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LC Oscillator Design Help Existing Circuit

LC Oscillator Design Help Existing Circuit

(OP)
I am trying to rebuild a design and change the oscillator. Basically this product drives a non contact inductive sensor and measures the presence of metal near the sensor. I have drawn out the schematic, but am not completely understanding what type of oscillator design this uses. From what I can tell, there is an oscillator circuit that connects to the inductive loop sensor through what is measuring as a 1:1 signal transformer. I can't tell the value of the caps in the circuit. C2 and C3 are not always in circuit and look to be a way to change the frequency it oscillates at. The output is a square wave converted by the transistors to go into a PWM measure pin on a micro.

Any help with this would be greatly appreciated. I am curious how to isolate the oscillator portion from the portion that converts to square wave. I would also like to know the oscillator type so that I can calculate frequencies and values.

RE: LC Oscillator Design Help Existing Circuit

(OP)
Correction. That is not a 1:1 signal transformer. I am trying to figure out what it is now.

RE: LC Oscillator Design Help Existing Circuit

It's difficult to visually recognise a common circuit when it's not drawn in the textbook arrangement. I've seen examples where two resistors were so widely scattered that one wouldn't see the voltage divider without redrawing it.

So I'd recommend simplifying and redrawing the schematic in textbook format, and then comparing it to http://en.wikipedia.org/wiki/Electronic_oscillator .

RE: LC Oscillator Design Help Existing Circuit

I agree, the schematic is confusing, although I do recognize a current mirror buried in there. I was about to say that I couldn't find the power source, but I finally found it; there's definitely something disturbing about the Vcc being visually below the commons.

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RE: LC Oscillator Design Help Existing Circuit

(OP)
I have simplified the circuit and made it more readable. I am still not sure what type of oscillator is used and am therefore unable to calculate it. I am unsure how the transformer will work into the circuit too. I have measured the transformer to be a 2:1 signal transformer instead of the 1:1 I originally thought.

RE: LC Oscillator Design Help Existing Circuit

(OP)
Sorry about that. Sketched it out based on how the board is laid out. For some reason it never really bothered me to have Vcc below COM. I guess I am strange that way.

RE: LC Oscillator Design Help Existing Circuit

I don't believe that the schematic is correct as drawn.

For example, what's the purpose of Q1? Going from +5v rail to Com, there doesn't appear to be any output, or any purpose, for Q1. All it can do is heat up R1.

RE: LC Oscillator Design Help Existing Circuit

(OP)
Sorry about that. When I rearranged that schematic and screwed up that part. I have double checked it again and simplified a little more by removing multiple parts such as the caps. They were in parallel so I just combined them down into one. This one still has me stumped though.

RE: LC Oscillator Design Help Existing Circuit

Still doesn't look right. For one thing, Q1 is upside down (+ rail connector to an NPN's Emitter). And its Base is quite firmly grounded through R1 (47), so it doesn't seem do anything. It doesn't seem to be a common base amplifier.

You could try redrafting it with Q5 as the primary active element, draw Q4 as a diode, and see if Q1 is being used as some sort of bizarre feedback network around Q5. Don't bother including Q2 and Q3 at this point; they're just output.

It still doesn't match any of the text book oscillators.

Edited as shown.

RE: LC Oscillator Design Help Existing Circuit

(OP)
I have been able to strip out everything that isn't used to make this oscillate. It is starting to make a little more sense to me now. I did have the transistor drawn wrong on this. It looks like the capacitor is used to slow down the oscillator, but not necessary to make it oscillate.

RE: LC Oscillator Design Help Existing Circuit

Let's call your circuit drawing attempts A, B, C, and D in order that you produced them.

You'll need C1 back in Rev D, as C1 and the loop define the resonant frequency (f = 1/sqrt(2*PI*L*C). I'm assuming Loop1 and Loop2 are two ends of an inductive loop.

in Rev D it appears the current mirror is set up to toggle Q1 (when Q1 is off R3 is biased and the current mirror flows current that turns on Q1 that shuts off the current so Q1 is off...). The circuit appears to have improper polarity still (when Q1 is on the current mirror is off).

On another thought track, Rev D makes it look like Q1 is a current dump for some type of automatic amplitude control. Going back to Rev A that appears to make sense, as the capacitance is wired in series with the inductor and resistor on one side of the current mirror. I believe the current mirror is really the oscillator and Rev D is still not correct.

So I'm seeing two possible oscillator circuits depending up on the schematic revision. Typically a transistor lets current flow through a RLC circuit define your oscillator frequency and quality at resonance. A feedback path turns off the transistor at the resonant frequency to keep the oscillation from dampening out.

My \$0.02.

Z

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