feedthrough from attenuation control inputs to RF ports

Question

I need to know how much feedthrough to expect from the attenuation control inputs to the RF ports. I am concerned that noise on the digital attenuation control inputs may feed into the RF signal. 
 The data sheets for the HMC1119 and HMC470 show similar equivalent circuits for the attenuation control inputs. The data sheet for the HMC1119 evaluation board shows 100pF bypass capacitors on the attenuation control inputs. Their presence suggests that the designer was aware of the possibility of noise feed-through. Possibly there are other reasons for putting these rather large capacitors on the attenuation control inputs. 
 Can you provide data on the amount of feedthrough to be expected. Expressed in S parameters, this informatoin would be expressed as: 
 S( ATTNx, Dy ) where x=IN/OUT and y=1,2,3,4, ... 
 
 Thank you very much! 
 
 Max Nielsen 
 Staff Engineer RF Microwave Design 
 Northrop Grumman Mission Systems

zwang · Accepted Answer

Hi Max, 
 As far as I understand, you are interested in the signal coupling from control inputs to the RF ports. This would be very difficult to measure on the bench since RF ports are 50ohm, while control inputs are high impedance. 
 We did a simulation on another similar digital attenuator and found that the signal coupling is below -45dB typical up to 6GHz (HMC1119 is up to 6GHz max). Therefore, I would expect a similar signal coupling value for HMC1119/HMC470A as well. 
 Do you have a targeted value to achieve? Is -45dBm typical a reasonable value in your design? Moreover, you can still apply the 100pF bypass caps on the control inputs to further improve the isolation. Will this be an approach in your layout? 
 Zheng)

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feedthrough from attenuation control inputs to RF ports