I was working one of my project wherein i was supposed to connect inductor to an inverter and inverter's output would be fed back to inductor. However my observation is that whenever inductor is connected to inverter, inverter output frequency reduces. I am unable to understand what is causing that. Can somebody help me understand this.
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1\$\begingroup\$ If the inductor wasn't connected then why would there be any output frequency? Are you missing some of your logic-gate schematic? \$\endgroup\$– Andy akaCommented May 9, 2021 at 15:31
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\$\begingroup\$ Output of inverter would be connected back as input to inverter. Assuming output as 0, it would be inverted and hence output would be 1 which would be fed back as input. This continues giving oscillation on inverter output \$\endgroup\$– ManuCommented May 9, 2021 at 15:35
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1\$\begingroup\$ That's not true of all inverters. \$\endgroup\$– Andy akaCommented May 9, 2021 at 15:40
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\$\begingroup\$ However, that's how it's working in my case. If inductor is not connected, inverter o/p would be around 10mhz which reduces to around 6mhz with inverter connected. \$\endgroup\$– ManuCommented May 9, 2021 at 15:45
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1\$\begingroup\$ millihertz? Also, your inverter has no frequency of its own, as Andy said already. "The frequency reduces" hence really makes no sense. \$\endgroup\$– Marcus MüllerCommented May 9, 2021 at 15:47
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1 Answer
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As a thought experiment, if the inverter gate has a signal propagation delay of 50 ns, then, if it oscillates when you connect output to input directly, the propagation delay is equivalent to an extra 180° of phase shift. So, if \$t_P\$ is 50 ns, the inverter would oscillate at 10 MHz because 180° of the period of 10 MHz is 50 ns.
If you fed the output back via an inductor, the inductor and the input capacitance of the inverter (a few pF) will add some more delay. Maybe that delay is 10 ns. Now, that means the frequency that has half a period of 10 ns + 50 ns is 8.3333 MHz.
Thus, if an inverter can be made to oscillate when output is fed directly to input, then replacing the direct connection with an inductor will naturally reduce that oscillation frequency.
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\$\begingroup\$ Thanks @andy aka. If a metal is brought close to the inductor in this setup, will it have any impact on frequency? I know it would generate eddy currents. However, since it's a digital circuitry, I just want to know the impact on output frequency \$\endgroup\$– ManuCommented May 9, 2021 at 16:35
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\$\begingroup\$ @Manu if eddy currents are generated in the metal sample and the metal sample has got good conductivity and isn't ferrous then the eddy currents act to reduce the inductance value. A poorer conductivity sample might reduce the inductance less but, to a certain point, the losses in the poorer conductance might appear to barely alter the inductance. Ferrous will have a mixed bag of responses based on frequency and conductivity. A lower value of inductance means a higher oscillation frequency. \$\endgroup\$– Andy akaCommented May 9, 2021 at 17:26