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Backplane

Figure 1: Backplane (front view)

Backplane

After several attempts of floating wiring, I decided on a backplane. This board was designed with the program “EAGLE 5.2”. It was quite simple except for the mixing of metric and inch dimensions. The financial effort is worth it because many mechanical works are omitted.

The spacing allows the use of cartridge modules in the standard dimension 3HE 8TE. A MOLEX connector for computer power supplies was mounted directly on the backplane. The voltages 3.3 V, 5 V, 12 V, and −12 V could thus be distributed directly to the PCB slots. Each slot also has an individual connector area for a USB port, which is not yet assembled here in the picture.

Figure 2: Backplane (conductor side)

In our test model, we used a small ITX power supply (picoPSU‑120), which is plugged directly onto this MOLEX connector and only needs an external power supply of +12 V / max. 120 W. Please note that this power supply has 90° angled connectors for the primary power cable because the MOLEX connector is quite close to the sidewall. The large additional holes in the backplane are used to accommodate 15 mm spacers, which are also screwed into the threaded strip of the housing. On these spacers, an additional base plate made of aluminum sheet metal is to be attached, on which the processor board (Pico-ITX or Raspberry Pi) is later mounted. The back wall of the 10-inch laboratory housing is to accommodate a small touch screen in a later stage of upgrading, which is to show a display like an A-scope in addition to the handling of the computer.

Note: Switching power supplies usually only work if there is a minimum load on the 5 V line. The picoPSU‑120 is no exception, but most of the consumers in self-built radar operate at 3.3 V and 12 V. However, connecting a small hard disk has put sufficient load on the power supply.


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