Frequency drifts

Population genetics has produced a detailed theory of the relationship between frequency, drift, and selective value of genetic variation (Nei, 1975 Hard and Clark, 1989 Li, 1997). The problem with its straightforward application is that the main parameters, such as history of population size, fixation rate, and selective pressure, are difficult to establish. 

Because the initial emphasis of this study was on extending ACC to liquid-fueled combustors, a simple closed-loop controller, which had been well tested in the previous studies involving gaseous fuel, was utilized. Such a controller, however, may not be effective in a combustor where the oscillation frequencies drift significantly with the control. The main problem was the frequency-dependent phase shift associated with the frequency filter. For such a case, it would be more useful to employ an adaptive controller that can rapidly modify the phase setting depending on the shift in the dominant oscillation frequencies. 

Development of Dosimeter Materials. - 4.2.1 Quantitative ESR and Intensity Standards of Mn1+ and CuS04.5H20. An ESR spectrometer consists of vacuum tube amplifier and a Klystron oscillator with thermal noise and frequency drift. Hence, an inherent standard of Mn2+ was used in ESR dating of carbonate stalactites,8 and patented as a standard in ESR radiation dosimetry in 1980.102 The standard sample of MgO with Mn2+ is frequently used for calibration of -factor and the magnetic field as well as for radiation dosimetry.103... 

The data recorded as the laser frequency is scanned consists of the fluorscence signal from the PMT, a Doppler-free I2 spectrum and frequency markers from the etalon. The etalon provides a calibration of the frequency scan. The Doppler-free I2 spectra provides an absolute frequency reference used to correct for small laser frequency drifts, separator voltage drifts and to determine the absolute acceleration voltage of the separator for the Doppler shift corrections. We are thus able to record data over long periods of time, e.g. 3 hours, and maintain a reasonable resolution of 100 MHz. Some of the first online data recorded with this system is shown in Figure 2. The overall detection efficiency has been measured to be 1/1000, i.e. one detected photon per 1000 atoms, for the largest transition in the nuclear spin 1/2 isotopes. 

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. 

The cell and oscillator circuit should oscillate stably in the range 1 to 3 MHz and should have a frequency drift of less than 1 ppm per minute. The cell temperature should be held constant to 0.1°C by submerging it in a bath of water (or acetone and Dry Ice for low temperatures) contained in a Dewar flask. If the frequency is sensitive to placement of nearby objects (including the experimenter s hands), look for defects in the electrostatic shielding all visible parts of the apparatus should be at ground potential. 

A coordinated time base is essential. This simple requirement is not necessarily trivial on-board ship. Many analytical instruments and recording devices keep time by counting electrical cycles. This timekeeping method uses the excellent frequency control commonly found in U.S. electric power systems. At sea, however, frequency drifts, thus, clocks drift too. If all laboratory data were logged immediately into a computer data base, it would be relatively easy to coordinate computer time and real time. However, the situation can be confused when signals first go to strip-chart recorders for subsequent evaluation and then transfer to the general files. Provision then must be made for regular time marks on the chart. 

The instrument is built on the heterodyne principle with 10 MHz as the intermediate frequency. Both the lock frequency and the spectrometer frequency are derived from the synthesizer, which ensures that any source frequency drift would appear equally in both the lock and the observe channels thus resulting in maintaining the resonance condition. 

The CPU clock is monitored against possible frequency drift. 

A dual SAW delay line oscillator is illustrated in Figure 3. It consists of two SAW delay lines fabricated on the same ST-quartz substrate. One delay line is coated with the chemically sensitive film and the other is left uncoated. The frequencies of the two delay line oscillators are mixed and filtered to provide a frequency equal to the difference of the two oscillator frequencies. This scheme has several advantages when used in chemical vapor sensor applications. First, it helps to compensate for SAW frequency drift caused by ambient temperature and pressure fluctuations. Additionally, the difference frequency is much lower than the frequency of the oscillators themselves. This permits SAW vapor response measurements using inexpensive digital circuitry. 

The device can be used to replace the tuned circuit in an oscillator by providing the resonant frequency or it can be coupled to the oscillator circuit, which is tuned approximately to the crystal frequency. In this type, the crystal prevents frequency drift. The device is widely used in quartz clocks and watches. 

The optical setups for these experiments are very similar to the setups described in the previous chapters. The pentacene study used a pinhole to reduce the excitation volume. The setup consisted of a home-buflt pressure cell and an external home-built reference cavity, to account for the slow frequency drifts of the commercial dye laser. In the terrylene study a magnet controlled lens, immersed in liquid helium, focused the laser light to a tiny spot and an ellipsoidal mirror collected the fluorescence. 

Guan, S., Wahl, M.C., and Marshall, A.G. (1993) Elimination of frequency drift from FTICR mass spectra by digital quadramre heterodyning ultrahigh mass resolving power for laser-desorbed molecules. Anal. Chem., 65, 3647-3653. 

Estimate the frequency drift of a laser oscillating at A, = 500 nm because of thermal expansion of the resonator at a temperature drift of l°C/h, when the resonator mirrors are mounted on distance-holder rods (a) made of invar and (b) made of fused quartz. 

Mode selection in an argon laser is often accomplished with an intracavity etalon. What is the frequency drift of the transmission maximum... 

Compensation for temperature-related frequency shifts can be accomplished by connecting a reactance (e.g., a variable voltage capacitor called a varactor) in series with the resonator. The effect of this reactance is to puU the frequency to cancel the effect of frequency drift caused by temperature changes. An example of a frequency vs. temperature characteristic both before and after compensation is shown in Fig. 3.8. Notice that the general form for the characteristic before compensation follows the cubic variation given earlier in this section. 

Crystal-controlled oscillator An oscillator in which a piezoelectric-effect crystal is coupled to a tuned oscillator circuit in such a way that the crystal puUs the oscillator frequency to its own natural frequency and does not allow frequency drift. 

Normalized frequency drift The frequency drift divided by the nominal frequency value. 

---