Oscillation circuit

Most NC-AFMs use a frequency modulation (FM) teclmique where the cantilever is mounted on a piezo and serves as the resonant element in an oscillator circuit [101. 102]. The frequency of the oscillator output is instantaneously modulated by variations in the force gradient acting between the cantilever tip and the sample. This teclmique typically employs oscillation amplitudes in excess of 20 mn peak to peak. Associated with this teclmique, two different imaging methods are currently in use namely, fixed excitation and fixed amplitude. 

Foxboro s Model 823 transmitter uses a taut wire stretched between a measuring diaphragm and a restraining element. The differential process pressure across the measuring diaphragm increases the tension on the wire, thus changing the wire s natural frequency when it is excited by an electromagnet. This vibration (1800—3000 H2) is picked up inductively in an oscillator circuit which feeds a frequency-to-current converter to get a 4—20 m A d-c output. 

Schwing-kreis, m. oscillating circuit circuit, -kristall, m. crystal oscillator, -neigung, /. oscillating tendency. -lohr, n. swing pipe, -rohre, /. oscillatory valve. 

This is merely an oscillating circuit in which one of the parameters (L or C) is made to vary at a frequency twice as great as the natural frequency of the circuit. 

Heinrich Hertz in 1887 who used an oscillating circuit of small dimensions to produce electromagnetic waves which had all of the properties of light waves... 

AT-cut, 9 MHz quartz-crystal oscillators were purchased from Kyushu Dentsu, Co., Tokyo, in which Ag electrodes (0.238 cm2) had been deposited on each side of a quartz-plate (0.640 cm2). A homemade oscillator circuit was designed to drive the quartz at its resonant frequency both in air and water phases. The quartz crystal plates were usually treated with 1,1,1,3,3,3-hexamethyldisilazane to obtain a hydrophobic surface unless otherwise stated [28]. Frequencies of the QCM was followed continuously by a universal frequency counter (Iwatsu, Co., Tokyo, SC 7201 model) attached to a microcomputer system (NEC, PC 8801 model). The following equation has been obtained for the AT-cut shear mode QCM [10] ... 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

Figure 3.24 depicts a piezoelectric sensor consisting of two oscillator circuits a detector crystal oscillator and a reference crystal oscillator. The two are identical except for the fact that the reference oscillator is not coated with biological material and is intended to correct for temperature and humidity fluctuations, as well as other interfering effects. The two oscillator frequencies are fed to a mixer that provides the difference in frequency between the two crystals. In order to use the piezoelectric effect to detect a target dissolved substrate it should be reacted with a suitable biocatalyst immobilized on the crystal by entrapment (deposition from an acrylamide solution), cross-linking, irradiation or pre-coating. 

Figure 3.24 — Typical system for piezoelectric crystal detector incorporating reference (C,) and test (CJ crystal sensors individually held in oscillating circuits (Or and 0 respectively) serviced by separate frequency counters (FC, and FCj, respectively) interfaced to a common microprocessor or other readout device. (Reproduced from [167] with permission of the American Chemical Society).

All unite developed up to now are based on use of an active oscillator, as shown schematically in Fig, 6.5. This circuit keeps the crystal actively in resonance so that any type of oscillation duration or frequency measurement can be carried out. In this type of circuit the oscillation is maintained as long as sufficient energy is provided by the amplifier to compensate for losses in the crystal oscillation circuit and the crystal can effect the necessary phase shift. The basic stability of the crystal oscillator is created through the sudden phase change that takes place near the series resonance point even with a small change in crystal frequency, see Fig. 6.6. 

Normally an oscillator circuit Is designed such that the crystal requires a phase shift of 0 degrees to permit work at the series resonance point. Long-and short-term frequency stability are properties of crystal oscillators because very small frequency differences are needed to maintain the phase shift necessary for the oscillation. The frequency stability Is ensured through the quartz crystal, even If there are long-term shifts In the electrical values that are caused by phase jitter due to temperature, ageing or short-term noise. If mass Is added to the crystal. Its electrical properties change. 

Fig. 6.7 shows the same graph as Fig 6.6, but for a thickly coated crystal. It has lost the steep slope displayed In Fig. 6.6. Because the phase rise Is less steep, any noise In the oscillator circuit leads to a larger frequency shift than would be the case with a new crystal. In extreme cases, the original phase/frequency curve shape Is not retained the crystal Is not able to carry out a full 90 ° phase shift.

This time-variant signal (usually referred to as an AC signal) is found in a multitude of electronic circuits. Power delivered to homes and businesses is nearly universally transmitted using an AC signal. Communications circuits require exact sine waves in order to transmit information over large distances with low loss of signal integrity. Just as numerous as the amount of potential uses for oscillator circuits is the amount of circuits that can create these oscillators. In this chapter we will examine several oscillator circuits in detail. 

Note that the Q of the op-amp stage in an oscillator circuit is designed to be large in order to create an oscillation, while the Q of the op-amp stage of a filter is designed to be small in order to suppress the possibility of oscillation. 

The crystals are placed between the plates of a condenser which forms part of an oscillating circuit. An audio-frequency amplifier, with headphones or speaker, is connected to the oscillator. When the frequency of the oscillator is changed continuously by means of a variable condenser in the circuit, clicks (or, for a large number of small crystals, rustling noises) are heard. The reason is that whenever the frequency of the oscillator happens to coincide with a natural frequency of one of the crystals, there is a sudden change of current through the condenser and consequently an impulse which is amplified by the audio-frequency amplifier. For a suitable circuit see Wooster (1957). 

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