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This is a part of an Agilent signal generator output attenuator. These are 4 or 5 of these stages with the resistor setup as shown, different values at each stage to set different attenuation levels and these can be combined using relays to create several combinations. For example one stage is -2dB, another stage is -4dB but they can be both inserted inline creating (surprise) -6dB attenuation.

What I don't really get is the resistor arrangement. What's up with the values and why the network instead of 3 simple resistors?

enter image description here

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My first thought is that although 4x 11.8 ohms in series/parallel still equals 11.8 ohms, you get 4 times the power rating and, if the resistors are high tolerance and/or low drift types (like the 25pp/degC used on the right-hand attenuator), finding a resistor that has this "spec" at 4x the power rating may have been impossible.

A second thought is that 4 in series/parallel as the series pass element of the attenuator may also give more options when it comes to making the attenuators "natural impedance" something other than 50 ohms, say 75 ohms (as may be used on the same PCB but in a different product).

The attenuators above are for 50 ohm systems - I did a quick calc to confirm the left-hand pi attenuator is 2.03dB when 50 ohm loading is used on the output (right hand side of the circuit) and it looks the same for the right hand circuit.

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  • \$\begingroup\$ I can see that with a 50 Ohm load the overall impedance of the stage is 50 Ohms. How did you calculate the attenuation? \$\endgroup\$ – user34920 May 7 '14 at 21:06
  • \$\begingroup\$ @user34920, 430||50 ohms = 44.79167 ohms and, the 11.8 ohms and this resistance forms a divider of 0.791488. Take the log and multiply by 20 to get -2.031dB \$\endgroup\$ – Andy aka May 7 '14 at 21:15
  • \$\begingroup\$ 20 not 10! That was my mistake. Thank you! \$\endgroup\$ – user34920 May 8 '14 at 15:32

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