# Oscillator circuit LC

Discussion in 'General Electronics Discussion' started by graemeian, Oct 2, 2014.

1. ### graemeian

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Oct 2, 2014
I want to build a simple battery powered oscillator circuit that has an output frequency that follows w=sqrt(1/LC). I need to build an AC susceptometer. My goal is to build a circuit with an air core inductor. I want to monitor the frequency of the tank circuit and pass a sample into the coil. If the sample contains ferromagnetic impurities, the inductance increases and the frequency increases. We fabricated one in graduate school using a tunnel diode. However, tunnel diodes are hard to find.
I would like to find something like a 555 chip and instead of changing R and C , I would like to include L. I would like to get a square wave output. I have frequency counters available. Any help would be appreciated.
Thanks.

2. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Hello and welcome to Electronics Point

I'm not aware of any IC that's designed to make an LC circuit oscillate, though there are many circuits using op-amps and other devices.

But can you start by telling us the frequency range that the circuit needs to operate over?

3. ### graemeian

13
0
Oct 2, 2014
The frequency range will be ~10K to 100K Hz. I could add more turns on the coil to reduce frequency. A typical counter should have no trouble counting 100K Hz.

4. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
That's fine. Well, there are lots of ways to make an LC tank circuit oscillate. Check out this image search: http://www.google.com/search?q=lc+oscillator&tbm=isch

A common result there is the op-amp-based circuit, which is the op-amp-based "negative resistance" circuit (see http://www.google.com/search?q=negative+resistance+circuit&tbm=isch for other ways of making a negative resistance circuit) connected across the LC tank.

The op-amp circuit normally uses a split supply, i.e. a positive supply rail, a 0V rail, and a negative supply rail; typically these are ±15V though ±12V and ±9V are OK too. If a split supply isn't available, the circuit can be modified. Suitable op-amps for that circuit would be TLE2071 (http://www.digikey.com/product-detail/en/TLE2071CP/296-10419-5-ND/380933), TLE2081 (http://www.digikey.com/product-detail/en/TLE2081CP/296-10438-5-ND/380941), OPA604 (http://www.digikey.com/product-detail/en/OPA604AP/OPA604AP-ND/251162), OP42 (http://www.digikey.com/product-detail/en/OP42GPZ/OP42GPZ-ND/1007334), or dual versions of those ICs.

The oscillator can then be fed through a comparator to convert its output into a clean square wave.

If you need an exact 50% duty cycle, the simplest option is to feed the comparator's output signal into a flip-flop connected as a divide-by-two circuit or "T" ("toggle") flip-flop. For example, half of a CD4013 with the D (data) pin connected to the Q-bar output will do this.