Generate square or triangular wave 1 Mhz or higher

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dark_soul

New Member

• Oct 3, 2010

• #1

Greetings, I am trying to generate a square wave or sine wave frequency of 1 MHz and if you can rather than better. For this I'm using the LF347 OPAM that according to the maximum bandwidth is 4 MHz, but when I do the calculations to generate 1 MHz does not make me anything, as much 300 kHz without losing the waveform.

I thought that I have no adequate response by the gain that is handling the OPAM, but can not find or do not how to reduce the gain to the unit if necessary.

Attached two images of the settings I use to generate the waves.

Hopefully you can help me, thank you very much for everything.

 

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audioguru

Well-Known Member

Most Helpful Member

• Oct 3, 2010

• #2

The slew rate of the LF347 opamp reduces its output swing at frequencies above only 80kHz because it cannot swing fast enough. The graph on the datasheet shows a max output swing of 9V p-p at 500kHz and almost nothing at 1MHz. That is the fundamental so harmonics will not happen.

 

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dark_soul

New Member

• Oct 3, 2010

• #3

Thanks for the help,which OPAM work at that frequency.

 

audioguru

Well-Known Member

Most Helpful Member

• Oct 3, 2010

• #4

I don't know opamps that work at radio frequencies. Some do.

 

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dark_soul

New Member

• Oct 3, 2010

• #5

thanks again, just one more thing, mention the slew rate and output voltage referred to often as I can know that I need slew rate, as calculated.

I had understood that reducing the gain can reach more bandwidth, you could apply this right here?.

 

Last edited: Oct 3, 2010

audioguru

Well-Known Member

Most Helpful Member

• Oct 3, 2010

• #6

All opamps have frequency compensation so that they have no gain at high frequencies that cause a 180 degree or more phase shift. Then they do not oscillate when negative feedback is applied. The frequency compensation reduces the gain at 6dB per octave.
The LF347 has a max gain of 1 at 4MHz and a max gain of 4 at 1MHz. Some LF347 opamps have a max gain of 1 at 2.2MHz.

The slew rate limitation converts a sine-wave or square-wave into a triangle-wave. The output level reduces at higher frequencies because the output cannot swing fast enough.

 

gramo

New Member

• Oct 3, 2010

• #7

Depending on the accuracy required, a R2-R ladder connected to an overclocked PIC (upwards of 100Mhz) would (should) perform quite well as a programmable signal generator.

You would have to compensate resolution for frequency, though that's a given.. Slew rate might reduce the quality on the high end as well, but it should still look *reasonable* I'd image?

 

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dark_soul

New Member

• Oct 5, 2010

• #8

Hi again, i was looking for some OPAMS available in my country and I found the NE5539 which has a bandwidth of 350 MHz unity gain and high slew rate, but I have a little problem, now I get 1 MHz but i have a distorted waveform, how can I do to improve the waveform. I don't know if it is offset in frequency and if so, what i need to do?.

Thanks for everything.

 

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BrownOut

Banned

• Oct 5, 2010

• #9

I haven't looked at the spec sheet, but it appears to me your opamp doesn't have symmetric output buffers.

 

Roff

Well-Known Member

• Oct 5, 2010

• #10

You are trying to use an op amp as a Schmitt trigger. Op amps take a long time to recover from saturation, so will only work at low frequencies in this application. If you want to use this configuration, you will need to use a high speed comparator (Google).