Frequency synthesiser

To sweep a frequency band the synthesiser generates a series of incremental frequency steps at programmed intervals, say 1000 steps of 50 Hz to the -th harmonic of which the Gunn source will follow, as explained earlier. As important as frequency stability and resolution is the settling time between steps of the frequency synthesiser and indeed the other parts of the control circuits, which dictates the scan rate and the ability to step between spectral lines quickly. In the example above, a settling time of 1 ms would dictate a sweep time not less than... 

Taking into account that it is necessary to make vary the flaw detector settings during the main part of the verifications, the total number of verifications is rather important. So, Technical Center for Mechanical Industries (CETIM) began the development of an electronic system enabling to benefit of the current possibilities of generation of synthesised radio frequency signals and help of personal computer for operator assistance and calculation. 

NPY is primarily (but not exclusively) synthesised and released by neurons, which in the peripheral nervous system are predominantly sympathetic neurons [1]. In most cases, NPY acts as a co-transmitter that is preferentially released upon high frequency nerve stimulation. NPY can be metabolised by the enzyme dipeptidylpeptidase IV (also known as CD26) to generate the biologically active fragment NPY3 36. 

Kochenburger, R. J. Trans. AIEE 69 (1950) 270. A frequency response method for analysing and synthesising contactor servomechanisms. 

Within the frequency range up to 200 GHz the klystron can be replaced (at a price) by synthesisers, coupled with solid state microwave amplifiers and passive or active frequency multipliers. These devices have very high frequency stability, are easily modulated either in frequency or power, and are readily compatible with computer control of all their main fbnctions. The klystron is gradually becoming redundant, but has an honoured place in the development of microwave spectroscopy. 

The simplest way to obtain a subset of a dataset is by means of simple random sampling but this is most unlikely to provide a subset that encompasses all of the structural classes present within that dataset. Instead, classes that are heavily populated in the dataset, such as the very large sets of analogues that characterise many corporate databases, will be represented proportionally in the subset, while low-frequency classes, where only a few molecules have been synthesised or otherwise acquired, are unlikely to be represented. Thus, while random selection samples the molecules that are present within a dataset, the selection methods discussed in this chapter are intended to sample the classes of molecules that are present within that dataset. The first question to be addressed when considering the effectiveness of the various methods is thus whether they do, in fact, perform better than random only when an affirmative answer has been received to this question is it appropriate to consider which method (or class of methods) is the best ofthose that are available. 

Polymers were formed upon reaction of a 1 1 mixture of 4,4 -diisocyanatodiphe-nylmethane and 30% phenyltriisocyanate with bisphenol A (2,2-bis(4-hydroxyphe-nyl)-propane), diluted 1 20 with solvent, either CHCI3 or THF (for structures see Fig. 20.4). A MIP synthesised in THF showed no (frequency) response to ammonia vapour, whereas a non-imprinted polymer showed a response to ammonia (a much stronger electron donor) twice that to THF. 

The frequency required to observe a particular nucleus is the first consideration. This frequency was formerly synthesised by analogue methods. Frequencies below 200 MHz... 

The initially amplified signal is then usually detected in the receiver using a three-port device acting as a double-balanced mixer. Three port devices, which can be used as gates, mixers or phase-sensitive detectors, consist of a series of diodes and transformer coils. The phase-sensitive detector has two input signals, the NMR signal (wq) and the frequency of the synthesiser (Wref)- The output is proportional to... 

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