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Home Brew Analog Computer System

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johnwa:

--- Quote from: GK on February 03, 2014, 12:41:01 pm ---
Of course my real world implementation does not use a floating voltage source to set the IC of integrator 1 and there is some simple control logic involved, but I'll be able to share that with you another evening - got to go to bed now, unfortunately. I think this is probably close to what you were contemplating in your "sample and hold" idea?


--- End quote ---

Yes, this was basically what I was thinking of, though I hadn't worked out the details. I might give your circuit a go for my application, when I eventually get round to looking at it.

I have ordered a load of LM13700 OTAs for this project, primarily for use as multipliers (I would have liked to use AD833s, but they are $$$  :( ) Though I think they can be used in an oscillator configuration that will give simple frequency control and amplitude stabilization, without problems with matching the phases.  (I will also want a three phase oscillator at some point). I will see what I can come up with when they get here.

GK:
A quick e-bay search came up with this:

http://www.ebay.com/itm/10pcs-XR2208CP-XR2208-DIP-16-EXAR-/180915775470

Not my favorite multiplier IC, but that's pretty cheap at 10 devices for around about the price of a pair of AD633's. They are NOS, being long obsolete now, but fine for experimenting with. Might be worth a punt.

Datasheet attached.
 

mikejp56:
Hi GK,
I started an analog computer project about 2 years ago, but it wasn't nearly as ambitious as yours.
After much research I decided on 4 integrators, 4 summing amps, 4 inverters, and 2 4 quadrant multipliers. I also decided to use 8 potentiometers for coefficients, +/- 1V reference supplies, and a zero center analog meter for output.
This system will use a handful of op amp ICs and logic ICs for switching, and several 4066 quad analog switches to actually do the switching.
I had started to build it, but then life intervened and I put it aside for 2 years. I have since resurrected it, and read your posts with great interest.
I wish you luck with your design, and when mine is finished and running, I will post the schematics and a writeup.
Regards,
Mike

GK:
Hi Mike,

Sounds good; will be interesting to see.

IanB:

--- Quote from: GK on February 03, 2014, 12:41:01 pm ---All there is to it is two integrator stages with initial conditions in a feedback loop with an inverter.
--- End quote ---

This is fundamentally neat and elegant. You have a circuit that solves the following differential equations:

y = Tc dx/dt
x = -Tc dy/dt

Set x = sin(t/Tc), then y = Tc d[sin(t/Tc)]/dt = cos(t/Tc)
and x = - Tc d[cos(t/Tc)]/dt = sin(t/Tc)

However, I think in practice an automatic gain control circuit is required to keep the amplitude constant, otherwise any small errors in the system will be integrated without limit.

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