RF电阻衰减器设计.docx

RF Resistor Attenuator Pad DesignABSTRACT- 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.RF attenuators includes: 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 pads 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.1 Attenuator pad typesThe main formats for RF resistor attenuator pads 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.2 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.3 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 section RF resistor attenuator padSimilarly the values for the pi section attenuator can be calculated:4 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.The 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.5 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 section 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 section RF 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 Pad6 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.Loss in dBR1R2R3R4R5 R6 1 2.9 433 870 5.8 6.1 410 2 5.7 215 436 11.6 12.9 193 3 8.5 142 292 17.6 20.6 121 4 11.3 105 221 23.8 29.385.4 5 14.0 82.2 179 30.4 38.9 64.3 6 16.6 66.9 151 37.3 48.9 50.3 7 19.1 55.8 131 44.8 61.9 40.4 8 21.5 47.3 116 52.8 75.6 33.0 9 23.8 40.6 105 61.6 90.9 27.5 10 26.0 35.1 96.2 71.2 108 23.2 11 28.0 30.6 89.2 81.7 128 19.6 12 29.9 26.8 83.5 93.2 149 16.8 13 31.7 23.6 78.8 106 173 14.4 14 33.4 20.8 74.9 120 201 12.4 15 34.9 18.4 71.6 136 231 10.8 16 36.3 16.3 68.8 154 265 9.4 17 37.6 14.4 66.5 173 304 8.2 18 38.8 12.8 64.4 195 347 7.2 19 39.9 11.4 62.6 220 396 6.3 20 40.9 10.1 61.1 248 450 5.6 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.7 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.Balanced 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.8 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.Electronically 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.9 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.10 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.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.11 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.12 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.13 Use optimum components for the attenuator constructionThe choice of compon