Introduction

The next section contains a design tool for L, U, PI, T, and H-Attenuators.

  • L-Attenuator - Consists of a single series arm resistor (R1) and a single parallel resistor (R2). This makes this type of attenuator a simple voltage divider. R1 and R2 are connected in series across the whole of the input signal, Z1. While the output signal is taken across just one resistance, R2. This type of attenuator is commonly used in audio applications, to reduce a signal, while at the same time, matching the impedances Z1 and Z2. This assumes that Z1 and Z2 are equal.

    Note: If Z1 or Z2 are different, you need to choose whether you are matching the attenuator to Z1 or Z2, not both. If Z1 Matching is selected, and Z1 is small, compared to Z2, very small (fractional) resistances can be calculated. Selecting Z2 Matching will produce more reasonable resistor values, but it will be matching the network to Z2.

  • PI-Attenuator - Consists of two series arm resistors (R1 and R2) and one parallel resistor (R3). This type of attenuator is considered unbalanced and is commonly designed with Z1 and Z2 equal. This configuration is then symmetrical as R1 and R2 are equal values.
  • T-Attenuator - Consists of two series arm resistors (R1 and R2) and one parallel resistor (R3). As with the PI-Attenuator, this type of attenuator is considered unbalanced. If this attenuator is designed with Z1 and Z2 equal, R1 and R2 will also be equal, and the configuration will be symmetrical. Being symmetrical makes the T-Attenuator ideal for insertion between two equal impedances ( Z1 = Z2 ), to reduce signal levels.
  • H-Attenuator - Consists of two series arm resistors (R1 and R2, R4 and R5) and one parallel resistor (R3). This type of attenuator is considered Balanced and is commonly designed with Z1 and Z2 equal.

For some attenuation values, making Z1 not equal to Z2 can cause the calculated circuit values for the series arm resistors to be negative. In these cases you will need to increase the Attenuation, high enough to make the values positive.

L, U, PI, T, and H-Attenuator Calculator

The calculator is split into four parts. The area on the right is for selecting the type of attenuator and defining the Input Impedance (Z1 - Ohms), Output Impedance (Z1 - Ohms), and Attenuation (A - dB). Any changes made in this section will be immediately reflected in the other sections.

The top-left of the calculator contains a schematic of the selected attenuator. The exact calculated resistor values, along with the specified impedances and attenuation, are shown on the schematic. The equations used to calculate the resistor values are shown below the schematic.

Note: Negative resistor values are an indication of an error. These are unrelizeable attenuations, for the input and output impedances specified.

The bottom-right of the calculator contains real world resistor values that can be used to achieve the specified attenuator. The more exact the resistor values are to the design values in the schematic, the closer your attenuator will be to the specified impedances and attenuation.

OR
Attenuator Type PI-Type T-Type H-Type
Input
Impedance
 Output
Impedance
 Resistor
Tolerance

If you would like a better overview of various attenuation values verses resistor values, might find the chart on the right useful.

The calculator for the chart is on the left. At the top of the calculator, specify the type of attenuator and the input and output impedances. You can also select the tolerance for the resistors you would like to use. The chart will list the resistances required for attenuations from 1 to 40 dB. The attenuation values have a light background color for visability purposes. The attenuation gets grainer at 25 dB, because the inbetween attenuations are tougher to obtain. If you want something in the middle, in the 25 to 40 db range, like 37 db, use the caculator at the top of this section.

Should any of the calculations turn up negative value, it will be marked in the chart with a red box marked with "Err". These are unrelizeable attenuations for the input and output impedances specified.

K-Table