Tuesday, September 22, 2009

N2PK VNA Power supply module

I mounted the power supply module onto a piece of double-sided FR4 board. The board is exactly the same width as the main VNA PCB and just slides in the lowest channel of the Hammond enclosure - the same way the main VNA PCB is mounted.

This makes up for a very nice mounting solution with no drilling and no screws on the bottom of the enclosure - I really wanted to keep the enclosure clean and free of unsightly screws on the outside. The actual PS PCB is screened with a tin-plated brass RF shield (soldered to the top copper layer of the FR4 board). Copper tape strips on each side of the module are used to improve the electrical connection between the aluminum enclosure and the two copper planes of the FR4 board, once it is inserted in its channel. The lid of the RF shield is attached with self-adhesive copper tape (with conductive adhesive). This allows for an easy removal of the lid should a fuse change is necessary. If needed, the whole power module can be completely removed for servicing just by detaching the connectors and sliding it out of the channel.

The PCB is bolted to the FR4 board inside the shield using small brass stand-offs/spacers (there are components on the bottom side of the PS PCB and some clearance is needed between the solder joints/components and the top copper plane).

The bottom side of the FR4 plate. I decided to move the +12V linear voltage regulator from the bottom side of the PS PCB to the bottom side of the FR4 mounting plate. When the plate is inserted in the very bottom channel of the enclosure, there is just enough space (aprox. 5mm) for the linear regulator to fit in. This, actually turned out to be a pretty good cooling solution. On the right side of the board are visible some current limiting resistors for the two LEDs and the VNA Detect circuit as well as by-pass and filter caps. There is also a second LDO voltage regulator for +9V line (along with some filter caps). The +9V line is wired to the Accessory connector on the back, powering Transverters, RF-IV sensor or S-parameter test set.

The linear regulators are using the bottom side of the aluminum enclosure as a giant heatsink. Some thermal grease and a small copper insert (shim) ensures the good mechanical contact between the aluminium wall and the IC. The bottom copper layer of the FR4 board serves as a secondary heatsink - the regulator is mounted with its metal tab facing down and it is "sandwiched" between the FR4 mounting plate and the bottom wall of the enclosure. I used thermal double-sided self-adhesive tape to attach the regulator to the FR4 board (to the copper shim actually, the shim is soldered to the FR4 along one of its edges so it can flex). Soldered to the board is a little brass stub that goes into the regulator's mounting hole. This provides extra mechanical stability when the board is installed/removed.

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