Harvey invents a device PCB that keeps a record of the telephone calls made from his phone. The design is relatively simple and is shown in Figure 15. However, PCB when it is hooked up to the phone line, radiation from the device interferes with his TV reception.
Redesign Harvey's board in order to reduce the radiated EMI. You may move PCB components and/or add components, but you must use a single-sided board.
We should PCB begin by identifying the potential sources and antennas. Certainly, the 8-MHz clock signal is a potential source and so are the data lines. This device may also put significant noise on the power traces. Potential antenna parts are the three connectors. Nothing else on this board is large enough to be an efficient PCB radiation source.
When we start to rearrange the components, we should try to put all of the antenna parts (i.e. the connectors) on one side of the board. We should also reorient the components to minimize the length of the traces. Finally, we should fill the empty space on the board with PCB ground and make sure that each signal trace has a nearby return path.
One solution to this problem is shown in Figure 16. Try to trace the path of the 8-MHz signal current in the layout of Figure 15 compared to the same path in Figure 16. This current flows out the clock output pin PCB of the oscillator, into the clock input pin of the upper IC, out the ground pin of the upper IC, and into the ground pin of the oscillator. This loop area is significantly smaller in the Figure 16 layout. Also note that no high-frequency current returns on the portion of the plane between any two connectors in the Figure 16 layout.
The design in Figure 15 is unlikely to meet a radiation emissions specification and therefore cannot be marketed or sold. The design in Figure 16 should meet virtually PCB any country's radiated emissions specification without the need for any PCB shielding or high-cost components. Note that we could have provided pads for PCB mounting filter components on the phone lines if we felt that might be necessary.
