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I am trying to build an LC meter for measuring capacitance variations based on an LC tank Oscillator and an LM311 comparator. I took the circuit from this source (https://www.schiessle.de/emt1/MessKleinKap/MessKleinKap1.htm), and I would like to stick with that circuit as much as possible. I first simulate the circuit in LTspice to get an idea on how the circuit works. I want the LC circuit to oscillate around 100 kHz, and in the simulation, the results are good.

enter image description here

However, when I built the circuit, the results differ from the expected values. As a first step I made the circuit on a protoboard, and I checked it with a nScope USB oscilloscope.

I included a bypass capacitor from the power supply to ground, I connect the balance and strobe pins of the LM311, and I also used a different L1 inductance value of 2.7mh (therefore the oscillating frequency should be around 96Khz) that is not ideal (it has a 23ohms component). The components that I am using have, except the capacitor C3, 10% tolerance value.

The problem is that the circuit becomes damped and noisy after a few seconds, and I can not find why. I try modifying the C3 value. The bigger the value of the C capacitor, the longer it lasts in a stable way. I tried placing another 1mF capacitor in parallel to C3 and it works.

video here: https://youtu.be/F3j8kqEoxcw

Could these issues be caused because of building the circuit in a regular protoboard and not on a PCB? What else could be the problem? Any comments will help!

Thanks in advance!

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  • \$\begingroup\$ Why is the output pin of the LM311 connected to the LM311's Vcc pin? Don't use this oscillator design - it is unorthodox and brings nothing to the party in terms of theory or transferability to a different device. Someone has found it works and hey presto, "it must be a good design". It isn't so don't use it. There are much more orthodox and "proven" oscillator circuits that use a comparator and a tuned circuit to generate a sine wave. \$\endgroup\$
    – Andy aka
    Commented May 6, 2020 at 11:30
  • \$\begingroup\$ According to your schematic C3 would be a polarized i.e. electrolyte capacitor and in you picture it looks like it - is this really the case? Are you aware of the nature of such caps? \$\endgroup\$
    – Christian B.
    Commented May 7, 2020 at 9:06
  • \$\begingroup\$ Yes, I am using a polarised capacitor for C3 but on the simulations worked fine. Would this have such an impact? And what is weird is the behaviour after a few seconds. At the beginning works fine (as shown in the video) \$\endgroup\$
    – Mahuertar
    Commented May 7, 2020 at 20:58

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using a very-high-gain high-current-surges LM311 on a protoboard...... is asking for constant mis-behavior.

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By the way, simulators will do NOTHING with a polarized cap, at least in my experience. That is not modeled in the CAP model within SPICE.

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  • \$\begingroup\$ I've removed unnecessary cables and the frequency value improved, now it is close to the desired value. However, the signal keeps becoming noisy and damped after a few seconds. I will try to solder the components on a PCB in the next days. \$\endgroup\$
    – Mahuertar
    Commented May 6, 2020 at 21:23
  • \$\begingroup\$ surprisingly the problem was solved by connecting directly the +Vcc pin of the LM311 to 5volts. In the source (schiessle.de/emt1/MessKleinKap/MessKleinKap1.htm) and in several approaches I found in internet, they connect it through a 1K resistor. \$\endgroup\$
    – Mahuertar
    Commented May 11, 2020 at 21:05

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