Discussion in 'The Projects Forum' started by peter_morley, Jun 5, 2011.

1. ### peter_morley Thread Starter Member

Mar 12, 2011
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I have attached a quadrature oscillator that I want to use to display LEDs slowing turning on and off. Basically I want to get a sine wave form output for my led display. In the schematic I understand that the circuit has a feedback loop which causes the circuit to oscillate. I don't understand why the circuit does not contain a voltage source. How will the circuit oscillate if I don't have electrons flowing through. Maybe i'm overlooking something. If you have any easier ways to achieve what I want I would appreciate the ideas. Basically I want to create a crude signal generator that only outputs sine wave forms.

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Dec 26, 2010
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In fact, the ICs are shown linked to two power supply rails, Vcc and Vee. Typically Vcc might be +15V, and Vee might be -15V.

The oscillation itself arises because there is, or should be, a net gain around the loop which is (initially) greater than one at some frequency where the signal comes round the loop in-phase. Either the initial switch-on transient, or random noise impulses tend to generate a starting signal which then builds up around the loop. The signal amplitude then builds to a level where the large-signal gain is exactly unity.

3. ### peter_morley Thread Starter Member

Mar 12, 2011
179
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I understand that the Vcc and -Vee are hooked up to voltage sources. My question was within the actual circuit why isn't there a voltage source? I've always dealt with op amps whether used as comparators or amplifiers with a voltage source within the circuitry (not talking about Vcc or -Vee).

Dec 26, 2010
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This is an oscillator, a system applying positive feedback around a closed amplifying loop. The whole point of this is to originate a signal, normally without any external signal being required.

Some oscillators may receive inputs for special purposes such as synchronisation, but normally oscillation is initiated either by random noise, or in some cases by a transient applied at start-up. Once the oscillator is running, the amplification around the feedback loop perpetuates the signal for as long as power is applied.

Other types of oscillators such as neon or uni-jnction relaxation devices may not have such obvious feedback loops, but they all have in common conditions which bring about a cyclic change, without necessarily having to be given an external drive signal.

If you think about it, such circuits are obviously necessary. Without a free-running device somewhere, how could you ever obtain the drive signal you expected to find within the oscillator in the first place? This would be a chicken-and-egg situation.

5. ### peter_morley Thread Starter Member

Mar 12, 2011
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I looked online and found an easier example to understand. I calculated my frequency to be 0.5 Hz about and the output should be a sine wave. I get a wave that looks like a sine initially but then saturates at about plus or minus 15 volts which is my Vcc and -Vdd values. I have the circuit as an attachment with the wave form being produced. Do I need to change R3 and R4 to make this smoother? Any advice? When I implement this circuit on my breadboard the 741 op amp heats up really fast and I don't get any frequency.

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6. ### #12 Expert

Nov 30, 2010
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Page 12 has the antique diagram with the light bulb regulator. It works because the light bulb changes resistance as the wave tries to saturate.

(I hope that's a smiley face that I just attached.)

• ###### oscillators.pdf
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Dec 26, 2010
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Getting a good sine wave from a Wien bridge or similar RC oscillator requires a level regulating system, otherwise the amplitude will rise until clipping occurs. A web search will probably turn up something, but note that some circuits use parts like thermistors that may not be so easily available.

I would guess that your overheating hardware circuit probably contains a wrong connection, or a faulty part.

Ah, someone has done some web searching. They have also mentioned the old lamp method. Not so good for very low frequencies, but most educational.

Last edited: Jun 5, 2011
8. ### peter_morley Thread Starter Member

Mar 12, 2011
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I looked at a schematic done by bill marsden and it worked great! Thanks for the help!