Marginal oscillation

In ion cyclotron resonance spectrometry, a signal results when the cyclotron frequency of an ion, co ( = qH/mc), equals the frequency coq of a marginal oscillator detector. At this point, the ions of a particular mass m and charge q are in resonance, and they absorb power from the marginal oscillator when the... 

The cyclotron motion of ions can also be excited by a weak field without sweeping them out from the swarm, which enables the absorbed energies at the different resonance frequencies to be obtained. This can be done by using either an rf bridge circuit whose imbalance in resonance is displayed, or a marginal oscillator, extremely sensitive to small changes in impedance in the resonant circuit. 

Another scheme involves modulation of the drift voltages which, by varying the ion transit time, produces modulations in the density of ions in the cell the marginal oscillator records a periodic change in energy absorption which is detected as sm absorption signal at the modulation frequency. 

A substantial improvement in sensitivity was obtained by Lle-well5m 132a) who connected the irradiating transmitter and the marginal oscillator in series to the opposite drift plates of the analyzer region. 

Marginal oscillation is a state rather than a device characteristic. It refers to the initial phase in the build-up of oscillation in an electronic circuit. Figure 3.4 shows the typical MMW power output vs. bias voltage of a Gmm oscillator exaggerated for clarity. A conventional oscillator would work at V it is possible, however, to work in the marginal oscillation region and this has special characteristics that make it attractive for simple MMW structure designs. 

The work of Dumesh, Surin et describes their Orotron devices where power absorbed by the gas is measured through a change in the electron beam current within the structure, when it is in a state analogous to marginal oscillation. The sensitivity and other attractive characteristics of those devices are discussed in Chapter 5. 

Detection of absorption was achieved by source FM and monitoring changes in the current when the Orotron was in a state of marginal oscillation using the usual phase-coherent detectors. The device under those conditions operated as a square law detector with sensitivity 1 AW . The resulting sensitivity of absorption was 3-5 X 10 m with a 1 Hz bandwidth receiver and they report analysis of CO, CH3OH, CH3COCH3 and OCS in admixture with water or air. 

However, for such a calculation, the sensitivity of the marginal oscillator must be known (i.e., the response for a known ion current at some m/e must be known). It is usually easy enough to calibrate a given marginal oscillator at a given frequency, but it is also true that the sensitivity of... 

When the marginal oscillator frequency a>i is held constant and the magnetic field is swept to obtain a spectrum, all ions pass through resonance at the same natural cyclotron frequency (i.e., co = Solving Eq. 

As mentioned previously, a serious problem associated with the rf-field-swept experiment is the difficulty of constructing a marginal oscillator whose sensitivity is independent of frequency. Even relative intensities are obviously of little use if the sensitivity of the oscillator changes between peaks. 

Continuous wave techniques CW NQR spectrometers are based on the use of oscillator-detectors, which are built around the circuits of a marginal oscillator or a limited oscillator (Robinson oscillator). Such an oscillator-detector includes a tank circuit with a coil, into which the studied sample is inserted. When the frequency of the oscillator-detector coincides with the NQR frequency in the sample, the... 

In the literature one can find many descriptions of CW NQR spectrometers and oscillator-detectors for various purposes and frequency ranges. Particular interest is evoked by limited oscillators, which have a low noise level similar to that of marginal oscillators but do not need continual critical adjustment of circuit parameters. Figure 2 shows a simplified block diagram of a CW spectrometer, containing an oscillator-detector with a Zeeman modulation system and a computer for signal processing and frequency control. 

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