Digital waveform synthesizer for nmr phase control

Abstract

In an RF source, a digital waveform synthesizer comprises a computational module to synchronously determine a desired periodic function, f(θ) within a first bandwidth portion, to which computational result there is combined an injected digits noise increment in an adjustable range of hounded amplitude, specifically selected to average over discontinuities of the DAC transfer characteristic. The combination is effected after passing the injected noise increment through a programmable digital filter forming a composite tuning word having a total bandwidth at a selected Nyquist zone and thence passing the composite timing word through a truncation component to a DAC. The programmable digital filter is constructed to displace the spectral distribution of the injected noise increment to a portion of the total bandwidth remote from the first bandwidth portion.

1 . In NMR pulse sequencing apparatus, a method of phase change selection of any RF pulses of a sequence of RF pulses, comprising the steps of:
 (a) directly synthesizing a first RF wave train of frequency w, bandwidth β and of initial phase angle θ initial =θ 0 ;   (b) gating said first wave train of initial phase angle θ 0  to an NMR probe for a first desired duration;   (c) directly synthesizing a second RF wave train of initial phase angle θ initial =θ 1 , θ 1 ≠θ 0 ; and   (d) gating said second wave train of initial phase angle θ 1  to an NMR probe for a second desired duration.   
     
     
         2 . The method of  claim 1 , wherein said steps of directly synthesizing comprise:
 (e) initiating a representation of a sinusoid from a set of tuning words by selecting an initial tuning word for the value θinitial and selecting succeeding tuning words for the values θ comprising the digital representation of the respective RF pulse;   (f) perturbing said initial tuning word and succeeding timing words by (i) generating a bounded random number within selected bounds and passing the bounded random number through a digital filter to constitute filtered noise;   (ii) combining the filtered noise with each tuning word while said digital filter displaces the spectral distribution of said filtered noise to a selected spectral region remote from said bandwidth β; and   (g) repeating steps (e)-(f) for the RF wave train of step (c), to obtain corresponding portions of the representation of the sinusoid, whereby each directly synthesized RF wave train exhibits enhanced relative phase fidelity.   
     
     
         3 . The method of  claim 2 , wherein the initial tuning word specifies the desired initial phase angle θ initial , and the step of perturbing comprises combining the filtered noise with each said tuning word. 
     
     
         4 . The method of  claim 2 , wherein the initial tuning word specifies the desired initial phase angle θ initial , and the step of perturbing comprises combining the filtered noise with said portions of the representation of the sinusoid to form a perturbed representation portion. 
     
     
         5 . The method of  claim 3 , wherein said steps of directly synthesizing are accomplished by presenting each perturbed representation portion to a DAC comprising N bits and having a dynamic range of 2 N  and the bound of the noise amplitude is less than 2 least significant bits of said DAC. 
     
     
         6 . The method of  claim 4 , wherein said steps of directly synthesizing are accomplished by presenting each perturbed representation portion to a DAC comprising N bits and having a dynamic range of 2 N  and a transfer characteristic comprising P discontinuities of a constant valued slope and the bounds of the noise amplitude are 2 N+1-log2P <noise amplitude<2 N-log2P . 
     
     
         7 - 16 . (canceled)