Sample synthesis for matching digitizers in interleaved systems

Details Sample synthesis for matching digitizers in interleaved systems Sample synthesis for matching digitizers in interleaved systems

2002-02-27

2003-05-20

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Analog to digital conversion

C341S126000, C341S143000

Reexamination Certificate

active

06567030

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to a system for matching digitizers in interleaved systems using a sample synthesis process.
Waveform digitizing technology is subject to the principle that the maximum speed (or sampling rate) of the fastest digitizing elements—generally in the form of analog-to-digital converters (ADCs)—lags behind the speed of the maximum bandwidth of the highest bandwidth systems. This means that in high bandwidth systems, the highest speed ADCs are simply incapable of sampling the analog signal at a rate sufficient to use the full resources of the high bandwidth systems.
Those skilled in the art of digital signal processing are familiar with Nyquist's criteria. This Nyquist criteria requires that a continuous signal sampled at a rate such that no frequency component is present in the analog signal above one half the sampling rate can be completely described by the sampled signal. See H. Nyquist,
Certain Factors Affecting Telegraph Speed
, Bell System Technical Journal, April 1924, p. 324; H. Nyquist,
Certain Topics in Telegraph Transmission Theory
, A.I.E.E. Trans., Vol. 47, April 1928, p.617. When Nyquist's criteria is not met, a phenomena called “aliasing” occurs. Aliasing results in some frequencies in the analog signal not being distinguishable from other frequencies in the sampled signal (i.e. some frequencies appear as other frequencies, hence the term “aliasing”).
Because ADCs cannot satisfy the Nyquist criteria for the highest bandwidth systems, aliasing will occur if the analog signal is attempted to be sampled by a single ADC. This is because the bandwidth of the system being greater than one half the sample rate of that ADC. A single sampled waveform in which aliasing has occurred is generally not useful in any further digital device that would utilize such a sampled signal.
Many techniques have been developed to produce sufficiently oversampled, high-bandwidth waveforms, particularly in the area of high performance digital sampling oscilloscope (DSO) design. Each of these techniques is intended to overcome the sample rate limitations of ADCs. The present invention relates to a technique called “interleaving.” Interleaving is the most widely used technique in digitizing systems designed for high bandwidth, high sample rate, long memory length, single-shot applications. Interleaving involves delivering an analog signal to multiple ADCs, each of which samples the waveform at precisely offset, but different interleaved times.
FIG. 8
shows a block diagram of a traditional interleaved system. In
FIG. 8
, a continuous analog signal 80 enters the system through a front-end amplifier
81
. In a DSO, the front-end is designed to provide the appropriate input coupling, offset, and signal amplification. The analog signal is distributed from the front end to D independent samplers (or ADCs)
84
. In
FIG. 8
, the transfer characteristics of each ADC path are shown as H[d]
83
, where d is the digitizer number. Since the primary transfer characteristics of each path are sufficiently linear and because the effects of the ADC or other components in the signal path are indistinguishable from each other, the response characteristic of the entire path from front-end to digitizer is lumped into H[d]. Usually, the analog signal is also fed to trigger circuitry (not shown).
Each of the ADCs
84
in this system sample at a rate F, which results in an effective sample rate of D times F for the combination shown in FIG.
8
. As can be seen, ADC 0 produces the first point in the waveform, ADC
1
produces the second point, etc . . . . The D-1 sample is generated by the last ADC, after which the sampling repeats with ADC 0 providing the next sample. Hence, in this system, ADC 0 provides samples 0, D, 2D, etc . . . . An example of this type of system is the LeCroy WavePro™ 960. This DSO takes a single-shot acquisition at a bandwidth of 2 GHz and a sample rate of 16 GS/s by-utilizing 16 ADCs each sampling at 1 GS/s.
While interleaving seemingly offers an unlimited sampling rate, practical design problems are numerous. Interleaving problems are manifest as degraded waveform integrity. This degradation appears as “interleaving artifacts.” These artifacts are easily discernable from other types of degradation, such as random noise.
In order for an interleaved system to work properly, the signal delivered to each digitizer must be identical. Furthermore, the characteristics of each ADC must be identical. Matching signal path and ADC characteristics is not an easy task. In addition, the sample clock delivered to each ADC must be precisely timed.
Many commercially available ADCs designed for very high sample rates provide voltage controlled delay, gain, and offset adjustments so that each ADC can be adjusted (or matched) to one another. Despite these controls, a perfect match cannot be achieved. Furthermore, these ADCs ignore the fact that while the delay control is an absolute time delay, the gain control is a DC gain. Thus, it is not possible to provide a single control for both the gain and delay as a function of frequency. There is currently no way to control a frequency response mismatch between digitizers and the analog paths leading to each digitizer.
FIG. 9
illustrates an example of frequency mismatch between two ADCs. The top plot shows the responses
90
,
91
of each ADC as a function of frequency. The bottom plot shows the alternating samples generated by the respective ADCs. Each succeeding sample is taken by the other ADC. The frequency mismatch results from the different response characteristics of the two ADCs. Because of this response mismatch, the sampled waveform is severely degraded. Note that adjustment of the time delay or gain of either digitizer will not correct this problem because the problem is the response of each respective ADC.
The problem is that the transfer characteristics (H[d]) for each ADC are not identical. For example, although the WavePro™ 960 is a DSO designed with advanced technologies to minimize variation in the ADC paths, the characteristics of the signal path to each ADC and certain attributes of the individual ADCs are still not matched. FIG.
10
and
FIG. 11
show how multiple traces are easily distinguishable when the WavePro™ 960 is run in persistence mode using interleaving. (
FIGS. 10 and 11
are described in more detail below) This is because each path from front-end to digitizer contains many different components having varying response characteristics. Furthermore, the digitizers themselves differ in their response characteristics due to an effect called sampling efficiency. While sampling efficiency is correctable by independently considering the digitized waveform from each ADC, for the purposes of this discussion, it is considered as part of the path response characteristics.
SUMMARY OF THE INVENTION
In summary, many techniques exist in the design of sampled systems, particularly in the design of digital sampling oscilloscopes. The most widely used technique in the design of high-bandwidth, high-sample rate, single-shot DSOs is interleaving. Interleaving can create artifacts that serve to degrade the quality of the resultant digitized waveform. These artifacts are difficult if not impossible to remove through traditional methods of electronics design.
Therefore, in accordance with this invention, a system is provided that is capable of digitally matching the sampling characteristics of each ADC in an interleaved system.
The preferred embodiment of the invention provides a synthesizer for matching a plurality of interleaved digitizers in an acquisition system operating at an interleaved sampling rate. The synthesizer has a digitizer input port for inputting a digitized waveform sample output from a corresponding interleaved digitizer. A synthetic input port is used to input synthetic output samples corresponding to the synthesizer. These synthetic output samples are generated by synthesizers corresponding to the other interleaved digitizers. T

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