NMR meets MUSICIANS

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4. Hardware Modifications and Recording

Note: Any descriptions of hardware modifications to existing spectrometers given in this section are provided without any warranty concerning malfunction or hardware damage!

How difficult is it to modify an existing spectrometer for auditory FID monitoring? If you are familiar with electronics it takes not more than half an hour of soldering and wiring. You simply have to tap the analogue FID signal immediately before it enters the analogue-digital converter (ADC). Since spectrometers usually work in quadrature detection mode, two FID signals are present, one of them being referenced with a 90^o phase shift of the intermediate frequency (IF) reference signal. These two signals may be fed into the inputs of a stereo amplifier.

For our JEOL Alpha500 spectrometer, the schematic wiring diagram and the necessary modifications are indicated by the scheme presented below.

Initially, for precautional reasons I employed a stereo amplifier working with tubes. This device has a very high input impedance, thus avoiding any disturbance of the FID signal. However, I noticed that on our spectrometer the output impedance of the final stage FID amplifier is extremely low (order of some Ohms). Hence, virtually any modern HiFi amplifier based on transistor or IC input may be employed as well.

When you try to record your FID sounds I strongly recommend to employ a digital device such as DAT tape or hard disk recording. This is due to the following: tones obtained from an FID are among the (if not the) purest tones obtainable. They are produced by atomic nuclei and, hence, are inherently as pure as time standards obtained from cesium clocks. The only sources of imbalances that may be introduced are:

• Jitter in the master reference frequency: not noticeable since based on quartz. Of course, at the same time this makes the NMR tones "only" as pure as tones based on a quartz clock.

• Instabilities of the magnetic field: modern superconducting magnets are extremely stable. Long-term drifts usually are some Hertz per week. Short-time fluctuations practically do not occur in a "quiet" environment (i.e., no nearby moving steel or iron objects like subway trams, elevators etc.).

• Instabilities of the lock system: usually not noticeable on modern instruments. In cases of doubt, measure in an unlocked mode.

When compared to the above sources of "jitter", any analogue recording device (high end cassette recorder and even professional studio hardware) is much more instable, resulting in an audible and more or less disgusting jitter of the tones.

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August 20, 1996