The decrease in the amplitude of any kind of signal by an electronic active or passive device. Normally expressed as a ratio or in decibels, see dBm and the power conversion table.

Automatic matching unit

A device to transform a varying load impedance, for example an ionised gas, into a constant, defined impedance, usually the characteristic impedance of the system, e.g. 50Ω. The need for matching arises because the generator operates most efficiently, i.e. the point of maximum power transfer between the generator and load, when it is driving a load that is equal to the conjugate of its internal impedance.

Bipolar Transistor

A generic term for NPN and PNP junction transistors. See JFET and MOSFET.

Bleed resistor

A resistor placed across an energy storage device, such as a large capacitor, to ensure the energy is quickly dissipated when the power is switched off.


Chemical Vapour Deposition (CVD) is process for depositing thin solid films. A substrate is heated in a vacuum chamber and process gases are introducted which breakdown, due to the heat, into their reaction products and are deposited as a crystal or amorphous film. See PECVD

Characteristic impedance

The impedance which when terminating a line, such as a coaxial cable, gives rise to an impedance equal to itself. When a line is terminated by its characteristic impedance, e.g. 50Ω, the VSWR will be unity and the line will be free from reflections which means the reflected power will be zero. See Impedance and VSWR.

Circular Mils

A circular mil is a unit of area, used mainly in the USA, and equal to the area of circle with a diameter of one mil, or one thousandth of an inch. It is convenient because it can be calculated without using Pi.
The area in circular mils of a circle with a diameter of d mils is: area = d2.
The US National Electrical Code (NEC) uses the circular mil to define wire sizes larger than 4/0 AWG and is often used in tables showing the current carrying capacity of cables.

Core Saturation

The limiting value of magnetic flux in a transformer or inductor core. If the flux is forced beyond this point, the permeability of the core will decrease to the permeability of air and the losses will increase.

Directional Coupler

A directional coupler samples the voltage and current on a transmission line. If a transmission line is terminated by an impedance that is not its characteristic impedance a standing wave will be set up along the line. This can be resolved into two travelling waves, one moving toward the load and the other towards the generator. The directional coupler produces two voltages proportional to the amplitude of these waves, the forward voltage Vfwd, and the reverse or reflected voltage Vrev, or Vref. These are squared and scaled to represent the power into the characteristic impedance, typically 50Ω, and are referred to as Pfwd, Prev or Pref.

If the line is correctly terminated, for example by a 50Ω resistor, there will be no reflected wave and Pfwd will be equal to the power in the load. If the termination is not the characteristic impedance, i.e. not 50Ω, the power in the load will be equal to Pfwd-Pref.

DC-DC Converter

A circuit for changing the voltage of a dc source to another value by converting it to ac, transforming it, and then rectifying the output to produce desired level.


Decibel reference to 1 milliwatt; can be calculate from the formula:
dBm = 10 log (Power /1mW)

e.g 10W can be expressed in dBm as 10log (10/0.001) = 40dBm

Foldback Current Limiting

A circuit often used in power supplies that reduces the voltage if the current exceeds a defined value. It is used to protect the supply from short circuits.

Forward power

Often abbreviated to Pfwd, see Directional Coupler.

Ground Fault detector

See Residual Current detector.


High frequency (HF) refers to frequencies between 3 and 30MHz.


An ac circuit may contain resistance, inductance and capacitance, each of which opposes the flow of current. The combined effect of these elements is called impedance and is the ratio of the voltage to the current flowing in the circuit, the unit is Ohms (Ω). Since inductive and capacitive components have a phase shift between the voltage and current impedance is normally expressed as a complex number. See Characteristic impedance.


Junction gate Field Effect Transistor. A field effect transistor is a device that depends for its operation on the control of current by means of an electric field in the semiconductor. They are easier to fabricate in IC form compared to bipolar transistors and also occupy less space. Usually low power devices and often used in the front end circuitry of amplifiers where the inherent high impedance is useful.

Load power

Simply the power dissipated in the load. If power is indicated by a directional coupler, which is typically the case with a RF generator, the load power is equal to the forward power minus the reflected power.


Metal Oxide Semiconductor Field Effect Transistor. Similar to the JFET insomuch as it depends on the control of an electric field for its operation, but its configuration allows it to be more easily scaled making it suitable for high power devices. In general, MOSFETs are more robust than bipolar devices and have supplanted them for use in power amplifiers.

Peak reverse voltage

Also known as Peak Inverse Voltage. The maximum reverse bias voltage specification for a semiconductor device. Exceeding this value can cause junction breakdown and device failure.


In plasma enhanced CVD (PECVD) processes the process gases dissociate themselves under the influence of the plasma. They then react to create the film material which is deposited on the substrate. The advantage of PECVD over CVD is faster film growth and a higher utilization of process gases. Also, because PECVD typically operates at below 500C the substrate is less likely to be damaged compared to CVD. See CVD


A gas plasma is a gas in which a portion of the molecules are ionized. The presence of charged particles makes the plasma electrically conductive so that it responds to electric and magnetic fields. This means plasma has unique properties and is considered to be a distinct state of matter.
Plasmas used in industrially applications are termed cold plasmas. Only a small percentage of the molecules are ionised, the heavy particles, ions and neutrals, are not very hot, but the electron temperature is much higher, typically several thousands degrees C.

Plasma cleaning

By exploiting the properties of a plasma that cause ion bombardment and the production of gaseous compounds from the surface material of a substrate or object the process can be utilised for cleaning. Instead of using environmentally unfriendly solutions based on hydrocarbons substances such oil, grease, and silicon can be removed with a low pressure plasma. The temperature is typically below 100C so it is suitable for use on heat sensistive items.

Power amplifier

An amplifier, usually an electronic circuit, designed to increase the power of a signal applied to its input. Different layouts are classified into Types; A, B and C are linear configurations while D, E, and F are switched mode versions.

Radio frequency (RF)

A general term to describe any frequency useful for radio transmission.

Reflected power

Also referred to as reverse power. See Directional Coupler.


Often part of a power supply, a circuit that maintains a constant output over a range of loads and input supply variations.

Residual Current detector (RCD)

Also referred as a Ground Fault (Circuit) Interrupter (GFI or GFCI) It is a safety device usually installed at the mains distribution panel. It senses any current flowing directly to ground and immediately switches off the power.


Reactive Ion Etching is a precise method of removing surface material. Molecules of a gas, or gases, called the process gas are ionized in a low temperature plasma. This creates ions that are accelerated by an electric field collide with the surface of the material to be etched.

The etching process involves two processes: the accelerated ions impact the surface and errode the surface. The ions also react chemically with the material to produce gaseous compounds that are pumped away.

Some materials etch faster than others. By using a lithography process to print a pattern on the surface using a slow etch material, called a resist, fine structures can then be etched into the surface. Structures in the micrometer and sub-micrometer range can be etched by plasma dry etching, and the process has widespread use in the manufacture of semiconductors.


A residual, and usually unwanted, ac signal imposed on another signal. For example, the dc output from a simple linear power supply will have a small ac signal related to the mains supply frequency.


Occasionally called the effective value, RMS is the root-mean-square and describes the method of calculating a value of an alternating voltage or current that corresponds to a dc voltage or current that would cause the same heating effect.

Smith chart

The Smith Chart is a plot of the complex reflection coefficient in two dimensions on a circle diagram. It is commonly used as a graphical aid to solve impedance matching problems. It is scaled for normalised impedance allowing it be used for circuits with any characteristic impedance which is represented by the centre point of the chart. Over recent years Smith Chart programmes for PCs have become available making their use much easier and giving more accurate result, e.g. winSMITH2.0. Also see Tonne Software.


Safe Operating ARea, often displayed as a graph showing a permissible area representing a range of parameters over which a device, such as a transistor, may be safely operated.


A short transient or disturbance on a signal or power line, typically lasting microseconds. Although the energy contained in a spike is usually quite small they can often trigger sensitive circuitry and cause measurement or operational errors.


Similar to a spike but lasting longer, perhaps fractions of a second to several seconds. The term is commonly used to describe problems with supply from the power utility.


A term applied to a phenomena, usually a damped oscillatory voltage or current, which takes place in a system following a sudden change in conditions, and which persist for a relatively short time after the change has occurred. An example is the response of a RF generator connected an impedance matching network during the match period. See Automatic matching unit.

Volt amperes (VA)

The product of voltage and current in an ac circuit without regard to their relative phase.


Voltage Standing Wave Ratio. When a transmission line, e.g. a coaxial cable, is not terminating by its characteristic impedance a standing will be setup along the line. The ratio of the maximum voltage on the line to the adjacent minimum voltage is the VSWR. It is good practice to ensure transmission lines are correctly terminated so there is a uniform voltage and current along the line, i.e. VSWR =1. Standing waves can damage the line due to high voltages and higher current will increased losses.


The distance travelled by a wave in one period. A period is the time for a single cycle and is equal to the reciprocal of frequency. The wavelength of a sinusoidal waveform travelling at a constant speed can be calculated:

wavelength = V/f where V= phase velocity , f = frequency, =wavelength

For propagation in free space v is the speed of light, 3 x108 m/s but in all other mediums the velocity is reduced. For example coaxial cable often has a phase velocity of 2 x108 m/s.

The wavelength of a 13.56MHz signal in a coaxial cable is: 200,000,000/13.56MHz = 14.75 m