Accurate Frequency Measurements Research Articles

Accuracy of frequency measurement in the optical range

Accuracy of frequency measurement in the optical range

The role of time-frequency in satellite position determination systems

Many present-day radio navigation satellite systems employ the use or measurement of time or frequency to provide a position fix for the user vehicle. For example, the position fix obtained by the U. S. Navy's Navigation Satellite Systems (Transit) is based on the precise measurement of the Doppler shift from a single satellite observed by the user vehicle, plus the satellite's orbital elements and time. The time of the satellite's orbit parameter transmission is controlled by a crystal oscillator having a short-term stability of 2 parts in 1011. Multiple geostationary satellites (36000-km altitude) have the capability to provide a near instantaneous position fix using range measurements of the two-way propagation time of an RF signal initiated at a ground station and sent to a user vehicle by one satellite. The user retransmits this signal through the satellites to the ground where a position fix is obtained. Knowledge of user altitude, in the case of an aircraft, also is required. Low-altitude satellites can also be employed to provide a position fix via two-way ranging, but with a significant delay in obtaining a position fix. Inherent to this and many other navigation concepts is the accurate measurement of frequency, time, or time interval. If range is to be described by the time required for an RF signal to propagate between the satellite and the user or ground station, time interval becomes significant. For example, it takes approximately 1 µs for a radio wave to travel 0.3 km in free space. Time interval is often measured by counting cycles or by phase comparisons using highly stable and accurate frequency sources. NASA conducted tests of these concepts employing the ATS 1 and 3 satellites, and Nimbus 3 and 4 satellites. The test results indicate that locations of ships and aircraft can be determined to within 3-5 km, one sigma, by these satellite methods, and that location accuracy is both a function of the RF employed as well as the extent of knowledge of the satellite orbit.

Monomeric OH Stretching Frequencies in Monohalogenated Monocarboxylic Acids

In a recent publication from these laboratories we discussed spectra-structure correlations for pure monohalogenated monocarboxylic acids and noted that the bonded OH stretching vibration appeared to undergo a frequency shift in certain of the compounds studied. The extreme broadness of these absorption bands prevents accurate peak frequency measurements. We have now measured the frequencies of the monomeric OH stretching frequencies of these compounds in carbon tetrachloride solution, along with those of a few unsubstituted acids for comparison, and report the results here.

An integrated circuit frequency meter.

Impulse frequency meters available at present measure the interval between successive pulses and use a successive approximation technique to build up the hyperbolic transfer function needed to obtain the reciprocal of time. In this paper is presented a method for impulse frequency measurements with four operational amplifier units. Use is made of the mathematical relationship d/dx (logex)=1/x., The four functional stages of the instrument are thus an integrator, a logarithmic amplifier, a level restorer, and a differentiator. Measurements over 2·5 decades are possible without necessitating any switching of ranges. By switching in different values of capacitors for integration and differentiation, accurate frequency measurements over any desired range may be selected.

Observation of Singular Points in Defect-Induced Far-Infrared Spectra of KBr

Far-infrared absorption spectra of KBr crystals containing ${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, ${\mathrm{Sm}}^{++}$, ${\mathrm{OH}}^{\ensuremath{-}}$, and ${\mathrm{F}}^{\ensuremath{-}}$ are shown all to have sharp discontinuities at 74.8, 75.2, 85.5, and 89.5 ${\mathrm{cm}}^{\ensuremath{-}1}$. It is shown by comparison with dynamical models for the lattice and the defect that these frequencies correspond closely to expected van Home singularities in the host lattice. These experiments, when extrapolated to zero defect concentration, provide accurate measurements of certain phonon frequencies in the pure lattice.

Repetitive Q-modulated HCN gas maser

Repetitive modulation of the Q of an HCN gas maser resonator has enabled the effective lifetimes of the rotational levels to be studied as a function of the discharge parameters. Investigation of a Q-modulated maser with pulsed electrical excitation has shown that under certain conditions useful increases in the peak output power at 337 μm can be achieved. The motivation of the investigation is to produce a sub-millimetre radiation source with a high peak power. Such a source will have many future applications in the investigation of phenomena in solid-state and plasma physics. Interest has been confined to a group of maser wavelengths near 337 μm, which are strongly excited when an electrical discharge is run in gases or vapours containing compounds of hydrogen, carbon and nitrogen. The emissions centred at 30 cm-1 have recently been assigned to known vibration-rotation transitions in the HCN molecule by Lide and Maki in 1967. The radiation frequencies calculated from their theoretical assignment are in good agreement with accurate frequency measurements made by Hocker and Javan in 1967. The proposed energy level scheme is also partially confirmed by the observation of competition effects between transitions which are reported here. These effects have also been observed by Hocker and Javan. Further details concerning spectroscopic measurements are given in the paper by Matthias et al. in 1968. The results of two experiments are reported. Firstly, certain conditions necessary for the emission of the radiations at 310 μm, 311 μm, 337 μm and 373 μm are discussed. Secondly, experiments in which there is Q modulation of the rotational levels are described.

Large storage box hydrogen maser

An atomic hydrogen maser has been operated that confines atoms in a volume whose linear dimensions are approximately ten times larger than those of previous masers of this type. The uncertainties associated with the wall frequency shift, presently the principal limiting factor in the absolute accuracy of frequency measurements with the hydrogen maser, should be reduced with this device, since the fraction of time an atom spends on the storage box wall is inversely proportional to the diameter of the box. Oscillations are achieved by the use of two resonant cavities coupled by a high-gain amplifier. The strong field maintained in one cavity prestimulates the atoms to radiate at an enhanced rate in the weak field of the other. With enough gain, self-sustained oscillation can be achieved at normal hydrogen fluxes and normal cavity Q -factor values. The theory of the two-cavity large storage box maser is considered and preliminary results are discussed.

338. Vacuum technology: D Santeler, Virginia Polytech Inst Eng Ext Ser Circ, 1965

A set of quartz crystal microbalances (QMB) was used at JET with full carbon wall to monitor mass erosion/deposition rates in the remote areas of the divertor. After introduction of the ITER- like wall (ILW) in JET with beryllium main wall and tungsten divertor, strong reduction of the material deposition and accompanied fuel retention was observed. Therefore the existing QMB electronics have been modified to improve the accuracy of frequency measurements by a factor of ten down to 0.1 Hz which corresponds to 1.4 ng cm−2. The averaged deposition rates of 1.2–3 ng cm−2 s−1 and erosion rates of 5.6–8.1 ng cm−2 s−1 were observed in the inner divertor of JET -ILW with the inner strike point positions close to the bottom edge of vertical tile 3 and at the horizontal tile 4 respectively. The erosion with averaged rates of ≈2.1 ng cm−2 s−1 and ≈120 ng cm−2 s−1 were observed in the outer divertor for the outer strike point positions at tile 5 and tile 6 respectively.

Q Measurement of Strongly Coupled Cavities (Correspondence)

The long-line method herein described provides a rapid, simple, and fairly accurate measurement of resonant frequency and loaded bandwidth of a one-port cavity which is tightly coupled to its transmission line. Such tightly coupled cavities are often used as signal couplers in beam-type electron tubes. The cavity itself has a very high unloaded Q, and the beam loads the cavity to give a loaded Q approximately equal to the external Q (i. e., the beam is matched to the coupler's transmission line).

Frequency and Time Standards

The gradual development of quartz and atomic clocks has resulted in an improvement in the accuracy of frequency measurement by a factor of at least 104 in the past 30 years. The best quartz clocks drift upwards in frequency at an average rate of about 1 part in 109 per month and operate with a day to day stability of a few parts in 1011. A caesium standard based on the magnetic deflexion of an atomic beam has been in operation since 1955 and recent models of this type of standard enable frequency and time to be defined in terms of atomic constants with an accuracy of a few parts in 1011. It has revealed irregular and seasonal variations in the length of the day of about 1 msec. The techniques of optical pumping and detection, of the use of buffer gases, and of wall coatings which enable atoms to be reflected without undergoing transitions have made it possible to make working atomic standards which are already of great stability and may well be capable of still further improvement.

Phase Stabilized Klystron System and Its Application to Microwave Spectroscopy and Microwave Frequency Standards

A system is described for stabilizing the frequency and phase of a klystron where its frequency is to be varied continuously at any predetermined rate. This system, when applied to a microwave spectrograph and frequency standard, greatly improves the resolution, sensitivity, and accuracy of frequency measurements. The system also has application to superheterodyne detection systems in which the local oscillator is phase locked to the signal klystron.

Double Resonance Spectra of N14 and N15 Ammonium Ion

The double resonance spectra of N14 and N15 ammonium ion are in good agreement with the spectra calculated with a Hamiltonian that is made stationary by transformation to a rotating coordinate system. No effects of cross relaxation are apparent. Terms in the interaction of the strong radio frequency field with the proton magnetic moment, and off-diagonal elements in the nitrogen-proton spin-spin coupling can be neglected. The effects of sweeping magnetic field rather than frequency permit a very accurate measurement of the nitrogen resonance frequency, and the ratio of the N15 and N14 magnetogyric ratios.

Tracking in Space by DOPLOC

A satellite and space vehicle tracking system of the Doppler type, known as DOPLOC, is described. The characteristics of the heart of the system, a phase-locked tracking filter, are discussed from the viewpoints of bandwidth, signal-to-noise ratio, and accuracy of Doppler frequency measurement. System sensitivity to low energy signals is shown to be 2 × 10-29 watts at a bandwidth of 1 cps. Tracking ranges vs frequency are given for constant gain and constant aperture antennas. The advantages of DOPLOC in satellite tracking programs are briefly discussed.

A 10-cm mechanically swept spectrometer

The paper describes an instrument which was developed for monitoring continuously the spectrum of a 10-cm naval radar transmitter. The unit consists of a hybrid H011 coaxial mode cavity, which can be mechanically tuned over a ± 8 Mc/s frequency range at any predetermined point in the 10-cm band. The spectrum so obtained is displayed on a cathode-ray tube. The accuracy of frequency measurement with the components used is about ± ¼ Mc/s. The paper describes apparatus which would not necessarily be used in future equipments.

The technique of frequency measurement, and its application to telecommunications

Rigid frequency control has become a necessity in radio broadcasting and indeed in all forms of telecommunication, and the art of frequency measurement has likewise become of great importance. The improvement in frequency standards which has taken place in recent years is briefly outlined in this paper, and some of the methods which may be used to obtain accurate frequency measurements in terms of the improved standards are described.

Manufacture of Quartz Crystal Filters

Quartz crystal filters used in modern carrier systems present new problems in manufacturing technique. In the assembly and testing of the filters and in the production of component crystals, coils and condensers, special factory facilities are required for accurate measurement of frequency and control of atmospheric conditions. The manufacture of quartz crystal plates in particular combines several fields of applied science, including crystallography, precision grinding, vacuum technique and high frequency electrical measurement. Inductance coils and fixed and variable condensers for use in crystal filters must consistently meet advanced requirements, especially in regard to stability. The assembly of these components into filters resembles the manufacture of radio receivers, differing mainly because of smaller quantity requirements. Testing equipment must permit rapid shop adjustment and test of the completed filters with laboratory precision.