Radiofrequency characteristics
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In order to import radio devices into Russia such equipment should conform to certain emission standards established by the State Commission for Radio Frequencies. Product samples are actually tested in the Main Radio Frequency Center (MRFC)[1].
This article describes the general terms and radiofrequency characteristics, being measured or used during evaluation of conformity.
Units of Measure
Hertz (Hz)
Hertz is a unit of measure of periodic processes. It shows how often a measured process is performed in one second. For measurement of RF waves, it shows their oscillation frequency: 1 Hertz corresponds to 1 oscillation of RF waves a second.
This is a relatively small value with respect to radio waves, therefore multiple units are used: kilohertz (kHz), megahertz (MHz), gigahertz (GHz), and in rare cases terahertz (THz).
Communication systems most commonly use high frequencies about hundreds of thousands of megahertz. For example, mobile phones operate in the frequency range from 900 (GSM) to 2600 MHz (LTE), and microwave ovens emit RF waves at a frequency of 2400 MHz.
What is measured in Hz in the MRFC:
- Frequency bands of radio-electronic equipment (REQ)
- Emission frequency of a high frequency device (HFD)
Watt (W)
Watt is the main unit of measure of power. It determines the quantity of energy consumed or emitted in a certain period of time.
What is measured in W in the MRFC:
- Transmission power in REQ and HFD
- Power consumption of a device
Decibel (dB)
Decibel is a logarithmic unit of measure used to express the ratio of two values of the same measure. This unit is not included to the System International of units (SI), but it is widely used in communication systems and many other technical fields, as it allows to compare two values of any nature, provided that these values are expressed in the same unit.
Decibel is a decile of the main, larger unit — Bel. Bel is a common logarithm of the ratio of two energies. But in practice Bel was a far too big value, therefore its tenth part is used – decibel.
The logarithmic method of presentation of numbers often can be very useful, as it allows to replace multiplication with addition, division with subtraction, raising to power with multiplication, and extracting a root with the division.
What is measured in dB in the MRFC:
- Gain constant
- Sound level
- Signal/noise ratio
- Signal attenuation
Decibel with a reference level (dBm, dBW)
If certain value is taken as a zero level, it is possible to compare any other value with it and express the ratio in decibels. Unlike bare decibels this value will measure an absolute value of the corresponding physical quantity. Such an approach gives rise to a variety of dB-based units: for example, dBm is a unit with a reference level of 1 milliwatt, dBW is a unit with a reference level of 1 watt. These units are widely used in practice along with Watt, mainly to measure signal strength.
If it is necessary to quickly convert dBm to W or vice versa, you may use one of the online calculators[2].
What is measured in dBm in the MRFC:
- Signal strength in cellular networks
- Receiver sensitivity
- Output of a Wi-Fi router
Characteristics
Radiated power
Radiated (output) power is a value that describes the amplitude of emission of radio waves. In most cases it completely defines the range of coverage of a device. Usually it is measured in W or dBm.
Effective isotropic radiated power
Effective isotropic radiated power (EIRP) is a characteristic of a transmitter power that considers characteristics of its antenna and losses in transmission of a signal to it. It is a product of multiplication of power of the signal delivered to the antenna by its gain ratio, and measured in units of power (W, dBW, dBm).
This characteristic allows to evaluate the real output radiation level.
Primary emission
Primary emission is emission in the frequency band necessary to transmit a message with a required speed and quality. Primary emission is made on the operation frequency selected by the radio-electronic system manufacturer.
Off-frequency emissions
Besides wanted radiation there are also off-frequency emissions — those are emissions outside the operational frequency band, but adjoin immediately to it. They result from distortion of the modulating signal and nonideality of modulator characteristics. Off-frequency emissions are unwanted, as they create load on the radio frequency resource, but any radio station has them.
Side emissions
Side emissions are unwanted emissions outside the primary emission on frequencies that are multiples of the base frequency, and resulting from any non-linear processes in radio receiving devices, except modulation. Side emissions from any unit except the antenna and its feeder, should not have more impact that would be discovered if the antenna was fed with a maximum permitted power at the frequency of such side emission.
Bandwidth
Bandwidth or pass bandwidth is a frequency range of radio waves used for primary emission of the radio-electronic device or high-frequency device. For any device the frequency bandwidth is set so that it would contain not less than 90% of power of the wanted signal.
Modulation
In order to make transmission of information by radio communication simple, and ensure its immunity to interference, signal processing is used – modulation (manipulation) — modification of characteristics of a high-frequency carrying signal based on a low-frequency one bearing information (sound, video, data).
There are several types of modulation: amplitude, frequency and phase.
Modulation of a digital signal is called manipulation.
Spectral power density
Spectral power density is a radio signal characteristic that describes distribution of signal power in the range of primary emission. It shows power spectrum of the signal, i.e, which level of emission power pertains to each frequency.
Class of emission
The multitude of emission characteristics is designated with an alphanumeric code called Class of emission. This parameter is adopted by the regulation of the International Telecommunications Union, it describes 3 mandatory characteristics, with optional provision of 2 additional characteristics:
- Carrier modulation type (the first digit in the code)
- Modulating signal nature (the second digit)
- Type of transmitted information (the third digit)
E.g., sound broadcasting in AM has the class of emission, sound broadcasting in FM — F3E.
Magnetic field strength
In some low frequency communication systems that use magnetic field properties (e.g., RFID), maximum magnetic field strength at 10 meters is measured, as it characterizes power of the device.
References
See also
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