Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1998-12-29
2001-10-02
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06297637
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a high-frequency receiver, particularly for a nuclear magnetic resonance apparatus, of the type having an analog input part with at least one analog mixer stage, the mixer stage being operable with an injection frequency, and having a following digitalization stage that includes an analog-to-digital converter for the digitalization of a signal from the analog input part with a sampling rate prescribed by a sampling frequency.
2. Description of the Prior Art
Diagnostics nuclear magnetic resonance apparatuses (magnetic resonance apparatus, MR apparatus) currently employ analog-to-digital converters in the receiver part in order to sample a high-frequency reception signal received from a reception antenna and supply it to an image computer. The dynamics, immunity to interference and bandwidth of the high-frequency receiver critically enter into the quality and flexibility obtainable with the nuclear magnetic resonance apparatus with respect to various applications.
Regardless of whether they are implemented as analog or digital receivers, such receivers are often realized as superheterodyne receivers wherein mixers transform the received nuclear magnetic resonance signal into an intermediate frequency range or into a base frequency range.
U.S. Pat. No. 5,170,123 discloses a high-frequency receiver of this type. The receiver is connected to a high-frequency antenna for the reception of nuclear magnetic resonance signals. The frequency of the reception signals lies on the order of magnitude of 20, 40 or 60 MHZ. After a pre-amplification, the reception signal is supplied to an analog mixer. The mixer converts the reception signal to a fixed intermediate frequency with 125 kHz. This means that the frequency of the local oscillator or the injection frequency for the mixer must differ from the center frequency of the reception signal by this constant intermediate frequency. After low-pass filtering, the reception signal converted to the intermediate frequency is supplied to an analog-to-digital converter that samples the signal with 500 kHz, i.e. a four-fold over-sampling with reference to the intermediate frequency. In particular, frequencies within a frequency band from 0 to 250 kHz can thus be digitalized. The injection frequencies supplied to the mixers and the sampling frequency of the analog-to-digital converter generate harmonics and harmonic combinations that, without anti-interference measures, are superimposed on the reception signal. The necessary anti-interference measures can be considerable.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a high-frequency receiver that is simple to screen with respect to internally generated frequencies.
This object is achieved in a high-frequency receiver of the type initially described having an injection frequency or sampling frequency generator connected to the mixer stage and to the analog-to-digital converter, which generates the injection frequency or sampling frequency only as a whole-number multiple of a basic frequency. All frequencies generated in the analog part of the receiver itself, including their harmonics and their mixed products with one another, are thus whole multiples of this basic frequency. Interference can occur only at reception frequencies that are a multiple of the basic frequency. Anti-interference measures are thus substantially simplified.
The injection frequency and the sampling frequency can be the same or different. Many possibilities for defining the frequency plan of the high-frequency receiver are thus made available.
Anti-interference measures are especially simple in an embodiment wherein the basic frequency and all multiples thereof lie outside the reception signal band. No whole multiples of the basic frequency can fall into an intermediate frequency band. Given arbitrary mixing, all frequency bands that contain an information-carrying signal are free of frequency components generated in the receiver itself.
After an analog-to-digital conversion, the digitalized intermediate frequency is converted into a baseband. Digital multipliers and a digital local oscillator signal whose frequency is determined by the exact center frequency of the payload signal, for example the center frequency of the nuclear magnetic resonance signal, serve this purpose. The digital local oscillator signal is generally not a multiple of the basic frequency. The interference generated in the analog part itself thus also is converted to a frequency outside the baseband.
Following the digital frequency conversion, a reduction to a lower sampling rate is normally undertaken for subsequent signal processing, for example in an image computer. To that end, the baseband signal is conducted over an adjustable, digital interpolation low-pass filter that filters out the noise parts and interference outside the baseband. Since, however, only noise parts and no coherent disturbances are present, relatively slight blocking attenuations around 30 dB suffice for the suppression. Compared thereto, significantly greater values, for example greater than 80 dB, would typically be required for the suppression of coherent disturbances.
In order to keep the outlay for the aforementioned low-pass filter low, the analog-to-digital converter in a further embodiment can be followed by a digital band rejection filter that has a high attenuation at all multiples of the basic frequency, and which eliminates coherent disturbances before the digital frequency conversion. This filter is a type generally referred to as a notch filter. This filter can be constructed simply because of the whole-number relationship between sampling rate and basic frequency. It is advantageous for the sampling rate of the analog-to-digital converter to amount to a relatively small multiple of the basic frequency, for example four.
The digital band elimination filter can be realized as time-discrete system with a finite impulse response (FIR filter—finite impulse response filter) or as a time-discrete system with an infinite impulse response (IIR filter—infinite impulse response filter). The delay elements contained in these filters thereby have a transit time that is equal to the reciprocal of the basic frequency.
High demands as to the flatness (constancy) of the frequency response in broad information signal bands, for example ±0.1 dB given a bandwidth of 500 kHz, cannot always be met with short FIR filters. However, an equalization of the frequency response can then ensue in the following signal processing. For example, a weighting of the spectral components by Fourier analysis is possible in the image computer in nuclear magnetic resonance tomography apparatus. Since coherent disturbances, which are far smaller than the least significant bit (LSB) of the analog-to-digital converter, are also boosted from the noise by the image calculation in the nuclear magnetic resonance apparatus, and since the rounding errors given an IIR filter contain systematic (i.e. coherent) parts, the word width of an IIR filter should be greater than the word width of the analog-to-digital converter.
Even when multiples of the basic frequency fall into the information signal band, the information signal can be equalized when the injection frequency and the sampling frequency are selected as whole multiples of a basic frequency. However, the coherent disturbances can no longer be eliminated by a filter. Since, however, the disturbances are then periodic in the basic frequency and change only insignificantly over the duration of a reception cycle that, for example, has a length up to 100 ms, they can be subtracted from the received signals following the analog-to-digital converter according to a further embodiment. To that end, samples of an estimate of the interference are retained in a memory. The samples are dependent on the selected injection frequencies and must, for example, be reloaded given frequency changes. Since the same rounding errors are always periodically suppl
Feld Peter
Kroeckel Horst
Vester Markus
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Vargas Dixomara
Williams Hezron
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