Once in common use in radio, the passive attenuator is used less frequently today because of active circuitry and bridging impedances instead of matched impedances.
The basic attenuator is a T pad, which provides an unbalanced attenuator path referenced to ground. The balanced version is an H pad.
A pad can have equal or unequal input and output impedance.
To calculate the values for a pad, you must know the desired input impedance, output impedance and loss in dB.
To calculate the values for an H pad, first calculate the values for a T pad, and then divide the values of the input and output resistors (R1 and R2) in half to equally divide the load.
The value of loss in dB must be converted to a ratio of current, voltage or power. The formula to determine K is
K = 10(desired attenuation in dB/20)
Note: For all these formulae, Z and R values are expressed in ohms.
Equal Impedance Pad Calculation
The formulae to calculate a pad of equal input and output impedance is simplified because some of the values effectively cancel each other. Use the formulae in the next section to calculate values for pads with unequal impedances.
When Z1 equals Z2, R1 will equal R2.
Unequal Impedance Pad Calculation
The value of R3 can be used to determine the values of R1 and R2, so it is easier to calculate it first. The equation for R3 can be substituted in the equations for R1 and R2 if you prefer to repeat the work.
T and H pads are bidirectional, so either side can be used as the input. When calculating the values for a pad with unequal impedances, the value of Z1 must be greater than the value of Z2.