3/6/08 888-2463-001 4-35
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Diamond Series™
An adjustable attenuator can be made from a 3 dB hybrid and two 50 ohm rheostats or
variable resistors ganged together. Power is input to port 1, and ports 2 and 3 are each
terminated with one section of the rheostat, so that each sees the same impedance (see
Figure 4-1). Port 4 is taken as the attenuator output.
Figure 4-1 3dB Hybrid
If the pots are both set for 50 ohms, ports 2 and 3 see matched terminations, and the pots
absorb all of the power, with no power being reflected from them back into the hybrid;
this gives maximum attenuation (see Figure 4-2). If the pots are set for zero ohms (a
short circuit), the signals that appear at ports 2 and 3, still in quadrature and equal in
amplitude, are completely reflected back into the hybrid. Thus all of the input power
appears at port 4, and the attenuation is minimum. Intermediate settings give
intermediate values of attenuation.
Figure 4-2 Maximum Attenuation
Input
Output
Variable R
Variable R
3dB Hybrid
1
2
34
Input
Output
3 dB hybrid
-90
50 ohm
50 ohm
0
º
1
2
34
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