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Extending Bandwidth of Oscillator

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jonnybgood

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Attached is schematic of oscillator whose output frequency depends on the LC tank circuit. The inductance by which this oscillator seems to be 'stable' is 1nH - 10mH. I need to know what is the purpose of the negative feedback with a bypass capacitor to ground and the purpose of the resistor network on top. Is there an effective way to modify the circuit to handle higher inductor values?

thanks
View attachment osc.bmp
 

It looks like a LC oscillator by accident. The impedance of the input node, and in turn the exact oscillation frequency, is e.g. depending on the OP bandwidth, gain, the RC feedback. There's really a lot of pleasant, almost simply calculable oscillator circuits available, why should we refer to scrap circuits like this?
 

This circuit will be used to find the value of an unknown inductor or capacitor connected, by measuring the frequency with a micro controller. What type of oscillator do you suggest that has the same properties?

thanks
 

I would use a linear oscillator with programmable negative impedance.

Now that you mention the origin of the circuit, I remember that it's been used in a LCR meter from electronics-diy.com, driven by a comparator LM311. I guess, it serves a purpose in this application, but I keep my opinion, that the circuit behaviour is rather obscure. The original circuit can be found at edaboard: https://www.edaboard.com/threads/101496/#post442750

To extend the operation to lower frequencies, it may be necessary to increase the values of both electrolytic capacitors. But accepted inductance is also restricted by the impedance level.
 

trojsi said:
Attached is schematic of oscillator whose output frequency depends on the LC tank circuit. The inductance by which this oscillator seems to be 'stable' is 1nH - 10mH. I need to know what is the purpose of the negative feedback with a bypass capacitor to ground and the purpose of the resistor network on top. Is there an effective way to modify the circuit to handle higher inductor values?
thanks
View attachment 64309
Click to expand...

I think, in principle the shown circuit represents one of the classical oscillator topologies: A positive feedback loop with bandpass properties (Meissner principle).
Regarding the negative feedback loop: This is necessary because of the single supply concept.
The voltage divider at the pos. input provides a dc bias of half of the supply voltage. To transfer this operating point (mid of supply voltage)
to the opamp output as well as to the invering opamp input a dc gain of unity is necessary. This is established by the R-C feedback.combination.
 

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