Balanced Resistive Attenuators

Balanced attenuators are used in systems where a balanced line is required: antenna feeders; balanced microphones, etc . ..

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RF Attenuators Includes:
Attenuator basics Attenuator specs Resistive attenuator design Attenuator resistor values table Balanced resistive attenuator pads Variable PIN diode attenuator SMA attenuator

Balanced resistive attenuators are not a widely used as their unbalanced cousins. Their major use is in conjunction with balanced lines of various forms. These include balanced audio lines for microphones (where 600Ω is a common impedance value) and also balanced transmission lines for antennas.

Determining the values for balanced attenuators is quite straightforward. Also the same rules for maximum attenuation in any given section also applies.

As can be seen from the circuits for the balanced attenuators below, the main difference is that the series resistance is split between the two lines - the resistors between the lines remain the same as they were from the un-balanced attenuators.

Balanced attenuator basics

There 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 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.

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.

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 section attenuator pad — Balanced Pi section resistive attenuator pad

On the occasions when balanced attenuators are required, they are simple to design and construct. The design can be based upon the unbalanced attenuator designs and then altered accordingly.