signal.
4. The apparatus of claim 1, further comprising one or more
band-pass filters for filtering a desired digital signal into a
plurality of sub-bands, each sub band corresponding to one of the
plurality of digital-to-analog converters.
5. The apparatus of claim 4, wherein the desired digital signal is
supplied by a user.
6. The apparatus of claim 4, wherein the desired digital signal is
selected from one or more predetermined signals.
7. The apparatus of claim 1, further comprising a local oscillator,
the upconverter utilizing at least a signal generated by the local
oscillator for producing the upconverted signal.
8. The apparatus of claim 7, wherein the local oscillator is
synchronized to a sample clock of the plurality of
digital-to-analog converters.
9. The apparatus of claim 1, further comprising a digital signal
processing element for processing a signal prior to conversion by
the plurality of digital-to-analog converters.
10-20. (canceled)
21. An apparatus for generating signals comprising: a plurality of
digital-to-analog converters, each having a bandwidth and each
producing a D/A output signal; at least two upconverters, each
upconverter receiving a D/A output signal from one of the plurality
of digital-to-analog converters and producing an upconverted signal
at a different translated frequencies; and a combining element that
receives and combines the at least two upconverted signals at
different translated frequencies and produces a final output
signal; whereby the final output signal comprises a synchronized
signal, and includes spectral content occupying a substantially
contiguous frequency band whose width is greater than the bandwidth
of any one of the plurality of digital-to-analog converters.
22. The apparatus of claim 21, wherein the final output signal is a
substantially accurate analog representation of a portion of a
desired digital signal.
23. The apparatus of claim 22, wherein the final output signal
spans a frequency range substantially similar to that of a
designated portion of the desired digital signal.
24. The apparatus of claim 21, further comprising one or more
band-pass filters for filtering a desired digital signal into a
plurality of sub-bands, each sub band corresponding to one of the
at least one upconverters.
25. The apparatus of claim 24, wherein the desired digital signal
is supplied by a user.
26. The apparatus of claim 24, wherein the desired digital signal
is selected from one or more predetermined signals.
27. The apparatus of claim 21, further comprising a local
oscillator, the at least two upconverters utilizing at least a
signal generated by the local oscillator for producing the
upconverted signals.
28. The apparatus of claim 27, wherein the local oscillator is
synchronized to a sample clock of the plurality of
digital-to-analog converters.
29. The apparatus of claim 21, further comprising a digital signal
processing element for processing a signal prior to conversion by
the plurality of digital-to-analog converters.
30. The apparatus of claim 29, wherein the digital signal
processing element further comprises: a plurality of band-pass
filters for extracting a plurality of frequency portions of a
waveform; and a mixing element for mixing a local oscillator with
one or more of the extracted portions of the waveform to generate a
lower frequency version of each of the one or more of the extracted
portions of the waveform.
31. The apparatus of claim 30, wherein the digital signal
processing element further comprises a plurality of low-pass
filters for separating the lower frequency versions of the
extracted portions of the waveform from the one or more of the
extracted portions of the waveform.
32. The apparatus of claim 31 wherein the digital signal processing
element further comprises a plurality of summing elements for
summing each of the lower frequency versions of the extracted
portions of the waveform with a reference tone phase synchronized
with the local oscillator to fix the phase relationship between the
one or more extracted portions of the waveform.
33. The apparatus of claim 21, wherein the frequency content of the
upconverted signals substantially overlap.
34. The apparatus of claim 21, wherein the resultant sum of the
frequency content of the upconverted signals results in a waveform
occupying a substantially contiguous bandwidth.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to an arbitrary
waveform generator (AWG) and in particular to a high-speed AWG.
BACKGROUND OF THE INVENTION
[0002] In the generation of high speed analog signals, it is often
useful to generate these signals from digital signals. This is
because digital signals are in a form most easily manipulated by
digital computers and digital signal processors. In this situation,
a device called a digital-to-analog converter (DAC or D/A
converter) is utilized to convert digital waveforms to analog.
These devices have basic limitations on speed and signal-fidelity.
The speed limitations are expressed by two parameters: bandwidth
and sample-rate. Sample-rate limitations are traditionally overcome
through time-interleaving. There have been no easy ways to overcome
bandwidth limitations. What is needed are waveform generators with
high bandwidth and high sample-rate.
OBJECTS OF THE INVENTION
[0003] It is an object of this invention to overcome the bandwidth
limitations encountered in the design of high-speed waveform
generators.
[0004] Still other objects and advantages of the invention will in
part be obvious and will in part be apparent from the specification
and drawings.
SUMMARY OF THE INVENTION
[0005] In order to overcome the bandwidth limitations of high-speed
waveform generators, a novel method is utilized whereby a digital
waveform is preferably processed and separated for delivery to
multiple D/A converters. Each D/A converter is inherently limited
in bandwidth. Each waveform delivered to a particular D/A converter
contains a portion of the total spectral content of the original
waveform, but processed in such a manner such that it meets the D/A
converters bandwidth criteria. These multiple D/A converters
generate signals whereby each signal is processed in an analog
fashion and combined such that the combined signal occupies the
desired bandwidth, and the spectral content of the output signal
substantially matches the spectral content of the original digital
waveform despite the fact that it was generated using D/A
converters each having insufficient bandwidth to independently
generate the waveform.
[0006] The invention accordingly comprises the several steps and
the relation of one or more of such steps with respect to each of
the others, and the apparatus embodying features of construction,
combinations of elements and arrangement of parts that are adapted
to affect such steps, all is exemplified in the following detailed
disclosure, and the scope of the invention will be indicated in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] For a more complete understanding of the invention,
reference is made to the following description and accompanying
drawings, in which:
[0008] FIG. 1 is a drawing of a high-speed waveform generator
according to the present method;
[0009] FIG. 2 is a drawing of a basic upconverter;
[0010] FIG. 3 is a block diagram showing digital signal processing
that produces the digital waveforms;
[0011] FIG. 4 is a drawing of local oscillator generation using
shared references;
[0012] FIG. 5 is a drawing of sample clock generation using a
divided down local oscillator;
[0013] FIG. 6 is a drawing of local oscillator generation using a
divided down sample clock;
[0014] FIG. 7 is a drawing of local oscillator generation using
reference tone injection;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] FIG. 1 shows an arbitrary waveform generator constructed in
accordance with to the present invention. The generation of
high-speed signals starts with designation of a desired waveform
[1] where it is understood that the desired waveform [1] is in
digital form or has a possible digital representation. It is also
understood that the desired waveform [1] is shown as spectral
content (i.e. in the frequency-domain) as opposed to an equivalent
time-domain representation as will all waveforms described. This is
only because the present method is best understood by examination
from a frequency-domain perspective. The invention may also be
applied to a time-domain defined signal.
[0016] It should be pointed out that traditionally, this digital
waveform would have been presented to a D/A converter such as D/A
converter [5] either directly or through a high-speed memory
element such as [3]. But the response of the D/A converter [7]
shows that it has insufficient bandwidth to generate the desired
signal. Furthermore, by utilizing multiple D/A converters and
memories in a traditional manner, while capable of increasing
sample-rate through the well known technique of time interleaving,
cannot overcome the bandwidth limitations. To clarify, when one
says that a given D/A converter has a given bandwidth, it defines a
characteristic of the D/A converter; the characteristic being the
distance, in frequency, between the highest frequency and the
lowest frequency that a D/A converter can produce. For example, if
a D/A converter can produce waveforms with spectral content from DC
to 2 GHz, one says that the D/A converter has a bandwidth of 2
GHz.
[0017] Therefore, in accordance with the present method, the
desired waveform [1] is processed utilizing digital signal
processing (DSP) indicated by the DSP processing block [2] in a
manner that will be described subsequently in detail to produce two
digital waveforms. One waveform is presented either directly or
through a memory element [3] to a D/A converter [5] designated as
low-frequency, or LF. Another waveform is presented either directly
or through a memory element [4] to a D/A converter [6] designated
as high-frequency, or HF, and is a downconverted portion of the
desired waveform [1].
[0018] The LF D/A converter [5] is either physically limited to, or
has been restricted by the DSP processing [2] to have a given
transfer characteristic [7]. This means that the output of the D/A
converter [5] has frequency content [8] corresponding to only a
portion of [1]
[0019] Furthermore, HF D/A converter [6] is either physically
limited to, or has been restricted by the DSP processing [2] to
have a given transfer characteristic [9]. This means that the
output of the D/A converter [5] has frequency content [10]
corresponding to only the downconverted portion of [1].
[0020] The output of HF D/A converter [6] is presented to an
upconverter [11]. Upconverter [11] utilizes a local oscillator (LO)
[12], whose content is indicated by [13] and whose generation will
be explained subsequently, to produce images [14], at least one of
which corresponds ideally to a portion of [1] in the correct
frequency locations.
[0021] All processed D/A converter outputs are presented to a
diplexer [15] which has a low frequency side [16] with ideally a
low-pass response characteristic [18] and a high frequency side
[17] with ideally a band-pass characteristic [20]. The diplexer
[15] serves to combine the signals [19] and [21] shown at [22]
thereby producing an output waveform [23] that is an analog
waveform that substantially represents the digital desired waveform
[1].
[0022] FIG. 2 represents detail of a typical upconverter element,
such as that shown at [11] in FIG. 1. Intermediate frequency (IF)
or baseband frequency signals enter the IF input [32]. The signals
may be filtered using a low-pass filter [28]. Usually, the signals
are attenuated with an attenuator [29] to reach a power level
sufficient for low distortion mixing by the mixer [24]. The
attenuator [29] also pads the input to provide a better impedance
match between the IF input [32] and the mixer IF port [25]. If the
power level at the IF input [32] is variable, then there may also
be some form of variable attenuation or variable gain to supply the
mixer IF input [25] with the correct power level. Depending on
impedance matching requirements attenuators may also be placed
before the low-pass filter [28]. Many different arrangements are
possible in the path between the IF input [32] and the mixer IF
port [25] and the tradeoffs of various configurations are well
known to those skilled in the art of microwave and RF design. An LO
is applied to the LO input port [31]. The LO is a periodic
waveform, usually a sinusoid at a single frequency. It can also be
a train of pulses in a sampler arrangement. A pad [30] is also
usually placed between the LO input port [31] and the mixer LO port
[26] to provide a better impedance match. Usually, to avoid
distortion, the power level at the LO input port [31] is high. If
the power level at the LO input port [31] is insufficient, gain or
attenuation may be provided. Also, if the spectral content of the
signal provided at the LO input port [31] is inadequate, filtering
may also be provided. Many different arrangements are possible in
the path between the LO input [31] and the mixer LO port [26] and
the tradeoffs of various configurations are well known to those
skilled in the art of microwave and RF design.
[0023] The mixing action of mixer [24] causes two images or
sidebands of the signal present at the mixer IF port [25] to appear
at the mixer RF port [27]. These images are at sum and difference
frequencies between the spectral content of the signals at the LO
port [26] and the IF port [25]. In a preferred embodiment, a
band-pass filter [35] may be provided to retain only a desired
portion of the spectral content of signal at the mixer RF port
[27]. There may be a large amount of leakage between the LO port
[26] and the RF port [27], which may require filter [35] to at
least filter out the spectral content of the LO present in the
converted signal. In addition to some filtering, a pad [33] may be
supplied to improve the impedance match at the RF port [27]. In a
preferred embodiment, a variable gain amplifier (VGA) [34] may be
provided so that the output power of the signal at the RF output
port [36] can be varied. Some other options include variable
attenuation and fixed gain as well as additional filtering and
padding to reduce spurious and reflections. The features and
tradeoffs involved in the various options are well known to those
skilled in the art of microwave and RF design.
[0024] The DSP processing element [2] in FIG. 1 is shown in a
preferred embodiment and in detail in FIG. 3. Processing begins
with a desired output waveform [38] expressed or capable of being
expressed in digital form. Desired output waveform [38] may either
exist in memory, or as a formula or function, or can be supplied by
up-stream digital signal processing. It is presented to the DSP
input [37]. The waveform preferably enters a pre-compensator [39].
Pre-compensator [39] processes the waveform to account for the
effects of all downstream processing of the waveform, both digital
and analog, and is contrived to alter the waveform in advance, so
that after all processing, the output waveform is a substantially
correct analog representation of the digital input waveform [37].
The pre-compensation comprises, but is not limited to, magnitude
compensation, phase compensation, and non-linearity compensation.
In a preferred embodiment, the pre-compensation performs
corrections on the digital waveform best performed on the aggregate
desired waveform, prior to separation. After pre-compensation, the
processing follows two paths of processing. One path, designated
the LF path, involves generation of the digital signal to be
provided to the LF D/A converter [60]. The other path, designated
the HF path, involves generation of the digital signal to be
provided to the HF D/A converter [61].
[0025] On the LF path, the waveform undergoes low-pass filtering
using the low-pass filter (LPF) [40] having ideally a low-pass
response characteristic [41]. The LPF extracts the low frequency
portion [42] of the waveform to restrict the spectral content to
that which can be physically transmitted by the LF D/A converter
[60]. Since LF D/A converter [60] has physical limitations, LPF
[40] can sometimes be eliminated, but its presence helps in
understanding the overall concept. LPF [40] produces a waveform of
low baseband spectral content. This waveform then enters preferably
an LF compensator [56]. LF compensator [56] is contrived to perform
pre-compensation to account for the effects of all downstream
processing of the waveform, both digital and analog. It is utilized
to compensate for effects that are best compensated for the LF
path. These may include, but are not limited to, integral
non-linearity (INL) and differential non-linearity (DNL) of the LF
D/A converter [60]. Furthermore, even though LF D/A converter [60]
is shown as a single converter, it may in fact consist of multiple,
interleaved converters, and the LF compensator [56] may also
compensate for interleave errors. Finally, phase distortion at band
edges may cause destructive signal summing at the diplexer [15]
thereby requiring some phase compensation to correct for this.
[0026] The LF path is shown with a memory element [57]. LF memory
element [57] is utilized as a circular buffer for the waveform, or
to provide pipeline delay. In an AWG, it is customary to play
waveforms over and over from memory, so the processing of the
waveform in the LF path may be performed once after the desired
input waveform [37] is known.
[0027] Regarding the HF path, the waveform enters a band-pass
filter (BPF) [43] having ideally a band-pass response
characteristic [44]. BPF [43] serves to extract a high frequency
portion [45] of the waveform. Sometimes, this filtering can be
avoided as long as images produced downstream do not overlap or
alias. Sometimes, also, a rate change is performed either through
upsampling (also used to avoid image overlap) or downsampling (to
reduce downstream processing requirements). Methods for upsampling
and downsampling and their effects are well known to those skilled
in the art of digital signal processing. The extracted high
frequency portion of the waveform is then mixed (multiplied) with a
LO waveform generated by a tone generator [49] at the mixer [51].
The LO generated by the tone generator [49] is generated in a
manner whereby it is phase locked in LO is synchronous with the
sample clock used to clock the HF D/A converter [61]. Usually, the
LO is a single tone or sinusoid, but it can also be a train of
impulses, as with a sampler. The intent is that the tone generated
anticipates how the analog LO signal [12] is generated and
synchronized with the sample clock. Methods for synchronizing the
LO signal and the sample clock are described subsequently.
[0028] The mixing action of mixer [51] produces images at sum and
difference frequencies of the LO waveform spectral content [52] and
the mixer input frequency content [45] thereby producing images
[53]. Preferrably, the lower frequency image is extracted utilizing
LPF [46] with a response characteristic [47] that causes the output
of LPF [46] to contain spectral content [48] that appears within
the physical bandwidth limitations of the HF D/A converter [61].
This waveform then enters preferably an HF compensator [58]. HF
compensator [58] is contrived to perform pre-compensation to
account for the effects of all downstream processing of the
waveform, both digital and analog, and is utilized to compensate
for effects that are best compensated for the HF path. These may
include, but are not limited to, integral non-linearity (INL) and
differential non-linearity (DNL) of the HF D/A converter [61].
Furthermore, even though HF D/A converter [61] is shown as a single
converter, it may in fact consist of multiple, interleaved
converters, and the HF compensator [58] may also compensate for
interleave errors. Finally, phase distortion at band edges may
cause destructive signal summing at the diplexer [15] thereby
requiring some phase compensation to correct for this.
[0029] The HF path is shown with a memory element [59]. HF memory
element [59] is utilized as a circular buffer for the waveform, or
to provide pipeline delay. In an AWG, it is customary to play
waveforms over and over from memory, so the processing of the
waveform in the HF path may be performed once after the desired
input waveform [37] is known.
[0030] LO Generator [49] is shown producing an LO [50] and also
optionally a reference [54]. This optional reference [54] is
preferrably a divided down, phase-locked version of the LO [50] and
is inserted into the HF waveform at a summing node [55]. The
purpose is that for certain methods for LO synchronization, that
will be described subsequently require a reference tone inserted in
the waveform. Note that this reference can just as well be inserted
in the LF path with the typical requirement being that the signal
not interfere with the spectral content of the actual waveform.
[0031] All of the DSP processing shown in FIG. 3 has been described
from a time-domain processing standpoint. It is understood that all
DSP processing can be performed entirely in the frequency-domain or
in a mixture of domains to have the intended described effect.
[0032] At this point it is important to describe how the local
oscillator is synchronized with the D/A converter sample clocks.
FIG. 4 shows a preferred method. In FIG. 4, a reference [62] is
supplied to two frequency multipliers; one multiplier [63]
generates the LO signal [64] and another multiplier [65] generates
the sample clock [66] supplied to the D/A converter [67]. The
multiplication factors are integer and therefore the LO signal [64]
and the sample clock [66] are synchronized to each other. This
method suffers from the fact that the phase-locked loops usually
present in the multipliers must not drift in absolute phase
relationship.
[0033] Other methods may include to derive the LO from the sample
clock or vice-versa either by multiplying or dividing one to
produce the other. These methods are shown in FIG. 5 and FIG. 6. In
FIG. 5, the LO is delivered to the upconverter [70] and a frequency
divider [68] divides the frequency delivered to the D/A converter
[69]. A frequency multiplier can also be used in place of the
frequency divider [68] when it is desirable to have the D/A sample
clock higher than the LO frequency. In FIG. 6, the sample clock is
delivered to the D/A converter [72] and a frequency divider [71]
divides the frequency delivered to the upconverter [73]. A
frequency multiplier can also be used in place of the frequency
divider [71] when it is desirable to have the LO frequency higher
than the D/A sample clock frequency. When using dividers a method
is needed to ensure that the correct phase is utilized, either
though the use of an overriding set or clear on flip-flops used to
divide a waveform in frequency, or through the use of hopping
circuitry.
[0034] FIG. 7 shows another method that does not suffer from the
problems inherent in the techniques previously mentioned. In FIG.
7, The spectral content [74] of the signal driven from the D/A
converter [75] shows a small reference tone in addition to the HF
signal portion. This was the intent of the optional insertion of
the reference tone [54] at the reference summing node [55] in the
dsp processing in FIG. 3. This reference tone is picked off from
the output of the D/A converter [75] through the use of, for
example, a coupler [76]. The picked off signal is preferably
filtered by BPF [77] to extract the desired reference tone and
multiplied by multiplier [78] to generate the desired LO. The LO is
preferably amplified by amplifier [79] and filtered by BPF [80] to
generate an LO with sufficient power and spectral purity and
applied to the LO port of the upconverter [81]. The signal applied
to the IF port of the upconverter [81] is filtered with a notch
filter [82] if it is determined that the existence of the reference
tone in the upconverted signal would cause a problem.
[0035] It should be noted that the HF D/A converter [6] generally
is not required to be DC coupled. AC coupling relaxes some
constraints on the design and usage of the HF D/A converter
[6].
[0036] While the description of the preferred embodiment involves
two spectral bands, one designated as LF and the other HF, with the
LF band undergoing no frequency translation, it should be
appreciated that this is not a requirement. It is possible for all
bands to undergo frequency translation whereby the result is not
only a higher bandwidth output waveform, but also a wider bandwidth
output waveform where the lower frequency does not extend to
DC.
[0037] All of the D/A converters utilized do not need to sample at
the same rate. Rate requirements are such that the D/A converters
and local oscillators can be synchronized and that the rates
utilized satisfy Nyquist's criteria.
[0038] While the method described utilizes two spectral bands, the
limitation to two bands in the description is artificial and only
intended to simplify the description. It should be apparent that
the method extends to any number of spectral bands and that it is
obvious how the methods disclosed can accomplish bandwidth
enhancement using more than two D/A converters.
[0039] It will thus be seen that the objects set forth above, among
those made apparent from the preceding description, are efficiently
attained and, because certain changes may be made in carrying out
the above method and in the construction(s) set forth without
departing from the spirit and scope of the invention, it is
intended that all matter contained in the above description and
shown in the accompanying drawings shall be interpreted as
illustrative and not in a limiting sense.
[0040] It is also to be understood that the following claims are
intended to cover all of the generic and specific features of the
invention herein described and all statements of the scope of the
invention which, as a matter of language, might be said to fall
therebetween.
* * * * *