Development of a measuring system for checking the parameters of variable capacitor with capacity 15-500 pF

Characteristic variable capacitor structure. The definition of electronic control features. Issdelovanie circuit AC bridge. Determination of the resistance measuring bridge capacitors parameters. Consideration device universal scheme of the bridge.

Рубрика Физика и энергетика
Вид реферат
Язык английский
Дата добавления 17.12.2015
Размер файла 590,4 K

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National Aviation University

Institute of Informational-diagnostic sysytem

TERM PAPER

by discipline

“Metrology”

on the theme: “Development of a measuring system for checking the parameters of variable capacitor with capacity 15…500 pF”

Made by:

Eugen Nagorniy,

group №237

Checked: Anatoliy Pavlovich Kozlov

Kyiv

2014

General theory

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore it is sometimes called a tuning capacitor or tuning condenser), or as a variable reactance, e.g. for impedance matching in antenna tuners.

Mechanically controlled

In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed.

The most common form arranges a group of semicircular metal plates on a rotary axis (“rotor”) that are positioned in the gaps between a set of stationary plates (“stator”) so that the area of overlap can be changed by rotating the axis. Air or plastic foils can be used as dielectric material. By choosing the shape of the rotary plates, various functions of capacitance vs. angle can be created, e.g. to obtain a linear frequency scale. Various forms of reduction gear mechanisms are often used to achieve finer tuning control, i.e. to spread the variation of capacity over a larger angle, often several turns.

A vacuum variable capacitor uses a set of plates made from concentric cylinders that can be slid in or out of an opposing set of cylinders (sleeve and plunger). These plates are then sealed inside of a non-conductive envelope such as glass or ceramic and placed under a high vacuum. The movable part (plunger) is mounted on a flexible metal membrane that seals and maintains the vacuum. A screw shaft is attached to the plunger, when the shaft is turned the plunger moves in or out of the sleeve and the value of the capacitor changes. The vacuum not only increases the working voltage and current handling capacity of the capacitor it also greatly reduces the chance of arcing across the plates. The most common usage for vacuum variables are in high powered transmitters such as those used for broadcasting, military and amateur radio as well as high powered RF tuning networks. Vacuum variables can also be more convenient since the elements are under a vacuum the working voltage can be higher than an air variable the same size, allowing the size of the vacuum capacitor to be reduced.

Very cheap variable capacitors are constructed from layered aluminium and plastic foils that are variably pressed together using a screw. These so-called squeezers can't provide a stable and reproducible capacitance, however. A variant of this structure that allows for linear movement of one set of plates to change the plate overlap area is also used and might be called a slider. This has practical advantages for makeshift or home construction and may be found in resonant loop antennas or crystal radios.

Small variable capacitors operated by screwdriver (for instance, to precisely set a resonant frequency at the factory and then never be adjusted again) are called trimmer capacitors. In addition to air and plastic, trimmers can also be made using a ceramic dielectric.

Electronically controlled

The thickness of the depletion layer of a reverse-biased semiconductor diode varies with the DC voltage applied across the diode. Any diode exhibits this effect (including p/n junctions in transistors), but devices specifically sold as variable capacitance diodes (also called varactors or varicaps) are designed with a large junction area and a doping profile specifically designed to maximize capacitance.

Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance would be affected by the change of signal voltage, precluding their use in the input stages of high-quality RF communications receivers, where they would add unacceptable levels of intermodulation. At VHF/UHF frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by noise rather than large signal handling requirements, and varicaps are commonly used in the signal path.

Varicaps are used for frequency modulation of oscillators, and to make high-frequency voltage controlled oscillators(VCOs), the core component in phase-locked loop (PLL) frequency synthesizers that are ubiquitous in modern communications equipment.

Digitally tuned capacitor

A digitally tuned capacitor is an IC variable capacitor based on several technologies. MEMS, BST and SOI/SOS devices are available from a number of suppliers and vary in capacitance range, quality factor and resolution for different RF tuning applications.

MEMS devices have the highest quality factor and are highly linear, and therefore are suitable for antenna aperture tuning, dynamic impedance matching, power amplifier load matching and adjustable filters. RF tuning MEMS are still a relatively new technology and has not yet been accepted broadly.

BST device are based on Barium Strontium Titanate and vary the capacitance by applying high voltage to the device. The tuning accuracy is limited only by the accuracy of the D-A converter circuitry that generates the high voltage. The limitations for BST are stability over temperature and linearity in demanding applications.

SOI/SOS tuning devices are constructed as solid state FET switches built on insulated CMOS wafers and use MIM caps arranged in binary-weighted values to achieve different capacitance values. SOI/SOS switches have high lineary and are well suited to low power applications where high voltages are not present. High voltage endurance requires multiple FET devices in series which adds series resistance and lowers the quality factor.

The capacitance values are designed for antenna impedance matching in multi-band LTE GSM/WCDMA cellular handsets and mobile TV receivers that operate over wide frequency ranges, such as the European DVB-H and Japanese ISDB-T mobile TV systems.

Transducers

Variable capacitance is sometimes used to convert physical phenomena into electrical signals.

In a capacitor microphone (commonly known as a condenser microphone), the diaphragm acts as one plate of a capacitor, and vibrations produce changes in the distance between the diaphragm and a fixed plate, changing the voltage maintained across the capacitor plates.

Some types of industrial sensors use a capacitor element to convert physical quantities such as pressure,displacement or relative humidity to an electrical signal for measurement purposes.

Capacitive sensors can also be used in the place of switches, e.g. in computer keyboards or “touch buttons” for elevators that have no movable parts.

Special forms of mechanically variable capacitors

Multiple sections

Very often, multiple stator/rotor sections are arranged behind one another on the same axis, allowing for several tuned circuits to be adjusted using the same control, e.g. a preselector, an input filter and the corresponding oscillator in a receiver circuit. The sections can have identical or different nominal capacitances, e.g. 2 Ч 330 pF for AM filter and oscillator, plus 3 Ч 45 pF for two filters and an oscillator in the FM section of the same receiver. Capacitors with multiple sections often include trimmer capacitors in parallel to the variable sections, used to adjust all tuned circuits to the same frequency.

Butterfly

A butterfly capacitor is a form of rotary variable capacitor with two independent sets of stator plates opposing each other, and a butterfly-shaped rotor arranged so that turning the rotor will vary the capacitances between the rotor and either stator equally.

Butterfly capacitors are used in symmetrical tuned circuits, e.g. RF power amplifier stages in push-pull configuration or symmetrical antenna tuners where the rotor needs to be “cold”, i.e. connected to RF (but not necessarily DC) ground potential. Since the peak RF current normally flows from one stator to the other without going through wiper contacts, butterfly capacitors can handle large resonance RF currents, e.g. in magnetic loop antennas.

In a butterfly capacitor, the stators and each half of the rotor can only cover a maximum angle of 90° since there must be a position without rotor/stator overlap corresponding to minimum capacity, therefore a turn of only 90° covers the entire capacitance range.

Split stator

The closely related split stator variable capacitor does not have the limitation of 90° angle since it uses two separate packs of rotor electrodes arranged axially behind one another. Unlike in a capacitor with several sections, the rotor plates in a split stator capacitor are mounted on opposite sides of the rotor axis. While the split stator capacitor benefits from larger electrodes compared to the butterfly capacitor, as well as a rotation angle of up to 180°, the separation of rotor plates incurs some losses since RF current has to pass the rotor axis instead of flowing straight through each rotor vane.

Differential

Differential variable capacitors also have two independent stators, but unlike in the butterfly capacitor where capacities on both sides increase equally as the rotor is turned, in a differential variable capacitor one section's capacity will increase while the other section's decreases, keeping the stator-to-stator capacitance constant. Differential variable capacitors can therefore be used in capacitive potentiometric circuits.

Practical part

For the measurement of parameters of capacitors and inductors balanced ac bridges are widely used.

In the general case, the measuring bridge shoulders AC (Fig. 3) have complex impedances Z1, Z2, Z3 and Z4, one of which, e.g. Z4, is a measurement object. Voltage of bridge made ??from the AC source F, which voltage is supplied directly or through a transformer Tr to one of the diagonals of the bridge. In another diagonal there is zero indicator of AC.

Fig. 3. AC bridge Scheme

As in the DC bridges, measurement process is reduced to a balancing AC bridge, which is characterized by the absence of potential difference between the nodes a and b; for this it is necessary that the voltage drop across the shoulders Z1 and Z4 (as well as the shoulders Z2 and Z3) were equal in amplitude and phase match. Equilibrium is achieved when the following two conditions occur:

1) Equality of products of modules of full resistance of opposing arms, i.e.,

Z4Z2 = Z1Z3; (1)

2)Equality of sums of phase angles of the same shoulder, i.e.

ц4 + ц2 = ц1 + ц3 . (2)

If the arm of the bridge has an active R and reactive (capacitive or inductive) X resistances acting in series, then module of impedance of shoulder equals to

Z = (R2-Х2)0,5, (3)

and its phase angle ц is determined by the formula

tg ц = X/R . (4)

Fig. 4a shows a circuit multirange resistance box bridge. Its equilibrated with variable capacitor C1 and a variable resistor R1. Applying this scheme, the equilibrium condition (1), we obtain

R2*( Rx2+ 1/(2*р*F*Cx)2 )0,5 = R3*( R12+1/(2*р*F*C1)2 )0,5

Given that ц2 = ц3 = 0, the second equilibrium condition (2) can be written as an equality цx = ц1 or tg цx = tg ц1 or, according to (4)

1/(2*р*F*Cx*Rx) = 1/(2*р*F*C1*R1).

Solving the above equation, we find:

Сx = С1(R2/R3) ; (7)

Rx = R1(R3/R2) ; (8)

With respect to fixed resistors shoulder R2/R3, capacitor C1 and resistor R1 can be equipped with scales with count values of capacitance Cx and resistances loss Rx. Measuring range extension is achieved by using the group switched resistors R3 (or R2) of various denominations, usually differing by 10 times. The bridge is balanced quickly as adjustment implemented by capacitor C1 and resistor R1, mutually independent. If the bridge is designed to measure the capacitance of less than 0.01 uF, for whom the loss at low frequencies is very small, the resistor R1 can be omitted.

Fig. 4 multirange resistance box bridge for measuring the parameters of capacitors

In order to simplify the design in some bridges measuring capacitor C1 is taken with constant capacity, and as regulated elements two variable resistors, for example, R1 and R2 (Fig. 4b) are taken. From the formulas (7) and (8) follows that both adjustment of such bridge are related, so its balancing, controlled by rectifier indicator, should be done by successive approximation to a minimum by alternative changing of resistance of R1 and R2. Cx capacitance values are on a scale of resistor R2 taking into account the factor, determined by the set of the switch B. Since direct evaluation of the loss resistance Rx is impossible, then the scale reading of the resistor R1 is typically performed in the values of the loss tangent:

tg д = 2*рF*Cx*Rx = 2*р*F*C1*R1,

At a fixed frequency F, tg д is uniquely determined by the value of resistance R1. The validity of the last formula can be easily verified by respectively multiplying the left and right sides of the equations (7) and (8).

Simple capacitance meter is made by the scheme of the reochord bridge, which usually provides the ability to measure resistance, and sometimes inductons. Universal reochord bridge scheme:

Fig.5. Universal bridge scheme

variable capacitor resistance bridge

To eliminate the influence of parasitic relationships and errors of the bridge, bridge method of measuring capacity is often combined with the substitution method. In this case to the input terminals of the bridge capacity box (or reference variable capacitor) is connected and by some value of its capacitance C1, knowingly exceeding capacity Cx, we balance the bridge. Then analyzed capacitor is attached to the capacity box in parallel and by reduction of capacity the box to a certain value of C2 again balance the bridge. Obviously, the measured capacitance Cx = C1-C2.

Automatization of quality control of a variable resistor in production

Capacitors are shifted down the pipeline and reach point A. They stop at a certain period of time in this point. It occurs with the help of automatic unit that captures their position. The controller, in turn, is programmed to give signals with specific frequency. It regulates the flow of capacitors. Measurements are performed in automatic measuring block and the results are sent to the PC. The control program manages the entire process and this information is stored in memory, processed, and displayed. In fact, based on the data, faulty capacitors removed from the transmission line. Distributor must be placed at the end of the flow, which will separate satisfactory and unsatisfactory devices on two sides.

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