[amsat-bb] Protecting DEM Downconverters

["Art Goldman" <artg@xxxxxxxx>]

This forwarded on behalf of Don, W4VQA, in the hope that it may prevent
others from
inadvertently cooking their downconverters.......Art N3OY

After looking at two DEM downconverters that were zapped by transmitting
into
the 144 Mhz (if) output I am suggesting a simple modification that may help
prevent extensive damage to the DEM unit.
The DEM IF config has a RFC at the IF output to supply DC to the unit.
Following that is a DC block cap (C12) into an adjustable "T" pad
(R10,R11,R12) to the output of U7 via C13.
Excessive RF into the "IF" connection will eat up the "T" pad components &
possibly U7 as well causing loss of sensitivity (or no sensitivity).
A suggested fix is to add two "back to back" diodes from the junction of
C12/R10 to ground.
The diodes at this point are removed from any DEM DC voltage going to the IF
connector, and are at an RF point where we could clamp excessive power from
the IF source (like transmitting on 144) before it has a chance to do
damage.
I used some old diodes from the front end of a (very old) 432 converter
(1N100), but most any good unit should work. There is ample gain in the DEM
output to compensate for loading.
Testing here in the WSFH shows no impact on rcv sens with this mod
installed,
and physical placement of the diodes uses existing "pad" locations on the
DEM
unit.

73
Don - W4VQA


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