Tektronix risetime calculator

[Leo Bodnar]

I have not seen such an elegant solution for ages, this is analogue computing at its best!
https://vintagetek.org/risetime-calculator/
I have cleaned up the schematic for essential clarity.
Values don't matter as long as all potentiometers and resistors have the same value and pots scales and taper are linear.
Leo

[tggzzz]

Why this works is at https://groups.io/g/TekScopes/message/163545

[RoGeorge]

this is analogue computing at its best

If you like analog computing, you may want to read about the V2 rocket guidance computer (see the attachment at https://www.eevblog.com/forum/chat/german-v1-and-v2-control-systems-and-electronics-discussion/msg1398713/#msg1398713 )

[Yansi]

Very cool. I should probably build one to calculate just the good old pythagoras theorem. Would be a cool demo!

[GerryR]

I have been following this discussion on groups.io and find this interesting, except that I am a little slow on the draw, so-to-speak.  The math is fairly easy to follow, but the application eludes me.  First why is the quadratic term important?  I assume the application requires it.  Albert (groups.io) tried to explain that it would be useful in a cascaded circuit situation.  Example please.  I can see where propagation delay would be important in a cascaded situation, but what circuit(s) would require a calculation of total rise time?  Like I said, I can be a little slow on the draw.  Any help will be welcomed.

[Yansi]

Cascaded amplifiers for example?

[GerryR]

Square wave or step response of cascaded amplifiers??   

[macboy]

Estimate rise time (and therefore useful bandwidth) of a measurement system. Oscilloscope (mainframe) has some rise time, the vertical amplifier plugin has some rise time, probe has some rise time.  Then the rise time of the system is the root sum of the components.  Of course today you don't have plugins and mainframes, but you also don't need an analog computer to estimate rise time, do you?

Another use is to work backwards:
You have a pulse generator, and you know its rise time. You know the scope rise time. You don't know the rise time of the device under test that is connected between the pulse gen and the scope.  You can dial in the pulse gen and scope rise times as two of the inputs, and the measured rise time (from the scope) as the output, then adjust the third input dial until you find the null. That dial then indicates your DUT rise time!

[GerryR]

Estimate rise time (and therefore useful bandwidth) of a measurement system. Oscilloscope (mainframe) has some rise time, the vertical amplifier plugin has some rise time, probe has some rise time.  Then the rise time of the system is the root sum of the components.  Of course today you don't have plugins and mainframes, but you also don't need an analog computer to estimate rise time, do you?

But, if you know the rise times of the components, taking the root of the sum of the squares becomes trivial, and you don't need a computer for that, though admittedly, the calculator is an elegant analog solution.

Another use is to work backwards:
You have a pulse generator, and you know its rise time. You know the scope rise time. You don't know the rise time of the device under test that is connected between the pulse gen and the scope.  You can dial in the pulse gen and scope rise times as two of the inputs, and the measured rise time (from the scope) as the output, then adjust the third input dial until you find the null. That dial then indicates your DUT rise time!

Now this makes sense!  Having two of the inputs and the result to find the unknown input.  This seems like a much more practical application of this calculator.

[barry14]

This circuit cannot work as shown.  The presence of R5 means the lower left side of the bridge cannot have a resistance less than 5k.  However the right side of the bridge, depending on the settings of pots A, B & C, can have a resistance anywhere from 0 to 7.5k. Thus the bridge cannot be balanced for most of its range. As an example, if all of the pots are set to 0, then the right side is a short circuit so that bridge balance is not possible.