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I'd like to double the frequency of a triangle wave oscillator. The frequency is variable, and the range is within audio range, so 20Hz to 20KHz. The oscillator output is about 1.5V peak to peak, 0V centered.

I think I just need to rectify the negative (or positive) side of the triangle, then restore the amplitude using a gain of 2x, then correct the offset to get the same kind of output of the original.

Can anyone show me a simple and elegant way to do that? I tried to do some experiment but failed, since I'm not an engineer but just a beginner hobbyist, so I still cannot use opamps properly.!

This is what I tried so far: enter image description here

But I get something like this: enter image description here

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    \$\begingroup\$ VTC - This site is not a circuit design hand out site. Instead show what you have tried and you may get some help with what is giving you trouble. \$\endgroup\$ – Michael Karas Nov 6 '17 at 0:41
  • \$\begingroup\$ Sorry, I edited the post with my schematic so far \$\endgroup\$ – Dimitri Petrucci Nov 6 '17 at 1:04
  • \$\begingroup\$ Why did you sum the offset? It should go on the ground terminal of the second op amp. Secondly, whenever you graph, you need to put labels on it and numbers. \$\endgroup\$ – Voltage Spike Nov 6 '17 at 1:49
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    \$\begingroup\$ @MichelKeijzers STOP MAKING SILLY EDITS TO OLD POSTS \$\endgroup\$ – Chris Stratton Dec 18 '18 at 20:12
  • \$\begingroup\$ @ChrisStratton I just read your previous comment, will not do old posts anymore. Just wanted to help. \$\endgroup\$ – Michel Keijzers Dec 18 '18 at 20:14
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You were almost there, all you had to do was to buffer the signal. That way you won't distort your source, which you saw.

Here's my go at it, it can probably be done with 2 op-amps or even 1 op-amp if you're clever. But this is my quick and dirty solution.

enter image description here

Here's the link if you want to understand more.

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At first be sure that your triangle wave is symmetrical around zero. In other words the positive and negative halves must be equally high in volts and equally long in time.

Your rectifier clips another half wave off. You need a full wave rectifier. It will turn both half waves to the same polarity. So, search for "opamp full wave rectifier".

The offset voltage is ok only for certain amplitude. You bypass this limitation, if you use highpass filter to remove the DC component.

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This style of x2 full wave rectifier should get you close, though the capacitance of the diodes may affect things as the frequency goes up.

Since your question implied the input signal has no bias there is no AC coupling shown on the front end or buffering. I assumed your signal was low impedance.

The output is AC coupled to bring the rectified signal back to a zero bias. However, if your triangle wave is switching on and off you may need to add another amp to add in a DC bias instead.

schematic

simulate this circuit – Schematic created using CircuitLab

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