Technique for improving the resolution of an A/D converter

Claim 2

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: generating an analog signal having a first component representing the intensity of the modulated beam of analytical radiation and a second component representing a dither signal having a time variation; sampling the analog signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 3

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: sampling the spectrometer output signal to generate an analog signal level whose value equals the value of the spectrometer output signal at the moment of sampling; holding the analog signal level during the sampling interval; superimposing a dither signal on the analog signal level to provide a dithered signal; sampling the dithered signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values to provide corresponding digitized values; and averaging at least some of the digitized values.

Claim 4

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: sampling the spectrometer output signal to generate an analog signal level whose value equals the value of the spectrometer output signal at the moment of sampling; holding the analog signal level during the sampling interval; superimposing a dither signal on the analog signal level to provide a dithered signal; sampling the dithered signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values to provide corresponding digitized values; and averaging at least some of the digitized values.

Claim 5

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: superimposing a dither signal on the spectrometer output signal to provide a dithered signal; sampling the dithered signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values, to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values.

Claim 6

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: superimposing a dither signal on the spectrometer output signal to provide a dithered signal; sampling the dithered signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values, to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values.

Claim 7

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: sampling the spectrometer output signal; superimposing a dither signal on the sampled signal to provide a dithered signal, the dither signal having a time profile during each sampling interval that is characterized by a plurality of rectangular pulses, each defining an interval during which the dither signal is constant; digitizing the dithered signal during at least some of the intervals during which the dither signal is constant to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 8

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: sampling the spectrometer output signal; superimposing a dither signal on the sampled signal to provide a dithered signal, the dither signal having a time profile during each sampling interval that is characterized by a plurality of rectangular pulses, each defining an interval during which the dither signal is constant; digitizing the dithered signal during at least some of the intervals during which the dither signal is constant to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 9

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: configuring the spectrometer so that its signal channel has a bandwidth that is a significant factor wider than the range of frequencies that characterizes the spectral range of interest of the spectrometer output signal so that the spectrometer output signal provides noise over this bandwidth; sampling the spectrometer output signal a plurality of times to provide a plurality of sampled values; and digitizing at least some of the sampled values to provide corresponding digitized values.

Claim 10

In the operation of a Fourier transform spectrometer where an optical path difference is scanned over a range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, a method of digitizing the spectrometer output signal, comprising the steps, carried out for each sampling interval of a scan, of: configuring the spectrometer so that its signal channel has a bandwidth that is a significant factor wider than the range of frequencies that characterizes the spectral range of interest of the spectrometer output signal so that the spectrometer output signal provides noise over this bandwidth; sampling the spectrometer output signal a plurality of times to provide a plurality of sampled values; and digitizing at least some of the sampled values to provide corresponding digitized values.

Claim 11

The method of claim 1 wherein all of the sampled signal values are digitized.

Claim 12

The method of claim 1 wherein all the digitized values are averaged.

Claim 13

The method of claim 1 or 2 or 3 or 4 wherein the dither signal is characterized by a range of frequencies outside the range of frequencies that characterizes the spectral range of interest of the spectrometer output signal.

Claim 14

The method of claim 1 or 2 or 3 or 4 or 5 wherein said steps are performed for a plurality of scans, and further comprising the step of averaging the output values generated for corresponding sampling intervals for the plurality of scans.

Claim 15

The method of claim 1 or 2 or 3 or 4 wherein at least one characteristic of the dither signal is varied from scan to scan.

Claim 16

The method of claim 1 or 2 or 3 or 4 or 5 wherein the spectrometer is operated in a rapid scan mode.

Claim 17

The method of claim 1 or 2 or 3 or 4 or 5 wherein the spectrometer is operated in a step scanning mode.

Claim 18

The method of claim 1 or 2 or 3 or 4 wherein the dither signal is random noise.

Claim 19

The method of claim 1 or 2 or 3 or 4 wherein the dither signal is a periodic waveform.

Claim 20

The method of claim 4 wherein the dither signal comprises pulses of random amplitudes.

Claim 21

The method of claim 4 wherein the dither signal is a stepped periodic waveform.

Claim 22

The method of claim 4 wherein said sampling step is performed once for each sampling interval.

Claim 23

The method of claim 4 wherein said sampling step is performed a plurality of times during each sampling interval.

Claim 24

The method of claim 5, and further comprising the step of digitally filtering the set of digitized values generated for at least one scan.

Claim 25

The method of claim 1 wherein said generating step comprises the substeps of: generating an analog signal representing the intensity of the modulated beam of analytic radiation; and superimposing thereon an analog signal having a time variation that defines the time variation of the dither signal.

Claim 26

The method of claim 1 wherein said generating step comprises the substeps of: providing the spectrometer with an auxiliary light source having a time varying intensity that defines the time variation of the dither signal; and causing the spectrometer to output a signal that includes a component representing the intensity of the auxiliary light source.

Claim 27

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: means for generating an analog signal having a first component representing the intensity of the modulated beam of analytic radiation and a second component representing a dither signal having a time variation; sample and hold means, responsive to said analog signal, for providing a plurality of sampled signal values during each sampling interval; means for digitizing at least some of said sampled signal values to provide corresponding digitized values, said means for digitizing having a characteristic resolution; and means for averaging at least some of said digitized values for each sampling interval to provide an averaged value having increased resolution relative to said characteristic resolution by a factor of up to the square root of the number of values averaged.

Claim 28

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: means for generating an analog signal having a first component representing the intensity of the modulated beam of analytic radiation and a second component representing a dither signal having a time variation; sample and hold means, responsive to said analog signal, for providing a plurality of sampled signal values during each sampling interval; means for digitizing at least some of said sampled signal values to provide corresponding digitized values, said means for digitizing having a characteristic resolution; and means for averaging at least some of said digitized values for each sampling interval to provide an averaged value having increased resolution relative to said characteristic resolution by a factor of up to the square root of the number of values averaged.

Claim 29

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: first sample and hold means responsive to the spectrometer output signal, for providing an analog signal level whose value equals the value of the spectrometer output signal at a particular moment for each sampling interval; means for superimposing a dither signal on said analog voltage level from said first sample and hold means to provide a dithered signal; second sample and hold means, responsive to said dithered signal, for providing a plurality of sampled signal values during each sampling interval; means for digitizing at least some of said sampled signal values to provide corresponding digitized values, said means for digitizing having a characteristic resolution; and means for averaging at least some of said digitized values to provide an averaged value having increased resolution by a factor of up to the square root of the number of values averaged.

Claim 30

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: first sample and hold means responsive to the spectrometer output signal, for providing an analog signal level whose value equals the value of the spectrometer output signal at a particular moment for each sampling interval; means for superimposing a dither signal on said analog voltage level from said first sample and hold means to provide a dithered signal; second sample and hold means, responsive to said dithered signal, for providing a plurality of sampled signal values during each sampling interval; means for digitizing at least some of said sampled signal values to provide corresponding digitized values, said means for digitizing having a characteristic resolution; and means for averaging at least some of said digitized values to provide an averaged value having increased resolution by a factor of up to the square root of the number of values averaged.

Claim 31

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: means for superimposing a dither signal on the spectrometer output signal to provide a dithered signal; sample and hold means, responsive to said dithered signal, for providing during each sampling interval a plurality of sampled signal values corresponding to the values of said dithered signal; means for digitizing at least some of said sampled signal values to provide corresponding digitized values, said digitizing means having a characteristic resolution; and means for averaging at least some of said digitized values to provide an averaged value having increased resolution by a factor of up to the square root of the number of values averaged.

Claim 32

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: means for superimposing a dither signal on the spectrometer output signal to provide a dithered signal; sample and hold means, responsive to said dithered signal, for providing during each sampling interval a plurality of sampled signal values corresponding to the values of said dithered signal; means for digitizing at least some of said sampled signal values to provide corresponding digitized values, said digitizing means having a characteristic resolution; and means for averaging at least some of said digitized values to provide an averaged value having increased resolution by a factor of up to the square root of the number of values averaged.

Claim 33

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: sample and hold means, responsive to the spectrometer output signal; means for superimposing a dither signal on the sampled signal from said sample and hold means to provide a dithered signal, said dither signal having a time profile during each sampling interval that is characterized by a plurality of rectangular pulses, each defining an interval during which said dither signal is constant; means, characterized by a digitizing increment, for digitizing said dithered signal during at least some of the intervals during which said dither signal is constant to provide corresponding digitized values; and means for averaging at least some of said digitized values to provide an averaged value having increased resolution by a factor of up to the square root of the number of values averaged.

Claim 34

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: sample and hold means, responsive to the spectrometer output signal; means for superimposing a dither signal on the sampled signal from said sample and hold means to provide a dithered signal, said dither signal having a time profile during each sampling interval that is characterized by a plurality of rectangular pulses, each defining an interval during which said dither signal is constant; means, characterized by a digitizing increment, for digitizing said dithered signal during at least some of the intervals during which said dither signal is constant to provide corresponding digitized values; and means for averaging at least some of said digitized values to provide an averaged value having increased resolution by a factor of up to the square root of the number of values averaged.

Claim 35

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: sample and hold means, responsive to the spectrometer output signal, for providing during each sampling interval a plurality of sampled signal values corresponding to the values of the spectrometer output signal; and means, characterized by a digitizing increment, for digitizing at least some of said sampled signal values to provide corresponding digitized values; wherein the spectrometer has a signal channel whose bandwidth is a significant factor wider than the range of frequencies that characterizes the spectral range of interest of the spectrometer output signal so that the spectrometer output signal includes noise over the bandwidth that is of greater amplitude than the noise in the portion of the bandwidth that characterizes the spectral range of interest and the noise in the portion of the bandwidth that characterizes the spectral range of interest is less than said digitizing increment.

Claim 36

In a Fourier transform spectrometer where an optical path difference is scanned over a range of values so as to provide a modulated beam of analytical radiation, the range of values defining a scan with the scan being subdivided into a sequence of sampling intervals, apparatus for digitizing the spectrometer output signal, comprising: sample and hold means, responsive to the spectrometer output signal, for providing during each sampling interval a plurality of sampled signal values corresponding to the values of the spectrometer output signal; and means, characterized by a digitizing increment, for digitizing at least some of said sampled signal values to provide corresponding digitized values; wherein the spectrometer has a signal channel whose bandwidth is a significant factor wider than the range of frequencies that characterizes the spectral range of interest of the spectrometer output signal so that the spectrometer output signal includes noise over the bandwidth that is of greater amplitude than the noise in the portion of the bandwidth that characterizes the spectral range of interest and the noise in the portion of the bandwidth that characterizes the spectral range of interest is less than said digitizing increment.

Claim 37

The apparatus of claim 22 or 23 or 24 or 25 wherein: said digitizing means is an A/D converter having a particular digitization increment; and said dither signal has maximum fluctuations that exceed said digitization increment.

Claim 38

The apparatus of claim 22 or 23 or 24 or 25 wherein said dither signal is characterized by a range of frequencies outside the range of frequencies that characterizes the spectrometer output signal.

Claim 39

The apparatus of claim 22 or 23 or 24 or 25 wherein the average value of the dither signal, taken over that portion of each sampling interval that corresponds to those sampled signal values that are averaged, is constant for all sampling intervals in the scan.

Claim 40

The apparatus of claim 22 or 23 or 24 wherein said dither signal is random noise.

Claim 41

The apparatus of claim 22 or 23 or 24 wherein said dither signal is a periodic waveform.

Claim 42

The apparatus of claim 22 or 23 or 24 or 25 wherein at least one characteristic of said dither signal is varied from scan to scan.

Claim 43

The apparatus of claim 22 or 23 or 24 or 25 or 26 wherein the spectrometer is operated in a rapid scan mode.

Claim 46

The apparatus of claim 22 or 23 or 24 or 25 wherein the dither signal is characterized by a range of frequencies outside the range of frequencies that characterizes the spectral range of interest of the spectrometer output signal and the average value of the dither signal, taken over that portion of each sampling interval that corresponds to those sampled signal values that are averaged, is constant for all sampling intervals in the scan.

Claim 47

The apparatus of claim 22 wherein all of said sampled signal values are digitized.

Claim 48

The apparatus of claim 22 wherein all of said digitized values are averaged.

Claim 49

The apparatus of claim 25 wherein said sample and hold means is clocked once for each sampling interval.

Claim 50

The apparatus of claim 25 wherein said sample and hold means is clocked a plurality of times for each sampling interval.

Claim 51

The apparatus of claim 22 wherein said generating means comprises: means for generating an analog signal representing the intensity of the modulated beam of analytical radiation; and means for superimposing thereon an analog signal having a time variation that defines the time variation of said dither signal.

Claim 52

The apparatus of claim 22 wherein said generating means comprises: an auxiliary light source having a time varying intensity that defines the time variation of said dither signal; and means for generating an analog signal representing a combination of the intensity of the modulated beam of analytical radiation and the intensity of said auxiliary light source.

Claim 53

In a method of operating of a Fourier transform spectrometer including the steps of directing a beam of analytical radiation into an interferometer, scanning an optical path difference in the interferometer over a range of positions defining a scan so as to provide a modulated beam of analytical radiation, directing the modulated beam of analytical radiation to a detector, and subdividing the scan into a sequence of sampling intervals, the improvement comprising the steps, carried out for each sampling interval of a scan, of: illuminating the detector with a modulated auxiliary beam; generating an analog signal representing a combination of the intensity of the modulated beam of analytical radiation and the intensity of the modulated auxiliary beam; sampling the analog signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 54

In a method of operating of a Fourier transform spectrometer including the steps of directing a beam of analytical radiation into an interferometer, scanning an optical path difference in the interferometer over a range of positions defining a scan so as to provide a modulated beam of analytical radiation, directing the modulated beam of analytical radiation to a detector, and subdividing the scan into a sequence of sampling intervals, the improvement comprising the steps, carried out for each sampling interval of a scan, of: illuminating the detector with a modulated auxiliary beam; generating an analog signal representing a combination of the intensity of the modulated beam of analytical radiation and the intensity of the modulated auxiliary beam; sampling the analog signal a plurality of times to provide a plurality of sampled signal values; digitizing at least some of the sampled signal values to provide corresponding digitized values; and averaging at least some of the digitized values to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 55

The improvement of claim 42 wherein said illuminating step comprises the substeps of: providing a narrow band auxiliary source; and directing a beam from the auxiliary source through the interferometer to produce the modulated auxiliary beam.

Claim 56

The improvement of claim 42 wherein said illuminating step comprises the substeps of: providing an auxiliary source; and modulating the auxiliary source outside the interferometer.

Claim 57

In a Fourier transform spectrometer including an interferometer, means for directing a beam of analytical radiation into the interferometer, means for scanning an optical path difference in the interferometer over a range of positions defining a scan so as to provide a modulated beam of analytical radiation, means for directing the modulated beam of analytical radiation to a detector, and means for subdividing the scan into a sequence of sampling intervals, the improvement comprising: means for illuminating the detector with a modulated auxiliary beam; means for generating an analog signal representing a combination of the intensity of the modualted beam of analytical radiation and the intensity of said modulated auxiliary beam; means for sampling the analog signal a plurality of times to provide a plurality of sampled signal values during each sampling interval; means, having a characteristic resolution, for digitizing at least some of said sampled signal values to provide corresponding digitized values; and means for averaging at least some of the digitized values provided during each sampling interval to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 58

In a Fourier transform spectrometer including an interferometer, means for directing a beam of analytical radiation into the interferometer, means for scanning an optical path difference in the interferometer over a range of positions defining a scan so as to provide a modulated beam of analytical radiation, means for directing the modulated beam of analytical radiation to a detector, and means for subdividing the scan into a sequence of sampling intervals, the improvement comprising: means for illuminating the detector with a modulated auxiliary beam; means for generating an analog signal representing a combination of the intensity of the modualted beam of analytical radiation and the intensity of said modulated auxiliary beam; means for sampling the analog signal a plurality of times to provide a plurality of sampled signal values during each sampling interval; means, having a characteristic resolution, for digitizing at least some of said sampled signal values to provide corresponding digitized values; and means for averaging at least some of the digitized values provided during each sampling interval to provide an output value with a digitization error reduced by a factor of up to the square root of the number of values averaged.

Claim 59

The improvement of claim 45 wherein said means for illuminating comprises: a narrow band auxiliary source; and means for directing a beam from said auxiliary source through the interferometer to produce said modulated auxiliary beam.

Claim 60

The improvement of claim 45 wherein said means for illuminating comprises: an auxiliary source; and means, distinct from the interferometer, for modulating said auxiliary source outside the interferometer.