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Constant Impedance Stepped Attenuator Networks
The constant-impedance stepped attenuator (herein abbreviated CISA) is an alternative to standard ladder or other resistive stepped attenuators which generally require many resistors and an expensive rotary contact switch with many positions. The CISA design minimizes the number of resistors and switches / relays needed to implement an accurate and precise logarithmic stepped attenuator. CISA networks are particularly suited to digital control, since attenuation values can be selected with a simple binary number.
The basic configuration is a chain of 2-resistor attenuation networks, terminated with a fixed resistor at the load end. Each network achieves either zero attenuation, or a fixed attenuation depending on the resistor values and, when switched on or off, does not alter the load impedance seen by the preceding stage in the chain. Thus, several stages can be chained together, each with a different attenuation value, and the input impedance to the network will always remain constant (the same as the termination resistance). It is possible to build a CISA network which will provide 0dB to -63dB attenuation in 1dB steps, and uses only 6 relays!
The following excel spreadsheet will perform the necessary calculations to determine the value of each resistor, and contains some diagrams to illustrate how the attenuator is constructed. Tolerance calculations are also included to help evaluate the effect of real resistor values and tolerances.
[CISA Calculator] - download a .zip file containing an Excel spreadsheet with calculations and diagrams for CISA volume control.
Where Is the file?