Photonic parallel analog-to-digital converter

Details Photonic parallel analog-to-digital converter Photonic parallel analog-to-digital converter

2001-05-03

2003-02-25

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Using optical device,

C341S155000

Reexamination Certificate

active

06525682

ABSTRACT:

FIELD OF THE INVENTION
The present application relates to an apparatus and method for analog-to-digital conversion, and more specifically to a parallel analog-to-digital converter system utilizing photonic sampling.
BACKGROUND OF THE INVENTION
Analog-to-digital conversion is well known as a process in which a continuous time analog signal, which theoretically has an infinite number of values or states, is converted to a digital signal, which has a finite number of values or states. Typically, in analog-to-digital conversion, the analog signal is first sampled. The sampled analog signal is represented as a series of pulses. Each pulse has a magnitude equal to the magnitude of the analog signal at a discrete moment in time. After sampling, the discrete time signal is then quantized by rounding the value of each pulse to the closest one of a finite number of values. The resulting signal is a digital version of the analog signal.
One byproduct of analog-to-digital conversion is quantization noise. Quantization noise occurs because the magnitude of the analog signals entering a quantizer can theoretically be equal to an infinite number of values, whereas the magnitude of the rounded signals leaving the quantizer can only be equal to a finite number of values. Therefore, the quantizer causes a rounding off error, or quantization noise.
One way to reduce quantization noise is through oversampling. It is well known that to recover a sampled analog signal, the signal must be sampled at a rate greater than or equal to twice the signal frequency. Oversampling refers to sampling the signal at a rate much greater than twice the signal frequency. Increasing the sampling frequency spreads the quantization noise over a larger bandwidth because the total amount of quantization noise remains the same over the different sampling bandwidths. Thus, increasing the sampling rate relative to twice the signal frequency, or oversampling, reduces the quantization noise in the bandwidth of interest.
One ADC architecture well known in the art that uses over-sampling is the delta-sigma modulator-based ADC. A delta-sigma (&Dgr;&Sgr;) modulator consists of an analog filter and a quantizer enclosed in a feedback loop. The filter, in conjunction with the feedback loop, acts to attenuate the quantization noise at low frequencies while amplifying the high-frequency noise. Since the signal is oversampled at a rate much greater than twice the signal frequency, a digital lowpass filter can be used to remove the high frequency quantization noise without affecting the signal.
A problem with &Dgr;&Sgr; modulator-based ADCs is the oversampling requirement, that is, the circuitry of the ADC must be designed to operate at a significantly higher frequency than the maximum frequency of the analog signal that is converted by the ADC. The greater the required accuracy of the &Dgr;&Sgr; modulator-based ADC, the larger the sampling frequency must be. Limitations in circuitry capabilities have, therefore, limited the use of the single channel &Dgr;&Sgr; modulator-based ADCs to relatively low signal frequencies. However, the sampling frequency may be reduced by using multiple &Dgr;&Sgr; modulators in parallel. An ADC using multiple &Dgr;&Sgr; modulators is disclosed in U.S. Pat. No. 5,196,852, “Analog-To-Digital Converter Using Parallel &Dgr;&Sgr; Modulators,” issued Mar. 23, 1993 to I. A. Galton.
As shown in
FIG. 1
, Galton discloses an all-electronic ADC comprised of multiple parallel channels, each of which operates on the same analog input signal, and the outputs of which are summed to produce the overall digital output. Each channel is comprised of a multiplier
101
that multiplies its input by an internally generated sequence u(n) followed by a &Dgr;&Sgr; modulator
111
. The output of each &Dgr;&Sgr; modulator
111
is filtered by a digital low pass filter
112
followed by an N-sample decimator
113
. Another multiplier
121
multiplies the decimated output by another internally generated sequence v(n). The first multiplier
101
and the &Dgr;&Sgr; modulator
111
can be considered as mostly analog functions, while the lowpass filter
112
, decimator
113
, and second multiplier
121
are digital functions. The internally generated sequences are derived from a Hadamard matrix, in which the multipliers use factors of +1 or −1. The ADC disclosed by Galton provides significant levels of ADC accuracy using a sampling frequency as low as twice the signal frequency. The ADC accuracy is increased by using additional channels.
A difficulty associated with sampling, however, is that temporal jitter in the occurrence of the sampling clock limits the performance of the ADC by causing non-uniform sampling and therefore increases the total error power in the quantizer output. If the clock jitter is assumed to contribute white noise, the total power of the error is reduced in an ADC by the oversampling ratio. Nevertheless, the clock jitter still can be a limiting factor for conversion of wideband signals.
Fortunately, sampling jitter limitations can be overcome by using photonic sampling. Photonic sampling makes use of ultra-short laser pulses with high temporal stability to sample an analog electrical input. Compared to electronic samplers, the photonic approach is capable of shorter sampling windows (sub-picosecond) and higher sampling rates, approaching
100
gigasamples per second (GSPS), and thus can sample wideband analog inputs.
A conventional photonically sampled A/D converter
200
is shown in
FIG. 2. A
series of optical impulses
201
from a mode-locked laser
203
are applied to an electro-optic modulator
205
. The analog electrical input X(t) is also applied to the modulator
205
. The optical impulses
201
sample the voltage on the modulator
205
electrodes. The resultant optical pulses
207
, with intensities determined by the modulator
205
voltage, are fed to a photodetector
209
. The photodetector
209
electrical output
211
is an electrical current, which can be supplied to the input of an electronic quantizer
212
.
The above approach achieves very high sampling rates because the pulse repetition rate of the mode-locked laser can be 40 GHz or higher. Even higher repetition rates for the optical sampling pulses can be achieved by combining several time-delayed copies of each laser pulse.
Another example of a photonic sampled A/D converter, described by P. E. Pace and J. P. Powers of the U.S. Naval Postgraduate School in “Photonic Sampling of RF and Microwave Signals,” Mar. 16, 1998, is illustrated in FIG.
3
. The A/D converter of
FIG. 3
uses a delta-sigma modulator architecture, which is well known for reducing quantization noise, as described by Galton, with a photonic sampler. The delta-sigma converter
300
contains a mode-locked fiber laser
302
to act as the source of sampling pulses, and two photonic samplers
304
, which are Mach-Zehnder interferometric modulators. The fiber-lattice structure
306
acts as an optical integrator. The photonic samplers
304
also serve as the analog summing point at the input of the delta-sigma loop.
One difficulty associated with the photonic A/D converters discussed above is high non-linearity and noise spurs. The dynamic range of the photonic sampler limits prior A/D converters using photonic sampling techniques. For example, an analog waveform with a 5 GHz bandwidth can only be sampled to a resolution of 7.5 bits because the spur-free dynamic range of such modulators is approximately 110 dB-HZ
2/3
. Therefore, what is needed is an A/D converter system that can utilize optical sampling without being adversely affected by the noise and distortion from the photonic sampler.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for using optical sampling to provide for analog-to-digital conversion of an analog signal. An additional object of the present invention is to provide such conversion while reducing the noise and distortion associated with optically sampling an analog signal.
The photonic ADC system of

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