By James Reinholm
Although most people probably haven’t given it much thought, the invention of the coaxial cable was probably one of the most important discoveries ever made. Telecommunications and radio broadcasting would not exist as they are today without the invention of the coaxial cable.
Coaxial cables first started to appear in various applications back in the 30′s as a need developed for more efficient cabling systems with less interference. As more coaxial cables were used, standardised versions became available. Probably the most important parameter used in coaxial cabling is the characteristic impedance.
This is the main electrical characteristic that determines the level of power transfer and attenuation along the cable length, and also controls the amount of reflected and standing waves. Any type of coaxial cable is typically chosen based on the characteristic impedance. The main consideration is that impedance levels should match both at the transmitting and receiving end.
Although there are many standard impedances levels, the most common ones by far are the 50Ω and 75Ω impedances. These two standards are used for most coaxial cable applications, but other standards are also available in lesser quantities. For ordinary signal and data transmission applications, the cable that almost always chosen is the 50Ω type, while the 75Ω type is almost exclusively used for video signal and high-frequency RF applications, such as VHF (Very High Frequency) and UHF (Ultra High Frequency).
The need for high capacitance can be fulfilled via the use of a Capacitance Multiplier. The operational amplifier circuit is used as a capacitance multiplier in such a way that multiple small physical capacitances are combined in the integrated circuit technology to yield a large overall capacitance. The aim is often to multiply the original capacitance value hundreds and thousands of times. For example, a capacitor of 10 pF capacitance could be upgraded by the use of capacitance multiplier to behave like a 100 nF capacitor.
Construction of Capacitance Multiplier Circuits:
The circuit construction of a capacitance multiplier is quite simple. Two operational amplifiers, two resistors and a capacitor are used. The second operational amplifier is an inverted amplifier. A voltage source connected to the first operational amplifier will make the amplifier operate as a voltage follower. The circuit will produce a capacitance via the load imposition created by the second amplifier acting as an inverted amplifier. The produced capacitance is isolated from the circuit with the help of voltage follower. In this way, no current flows into the input terminals of the operational amplifier – the input current will flow through the feedback capacitor of the capacitance multiplier circuit.
How Are Multiple Capacitances Produced Using An Operational Amplifier Circuit?
Critical to production of effective capacitance is the selection of good resistance values for the two resistors in the multiplier circuit. The effective capacitance produced will be the capacitance between the input terminal of the operational amplifier and the ground. This effective capacitance will be the multiple of the physical capacitance ‘C’ of the operational amplifier circuit being used as a capacitance multiplier. There is an option to limit the size of this effective capacitance by the use of an inverted output voltage limitation technique. This is a practical approach to limit the size of the effective capacitance.
Relation between Size of Effective Capacitance and Input Voltage:
The capacitance multiplication and the maximum input voltage avoiding saturation state in the operational amplifier are inversely proportional to one other. Effectively, the larger the size of the effective capacitance, the smaller the input voltage into the input terminals of the operational amplifier. Using a similar technique, a resistance multiplier circuit can also be implemented by configuring an operational amplifier circuit. Furthermore, the same operational amplifier circuit can also be designed to simulate inductance.
Raj from Embedded Lab describes in his latest tutorial the theory of a very basic digital capacitance meter and its implementation using a PIC microcontroller. It is based on the principle of charging a capacitor through a series resistor and determine the time required to charge it to a known voltage. The built-in analog comparator and Timer2 modules are used in this process. The meter can measure capacitance from 1nF to 99.99 uF.
Digital Capacitance Meter using a PIC Microcontroller – [Link]
An LCR meter is an extremely useful device for measuring three basic impedance elements, namely, Inductance (L), Capacitance (C), and Resistance (R). Recently, I got a TENMA 72-8155 digital LCR meter from Newark for review. I was very excited to receive it as I didn’t have a dedicated LCR meter in my home lab. Here’s my quick review of this product.
TENMA 72-8155 digital LCR meter - [Link]
This is very accurate home made LC inductance/capacitance meter built with very common components which are very easy to find all around . The range of this LC Meter is extremely good at measuring very low value of capacitance and inductance.
LC Meter’s Inductance Measurement Ranges:
- 10nH – 1000nH
- 1uH – 1000uH
- 1mH – 100mH
LC Meter’s Capacitance Measurement Ranges:
- 0.1pF – 1000pF
- 1nF – 900nF
Very Accurate LC inductance / Capacitance Meter - [Link]
Engineers at the University of California, Berkeley, have shown that it is possible to reduce the minimum voltage necessary to store charge in a capacitor, an achievement that could reduce the power draw and heat generation of today’s electronics. Shown is a rendition of an experimental stack made with a layer of lead zirconate titanate, a ferroelectric material. UC Berkeley researchers showed that this configuration could amplify the charge in the layer of strontium titanate, an electrical insulator, for a given voltage, a phenomenon known as negative capacitance.
“Just like a Formula One car, the faster you run your computer, the hotter it gets. So the key to having a fast microprocessor is to make its building block, the transistor, more energy efficient,” said Asif Khan, UC Berkeley graduate student in electrical engineering and computer sciences. “Unfortunately, a transistor’s power supply voltage, analogous to a car’s fuel, has been stuck at 1 volt for about 10 years due to the fundamental physics of its operation. Transistors have not become as ‘fuel-efficient’ as they need to be to keep up with the market’s thirst for more computing speed, resulting in a cumulative and unsustainable increase in the power draw of microprocessors. We think we can change that.” [via]
Negative capacitance – one day soon - [Link]
Microcontrollers are widely used in measuring various physical variables. The techniques involved in the measurements could be different for individual variable type and are mostly based on the characteristics of the variables to be measured. This tutorial describes some methods for measuring the capacitance of a capacitor using microcontrollers. The techniques use the characteristics of the capacitor itself and are therefore universal and can be easily implemented with any microcontrollers.
How to measure capacitance with a microcontroller? – [Link]
Giorgos Lazaridis writes:
Some time ago i wrote a theory explaining how the touch sensors work, covering the resistance, the AC Hum and the capacitance touch sensors. Microchip has developed the mTouch(tm) sensing solutions. Part of the mTouch(tm) sensing solutions, is a series of PIC microcontrollers with embedded capacitance module. A list of these PICs can be found in this link.
In this article, i will use the PIC 16F1937, a powerful microcontroller with a 16-channel capacitance module. I will only use the 4 of them, but the idea to expand it to all 16 channels is the same.
PIC Capacitance Sensor with 4 buttons and Multitouch Function – [Link]
There are many ways to determine the capacitance of a capacitor. You can use an oscillating circuit where the capacitor is a part of it and measure the frequency of oscillation to find the capacitance. Or, you can also use a resistor-capacitor network and measure the rate of voltage rise across the capacitor to determine the capacitance, if the value of the resistor is known.
Here’s a similar project where a PIC16F88 microcontroller measures the time required by a capacitor to charge through a known resistor from 0 to half of the reference voltage provided, and the capacitance is determined based on that information. [via]
Determine capacitance by measuring the charging time – [Link]