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An attenuator is used to introduce a predetermined attenuation within a specified frequency range. It is generally indicated by the decibel of the attenuation introduced and the ohm of its characteristic impedance.
Attenuators Explained
I What is an Attenuator?
An attenuator is used to introduce a predetermined attenuation within a specified frequency range. It is generally indicated by the decibel of the attenuation introduced and the ohm of its characteristic impedance.
Attenuators are widely used in cable television systems to meet the level requirements of multiple ports, such as amplifier input and output level control and branch attenuation control.
There are two types of attenuators: passive attenuators and active attenuators. The active attenuator cooperates with other thermal elements to form a variable attenuator, which is used in the amplifier for automatic gain or slope control circuit. The passive attenuator includes the fixed attenuator and adjustable attenuator.
II Technical Index
1. Working Frequency Band
The working frequency band of the attenuator refers that when the attenuator can reach the index value when used within a given frequency range. Because the radiofrequency/microwave structure is related to frequency, components in different frequency bands have different structures and cannot be used universally. The working frequency band of the modern coaxial attenuator is quite wide, so pay attention to it in design or use.
2. Amount of Attenuation
Regardless of the mechanism and specific structure of the power attenuation, the two-port network shown in the figure below can always be used to describe the attenuator.
In the figure, the power at the input end of the signal is P1, the power at the output end is P2, and the power attenuation of the attenuator is A (dB). If P1 and P2 are expressed in decibel milliwatts (dBm), the relationship between the power at both ends is
P2 (dBm) = P1 (dBm)-A (dB)
It can be seen that attenuation describes the degree of power reduction after the power passes through the attenuator. The amount of attenuation is determined by the material and structure of the attenuator. The attenuation is in decibels, which is convenient for the calculation of the whole machine index.
3. Power Capacity
The attenuator is a kind of energy-consuming element, which turns into heat after power consumption. After the material and structure are determined, the power capacity of the attenuator is determined. If the power of the attenuator exceeds this limit, the attenuator will be burned. When designing and using, the power capacity must be clarified.
4. Return loss(RL)
The return loss is the standing wave ratio(SWR) of the attenuator, and the input and output standing wave ratio of the attenuator should be as small as possible. The attenuator is a power-consuming component, which cannot affect the circuits at both ends, which means it matches the circuits at both ends. This factor should be considered in designing attenuators.
5. Power factor
When the input power changes from 10mW to the rated power, the attenuation coefficient of change is expressed as dB/(dB*W).
The specific algorithm of the change value of the attenuation is to multiply the coefficient by the total attenuation power (W).
For example, an attenuator with a power capacity of 50W and a nominal attenuation of 40dB has a power coefficient of 0.001dB/(dB*W), which means that when the input power is increased from 10mW to 50W, its attenuation will change by 0.001*40*50 = 2dB.
III Related Parameters
(1) Attenuation: It is used to describe the amount of attenuation signal from one end to the other during transmission. It can be expressed in multiples or decibels.
(2) VSWR: It's equal to the ratio of characteristic impedance to the load impedance connected to the output end of the transmission line.
(3) Maximum Average Power: It's the maximum power that can be added to the input of the attenuator for a long time at the specified highest operating temperature when the attenuator is connected to the characteristic impedance to the output terminal.
(4) Power coefficient of Insertion Loss: It's the change value of insertion loss (dB) when the input power is from 10mW to the rated power.
(5) Maximum Peak Power: it’s the maximum peak power added to the 5MS pulse width of the attenuator's input end within the specified time at the specified highest operating temperature when the characteristic impedance is connected to the attenuator output end.
(6) Temperature coefficient: The maximum variation of insertion loss within the maximum operating temperature range;
(7) Impact and vibration: The attenuator must withstand impact and vibration tests in three directions.
(8) Upper limit of operating temperature: the highest temperature when the attenuator works at the maximum input power.
(9) Connection life: the number of normal connections/disconnections; All electrical and mechanical indicators shall meet the target requirements within the specified lifetime.
(10) Intermodulation distortion: Intermodulation distortion consists of spurious signals and is caused by nonlinear factors in the device.
We should pay more attention to the third-order intermodulation distortion because it’s the largest and cannot be filtered.
The test method of the third-order intermodulation level is to inject two pure signals (F1 and F2) of equal amplitude into the device under test, and the third-order intermodulation will appear at 2F1-F2 and 2F2-F1 of the output spectrum.
The third-order intermodulation product is defined by its size relative to F1 or F2 and represented by -dBc.
IV Basic Structure
The basic material that constitutes the RF attenuator or the microwave attenuator is resistive material. A general resistor is a basic form of the attenuator, and the resistance attenuator network formed from it is a lumped parameter attenuator.
Through a certain process, the resistive material is placed in the radiofrequency/microwave circuit structure of different bands to form the attenuator of the corresponding frequency. If it is a high-power attenuator, the volume must be increased, and the key is heat dissipation design.
With the development of modern electronic technology, fast adjusting attenuators are needed on many occasions. This type of attenuator usually has two implementation methods, one is a semiconductor low-power fast-adjusting attenuator, such as a PIN tube or FET monolithic integrated attenuator; the other is a resistance attenuation network controlled by a switch. The switch can be an electronic switch or radiofrequency relay.
V Types of Attenuator
1. Displacement Optical Attenuator
When two sections of optical fiber are connected, a fairly high centering accuracy must be achieved so that the optical signal can be transmitted with a small loss. Conversely, if the centering accuracy of the fiber is appropriately adjusted, the attenuation can be controlled.
The displacement optical attenuator is based on this principle to deliberately cause a certain misalignment of the optical fiber when it is connected. Therefore, the optical energy is lost a little, so as to control the attenuation.
Displacement optical attenuators are divided into two types: lateral displacement optical attenuator and axial displacement optical attenuator.
The lateral displacement optical attenuator is a relatively traditional device. Since the magnitude of the lateral displacement parameters is all at the micron level, it is generally not used to make variable attenuators, but in the production of fixed attenuators with welding or bonding method. There is still a large market for lateral displacement optical attenuators, and its advantage lies in the high return loss, which is generally greater than 60dB.
In the process design of the axial displacement optical attenuator, attenuation can be achieved by mechanically pulling the two optical fibers a certain distance apart. This principle is mainly used in the production of fixed optical attenuators and some small variable fiber optical attenuators.
2. Thin Film Optical Attenuator
This attenuator is based on the principle that the reflected light intensity of light on the surface of the metal film is related to the film thickness. If the thickness of the metal film vapor deposited on the glass substrate is fixed, a fixed optical attenuator is made. If a series of disc-shaped metal thin film with different thicknesses are inserted obliquely into the glass substrates in the optical fiber, the intensity of the reflected light can be changed, and different attenuations can be obtained to make a variable attenuator.
3. Attenuation Piece Optical Attenuator
The attenuation piece optical attenuator directly fixes the attenuator with absorption characteristics on the end face of the optical fiber or in the optical path to attenuate the optical signal. This method can be used to make fixed optical attenuators and variable optical attenuators.
VI Precautions
1. Frequency Response
It also indicates the frequency bandwidth, generally expressed in megahertz (MHz) or gigahertz (GHz). General-purpose attenuators usually have a bandwidth of about 5 GHz, with a maximum bandwidth of 50 GHz.
2. Attenuation Range and Structure
The attenuation range refers to the attenuation ratio, generally ranging from 3dB, 10dB, 14dB, 20dB, up to 110dB.
The attenuation formula is: 10lg (input/output), for example: 10dB
Representation: input: output = attenuation multiple = 10 times.
The structure is generally divided into two forms: fixed proportional attenuator and step proportional adjustable attenuator.
A fixed attenuator is an attenuator with a fixed ratio multiple in a certain frequency range. The stepped attenuator is an attenuator with a fixed value (for example, 1dB) and an adjustable proportional multiple at equal intervals. It is subdivided into manual step attenuators and programmable step attenuators.
3. Connector Form and Connection Size
The connector types are divided into BNC type, N-type, TNC type, SMA type, SMC type, etc. At the same time, the connector shape has two types: male and female.
The connection size is divided into metric and Britsh systems, and the above is determined according to the requirements of use; if the types of connectors need to be connected, the corresponding connection adapters can be equipped, for example, BNC to N-type connector.
4. Attenuation Index
The attenuation index has many requirements, mainly the following aspects: attenuation accuracy, withstand power, characteristic impedance, reliability, repeatability, etc.
VII What does an Attenuator do?
Attenuators are widely used in electronic equipment, and their main uses are:
(1) Adjust the size of the signal in the circuit;
(2) In the comparison method measurement circuit, it can be used to directly read the attenuation value of the tested network;
(3) Improve impedance matching. If some circuits require a relatively stable load impedance, an attenuator can be inserted between this circuit and the actual load impedance to buffer the impedance change;
(4) Control the Power Level. In the microwave superheterodyne receiver, the output power of the local oscillator is controlled to obtain the best noise figure and conversion loss of the photosensitive attenuator to achieve the best reception effect. In the microwave receiver, it can realize automatic gain control to improve the dynamic range;
(5) Relative standard. As a relative standard for comparing power levels;
(6) Used in radar anti-jamming. it is a variable attenuator whose attenuation can be changed suddenly. It usually does not introduce attenuation, but suddenly increases attenuation when encountering external interference. From a microwave network point of view, the attenuator is a two-port lossy microwave network, which is a through-type microwave component.
VIII Optical Fiber Attenuator
1. Working Principle
A fiber attenuator is a kind of optical device that can reduce the energy of optical signals. It is used to attenuate the input optical power to avoid the distortion of the optical receiver due to the super-strong input optical power.
Optical fiber attenuator, as a kind of optical passive device, is used for debugging optical power performance, debugging and calibration of optical fiber instrument in an optical communication system, and optical fiber signal attenuation. It is made of attenuating fiber doped with metal ions, which can adjust the optical power to the required level.
The optical fiber attenuator test system has high attenuation accuracy, low additional loss, and good stability. It's mainly used in optical fiber communication systems, optical fiber CATV, and high-power optical device measurement.
Attenuators are like sunglasses, which protect your eyes from strong light by absorbing excess light energy. The optical fiber attenuator can be like sunglasses to protect the optical fiber by working in a specific wavelength range.
2. Manufacturing Technology
The standard for a good fiber attenuator is to replace reflective fiber by absorbing additional fiber. Because in optical fiber communication, it is necessary to use lower optical power without damaging the optical fiber attenuator.
(1) Air Isolation TechnologyRestricted by the law of total reflection, the light transmission in the optical fiber cannot be scattered, keeping the intensity relatively stable. Once the light is separated from the optical fiber and an air gap is added between the optical fibes, the light will scatter out, causing light attenuation.
Since the scattering of light from ordinary optical fibers into the air is strong, to control the attenuation within a certain range, it is necessary to ensure the isolation distance and maintain the alignment of the optical fibers at both ends.
Through this principle, flange-type fixed attenuators and adjustable attenuators can be made. The flange-type fixed attenuator adopts an isolation attenuator, and attenuator with a certain thickness is made according to the graph, and the attenuator is implanted in the flange to fix the light attenuation.
The flange-type adjustable optical attenuator adopts the principle of mechanical rotation, and the distance between the connectors at both ends can be adjusted by mechanical rotation. The optical attenuation can be between 0-30dB.
(2) Displacement Dislocation TechnologyIn this method, the cores of the two optical fibers are slightly translated and dislocated to achieve the effect of power loss.
By using ordinary pigtails, the cores of the two pigtails are spliced with a fusion splicer under the condition of misalignment, so that core shift occurs during the transmission process, and a connector-type fixed attenuator, also known as an online fixed attenuator, is obtained.
(3) Attenuation Fiber Optic TechnologyAccording to the absorption effect of metal ions on light, the attenuation fiber doped with metal ions has been developed. Like ordinary fibers, this attenuation fiber also has a fixed attenuation coefficient, but the attenuation coefficient is not calculated in kilometers but in millimeters. The attenuation fiber is inserted into the ceramic ferrule after a special process, to make it into a male and female fixed attenuator.
(4) Glass Absorption MethodOptically polished neutral absorption glass sheets can also be used in the production of optical attenuators. The light absorption characteristics of the substance are used to make sheet-like or strip-like neutral dark glass, which can be placed on the light path to attenuate the light intensity.
Absorbing glass is distinguished by transmittance T and attenuation rate expressed in decibels.
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Transmittance T=transmitted light intensity/incident light intensity; Attenuation rate η=1/T; Attenuation rate expressed in decibels β=10 x logη=- 10 x logT |
The attenuation rate of the combined attenuation film group can be botained by adding the decibels of the attenuation rate of each film. The absorbing gass sheet are made with a fixed attenuation value, and the strips also have different attenuation values according to the continuously increasing dark material inside.
A single piece of light-absorbing glass can be made into a fixed attenuator, multiple pieces of absorbing glass can be converted into a stepped adjustable attenuator by roulette, and an absorbing glass stripe can be made into a continuously adjustable attenuator through continuous displacement.
(5) Solid-state light attenuation technologyAdjustable optical attenuaton made with air isolation and absorption glass use mechanical methods to achieve attenuation tunability, and a small number of them are made with solid-state optical attenuation technologies, such as adjustable diffraction grating technology, MEMS technology, liquid crystal technology, Magneto-optical technology, planar optical waveguide technology, etc.
