4. Hardware Modifications and Recording
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 90o 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:
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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