Microwave frequency standard

Another possibility is to use optical atomic frequency standards. Any evolution of a in time would lead to a frequency shift. To establish the connection between a and u>, relativistic calculations of the a dependence of the relevant frequencies for Cal, Srii, Bail, Ybn, Hgn, Inn, Tin and Rail have been performed [1]. The a dependence of the microwave frequency standards (Cs, Hg+) has also been accurately calculated. 

One way an optical standard could be provided is by harmonic multiplication of a microwave frequency standard in a synthesis chain. By use of this technique, a laser at 88 THz (3.39 pm) has been made phase coherent with a microwave oscillator. - The best optical frequency standards may be made by locking a local oscillator (laser) to an atomic or molecular resonance line. State-of-the-art accuracies are characterized by measurements on methane stabilized He-Ne lasers in which reproducibilities in the 10— -- range have... 

Atomic hyperfine structure and Microwave frequency standards... 

P.F. Fisk Trapped-ion and trapped-atom microwave frequency standards. Rep. Prog. Phys. 60, 761 (1997)... 

ASTM D2520, 2001. Standard test methods for complex permittivity (dielectric constant) of solid insulating materials at microwave frequencies to 1650°C. 

Maser amplifiers are used where the requirement for a very low noise amplifier outweighs the technological problems of cooling 10 low temperatures. They have been used in passive and active radiostronomical work, in satellite communications, and us preamplifiers for microwave spectrometry The ammonia and the atomic hydrogen masers have been studied as frequency standards and have heen used in accurate tests of special relativity. 

ASTM 1986. Standard Methods Qf Test for Complex Permittivity (Dielectric Constant) of Solid Electrical Insulating Materials at Microwave Frequencies and Temperatures 1q 1650°C. Document D 2520-86 (Reapproved 1990). Philadelphia, PA. American Society for Testing and Materials (ASTM). 

It is now recognized that cold collision frequency shifts [32] is a crucial issue for every high precision atomic frequency standard, microwave or optical. For hydrogen at a density of 109 cm-3 the shift of the 1S-2S transition is about 0.4 Hz, [8], or a fractional shift of 1.7 x 10-16. For a rubidium hyperfine standard operating at the same density, the shift is about 6 xlO-14 [45,46]. 

Other potential concerns involve the two frequency references used in the experiment - the proton precession frequency forming the basis of the magnetic field determination, and the Loran-C 10 MHz frequency reference used for the NMR and microwave frequency synthesizers. The Loran-C standard is based on hyper-fine transitions in Cs with Wf=0, and so is insensitive to any preferred spatial orientation, and would not introduce a signature for Lorentz violation into the spectroscopic measurements. Bounds on clock comparisons of 199Hg and 133Cs [7,8] place crude limits on the Lorentz violating energy shifts in the precession frequency of a proton of 10-27 GeV, which imply the NMR measurements are free of shifts well below the Hz level. 

One interesting reference standard may be the methane stabilized helium neon laser at 3.39 pm. Its infrared frequency can be compared directly with the microwave cesium frequency standard with the help of a relatively short frequency chain [32], An accuracy of 1 part in 1012 or better appears feasible for a transportable secondary standard. 

In this paper, we address two aspects of this general problem. First, we discuss the problem of frequency standards in the optical spectrum. (An analogue in the microwave region of the spectrum is the cesium beam frequency standard.) If one or a few of these reference frequencies can be accurately calibrated (perhaps by a frequency synthesis chain- -) then it may be possible to compare optical spectra to these standards. As an example of the precision that might be achieved, we discuss only optical standards based on stored ions. Second, we discuss the problem of frequency comparison of unknown frequencies to the standards. Here we primarily restrict discussion to generation of wideband frequency "combs". 

Hydrogen masers are used as a high-precision frequency standard its frequency of 1,420,405,752 Hz (long-term stability = 1 part in 5 x 1016 in 5 years ) also corresponds to the most intense microwave source in the universe, due to the energy difference between the (S = 1/2,7 = +1/2) and (S = 1/3, 7=1/2) states the energy separation is due to hyperfine coupling in hydrogen. 

Microwave-assisted operations are a very promising type of electromagnetic field application to intensify chemical processes [110-116]. Microwave frequencies range from 0.3 to 300 GHz but, to avoid interference, industrial and domestic microwave appliances operate at standard allocated frequencies, most often at 2.45 GHz. 

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