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circuit

I am using a single 74HC14 (hex schmitt trigger) to implement three oscillators. I now have a test build ready and encounter problems. See schematic, it shows component values. Also

  • proper 5V power supply
  • 2u2 electrolytic bypass near chip
  • 100n ceramic disc bypass next to chip
  • unused schmitt triggers' inputs are on ground or vcc
  • outputs connected to inverting opamp amplifiers, each their own opamp with 100k and 500nF in series to the inverting input (the C is for DC blocking)

The problem is: each of the three oscillators interfere with the other ones, changing their frequency. E.g. if oscillator 1 is set at 50 Hz, turning up 2 or 3 will cause 1 to go up in frequency (wild changes, think +50%)

I'd think the schmitt triggers are not loaded very high (resistances are all >100k, problems occur with the potmeters at around 200k already) so surely I'm not pushing it in terms of sink/source current.

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  • \$\begingroup\$ Try a ceramic decoupling capacitor. \$\endgroup\$ – Andy aka Nov 14 '18 at 18:34
  • \$\begingroup\$ To be placed where? \$\endgroup\$ – Kay Zed Nov 14 '18 at 18:49
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    \$\begingroup\$ This does not make sense if your input power is good... Are there resets etc you need to check? Do you have a scope to check your vcc line? \$\endgroup\$ – MadHatter Nov 14 '18 at 19:33
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    \$\begingroup\$ So normally if you just post a photo of your breadboard you get yelled at for no schematic. Could you post a photo of your breadboard so we can see what you mean when you say the 100nF cap is "close to" the chip? I would expect a bit of cross-coupling between oscillators, but not that much unless your power to the chip is horrible. I second @MadHatter's recommendation to look at the power line -- look at it right across the power and ground pins on the chip. \$\endgroup\$ – TimWescott Nov 14 '18 at 20:17
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    \$\begingroup\$ Also, how are you measuring frequency? \$\endgroup\$ – MadHatter Nov 15 '18 at 2:31

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