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Analogue systems and concepts
Published in Geoff Lewis, Communications Technology Handbook, 2013
Clapp oscillator. The circuit shown in Fig. 1.29 uses a series tuned circuit to control the frequency of oscillation and an amplifier that operates in the common drain mode due to the inclusion of the bypass capacitor C4. Capacitors C2 and C3 form a potential divider network to couple the tuned circuit into the three electrodes of the amplifier. The circuit oscillates at slightly above the series resonant frequency of L1C1, making the impedance inductive and providing positive feedback. Because of the very low impedance of the resonant circuit, this oscillator is relatively immune to drift in the parameters of the amplifier and thus forms a very stable variable frequency oscillator.
Introduction
Published in Sibley Martin, Modern Telecommunications, 2018
Figure 1.13a shows a number of radio stations as picked up by the receiver antenna. Note that there are a large number as the receiver antenna need not be too selective. The first two amplifiers in the receiver are RF amplifiers and they select a fairly narrow range of stations. Care must be exercised here so that the RF amplifiers do not filter out the station required. For this reason, the post-amplifier is often more selective than the pre-amplifier. In the diagram, only four stations are selected to pass to the mixer. The other input to the mixer is a local oscillator which can vary in frequency. This variable frequency oscillator (VFO) is tuned to a frequency of 1.6 MHz (Station 1) plus 470 kHz (Figure 1.13c). This offset is the IF (intermediate meaning intermediate between the RF and AF). The resultant mixing products are the sum and difference frequencies and Figure 1.13d shows the difference components. Tuning of the IF amplifiers, using parallel tuned circuits, to the IF of 470 kHz means that they will reject all stations that are not at the IF. Thus, Station 1 passes through to the demodulator and is amplified prior to conversion back into a pressure wave in the loudspeaker. Note that there is a negative frequency here – Station 4. This negative appears as a positive frequency of 130 kHz because the cosine of a negative number equals the cosine of a positive number. In effect, the negative signals fold around zero to appear positive. Care must be taken to ensure that the “negative” frequency does not interfere wit h the station sitting at 470 kHz, otherwise interference will result.
Unprecedented Technologies found in Nature Led by Harvesting the Geometry of Singularity
Published in Anirban Bandyopadhyay, Nanobrain, 2020
One fundamental parameter for electromechanical oscillations is the phase modulation. The phase modulation behavior in the kHz and MHz domain, since the phase difference between input and the output ac signal in these regions are quantized. Statistical count of phase difference Φ shows peaks at 0°, 45°, 90°, 135°, and 180°; wherein, each resonance peak is associated with a distinct Φ. The kHz and GHz bands do not change the phase of the ac input signal while a quantized phase modulation by nπ/4 occurs during MHz transmission across the microtubule, thus, in the MHz band, microtubule acts as automated phase-locked-loop (PLL) oscillator. A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input “reference” signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector. The circuit compares the phase of the input signal with the phase of the signal derived from its output oscillator and adjusts the frequency of its oscillator to keep the phases matched. The signal from the phase detector is used to control the oscillator in a feedback loop. Frequency is the derivative of phase. Keeping the input and output phase in lockstep implies keeping the input and output frequencies in lockstep. Consequently, a phase-locked loop or PLL can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. The former property is used for demodulation, and the latter property is used for indirect frequency synthesis.
Performance Analysis of Maximum Power Point Tracking for Two Techniques with Direct Control of Photovoltaic Grid -Connected Systems
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Bahaa Saleh, Ali M. Yousef, Farag K. Abo-Elyousr, Moayed Mohamed, Saad A. Mohamed Abdelwahab, Ahmed Elnozahy
PLL is the instantaneous phase angles detected by synchronizing the PLL rotating reference frame to the utility voltage vector. The PLL is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types of PLL circuits; the simplest is an electronic circuit consisting of a variable frequency oscillator and a phase detector in a feedback loop. The oscillator generates a periodic signal, and the phase detector compares the phase of that signal with the phase of the input periodic signal, adjusting the oscillator to keep the phases matched. To control the voltage at the inverter via the PI and H∞C, a current regulator and VDC regulator (voltage source control) are employed as presented in Figure 10c and 10d, respectively.
Anisotropic plates identification through analyses of dynamic behaviour
Published in Mechanics of Advanced Materials and Structures, 2023
Arcangelo Messina, R. Nobile, N. I. Giannoccaro, A. V. De Nunzio
A relevant classification of the vibrational methods for identifying material properties of composite materials is reported in the review [9]. The vibrational methods may use impulse and continuous variable excitation depending on the type of excitation used to vibrate the specimen. In impulse excitation, an impulse is usually used to strike the specimen mechanically and elastically. In contrast, continuous variable excitation commonly involves loudspeakers or shakers fed by a variable frequency oscillator. The impulse technique is often the primary choice for studying freely vibrating systems due to its simplicity and inexpensive procedure. About this technique currently, there is only one ASTM standard procedure [10] provided for testing isotropic materials, but none for anisotropic materials; despite this, multiple studies [9] have been carried out applying this procedure to orthotropic and anisotropic laminated materials. For example, in [11], the impulse technique is applied to identify six elastic material moduli of generally thick composite plates by using a set of natural frequencies of flexural vibrations and a numerical model based on the finite element method optimized using analytical sensitivities. In [12], the impulse technique is used to obtain the five elastic constants of aluminum and carbon/epoxy composite materials using the measured natural frequencies and a combination method of finite element analysis and optimum design. In [13], the impulse technique is used to measure the resonance frequencies (five frequencies used) of rectangular plates of orthotropic materials (commercial 6082 Al-alloy and 304 stainless steel) and estimate the four elastic constants by iteratively updating a finite element model to generate the same measured frequencies. In [14], a hybrid genetic algorithm is proposed to inversely determine for a transversely isotropic material (a plate of unidirectional glass/epoxy laminate and a plate of symmetric cross-ply laminate) the optimum solutions and the five elastic constants from the vibration testing data (eight frequencies) and a finite element model.