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Variable attenuators

Updated May 19, 2012

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For variable attenuators, the resistors are replaced with solid state elements such as MESFETs or PIN diodes. By controlling the voltage across the FET or the current across a diode, their RF resistances can be almost infinitely varied.

Variable tee, pi and bridged-tee attenuators

For the tee, pi and bridged tee attenuators, because there are two resistive elements R1 and R2, two control voltages or currents are required to make the variable attenuator work properly. Note that if R1 and R2 are not properly in synch, not only the attenuation value will be off but the characteristic impedance Z0 as well. Let's look at the resistor values for tee, pi, and bridged tee attenuator types, versus attenuation:

 

The optimum control voltage values for a tee-type voltage-variable attenuator using MESFETs can be found empirically, by setting VC1 to a fixed voltage and then varying VC2 to achieve input and output impedance values of 50 ohms and constructing a table of attenuation value, VC1 and VC2. But who wants to do that when several vendors have solved this problem for you?

Variable reflection attenuators

For variable attenuator applications needing an octave bandwidth or less, the reflection or balanced attenuators make a good choice because only one resistor value is needed. Typically the resistive element will be realized with PIN or NIP diodes, while the quadrature coupler will be a microstrip Lange or a stripline broadside coupler. It is important to match the voltage/current characteristics of the diodes, so that they will present the same impedance. The resistance that the diode presents is equal to the instantaneous slope of its I-V curve. A schematic of a balanced attenuator using diode elements is shown below:

The requirement for matching diode characteristics can be reduced if series bias resistors are added to each diode. The figure below illustrates this point. Here a pair of diodes are mismatched by 100% (a very extreme case). By adding 50 ohm resistors, the current mismatch is reduced to 5%. The only thing you trade for this is that more voltage will be needed to drive the attenuator.


Another cool trick to try with balanced attenuators is to configure two diodes within each leg, at a quarter-wavelength spacing (four diodes total make up the attenuator). This can more than double the dB value of attenuation for a given current/voltage operating point.

The load resistors on the quadrature couplers are needed to present a good match at high attenuation levels. Their VSWRs have only a secondary effect on the attenuation characteristic of the attenuator.

 


 
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