RF Knowledge -- Basic Concepts and Terminology

First, the basics

Concept Discrimination: dBm, dBi, dBd, dB, dBc

1, dBm

dBm is a value of the absolute value of the measured power, calculated as: 10lgP (power value / 1mw).

[Example 1] If the transmission power P is 1 mw, it is 0 dBm after being converted into dBm.

[Example 2] For a power of 40 W, the value converted in units of dBm should be:

10 lg (40 W / 1 mw) = 10 lg (40000) = 10 lg 4 + 10 lg 10 + 10 lg 1000 = 46 dBm.

2, dBi and dBd

dBi and dBd are the values of the gain (power gain), both of which are relative values, but the reference datum is different. The reference reference for dBi is an omnidirectional antenna, and the reference for dBd is a dipole, so the two are slightly different. It is generally considered that the same gain is expressed by dBi, which is larger than that expressed by dBd.

[Example 3] For an antenna with a gain of 16 dBd, when the gain is converted into units of dBi, it is 18.15 dBi (the decimal point is generally ignored, which is 18 dBi).

[Example 4] 0 dBd = 2.15 dBi.

[Example 5] The GSM900 antenna gain can be 13dBd (15dBi), and the GSM1800 antenna gain can be 15dBd (17dBi).

3, dB

dB is a value that represents the relative value. When considering the power of A is much larger or smaller than the power of B, the following formula is calculated: 10lg (A power / B power)

[Example 6] A power is twice as large as B power, then 10 lg (A power / B power) = 10 lg 2 = 3 dB. In other words, the power of A is 3 dB greater than the power of B.

[Example 7] The 100-meter transmission loss of the 7/8-inch GSM900 feeder is about 3.9 dB.

[Example 8] If the power of A is 46 dBm and the power of B is 40 dBm, it can be said that A is 6 dB larger than B.

[Example 9] If the antenna is 12dBd and the antenna is 14dBd, it can be said that A is 2 dB smaller than B.

4, dBc

Sometimes you will see dBc, which is also a unit that represents the relative value of power, exactly the same as dB. In general, dBc is relative to carrier power and, in many cases, is used to measure the relative value of carrier power, such as to measure interference (co-channel interference, intermodulation interference, intermodulation interference, band). External interference, etc.) and the relative magnitude of coupling, spurs, etc. Where dBc is used, in principle, dB can also be used instead.

1, power / level (dBm): the output capacity of the amplifier, the general unit is w, mw, dBm

Note: dBm is the absolute power level in decibels with 1mw as the reference value. Conversion formula:

Level (dBm) = 10lgw

5W → 10lg5000=37dBm

10W → 10lg10000=40dBm

20W → 10lg20000=43dBm

It is not difficult to see from the above that the power value increases by 3dBm for every doubling of power.

2, gain (dB): that is, the magnification, the unit can be expressed in decibels (dB).

Namely: dB=10lgA (A is the power amplification factor)

3. Insertion loss: The attenuation increased when a device or component is connected to the transmission circuit. The unit is expressed in dB.

4. Selectivity: measure the gain in the working frequency band and the suppression ability of the out-of-band radiation. The -3dB bandwidth is the bandwidth when the gain is reduced by 3dB, which is the same as -40dB and -60dB.

5. Standing wave ratio (return loss): ratio of antinode voltage to node voltage (VSWR) when standing wave state

Attachment: Standing wave ratio - return loss comparison table:

SWR 1.2 1.25 1.30 1.35 1.40 1.50

Return loss (dB) 21 19 17.6 16.6 15.6 14.0

6. Third-order intermodulation: If there are two sinusoidal signals ω1 and ω2, many intermodulation components will be generated due to nonlinear action, and the two frequency components of 2ω1-ω2 and 2ω2-ω1 are called third-order intermodulation components, and their power The ratio of the power of P3 to the signal ω1 or ω2 is called the third-order intermodulation coefficient M3.

That is, M3 = 10lg P3/P1 (dBc)

7. Noise figure: Generally defined as the ratio of the output signal-to-noise ratio to the input signal-to-noise ratio, the actual use is converted to decibel. The unit is in dB.

8. Coupling degree: The power ratio of the coupled port to the input port, in dB.

9. Isolation: The ratio of the power of the local oscillator or signal leaking to other ports to the original power, in dB.

10. Antenna gain (dB): refers to the ability of the antenna to concentrate the transmitted power to a specified direction. Generally, the field strength E in the maximum radiation direction of the antenna is compared with the uniform isotropic radiation field strength E0 of the ideal isotropic antenna, and the gain is defined as a gain in multiples of the power density increase. Ga=E2/ E02

11. Antenna pattern: It is the range of electromagnetic waves radiated by the antenna in free space. The width of the pattern generally refers to the angle of the main lobe, that is, the angle between two points when the maximum value is decreased by half.

The E-plane pattern refers to an in-plane radiation pattern parallel to the electric field;

The H-plane pattern refers to the in-plane radiation pattern parallel to the magnetic field.

Generally, the wider the pattern, the lower the gain; the narrower the pattern, the higher the gain.

12. Antenna front-to-back ratio: refers to the ratio of the maximum forward gain to the maximum reverse gain, expressed in decibels.

13. Simplex: It is also called single-frequency simplex system, that is, the same frequency is used for receiving and transmitting. Since the same frequency is used for receiving and transmitting, the sending and receiving cannot be performed at the same time, which is called simplex.

14. Duplex: Also known as the inter-frequency duplex system, that is, sending and receiving using two different frequencies, either party can receive the other party's speech while speaking.

Both simplex and duplex are the working methods of mobile communication.

15. Amplifier: (amplifier) A circuit used to achieve signal amplification.

16. Filter: (filter) A component or device that suppresses unwanted frequency signals by using a useful frequency signal.

17. Attenuator: A four-terminal network consisting of resistive elements with a phase shift of zero in a fairly wide frequency range, whose attenuation and characteristic impedance are constant independent of frequency. Its main purpose is to adjust Signal size in the circuit, improved impedance matching.

Power splitter: A device that performs power distribution. There are two, three, four .... power splitter; joint type is divided into N head (50Ω), SMA head (50Ω), and F head (75Ω) three types, our company commonly used N head and SMA head.

18. Coupler: A device that extracts a portion of the signal from the backbone channel. According to the degree of coupling, it can be divided into 5, 10, 15, 20.... dB different specifications; the high-power coupler (300W) can be used to extract signals from the base station, and the coupling degree can be selected from 30~65dB; the connector of the coupler is mostly N. head.

19. Load: The terminal is in a circuit (such as an amplifier) or an electrical output port. The components/components, components or devices that receive electrical power are collectively referred to as loads. The most basic requirements for the load are impedance matching and the power that can be withstood.

20. Circulator: A device that transmits signals in one direction.

21. Adapter: A device that connects different types of transmission lines together.

22. Feeder: It is a transmission line that transmits high-frequency current.

23. Antenna: (antenna) converts energy in the form of high-frequency current or waveguide into electromagnetic waves and emits them in a prescribed direction or restores electromagnetic waves from a certain direction to high-frequency currents.

Second, fiber knowledge

1, optical power: measure the size of the optical signal, can be directly tested with optical power meter, commonly used dBm

2, optical transceiver: mainly composed of optical transmitter and optical receiver, the function is to timely and accurately convert the electrical signal to be transmitted into the optical fiber for propagation (optical transmitter); at the receiving end, the optical signal is timely Accurate recovery and reproduction into the original electrical signal (optical receiver). Since the communication is bidirectional, the optical transceiver performs both electrical/optical (E/O) and optical/electrical (O/E) conversion.

3. Laser: The electrical signal is converted into an optical signal and used in an optical transmitter. The main indicator is the amount of optical power that can be emitted.

4, optical receiver: the optical signal is converted into an electrical signal, used in the optical receiver, the main indicator is the receiving sensitivity.

5. Optocoupler: Optical coupling is a connection of light between active or passive or active and passive optics. There are many forms of contact: the channel of light, the accumulation and distribution of optical power, the combination and splitting of light of different wavelengths, and the conversion and transfer of light. A device that achieves this connection of light is called an optocoupler.

6. Wavelength division multiplexer: Optical splitter or optical multiplexer is collectively called optical multiplexer, which can split or combine multiple carriers to double the capacity of optical fiber communication. At present, there are many 1310nm/1550nm wavelength division multiplexers, which can combine and split optical signals with wavelengths of 1310nm and 1550nm.

7. Optical attenuator: A device that performs a predetermined amount of optical attenuation on optical power during optical information transmission. According to the attenuation value, there are five kinds of 3, 5, 10, and 20dB, which are selected according to actual needs.

8, optical flange head: optical flange head is also known as fiber optic connector. For devices that implement two fiber-optic connections, the company currently uses FC-type and SC-type active connectors, which can be connected or separated.

9. Optical fiber: The optical fiber that transmits optical signals is divided into two categories: multimode fiber and single mode fiber. The fiber material is a glass core/glass layer, the standard operating wavelength of the multimode fiber is 850/1310 nm, and the standard operating wavelength of the single mode fiber is 1310/1550 nm. The attenuation constant is:

Operating wavelength 850nm 1310nm 1550nm

Single mode fiber (Class A) ≤0.35dB/km ≤0.25dB/km

Multimode fiber 3~3.5dB/km 0.6~2.0dB/km

10. Optical cable: It consists of several optical fibers, with sheath and outer sheath and reinforcing members, which have strong mechanical properties and protective properties. There are outdoor optical cables, indoor optical cables, soft optical cables, in-device optical cables, submarine optical cables, and special optical cables.

11. Tail fiber: Single-core fiber optic cable with fiber optic connector on one end.

12. Jumper: Single-core optical cable with connectors at both ends.

Third, network knowledge

1. Mobile communication: refers to the use of a wireless channel for mutual communication between mobile bodies or between a mobile body and a fixed body.

2. The three basic elements of a communication network are: terminals, transmission systems, and switching systems.

3. Analog communication network (frequency division system): Terminals, transmission and switching systems are communication networks implemented in an analog manner.

4. Digital communication network (time division system): Terminals, transmission and switching systems are all digital communication networks.

5. CDMA: Code division multiple access digital mobile communication. Multiple access communication is implemented using different coding methods.

6, TDMA: Time Division Multiple Access Digital Mobile Communications. Multi-access communication is realized by means of time division. At present, the GSM900/1800MHz repeater developed and produced by our company belongs to the TDMA system.

7. Channel: The channel through which signals are transmitted.

8, base station (BS): also known as wireless base station / base station. A set of devices that serve a wireless cell (usually an omnidirectional or three sectored cell). The base station is in a dominant position in the call processing process. The call processing process includes three main contents: 1. Control of the mobile station in the control channel, providing common information of system parameters; 2. Supporting the mobile station to access the network; 3. In the voice The mobile station is controlled in the channel.

9. Repeater: The same frequency double-amplified relay station, also known as the same-frequency repeater, the transmission mode is transparent transmission. The function is to receive and forward signals between the base station and the mobile station.

10. Cellular: The entire communication plane service area formed by the adjacent hexagonal wireless cell (also known as the cell) is shaped like a cellular, so it is called a cellular network (Cellular System), also called cellular mobile communication. network.