ASSIGNMENT 2

IMPORTANCE OF SMITH CHARTS IN RF SYSTEMS

What's a Smith chart?

What is a Smith chart? It's really just a plot of complex reflection overlaid with an impedance and/or admittance grid referenced to 1-ohm characteristic impedance. That's it! Transmission coefficient, which equals unity plus reflection coefficient, may also be plotted (see below). You can find books and articles describing how a Smith chart is a graphical representation of the transmission line equations and the mathematical reasons for the circles and arcs, but these things don't really matter when you need to get the job done. What matters knows the basics and how to use them, like always.

The Smith chart contains almost all possible impedances, real or imaginary, within one circle. All imaginary impedances from - infinity to + infinity are represented, but only positive real impedances appear on the "classic" Smith chart. Yes, it is possible to go outside the Smith chart "unity" circle, but only with an active device because this implies negative resistance.

One thing you give up when plotting reflection coefficients on a Smith chart is a direct reading of a frequency axis. Typically, plots that are done over any frequency band have markers calling out specific frequencies.

Why use a Smith chart?

Use of the Smith Chart utility has grown steadily over the years and it is still widely used today, not only as a problem solving aid, but as a graphical demonstrator of how many RF parameters behave at one or more frequencies, an alternative to using tabular information. The Smith Chart can be used to represent many parameters including impedances, admittances, reflection coefficients, a Smith chart is the RF engineer's best friend! It's easy to master, and it adds an air of "analog coolness" to presentations, which will impress your friends, if not your dates! A master in the art of Smith-charting can look at a thoroughly messed up VSWR of a component or network, and synthesize two or three simple networks that will impedance-match the circuit in his head!

Reference:

1. Chris Bowick.RF Circuits design. 1982, Oxford, Boston.
2. www.wikipedia.org/smithcharts
   

Assignment 1

Transmission line circuit representation diagram

Role of RF lines on TV and radio transmission

RF is one of the most important sections in a TV or radio system. Its functions are many but we will just discuss the in passing. Radio frequency, or RF, is a frequency or rate of oscillation within the range of about 3 Hz and 300 GHz. This range corresponds to frequency of alternating current electrical signals used to produce and detect radio waves. Since most of this range is beyond the vibration rate that most mechanical systems can respond to, RF usually refers to oscillations in electrical circuits.

1. RF AS MODULATOR

RF modulators are used for coupling an audio signal into an audio system via its radio tuner when an auxiliary input to the audio system is not available. Radio frequency modulation techniques are used for transmitting information on a carrier signal, e.g. sound information on a carrier radio wave. Frequency modulated (FM) signals are widely used in many communication devices, such as two-way portable and mobile radios, for communicating messages and other types of information. RF modulation apparatus modulates video and audio signals supplied from a video apparatus to enhance a television broadcast. The modulator modulates the accessory audio signal according to the modulation scheme used by a radio reception band of the radio tuner so that an auxiliary input in the audio system becomes unnecessary.

2. RF POWER AMPLIFIER RF

power amplifiers are commonly used in numerous applications, such as base stations used in wireless communication systems. Amplifiers produce from an input signal, an output signal having an increased magnitude. gain. An amplifier produces a constant output power at a higher level. Different amplifiers are known and produce various types of gains. Power amplifiers are classified into different groups: class A, class B, class AB, class C, class E, class F, and class D sometimes referred to as digital amplifiers. The different classes of power amplifiers usually signify different biasing conditions. Each of these types of amplifiers, however, has significant problems when operating in the RF range. Modern wireless communication base stations transmit and receive radio frequency signals through the use of RF power amplifiers. Radio frequency signal is an electrical signal conveying useful information and having a frequency from about 3 kHz to thousands of GHz range, regardless of the medium through which such signal is conveyed.

3. RF & MICROWAVE ATTENUATOR

An attenuator is a device that passes an input signal while operating to attenuate the signal by a precise amount. Attenuators are used in applications that require signal level control. There are many applications for attenuators. These applications include not only communications satellites, but also other communications systems, measurement instrumentation, and electrical apparatus such as televisions and radios. In many microwave systems attenuators are required for automatic gain control of receiver and transmitter systems. Attenuators are also used for amplitude weighting in phased array radar and for temperature compensation of microwave amplifiers. An attenuator is an electronic device which attenuates electromagnetic energy. Radio receivers accept and process RF transmitted energy signals to yield, generally, audible or visual information. To avoid variations in gain, the gain is intentionally attenuated during an initial period. Various signal processing functions may occur at the intermediate frequency (IF) level, including signal attenuation as may be necessary to prevent the signal from exceeding a threshold level. The degree of attenuation is controlled by an input signal. Attenuation may be performed on a received signal to cancel a number of distortion products gained during transmission. A microwave attenuator may be implemented as a fixed attenuator or a variable attenuator. A variable attenuator allows the level of attenuation to be adjusted.

Reference:

• http://www.electronics-manufacturers.com/info/rf/microwave-electronics/rf

• http://en.wikipedia.org/wiki/Smith_chart