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FAQs: RF design

Posted: 04 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FAQ? RF? design radio frequency?

What is RF?
RF stands for Radio Frequency, but it often used in the sense of " anything related with Electromagnetic signals".

RF is the wireless transmission of data by analog / digital radio signals at a particular frequency. It maintains a two-way, online radio connection between a mobile terminal and the stationed host terminal.

What are different frequency bands of RF?
The different RF Frequency Bands are:

HF: High Frequency, 3MHz to 30MHz

VHF: Very High Frequency, 30MHz to 300MHz

UHF: Ultra High Frequency, 300MHz to 3GHz

SHF: Super High Frequency, 3GHz to 30GHz

EHF: Extra High Frequency, 30GHz to 300GHz

What is the relation between frequency and wavelength?
Wavelength: the distance a radio wave will travel during one cycle.


c = 3x10 pow(8) = speed of light (meters per second)

f = receive frequency (Hertz)

= wavelength (meters)

What is the relation between power in mW and decibels in RF?
Basically the power level of RF world is expressed in dB / dBm. The common standard is to refer powers to 1mW (0.001 Watts). Such power ratio, expressed in decibels, is called dBm.

P dBm=10log(P watts/1mW)

The advantage of using decibels instead of Watts to express the power of a signal along an RF chain is that instead of dividing or multiplying powers to take care of amplifications and attenuations, we just add or subtract the gains and the losses expressed in decibels. For link budget calculations, the dBm convention is more convenient than the Watts convention.

What is ASK Modulation?
Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.

The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

What is FSK Modulation?
Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency. Continuous phase forms of FSK exist in which there is no phase discontinuity in the modulated signal.

What are the different antennas used in the RF/Wireless Communication System and what are their characteristics?
There are three types of antennas: Big antennas, thin antennas, and very small antennas.

Big antennas are half a wavelength or bigger. These antennas are expensive, fragile, and predominantly used for longer range fixed point-to-point applications. The advantage with this type of antenna is that it has a gain factor of 10 or even higher. Big antennas are rarely used in license-free radio products.

Thin antennas are typically a single thin wire (or PCB track) structure between a half and a quarter wavelength at the frequency of operation. They are efficient and have a fairly omnidirectional nature, which is excellent for mobile type applications, and are often used on the receiver side, for instance, as a quarter wavelength wire dangling from the receiver box. The gain for this type of antenna is about 1.5 to 3. Of all antenna types they generally provide the best performance to price ratio, but larger size and relative lack of robustness (compared to small antennas) keep them from dominating short range radio applications.

Very small antennas are typically small loops printed on a printed circuit board. These antennas are also of an omnidirectional nature. Very small antennas have an extremely narrow bandwidth, and generally need a post-manufacturing step to tune them to the exact transmit frequency. However a typical small PCB loop antenna in a key fob converts 95% of the RF energy generated by the key fob into heat, radiating only 5% of that precious milliwatt of generated RF power and results in an antenna gain of 0.075 (-11 dB).

In spite of all these shortcomings, small loop antennas are extensively used when products require pocket-sized product dimensions, robustness, and low cost. Application note AN831 from Microchip describes the design and matching of a small printed loop antenna in detail.

To work on the antenna design you may simply download the EZNEC Antenna Designer free demo from and play with it. This program will almost instantly give you a good idea of any antenna's performance.

Do radio transmitters require a license?
No. Most countries have a number of license-free frequency bands. Products operating in these license-free bands do not require end users to obtain a license.

However, such products must still conform to performance standards required by the country of operation, and it is the responsibility of the product manufacturer to ensure that such standards are met. Compliance is usually confirmed by testing in a government certified lab that provides a third party test report that governments then accept.

These standards are not difficult to understand, but are not always reader-friendly or concise. Much of Microchip's various RF support documents attempt to help users understand potential engineering issues relating to regulations, which are not always clear in the various government publications, and also explain why these regulations exist and how to design products that may be certified in multiple countries with little or no hardware differences. Once these regulations are better understood, many features of system and circuit design that may have initially seemed puzzling will become clearer.

Source: Microchip Technology Inc.

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