Many practical circuitss can be laid out easily if the lumped element model holds for the particular circuit. In radio especially, we cannot make this assumption and we must minimize parasitic effects due to layout of components, or take them into account with a more general model and use simulation software such as SPICE.
PCB layout Basic guidelines:
- it is often a good idea to have made a prototype circuit using point-to-point construction or wire wrap, as you will have solved certain basic issues to do with component selection: (eg: should I use a 1/4 watt resistor here, or do I need 1/2 watt? etc.)
- consider physical constraints on the assembled board's size and heat dissipation requirements; choose your heat sinks if needed.
- consider carefully the physical size of the components you are laying out; the circuit schematic doesn't tell you this. Equivalent components often have different packages.
- How do the components attach to the board? Are they surface mount components? or do they require holes, screws, washers, etc?
- are there mechanical parts directly mounted to the board? eg: switches or variable resistors?
- How will the board mount in its container? What stresses (shock, strain, shear) will there be upon it and upon components?
- How will the board connect to its power source? What other connectors will be required (e.g: signal inputs, outputs)?
- use construction paper and a pencil and sketch the board in its actual size; or use component layout software that includes information about the component outlines.
- decide appropriate widths for each of the signal traces; this depends on the current each trace is expected to carry.
- decide whether you will have a single-layer board, 2-layer, or multi-layer based on the circuit complexity and fabrication costs.
- begin by placing component outlines, then by placing signal traces; leave a little room around each for tolerances.
- for a single layer board, spend more effort to avoid having traces cross each other; play with component placement or run traces underneath components; sometimes a jumper wire is needed.
- in 2-layer and multilayer boards simply run the traces on different layers, and use plated-through holes to jump from one layer to another.
- try to predict and avoid assembly errors: where there are multiple components of the same kind, or where pins have a polarity (eg: electrolytic capacitors), try to place them in parellel and orient the positive pin in the same direction.
- identify the critical parts of the circuit and lay them out first
- have one of the layers act as a continuous ground plane (usually the 'bottom' side).
- if signal traces are constant width and height above the ground plane, and are properly terminated, then their characteristic impedance is more well-behaved and may be calculated.
- avoid sharp corners.
- keep signal traces and component leads as short as possible.
- inputs and outputs should be far apart, so that RF energy will not leak back from output to input. stages should line up, rather than snake around.
- decouple the RF parts of the circuit from the DC parts of the circuit.
- shield AF and IF components from RF components.
