the single figure shows in schematic form the circuit which makes use of the fact that the pin diode acts like a variable resistor at frequencies above approximately 100 mhz while compensating for the detrimental effects of the diode junction capacitance on phase with a resistor in shunt with the series diode . the rf resistance of the pin diode is dependent upon the bias current , with typical resistance varying from approximately 10 , 000 ohms at 0 ma to 1 ohm or less at 30 ma bias . the diodes are arranged in a double π pad arrangement which is two pads sharing diode 3 . it is noted junctions 30 - 34 are used as tie points . input terminal 25 receives an rf signal and output terminal 26 provides the variable attenuated signal . half of the attenuator , the left side , simplifies the bias description . there are actually two dc current loops , one through resistor 9 and diode 2 , the other through resistor 8 , diode 3 , and diode 1 . both loops share resistor 6 . in the &# 34 ; off &# 34 ; state , the control voltage at terminal 15 is 0 and diode 2 is back - biased , having a value of 10 k ohms . resistor 11 in parallel with diode 2 gives the effective resistance value necessary for a 20 db pad , thus limiting the effect of the junction capacitance of the diode on the total impedance . this keeps the phase shift due to the shunt rc combination to a minimum . resistor 8 and resistor 9 set the current through diode 1 and diode 3 for the proper 20 db pad shunt values . as voltage is applied to the control voltage point , current flows through diode 2 and more voltage is dropped across resistor 6 , effectively reducing the bias of diode 1 and diode 3 . this causes diode 2 to decrease in resistance and diode 1 and diode 3 to increase . finally , as the control voltage increases , the drop across resistor 6 is so large that diode 1 and diode 3 are back - biased and are effectively 10 k ohms each . diode 2 is approximately 12 ohms at this point . for the combined double π , the current through diode 3 is always twice that through diode 1 and diode 5 . the two sections ( left and right ) are limited to 20 db attenuation per section by resistor 11 and resistor 12 , which is mentioned , keeps the phase shift low by keeping the shunt rc impedance highly resistive . resistors 9 , 10 , 13 and 14 are for rf isolation . the values of resistors 6 , 7 , and 8 must be calculated for the particular pin diodes to be used . this may be aided by graphs of diode rf resistance versus bias current . the values for resistors and capacitors for hp 8052 - 0012 pin diode chips are as follows : for capacitors 15 - 24 , each one is 100 pf ; for resistors 6 and 7 , each is 6 . 7 k ohms ; for resistor 8 , the value is 14 . 2 k ohms ; for resistors 9 and 10 , each value is 1 k ohms ; for resistors 11 and 12 , each value is 306 ohms ; and the value for each of resistors 13 and 14 is 100 ohms .