Pi & T Resistive Attenuator Pads: Circuit Design

Resistive Pi and T attenuator pads are easy to design and construct using simple formulas calculations and circuits.

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RF resistive attenuator pads are used in many RF circuit design applications. The resistive attenuator pads are very easy to design and can be incorporated into many RF circuits very easily.

They 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, and they are also used to ensure that a good impedance match is obtained.

Either Pi or T attenuator pads can be used and there is little to choose between them. Bridge T attenuator pads can also be used if required.

If the correct components and circuit techniques are used, then these attenuator pads can easily be used to operate at frequencies well into the GHz region. Surface mount resistors are particularly good, and modern PCB technology provides a good base for frequencies well above 1 GHz, although care may need to be taken when choosing the PCB material as some may be lossy and give the incorrect performance.

Resistive attenuator pad types

The main formats for RF resistor attenuator circuits are summarised below:

  • 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.
  • Pi attenuator pads:   As the name indicates the pi attenuator pad has a topology similar in shape to the letter Π. It has a single series resistor in the signal line and at the input and output a resistor is taken to ground.
  • 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 format

The 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.

Resistive T section attenuator pad

The 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.

N = V in V oou

R 1   = R 0 N - 1 N + 1

R 2 = R 0 ( 2 N N 2 - 1 )

Pi attenuator pad format

The 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.

Resistive Pi section attenuator pad
Resistive Pi section attenuator pad

Similarly the values for the pi section attenuator pad can be calculated:

N = V in V oou

R 3   = R 0 N + 1 N - 1

R 2 = R 0 ( N 2 - 1 2 N )

Bridged T attenuator

The 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 resistive attenuator pad
Bridged T attenuator pad

The 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.

In many respects there is little difference between the pi and t attenuator pads. The choice of which type of pad to used often results from the preference of the design engineer. It may even result from the convenience of the values that result from the calculations. If the values from one type of attenuator pad fall conveniently close to standard values or those already in a design, then this may be a good reason for choosing "Pi" rather than a "T" attenuator pad or vice versa.

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