Tuesday, September 22, 2009

N2PK VNA Main board RF shielding

I fabricated the custom DDS ICs heatsink out of a used old Pentium CPU heatsink (the green heatsink on the picture). Using a Dremel tool, I shaped the heatsink to fit in between the nearby taller components (ICs, inductors and a transformer). It was a tricky job! The two holes on the PCB provided for heatsink mounting are hardly in their optimal location. Because the holes are located only to one side of the DDS chips, it was very tricky to mount it in a way where the heatsink is pressing on both chips with equal force without tilting to the side of the screws. I used some washers as spacers to achieve this. A second heatsink (the black square heatsink) was attached to the side of the main DDS heatsink for cooling the Valpey-Fisher Master Oscillator. Two smaller heatsinks were attached to the ADC chips using self-adhesive heatsink interface.

I made the RF shield cans out of tin-plated brass sheet. This material is very easy to work with and solder. Due to the high density of components and the really narrow solder pads for the shield, a great level of precision is needed while fabricating the RF shields. There is no need to solder completely the shield along its whole length - just a few solder points per side is sufficient. The extra heat used during a complete soldering is unnecessary and dangerous to the components. In addition, if the shield ever needs to come off it will be much easier that way.

This is the finished board ready to be installed in the enclosure. The lids of the two RF screening cans are attached with self-adhesive copper tape (with conductive adhesive). Again, this method provides sufficient electrical connection and allows for an easy access to the the detector circuits if ever needed.

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