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Semiconductor Devices
Published in Dale R. Patrick, Stephen W. Fardo, Electricity and Electronics Fundamentals, 2020
Dale R. Patrick, Stephen W. Fardo
Varactor diodes are used primarily in resonant circuits where some level of tuning or frequency control is desired. Figure 3-37 shows a representative varactor tuned LC circuit. The resonant frequency of this circuit is determined by the inductor L and the series combination of CD and C1. C1 normally has a larger capacitance value than CD. C1 is also used to prevent the dc bias voltage of CD from going into the inductor. R1, R2 and R3 serve as the bias voltage control network for CD. Adjusting R2 will change the amount of reverse bias voltage to CD. Moving the wiper arm of R2up causes more reverse bias voltage to CD. This increases the capacitance of CD. Moving the wiper arm down reduces the bias voltage, which causes a reduction in capacitance. Used in this manner the varactor becomes a tuning component for resonant frequency. Other applications of the varactor diode are automatic frequency control AFC in FM radio and television receivers.
Capacitance and Capacitance Measurements
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
Voltage variable capacitors: These capacitors make use of the capacitive effect of the reversed-biased p–n junction diode. By applying different reverse-bias voltages to the diode, the capacitance can be changed. Hence, the name varicap or varactor diodes are given to these devices. Varactors are designed to provide various capacitance ranges from a few picofarads to more than 100 pF. For improved performances, it is also possible to make use of high-speed switching silicon diodes as voltage variable capacitors. However, they are limited by the very low maximum capacitance availability. Typical applications of these varactor diodes are in the tuning circuits in radio frequency receivers. Present-day varactor diodes operate into the microwave part of the spectrum. These devices are quite efficient as frequency multipliers at power levels as large as 25 W. The efficiency of a correctly designed varactor multiplier can exceed 50% in most cases. It is also worth noting that some Zener diodes and selected silicon power-supply rectifier diodes can work effectively as varactors at frequencies as high as 144 MHz. In the case of the Zener diode, it should be operated below its reverse breakdown voltage.
Oscillator Circuits
Published in Mike Golio, Commercial Wireless Circuits and Components Handbook, 2018
Tunable resonators are very important because they offer the ability to transfer a reference frequency, with or without modulation, through a PLL. Tunable resonators also offer direct modulation and frequency agility for communication and test purposes. Varactor diodes are the most common device for tuning an oscillator. These devices are inexpensive, available in a variety of packages, and can be used at almost any frequency of interest. Varactors also offer rapid tuning for frequency hopping and high speed direct modulation. The only disadvantages of a varactor diode are low Q at high frequencies, low frequency noise, and a nonlinear tuning characteristic [6].
A multiband frequency reconfigurable and bifunctional metasurface
Published in International Journal of Electronics Letters, 2022
B Anil Babu, B. T. P Madhav, K Srilatha, M C Rao, Sudipta Das
In this paper, a single layer, multiband, flexible, and bifunctional metasurface with frequency switching is proposed for simultaneous full space EM wave manipulation. The designed bifunctional surface unit cell is realised using a flexible PDMS substrate with 18x18x1 mm3 dimensions. Two varactor diodes are utilised for frequency switching using DC bias control. As an AMC reflector in reflection space, it provides >84% total efficiency with unidirectional E-field patterns. In the transmission space, it provides an absorptivity of >96% at all frequencies as an absorber. The achieved simultaneous full-space bifunctional concept of the realised metasurface is illustrated using the CST full-wave simulation tool. The experimental validation is done with a fabricated prototype. Thus, the demonstrated bifunctional metasurface provides a strategic platform for wearable antenna integration in the future wearable communication devices.
Wide-frequency tunable bandpass filter with high-frequency selectivity
Published in Electromagnetics, 2019
Kaijun Song, Xi Wang, Cuilin Zhong, Yuxuan Chen, Shema Richard Patience, Song Guo, Yong Fan
Based on the above-described design, a constant absolute bandwidth tunable bandpass filter is fabricated on a 0.508 mm RF-35 substrate manufactured in Taconic with and . According to the above design method, the dimensions of the tunable bandpass filter are determined as follows: D1 = 0.9 mm, D2 = 0.4 mm, D3 = 0.4 mm, W0 = 1.14 mm, W1 = 0.5 mm, W2 = 1.14 mm, W3 = 2 mm, L11 = 4.5 mm, L12 = 8.5 mm, L13 = 4.1 mm, L14= 2.5 mm, L15 = 2 mm, L21 = 4.49 mm, L22= 17.85 mm. Figure 9(a) shows the fabricated tunable bandpass filter, and the circuit size is 25 mm × 45 mm. Moreover, the bias voltage is applied to the varactors by microstrip line, and in order to avoid the influence of bias circuit to the filter, resistors with large resistance needs to be loaded at the end of the bias circuit like Figure 9(a). The varactor diode is SKYWORKSS’s SMV-1234-079LF and its equivalent capacitance is from 1.32pF to 9.6pF.
Voltage-controlled oscillators using CFOAs with correction terminal Z
Published in International Journal of Electronics, 2023
To start the process of occurrence of oscillations in the circuit, a minimum value is initially set for the resistance RS, gradually increasing until the desired amplitude and linearity of the signal shape is reached. From (7) and (8) it can be seen that the self-oscillation conditions are influenced by the parameters of the CFOA, as well as the values of the components in a resonant LC circuit and the resistance of the variable resistor RS, with which a positive and negative feedback is implemented. Based on the method proposed in (Pandiev et al., 2005) Fig. 5 presented simulation results for the performed statistical Monte Carlo analysis when parameter values are changed. To perform the simulations of the circuit, SPICE models AD844 (Bowers et al., 1990) and varactor diodes BB201 (BB201 SPICE model Rev 1.3 Jun 8, 2012, 2022) were used. From all 100 analyzes with changing parameters (a tolerance on the RS is set (for example, from ± 25% to ± 50%) and for the LC tank is used passive components with ± 10% tolerances, four variants 3, 7, 11, 12 and 32 were selected, respectively, and for variant 3, the maximum value of the loop gain was obtained with a value closest to the set oscillation frequency and the phase angle had a minimum value. For variant 3 the parameter values are: RS = 14.49kΩ (the loop gain is approximately 2), CV = 18.8pF, L1 = 10.47μH and L2 = 1.045μH. The capacitance value of the varactor diodes is achieved by fine-tuning the control voltage.