Setting the Spectral Window

Normally we set our spectral window (SW) to ensure that every frequency component of the net magnetization vector (from a particular nuclide) is observed. Sometimes the term sweep width is used when spectral window is meant. These two terms are commonly used interchangeably, but this is not strictly correct. The term spectral window denotes not only how wide a range of frequency is covered, but also where this range is centered relative to the frequency of a standard. 

We can control both the width and the location of the SW. The width is controlled by varying the sampling rate of the analog-to-digital converter (A/D). We can translate (move side to side) the center of the spectral window by varying the transmitter frequency. The transmitter frequency is also known as the transmitter offset or the carrier frequency, or sometimes simply the transmitter (this is an imprecise term and should be avoided unless the context is well understood). On a Bruker instrument, the transmitter frequency is determined by a coarse value listed in MHz (sfrq, for spectrometer frequency) 

Mixing down. Syn. mixdown. The reduction of an analog signal from a high frequency (typically tens or hundreds of MHz) to a lower frequency range (typically below 100 kHz). 

Transmitter glitch. A small spectral artifact often observed in the very center of the spectral window that is caused by a small amount of the RF generated in the console getting through to the receiver. 

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Because digital filtering can produce a brick wall frequency response, any peak that falls outside the spectral window is removed completely and will not alias. This can be a problem if you set the spectral window too narrow You will never be aware of the peaks you miss. If you accidentally set the spectral window to include nothing but noise, you will get just that in the spectrum nothing but noise The good news is that if we are only interested in a small part of the ID spectrum, we can cut out the rest of the spectrum using the digital filter. For example, in a 2D N-1 HSQC spectrum of a protein, we are only... 

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