Frequency drifts stability

To ensure stability against frequency drift with temperature, the temperature coefficient if must be close to zero, which implies control over TCe and aL. 

The frequency drift of the QCM at about 90 K (ca. 0.83 Hz/min) was larger than that observed at ca. 300 K (0.14 Hz/min). This is due to the better stability of the system at room temperature and to the fact that the crystals were cut to minimize their thermal coefficient at ca. 300 K. Based on the characteristics of the QCM used in these studies, a 1 Hz frequency change corresponds to 24 ng/ cm2. The uncertainty in the mass determination was estimated from the magnitude of the fluctuations of the QCM frequency (or period) readings during spectral acquisition, i.e., ca. 0.5 Hz or 12 ng/cm2, which is well below the range of masses examined in this work. 

For long-term stabilization, however, stabilization onto an external FPI has its drawbacks. In spite of temperature stabilization of the reference FPI, small drifts of the transmission peak cannot be eliminated completely. With a thermal expansion coefficient a = 10 of the distance holder for the FPI mirrors, even a temperature drift of 0.01°C causes, according to (5.86), a relative frequency drift of 10 , which gives 6MHz for a laser frequency of vr = 6x 10 " Hz. For this reason, an atomic or molecular laser transition is more suit-... 

A disadvantage of the LR-CPMG detection method is its total insensitivity to field/frequency offset which must be adjusted before a profile measurement and cannot be corrected by means of a simple procedure during an automatic profile measurement. This requires a higher degree of longterm field stability (including any thermal effects) than the other methods. Despite the insensitivity of the technique, in fact, the field may not be allowed to drift too far from resonance where the RF pulses would lose their efficiency (excursions up to about 5 kHz are, however, quite tolerable). 

The stabilization of the field-frequency ratio, which is required for HDMR experiments in the INDOR mode, can be obtained by locking the NMR spectrometer field-frequency ratio to the resonance condition of a strong and sharp signal in the sample under investigation. (14) Additional stabilization can be achieved by locking together the main frequency of the spectrometer and master oscillator of the synthesizer. (15, 16) In this case, the drift in the frequency of the synthesizer is balanced by the change in the main frequency of the spectrometer. 

The most common way to deal with the problem of stochastic drift is to modulate the exposure of the analyte to the sensor and to synchronously detect the sensor response. When the analyte is off (i.e., the sensor is zeroed ), the sensor signal can be recorded as the baseline value. Drift-corrected signals can be obtained by subtracting the baseline signal from that recorded when the analyte is on. If the frequency of the on/off modulation is much higher than the frequency of the baseline drift, then this scheme results in dramatically improved stability in the measured data. An implicit requirement in this measurement strategy is that the response kinetics of the sensitive film/analyte combination be sufficiently fast to allow on/off modulation at the desired frequency. 

Brown and Skrebowski [37] first suggested the use of x-rays for particle size analysis and this resulted in the ICl x-ray sedimentometer [38,39]. In this instrument, a system is used in which the difference in intensity of an x-ray beam that has passed through the suspension in one half of a twin sedimentation tank, and the intensity of a reference beam which has passed through an equal thickness of clear liquid in the other half, produces an inbalance in the current produced in a differential ionization chamber. This eliminates errors due to the instability of the total output of the source, but assumes a good stability in the beam direction. Since this is not the case, the instrument suffers from zero drift that affects the results. The 18 keV radiation is produced by a water-cooled x-ray tube and monitored by the ionization chamber. This chamber measures the difference in x-ray intensity in the form of an electric current that is amplified and displayed on a pen recorder. The intensity is taken as directly proportional to the powder concentration in the beam. The sedimentation curve is converted to a cumulative percentage frequency using this proportionality and Stokes equation. 

Instrumental precision is determined by the stability of the analytical signal and is made up of two components—noise, appearing as a random variation, and drift, which is a systematic change in signal level. The importance of drift will depend on the frequency of standardization and... 

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