RF Attenuators Includes:
Attenuator basics Attenuator specs Resistive attenuator design Attenuator resistor values table Balanced resistive attenuator pads Variable PIN diode attenuator Construction guidelines SMA attenuator
It is often useful to incorporate an RF attenuator into an RF circuit. Building the attenuator into the circuit, or making a stand-alone attenuator is often the best option.
When laying out a printed circuit board, or designing a housing for an RF attenuator, there are a few practical guidelines that can help ensure the best performance, even at frequencies well into the GHz region.
RF attenuator construction guidelines & tips
Here are a few simple guidelines that help wit the design and construction of RF attenuators. If these are incorporated into the early elements of the design, this will ensure superior performance is achieved.
- Components: The choice of components used in the attenuator construction can have a major impact on the performance. By using the correct components in the attenuator construction, it is possible to obtain high performance levels.
One of the key requirements is to ensure that non-inductive resistors are used. Surface mount resistors are particularly good, because they are small and are not manufactured using any spiral cutting techniques. Accordingly the levels of stray inductance are very low.
In terms of conventional components a variety of forms of resistor are available. Wirewound resistors are obviously not acceptable, but as many other types use spiral cutting techniques to trim the resistance levels, some level of inductance may be introduced. This may introduce some stray effects at higher frequencies, although at frequencies, certainly below 30 MHz most types should operate satisfactorily. Specialised non-inductive resistors can be obtained where higher frequency operation, flat frequency responses and accurate levels of attenuation are needed.
- Attenuator sections: One key element of attenuator construction and design, is not to attempt to achieve a very high level of attenuation in one stage. If high levels of attenuation are attempted in a single stage, then the stray effects such as inductance, capacitance and imperfections in earthing may lead to the signal effectively bypassing the attenuator itself and the required level of attenuation not being accurately achieved.
If high levels of attenuation are required, then it is far better to build the attenuator in several sections - cascading several sections - so that the overall level of attenuation is achieved in stages. In this way the stray effects are not as significant
In attenuator construction, it is generally good practice not to attempt to achieve any more than a maximum of 20 dB attenuation in any one attenuator section. When this is done the adjoining resistors can be combined. In the case of the T section attenuator this simply means the two series resistors can be added together. For the Pi section attenuators there are parallel resistors.
- Stray capacitance: There can be very small amounts of stray capacitance that occur between elements of the circuit. These can significant levels in terms of performance, especially when they occur between the input and output of the attenuator. The result is that the input and output of the attenuator, or other areas are bypassed, especially at high frequencies. In view of this it is necessary to ensure that the input and output are kept sufficiently far apart and that capacitance between them is minimised.
- Stray inductance: When building an attenuator, any leads can provide a path for inductive coupling. Like the capacitance, this is particularly important in terms of coupling the input to the output.
- Poor earthing: As attenuation levels rise, the importance of the earthing increases. Levels of resistance can result in signal leakage around the attenuator.
- Power dissipation: For long term reliability, ensure resistors that have a sufficient power handling capacity are used.
To ensure these problems are not encountered screening between the input and output may be required, along with solid earth lines.
RF attenuators are very useful and building them can be simple if a few straightforward guidelines and tips are used in their construction.
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