RF 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.

RF 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 types

The 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 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.

The basic attenuator T pad showing two series resistors and one to ground
T resistor 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.

The formulas for calculating the values for resistors in a T pad RF attenuator circuit

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.

The basic attenuator Pi pad showing one series resistors and two to ground, one at either end.
Pi resistor attenuator pad

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

The formulas for calculating the values for resistors in a pi pad RF attenuator circuit

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.

The bridged T attenuator pad showing how the circuit arrangement of the basic T attenuator pad has been altered.
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.

Each attenuator pad format has its own advantages and disadvantages. Often, the choice of the attenuator pad format used is down to the individual.

By Ian Poole

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