Wheeeee! Anything to avoid a 555. But he does hold a commanding lead in the Lame Circuits Competition and doesn't want to sacrifice any part of it >:-}
With _snap_ (as in a 555) Barkhausen's criterion has no meaning.
Have you seen my postings...
There are easy ways to add symmetry and amplitude control... if need be. ...Jim Thompson
Yeah, like wanting precise control of the min and max voltages.
And using modern parts that we have in stock.
But he does hold a commanding lead
Sure, just keep adding parts.
But ideally, don't you want to dynamically change the RC value of the integrator when you change the frequency a lot, when it's variable ?
How do the modern function generators make triangle waves ? Not the old ICL8038 method I don't suppose ?
I used to use the old two op-amp integrator and comparator method a lot. Always worked.
boB
The problem at hand wasn't variable frequency, but you'd probably make variable current sources/sinks.
YEARS ago I did just that, but added components to clean up the distortion, etc. If I can find the drawing, I'll post it.
...Jim Thompson
[snip] ...Jim Thompson
Already posted (designed 1986), see...
Function_Generator_OmniComp-GenRad.pdf
on the S.E.D/Schematics Page of my website.
...Jim Thompson
It should be possible to match against any distribution.
That's obviously not true but even if it were, why work around a goofy sorta-triangle wave when a good one is easy to make?
It might not be obviously true, but it certainly isn't obviously not true. The 'goofy' isn't obvious to me, at all; that 'jump' only occurs at the triangle apices, and it's 30 mV on a 3.3V triangle wave. The reason to use a triangle wave is the long straight sweep, not the pointy-top shape. Similarly, the reason to use a square wave is the abrupt rise/fall not the kinda-flat top. I think we all agree on that, because we all used a flip-flop for the square (faster risetime than an op amp used as Schmitt trigger).
Because it works. And the work-around is a no-op (to exercise an ADC, the sweep should exceed the input range, or it doesn't hit all codes - the excess just needs to be greater than 30 mV-before-offset-and-scaling).
It's more useful to improve the triangle by symmetry control (because that allows for a high duty cycle of the upsweep, with short 'reset' delay). I've also done jobs where different measurements on a slow upsweep and faster downsweep improved my throughput.
You can, of course, eliminate the 'jumps' without adding a third chip, by using a (CMOS) '555 and dual op amp, integrating with the first section and inverting-with-attenuation with the second section (and take feedback from that inverted second section). Oddly, though, that would be slightly limiting, because the second section has to be a compensated op amp. That can get you a +/- 5V square and +/- 4V triangle, and benefits from the triangle being amplitude-regulated (to the '555 power rails).
With a small change (triangle wave was +/- 1.33, now +/- 4V) here's some component values.
And, a valid LTspice netlist, enough to do a simulation; note the triangle wave crest 'anomaly', and there's also some opamp-induced ripple after each peak.
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