Javascripe Electronic Notebook
by Martin E. Meserve
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Attenuators | H-Pad
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The diagram below is a H-Pad Attenuator. It's called a T-Pad because the
components of the circuit form the letter T. It 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.
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.
In the boxes provide below,
enter the required Input Impedance (Z1), Output Impedance (Z2), and
required Attenuation (A), in db. The resistor values necessary, to
realize this configuration, will then be calculated.
Note: After entering your
data click anywhere outside of the boxes and the new outputs will be calculated.
The exact resistor values needed
for an attenuator, with a Input Impedance (Z1) of x,
an Output Impedance (Z2) of x, and
an Attenuation (A) of x, are
Real World Values
For the design specified above,
the nearest standard 5% resistors are listed below. These values, not being exact, will
cause the Input/Output Impedances, and the Attenuation, to be slightly higher.
Note: Standard resistor values may not
be accurate for resistances less than 10 Ohms.
If you require greater
accuracy in the resistor values, you can use the closest 1% tolerance resistors,
or you can use two standard 5% resistors in parallel to approximate each of the resistor
arm values. The resulting resistances will be within 1% of the specified values.
The Delta below shows the difference between the original calculated value and
the parallel resistor combination.