RF Attenuator Basics and Tutorial- for use in radio frequency circuits including receivers and transmitters, etcIN THIS SECTION RF attenuator basics Resistor attenuator Attenuator resistor values Balanced attenuator Variable PIN diode attenuator Building / construction of RF attenuatorsRF attenuators are a universal building block within the RF design arena. As the name implies RF attenuators reduce the level of the signal. This may be required to protect a stage from receiving a signal level that is too high, an attenuator may be used to provide an accurate impedance match as most fixed attenuators offer a well defined impedance, or attenuators may be used in a variety of areas where signal levels need to be controlled.

Types of RF attenuatorRF attenuators can be categorised in a number of ways according to their capabilities and the technologies they use. Fixed RF attenuator: As the name implies fixed attenuators have a specific value and this cannot be changed. They may come in a variety of formats from small in-line items in a similar format to connector adaptors to those in small boxes with connectors on the ends to those incorporated within equipments. Switched RF attenuators: Switched RF attenuators are widely used in test systems where levels may need to be changed. They are often seen as small boxes with a number of switches, typically with switches for 1, 2, 4, 8, etc dB changes. Switched attenuators may also be found in items of test equipment to change the levels, for example of a signal generator output. Variable RF attenuators: variable RF attenuators are normally used in applications where it is necessary to continuously vary the level of a signal. Typically they provide a continuous level change by varying an analogue voltage on the input control line. They are normally used where accuracy is not a prime requirement.There are a number of ways in which attenuators can be designed and made. The two main types are given below. Resistor RF attenuators: Resistor attenuators are sued to provide fixed levels of attenuation. Levels may be varied by switching in different attenuator sections to provide the levels that are required. PIN diode RF attenuators: PIN diode attenuators are normally used in attenuator designs where a continuously variable level is required. FET RF attenuators: FET attenuators are normally used in attenuator designs where a continuously variable level is required. Like a PIN diode attenuator, FET attenuators use an analogue control voltage to control the level of attenuation.These are only broad categories for RF attenuators - they can be categories in a variety of ways according to the application and the type of attenuator technology that is used.

RF attenuator specificationsWhen designing, purchasing or using an RF attenuator it is necessary to be able to specify it to ensure that an attenuator with the correct performance is obtained. While some of the major specifications are detailed below, for some applications other parameters may need to be specified. Attenuation: This is the primary specification for an RF attenuator. It is the ratio between the output and the input power levels and is typically quoted in decibels (dB). Attenuation accuracy : It is often necessary to know the accuracy of the level of the attenuation of the attenuator. Particularly in applications where equipment is being tested, the attenuation accuracy is likely to be important. In these cases a tolerance on the nominal level of attenuation will be given. Frequency response: The level of attenuation of an attenuator will vary with frequency. This can result from the frequency dependence of the resistors or other components used in the RF attenuator, or where coupling between the input and output may exist as this will be frequency dependent. Some RF attenuators where the absolute level of attenuation is important may be provided with calibration charts measuring the absolute attenuation at different spot frequencies over a frequency band. Impedance : RF attenuators will be designed for use in systems with a given characteristic impedance. 50 ohms is the most common, although 75 ohms is also used, and it may be possible to obtain RF attenuators with other impedance values should the need arise. Power dissipation: In order to reduce the signal level, RF attenuators dissipate or absorb the unwanted power. For many small signal applications, power dissipation is not an issue, but for other applications where signal levels are higher, it is necessary to ensure that the RF attenuator will satisfactorily be able to handle the power levels anticipated. Power capabilities for RF attenuators may be quoted in Watts (or milliwatts) or as dBW - decibels relative one Watt (or dBm - decibels relative to a milliwatt)> Mechanical details of the attenuator: The mechanical details may include aspects such as the size and weight. The connectors may be included in this area of the attenuator specification. Environmental details: Many applications for attenuators are for use within benign conditions such as a laboratory environment. Environmental conditions would not be an issue. However for some applications it is possible that an environmental is required to detail factors such as vibration, temperature, humidity and the like.ByIan PooleRF Resistor Attenuator Pads: Circuit Design- of the basics of the design and build of RF resistor attenuator pads including Pi, T and Bridged T section resistive pads and their calculations.IN THIS SECTION RF attenuator basics Resistor attenuator Attenuator resistor values Balanced attenuator Variable PIN diode attenuator Building / construction of RF attenuatorsRF resistor attenuators or resistor attenuator pads are used in many RF circuit design applications. The RF attenuator circuits reduce the level of the signal and this can be used to ensure that the correct radio signal level enters another circuit block such as mixer or amplifier so that it is not overloaded. With attenuator pads including the Pi attenuator pad, T attenuator pad and bridged T attenuator pad topologies, it is necessary to look at each one in turn.The RF resistive attenuator pads also enable the correct impedance levels to be seen in by particular circuits such as mixers that may be impedance sensitive. The resistor attenuator pad has the effect of reducing any mismatch, although naturally some signal is lost.While it is possible to buy ready made resistive attenuators, it is also easy to make attenuators for many applications. Here a simple resistor network can be used to make attenuators that provide levels of attenuation up to figures of 60 dB and at frequency of 1 GHz and more, provided that care is taken with the construction and the choice of components.

Attenuator pad typesThe main formats for RF resistor attenuator circuits are summarised below: Pi attenuator pads: As the name indicates the pi attenuator pad has a topology similar in shape to the letter Pi. It has a single series resistor in the signal line and at the input and output a resistor is taken to ground. T attenuator pads: In terms of topology the T attenuator pad (or Tee attenuator pad) is the opposite of the Pi section resistive attenuator. It has a single resistor to ground and has series resistors on the input and output, forming a T section. Bridged T attenuator pads: The bridged T attenuator can be thought of as a combination of the Pi and T attenuator pad topologies.Both the Pi attenuator pad format and the T attenuator pad format perform equally well. Often the preference of which type to use is a matter of personal preference for the designer.

T attenuator pad formatThe diagram below shows the format for the T attenuator pad format. As the name implies, the T attenuator pad is in the form of a letter T with two resistors in series in the signal line and a single resistor to ground at the junction of the two series resistors.

T resistor attenuator padThe two resistor values can be calculated very easily knowing the ratio of the input and output voltages, Vin and Vout respectively and the characteristic impedance Ro.

Pi attenuator pad formatThe pi attenuator pad topology is in the form of the Greek letter pi and has one in line resistor and a resistor to ground at the input and the output.

Pi resistor attenuator padSimilarly the values for the pi section attenuator pad can be calculated:

Bridged T attenuatorThe bridged T attenuator can be used in a number of scenarios for which it provides some distinct advantages.The bridged T attenuator can be thought of as a modified Pi attenuator. , There is one resistor in line and two, one at either end that connect to a common junction point that passes signal to earth via a four resistor.

Bridged T Attenuator PadThe bridged T attenuator pad is often the favoured format for variable attenuators, especially those using PIN diodes. The reason for this is that the bridged T attenuator pad only requires the use of two variable resistors against the three required for both the Pi and T attenuator pads.A further advantage is that as the bridged T attenuator pad has a tendency to match itself to the characteristic impedance Zo. At high attenuation levels R5 is at a high resistance and R6 is low. Accordingly the predominant resistor values at those labelled R which is equal to the characteristic impedance.Each attenuator pad format has its own advantages and disadvantages. Often, the choice of the attenuator pad format used is down to the individual.ByIan PooleRF Attenuator Resistor Values- a table or chart of attenuator resistor values for use in Pi and T format resistor RF attenuator pads.IN THIS SECTION RF attenuator basics Resistor attenuator Attenuator resistor values Balanced attenuator Variable PIN diode attenuator Building / construction of RF attenuatorsThe calculations for the "Pi" and "T" RF attenuator resistor values are relatively straightforward. However it is often convenient to have a chart that provides the attenuator resistor values in a tabular format.The most common format for RF attenuators is in a 50 ohm system, i.e. one with a characteristic impedance of 50 ohms. Accordingly the table given below is calculated for a system with an impedance of 50 ohms.

Attenuator resistor definitionsThe diagrams below show the different attenuator resistor definitions that relate to the attenuator resistor values in the table.One of the most popular forms of resistor attenuator pad, is the T section pad. It gains its name from the topology of the attenuator pad. The attenuator resistor values are given for this format - the resistor identification in the table relating to the numbers in the diagram.

T resistor attenuator padThe attenuator resistor values are also given for the Pi section attenuator pads. Often there is little to choose between the Pi and T section attenuator pads - often it is the personal preference of the designer.

Pi resistor attenuator padThe bridged T attenuator shown below is often used, although it has four resistors in each section rather than the three used in other attenuator pad formats. It is often used in variable attenuators because of the fact that only two resistor elements need to be varied.

Bridged T Attenuator Pad

RF attenuator resistor values chartThe table given below provides the resistor values for Pi and T pad RF resistor attenuator circuits. The values in this table have been calculated for a characteristic impedance of 50 ohms.

RF ATTENUATOR PAD RESISTOR VALUES TABLE

LOSS IN DBR1R2R3R4R5R6

12.94338705.86.1410

25.721543611.612.9193

38.514229217.620.6121

411.310522123.829.385.4

514.082.217930.438.964.3

616.666.915137.348.950.3

719.155.813144.861.940.4

821.547.311652.875.633.0

923.840.610561.690.927.5

1026.035.196.271.210823.2

1128.030.689.281.712819.6

1229.926.883.593.214916.8

1331.723.678.810617314.4

1433.420.874.912020112.4

1534.918.471.613623110.8

1636.316.368.81542659.4

1737.614.466.51733048.2

1838.812.864.41953477.2

1939.911.462.62203966.3

2040.910.161.12484505.6

Table of resistor values for 50 ohm attenuatorsResistor designations refer to diagrams aboveNB:Attenuator resistor values in the table are for a 50 ohm system.

Attenuator resistor values for other impedance systemsThe attenuator resistor values in the table are given for a 50 ohm system as this is the most likely impedance system required. However it is recognised that other impedance systems may also be used.To convert the values in the table to another value of impedance, they should be multiplied by the factor Z / 50, where Z is the characteristic impedance of the required system.The RF attenuator resistor values chart given above enables resistors to be chosen for popular values of attenuation more easily than having to calculate each one individually. It provides a quick, at-a-glance reference for the attenuator resistor values. While only covering integer dB steps, it is unlikely that a any intermediate values would be needed. Also any attenuators providing more than 20 dB are likely to be made up from several stages each having a maximum of 20 dB.ByIan PooleBalanced Attenuator Pad- an overview of the balanced attenuator pad used for balanced RF and other systems including audio voice and television applications.IN THIS SECTION RF attenuator basics Resistor attenuator Attenuator resistor values Balanced attenuator Variable PIN diode attenuator Building / construction of RF attenuatorsBalanced attenuator pads are can be seen in a variety of circuits. While the unbalanced formats for the Pi and T section attenuator pads are probably the most widely used, balanced attenuator pads need to be used for balanced systems.Balanced attenuators are used for balanced RF systems, but they are probably more widely used for balanced audio systems where the characteristic impedance is 600 ohms and for some television systems as well.

Balanced attenuator basicsThere are a number of formats that can be adopted for balanced attenuators. The most commonly used are the balanced Pi attenuator and balanced T attenuator - these are basically balanced versions of the familiar Pi and T attenuator pads.Balanced Pi attenuator pad:The balanced Pi attenuator is shown in the diagram below. It can be seen from this that the series resistor in the top of the Pi section of the attenuator is shared between the two lines, rather than being completely contained within the non-earth line in the case of the unbalanced version. As a result the value of the series resistor is half that of the value of the resistor in the equivalent position on the unbalanced Pi attenuator.

Balanced Pi attenuator padThe resistor numbers relate to those used in other pages of this tutorial. Resistor values can be taken from those used in the "Attenuator Resistor Values" table provided on another page of this tutorial.Balanced T attenuator pad:The balanced T attenuator has a total of five resistors. As may be imagined, the resistors in the top of the T section are half the value of the equivalent resistors in the unbalanced version of the attenuator pad.As there are two resistors that are effectively split between the two lines, the balanced T attenuator pad has one more resistor than the balanced Pi attenuator.

Balanced T attenuator padAs above, the resistor numbers relate to those used in other pages of this tutorial. Resistor values can be taken from those used in the "Attenuator Resistor Values" table provided on another page of this tutorial.ByIan PoolePIN diode variable RF attenuator circuit- a constant impedance circuit design for variable RF attenuators using PIN diodes for use in a variety of RF design applications including programmable RF attenuators.IN THIS SECTION RF attenuator basics Resistor attenuator Attenuator resistor values Balanced attenuator Variable PIN diode attenuator Building / construction of RF attenuatorsElectronically controllable variable RF attenuators are often used in RF design. For example, it is often necessary to be able to control the level of a radio frequency signal using a control voltage. These variable RF attenuators can even be used in programmable RF attenuators. Here the known voltage generated by a computer for example can be applied to the circuit and in this way create a programmable RF attenuator.Often when designing or using variable or programmable RF attenuators, it is necessary to ensure that the RF attenuator retains a constant impedance over its operating range to ensure the correct operation of the interfacing circuitry. This RF attenuator circuit shown below provides a good match to 50 ohms over its operating range.

RF attenuator circuit descriptionThe PIN diode variable attenuator is used to give attenuation over a range of about 20 dB and can be used in 50 ohm systems. The inductor L1 along with the capacitors C4 and C5 are included to prevent signal leakage from D1 to D2 that would impair the performance of the circuit.The maximum attenuation is achieved when Vin is at a minimum. At this point current from the supply V+ turns the diodes D1 and D2 on effectively shorting the signal to ground. D3 is then reverse biased. When Vin is increased the diodes D1 and D2 become reverse biased, and D3 becomes forward biased, allowing the signal to pass through the circuit.

PIN diode variable RF attenuator circuitTypical values for the variable RF attenuator circuit might be: +V : 5 volts; Vin : 0 - 6 volts; D1 to D3 HP5082-3080 PIN diodes; R1 2k2; R2 : 1k; R3 2k7; L1 is self resonant above the operating frequency, but sufficient to give isolation between the diodes D1 and D2.These values are only a starting point for an experimental design, and are only provided as such. The circuit may not be suitable in all instances.

Choice of PIN diodeAlthough in theory any diode could be used in variable RF attenuators, PIN diodes have a number of advantages. In the first place they are more linear than ordinary PN junction diodes. This means that in their action as a radio frequency switch they do not create as many spurious products and additionally as an attenuator they have a more useful curve. Secondly when reverse biased and switched off, the depletion layer is wider than with an ordinary diode and this provides for greater isolation when switching or providing higher levels of attenuation.ByIan PooleRF 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.IN THIS SECTION RF attenuator basics Resistor attenuator Attenuator resistor values Balanced attenuator Variable PIN diode attenuator Building / construction of RF attenuatorsWhen 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 methodsOne 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 attenuatorOne 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 significantIn 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 constructionThe 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.ByIan Poole