That’s definitely something I hadn’t thought of – going 3 dimensional. I was so in the mindset that this was gonna be “as cheap as possible” that I thought for sure it was a single sided/single layer board. Another reason I didn’t think of multi-layers was that I didn’t think there was a way (at least using commonly available manufacturing methods) to connect multiple layers on a PCB without vias. Are there? (were they available in the 80s?)
Wide traces for minimizing resistance makes sense, but I wonder… if you’re trying to get to that level of precision and sensitivity, would the capacitance you’re adding by going with the wide traces start causing a problem with ringing/signal oscillations? As I understand it, there’s a limit to the benefits of reducing resistance in exchange for capacitance.
The way I think about it is that there are electrons in the trace. A voltage difference induces those electrons to flow in order to try and reach an equilibrium point. As the electrons start to move, you have an inductive effect. This inductive effect is like momentum. So the electrons build up momentum. When the voltage changes, or if the voltage difference starts approaching zero, the electrons need a finite amount of time to react.
Depending on the impedance vs. capacitance you have, you get all those fun oscillation/dampening situations:
- critical damping (impedance/capacitance at proper levels to ensure a quick arrival at the signal voltage without overshooting/oscillating)
- underdamping (capacitance is too high for the impedance and your signal overshoots the target/equilibrium point and oscillates before reaching equilibrium)
- overdamping (impedance is too for the capacitance so your signal is slow to arrive at the equilibrium point).
(I could be muddling terms like voltage/charge/etc, but that’s basically the gist of how I understand it)
To me, there’s a (somewhat) similar analogy to how intake runners are designed for the engine. The air/fuel charge substituting for the electrons, the runners themselves substituting for the traces. The cross-sectional area of the runner is your resistance and the length of your runner is your capacitance. For a given RPM, you’re attempting to maximize the amount of charge that flows through without getting overshoots (back pressure being introduced on the intake side when the valve closes and there’s still charge momentum going towards the cylinder) or undershoots (pressure in the runner decreasing before the valve closes and getting some of the cylinder’s charge back into the runner). What you’d be going for is the “critical damping” situation where the runner’s charge pressure matches the cylinder’s charge pressure so the charge velocity is exactly zero when the valve closes. Again, very rough analogy, and being able to tune this with valve timing makes this aspect of the runner’s dimensions less critical in terms of concerns about damping, but still.
Yet another possibility is that the board was designed to for multiple possible configurations. Maybe the resistive material is laid on in a subsequent step to the traces and in some configurations some of those loose end traces get bridged by a resistor. Though I can already see one pair of pads that don’t seem to line up enough to support this theory.
By the way, I enjoy guessing/testing/guessing/blabbing about it, as you can probably tell, even if I never find “the answer”… so feel free to ignore. I won’t be offended. That said, I haven’t looked at some of this stuff for years… so anyone who feels so inclined to correct me, I’m all ears.