RF Attenuator Construction Assembly Guidelines
- once an RF attenuator has been designed, the construction is equally important - we have prepared some essential guidelines to ensure any attunuators work as expected.
When building RF attenuators, there are a number of practical aspects that should be considered to ensure the optimum performance is obtained. The practical elements of RF attenuator construction can make the difference between success and failure in terms of their performance. Even what may appear to be relatively small points within the attenuator construction, layout or build can affect the performance.
Avoiding signal leakage by attenuator construction methods
One problem that can occur when building an attenuator is associated with signal leakage. The signal leakage can occur for a variety of reasons:,/p>
- 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.
To ensure these problems are not encountered screening between the input and output may be required, along with solid earth lines.
Section the attenuator
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.
Use optimum components for the attenuator construction
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.
If the correct components and techniques are used within the attenuator construction / build, then good levels of performance are not difficult to achieve. Using conventional component techniques, it is possible to gain good performance into the UHF region, and using surface mount technology within the attenuator construction, excellent levels of performance are achievable well beyond frequencies of 1 GHz or more. As previously mentioned, the chief problem using surface mount technology is that of power dissipation, although some higher power surface mount resistors are available should these be needed.
By Ian Poole
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