Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-02-12
2002-06-25
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S300000
Reexamination Certificate
active
06411093
ABSTRACT:
This application claims Paris Convention priority of DE 100 07 679.3 filed Feb. 19, 2000 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a method of operating an NMR (=nuclear magnetic resonance) spectrometer, in particular a high-resolution NMR spectrometer, comprising a DDS (=direct digital synthesis) generator which contains an NCO
L
(=numerical controlled oscillator) for generating an LO (=local oscillator) frequency, e.g. the first LO frequency f
LO1
, wherein the frequency of the NCO
L
is defined in the DDS generator through input of a numerical value Z.
An NMR spectrometer comprising such a DDS is disclosed in the company leaflet “AVANCE/Digital NMR” of Bruker AG, Fällanden, Switzerland, dated March 1999, wherein in particular page 11 shows a functional unit “DDS” performing as “frequency and phase control” in the CPU.
Frequency generators which operate with direct digital frequency synthesis, so-called DDS generators (DDS=Direct Digital Synthesis) are described e.g. in “Frequency Synthesizers Design Handbook”, J. A. Crawford, Artech House, Boston, London, 1994, page 346 or in “Digital PLL Frequency Synthesizers. Theory and Design”, U. L. Rohde, Prentice-Hall Inc., Englewood Cliffs, N.J. 1983, page 110.
The DDS generators have the following positive characteristics:
They generate numerical values with a clock rate given by an externally supplied constant clock frequency f
s
, and the numerical values represent a signal of a desired frequency. This signal is subsequently converted in a DAC (=Digital to Analog Converter) into an analog signal which has actually the same frequency stability as the clock frequency and is therefore very stable. The frequency cannot be changed continuously, but only in discrete frequency steps which may be very small using current methods, i.e. in the milliherz range for output frequencies between 10 and 30 MHz thus allowing almost continuous frequency adjustment.
The DDS generators essentially require only digital IC components which keeps their manufacturing costs low. A very advantageous solution consists in that the entire DDS generator is integrated in one single ASIC component (ASIC=Application Specific Integrated Circuit) which can considerably reduce costs when a large number are produced and allows particularly dense packing of the functional digital elements. The latter is particularly advantageous in fast electronic processes which are increasingly required today.
These positive aspects of a DDS generator, however, face the serious drawback that the spectral purity of the output signal is no longer sufficient for today's standards. DDS generators have been successfully used for more than 10 years in NMR (=nuclear magnetic resonance) spectrometers. The demand for spectral purity of the LO signals has increased in such a way that these generators can no longer provide the high performance needed during the receiving phase of the NMR signal.
The insufficient spectral purity of the DDS generator is caused by the so-called quantizising noise which is due to the fact that the signal generated in the DDS generator is quantizised, i.e. represents a stepped approximation to the desired signal, wherein the numerical values of these steps are defined only with a finite accuracy given by the maximum number of available bits.
The quantizising noise decreases the larger the number of steps within one period and the higher the accuracy of the numerical values of said steps. The number of steps cannot be increased arbitrarily. There is a limit given by the maximum clock rate of the digital components.
NMR signals in high-resolution NMR often consist of very strong and at the same time very weak frequency components, wherein the weak components are frequently the significant ones. This means that the NMR signal has a large dynamic range. One of the most sensitive mixing stages in the NMR receiver is the first mixing stage which uses an LO signal (f
LO1
) derived from the DDS generator and thus includes quantization noise. If this LO signal is mixed with the NMR signal, the quantization noise will be transferred particularly to the strongest frequency components of the NMR signal and will thus generate in the NMR spectrum a base line disturbed by unwanted frequency components. This disturbed base line also includes the desired weak frequency components of the NMR signal which are difficult to distinguish from the disturbing components. As a result, proper spectroscopy is impossible.
During the relatively uncritical transmitting phase in NMR spectroscopy, DDS generators are still used today without any problems.
However, during the critical receiving phase, the demand for spectral purity is very high today such that the DDS generator which provides the variable LO frequency does no longer meet these demands due to the quantization noise described above. Up to now, no practicable method has been available to reduce said quantization noise. Therefore, in all critical experiments which required high spectral purity, one had only the choice to do without this elegant and powerful generator or accept its disadvantages.
It is therefore the underlying purpose of the present invention to present a method comprising the initially mentioned features utilizing a DDS generator even when very high spectral purity is required, wherein particularly the quantization noise is eliminated as much as possible in the frequency range of the NMR spectrum.
SUMMARY OF THE INVENTION
In accordance with the invention, this object is achieved in a simple and effective way in that the numerical value Z is selected such that it assumes only values which satisty the equation
Z=n·N/m
wherein Z, n, N, and m are positive integers, wherein N is a power of 2 with a positive integer exponent, said exponent representing the maximum number of bits during the calculation process, wherein m is approximately 2·f
s
/&Dgr;B, n is approximately m·f
out
/f
s
and m a common integer divisor of n·N and wherein f
s
is the clock frequency of the NCO
L
, &Dgr;B the desired bandwidth with high spectral purity and f
out
the output frequency of the NCO
L
.
According to the inventive teaching, it is not allowed to use arbitrary but only selected Z values for the input to the DDS generator. As a result, the lowest occurring disturbing frequency will always be larger than the repetition frequency &Dgr;f
Raster
at which the signal of the NCO
L
repeats itself exactly. In this way it is possible to select the above described disturbing components with a sufficient separation to ensure that the NMR spectrum in between remains undisturbed.
In a variant of the inventive method which is particularly easy to carry out and is thus used with particular preference, m is a power of 2 having a positive integer exponent. This considerably simplifies the calculations to be carried out in the inventive method with respect to the general case and as a result the amount of calculations needed is reduced considerably.
The method is particularly facilitated in a further development of the above-mentioned variant, wherein the calculation of Z is carried out in the following three stages.
(a) the value for m is determined by means of the equation
m=2
RndDwn{log[2fs/&Dgr;B)/log 2]}
wherein &Dgr;B is the desired bandwidth of high spectral purity, f
s
is the clock frequency of the NCO and RndDwn is a rounding-off process to the next smaller integer value;
(b) the value for n is determined through equation
n=Rnd(m·f
out
/f
s
)
wherein f
out
is the desired frequency of the NCO, m the value calculated in the first stage and Rnd a rounding-off process to the next integer value;
(c) the value for Z is determined through equation
Z=n·N/m
wherein N is defined in claim
1
and m and n are the values determined in stages (a) and (b).
The present invention also includes a DDS (=Direct Digital Synthesis) generator for application in NMR spectromete
Gosteli Christoph
Schwilch Arthur
Arana Louis
Bruker BioSpin AG
Vincent Paul
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