Detector System:

High frequency radio signal is first converted to an audio frequency signal which can be thought of as being made up of two components; a carrier signal which has the frequency of the oscillator that is used to produce it and a superimposed NMR signal from the analyte. An analyte signal differs in frequency from the standard through a few ppm. In case of proton spectrum, chemical shifts are typically in the range of 1-10 ppm. Thus proton magnetic resonance data generated by 200 MHz spectrometer would lie within the frequency range 200,000,000 Hz to 200,002,000 Hz. That is impractical to digitise such a little difference. In practice, a difference signal is obtained that lies in the audio frequency in kilohertz range. The modern spectrometers contain a quadrupole phase sensitive detector which is capable of sensing the sign of frequency difference. This permits the determination of positive and negative difference between the frequency of standard and sample.