My Analog Computer: Lessons So Far (publ. 2025-05-23)
Time constant
A point that I did not understand, while I was building my analog computer, is the importance of the time constant of the integrators. For accuracy, the integrators should all have the same time constant — or set of time constants — so that they all give the same results for the same rate of change input.
I mistakenly thought that it was important to get the integrator feedback capacitors all at the same capacitance. More fundamentally, though, what you want is for all the integrators to have the same time constant, which is the product of the input resistor and the feedback capacitance. So, if R=100 kΩ and C=1 µF, then RC=0.1. That means your simulation time is 100 ms, and that your integrators are running at 1/0.1 or 10x as fast as real time.
Integrators are expected to have multiple resistor inputs to their summing junction. Typically several are labeled 1x, and then at least one is labeled "10x" for 10x the resistance. My integrators have only a single resistor per integrator. This doesn't pay off in the end because then you find yourself having to use extra adder and inverter amplifiers to do what could be done with a single extra resistor on the integrator. In the wild, you will find that many model diagrams will tie in to extra resistors on the integrator.
Open amplifiers can be used for a lot of things
An open amplifier, allowing you to insert anything you want for the feedback, can be used for many things:
- simple inverter
- amplify or attenuate
- adder, if multiple SJ inputs are provided
- division, if paired with a multiplier
An integrator is fundamentally an open amplifier with a capacitor for the feedback, though extra circuitry is needed to handle modes like IC and RUN.
The value of a large patch panel, i.e., many jacks available
If I did a version 2 of this project, I would look into some solution to get a lot more jacks, at least double the amount, but fit them together more closely to keep the panel size as small as possible. I'm thinking it might be worth switching from 4mm to 2mm, though I have a lot of 4mm cables already. Jacks are not cheap, however, at least not the ones of decent quality.
Output jacks
It would be nice to have some dedicated output jacks which tie to output posts or hooks to which you could easily attach the oscilloscope probes. Right now I have to use alligator clips to tie probes and banana cables together, and that is rather messy and a bother.
Repetition mode
I can see now how having a fast repetition mode would be handy and fun, though it might be just too much work for me to design and wire up such a system. But I'm looking forward to having that feature once I buy my own THAT computer, God willing.
Copyright
This work © 2025 by Christopher Howard is licensed under Attribution-ShareAlike 4.0 International.
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