Timeline for How can I make a linearly tunable 50% duty astable multivibrator?
Current License: CC BY-SA 3.0
15 events
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| Dec 20, 2015 at 17:23 | history | edited | Andy aka | CC BY-SA 3.0 |
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| Dec 20, 2015 at 16:40 | comment | added | Andy aka | High slew rate.... I'll amend my answer accordingly and possibly make an opamp suggestion | |
| Dec 20, 2015 at 16:36 | vote | accept | Pepijn | ||
| Dec 20, 2015 at 16:36 | comment | added | Pepijn | Ah! Thanks a lot. I'll accept this answer and look at the datasheets of the opamps they sell at the University. scintilla.utwente.nl/nl/stores/search?searchterm=opamp The ones I have are just leftovers from previous projects. So I'm looking for low slew rate and low propagation delay? | |
| Dec 20, 2015 at 16:15 | comment | added | Andy aka | Another thing is that the 741 has an output slew rate of 0.5 volts per microsecond - to go from +12V to -12V and back again is 48 micro-seconds just in pushing the output pin up and down. The time difference between an 800 Hz period and a 756 Hz period is 73 us - half the error in crap slew rate. Add to this the propagation and amplitude errors mentioned in my previous comment and you have your answer. You need to be looking for an op-amp with much faster rise times and a propagation delay that is significantly below 1 us. | |
| Dec 20, 2015 at 16:06 | comment | added | Andy aka | Bad choice of opamp. The faster opamp add insignificant delay when wired as a comparator. Off the top of my head, a 741 takes several tens of microseconds to come out of positive output saturation and slew all the way to neg saturation. That's the first problem. The 2nd problem is that as you lower R to get a higher frequency you are changing the loading conditions on the crappy 741 and it's output voltage p/p droops a tad and it takes a little longer than it should to integrate on the next stage. Designing really accurate oscillators like this needs a bit more thought on opamp choice. | |
| Dec 20, 2015 at 15:56 | comment | added | Pepijn | I had some uA741 and LM358 in a box, I used the uA741, which is probably not a very good choice. I'm driving them with +15v and -15v, using R1=22k, R2=10k, R3=0-4.7k C=1uF. But then we're talking about sub-kilohertz frequencies here, so I thought speed would not be really important. That said, my triangular wave does look a bit like a tangent. | |
| Dec 20, 2015 at 15:40 | comment | added | Andy aka | what opamp did you use. It needs to be fast if you want accuracy. | |
| Dec 20, 2015 at 15:06 | comment | added | Pepijn | I tried to make the first circuit in your answer, with the Schmitt trigger and integrator. But I ran into the same behaviour where halving the resistance does not double the frequency. 2 parallel resistors tuned to 400Hz only give 756Hz combined, not 800Hz. | |
| Dec 19, 2015 at 12:49 | comment | added | Transistor | That'll be a binary logarithmic response then. I asked him to clarify. | |
| Dec 19, 2015 at 12:39 | history | edited | Andy aka | CC BY-SA 3.0 |
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| Dec 19, 2015 at 12:22 | comment | added | Andy aka | @transistor here's what the op defines as linear: "It's also not linear. If I halve the resistance, the frequency is less than doubled". It appears to me that he wants that type of response i.e. if resistance halves then frequency doubles. | |
| Dec 19, 2015 at 11:45 | comment | added | Transistor | Andy, I read his question as requiring a linear change in frequency with the pot. I don't think any of your examples satisfy this requirement. Do they? | |
| Dec 19, 2015 at 11:33 | history | edited | Andy aka | CC BY-SA 3.0 |
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| Dec 19, 2015 at 11:26 | history | answered | Andy aka | CC BY-SA 3.0 |