Wednesday, 7 August 2013

Adjustable attenuator

In order to control the output level of a transmitter one can use an adjustable attenuator. It is not difficult to build an attenuator consisting of several "pads" that can be switched in or off dependant on the amount of attenuation one is aiming at. 

The circuit below shows that the pads have values of 1, 2, 3 and 4 dB and two times 10 dB enabling the user to select any value within the range 0...30 dB in steps of 1 dB. 



Attenuator, build up 

If an attenuation of 6 dB is required, one can select the pads 2 and 4 dB to on.
Also switching 1, 2 and 3 dB will give the same result. 

Normal resistors from the E-12 range can be used. No exotic values are required. Use standard BNC connectors for entrance and exit. Use a metal housing. The max users frequency (VHF/UHF) depends on how the circuit will be build. Short connections, type of resistors (no wire-wound !). Shielding of individual pads (the pads can not "see" each other) helps  increasing the max usable frequency. Of course the max allowable dissipation depends on the dissipation of the resistors used (1/8, 1/4, 1/2, 1 Watt...).

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