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85046A directivity problem

Question asked by BrucePenrod on Nov 24, 2007
Latest reply on Jun 28, 2013 by Harke
I've recently acquired an older 85046A to use with my 8753C.  Unfortunately in checking it out with my precision loads, I'm finding directivity failing miserably above about 100 MHz, and reaching up to -10 dB at 3GHz.  Though not identical, both S11 and S22 fail badly.  These same loads measured on my 778D coupler, tracking generator, spectrum analyzer rig are well below 30 dB out to 2.7 GHz.

I've swept and verified power levels at all ports and even jumpered around the mechanical transfer switch and the attenuator to isolate to the bridges.  It seems to be the bridges, both of them.  Everything else checks out OK.

Does anyone in the group have any experience with failure modes of the bridges?  What clobbers them, how difficult to repair?  I take it that they are touchier than the 778D coupler, it always seems to work, but not to 300 kHz!  

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