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Need help with basic Oscillator design

I have some electronics theory questions and some electronics simulator questions pertaining to an Armstrong oscillator. I have done some reading and have tried to come up with a design on my own. Most books just show a general diagram with no specific values. I am trying to simulate my design in LTSpice:

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Plotting voltages at points A and B, the circuit will work for up to

13 seconds (.tran 13) but does something strange after 14 seconds? I speculate that L2 is going to keep adding more and more energy to the tank circuit until things get out of control. I don=92t think that this has happened quite yet though?

What do I need to add to the circuit to build the voltage at point A to a maximum altitude and keep it there?

Any help would be greatly appreciated. Thanks

Reply to
jalbers
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Right-click the .tran directive and tick the check box "start external DC supply voltages at 0V"

The changes the directive to ".tran 13 startup"

This is often necessary for oscillator simulations to run properly.

Reply to
Andrew Holme

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Thanks That seems to help with the simulator part. I still have the same theory questions along with some new ones.

The output of the amplifier (node B) is 180 degrees out of phase with the input of the amplifier (node B) . It seems to me that the connection to the tickler coil should be reversed but when I do this in the simulator it does not seem to help.

I believe that the voltage gain of the amplifier comparing node A to node B is around 240 which mease that the voltage at node A can't exceed more than about 25 mv before the amplifier starts clipping. Is this correct? How do keep the tank circuit under 25mv or is it desirable to have the clipping in the circuit?

I also think that my turns ratio L2:L1 is all messed up but I don't know what it should be either. After some thought, it would seem to me that the overall gain (transistor amp + tickler) should be slightly more than 1 to handle any losses in the tank circuit. Is this true?

I would like to know what values would make the circuit oscillate at

159Hz and why they are the correct values. Can anyone help? Thanks
Reply to
jalbers

Usually the tickler coil has a lot fewer turns (lower inductance) than the tank coil, to avoid saturating the transistor. Also, you usually want to capacitively couple to the base, to allow for high standing voltages on the tank without too much loading (I can't see your circuit right now, I'm on the wrong computer).

I'd go for a turns ratio of about 10:1 from tank to tickler (IIRC you have 1:10).

Normally for a standing current of 1mA or so I'd choose a tank coil and cap that have a reactance of 200 ohms or so at the design frequency. But

-- 159Hz?? For that frequency this just isn't the right oscillator, unless you're doing something very special. 159kHz would start to be reasonable, but at 159Hz you'll use more space, weight and money for the tank coil than would be taken up by a nice solid digital sine wave generator.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

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Thanks

I just pulled the 159Hz out of thin air. I wanted to do some experimenting with various types of oscillators. I have seen two different configurations of Armstrong oscillators. One type seems to have the tickler coil as the input to the amplifier and the other seems to have the tank circuit as the input to the amplifier. My design is using the tank circuit as the input to the amplifier, and the amplifier feeds the tickler coil.

Reply to
jalbers

Windows Mail sucks...

Anyway, as Tim mentioned, make L1 be 1mH, and L2 be 10mH. It then oscillates quite nicely at 782.6Hz. If you want a lower frequency, use proportionally larger inductors.

Also, look into a Wein bridge, which is a better circuit for low frequency sine waves. The parts for resonant LC circuits get a bit unwieldy at the frequencies you are looking at.

Regards, Bob M> > >

Thanks

I just pulled the 159Hz out of thin air. I wanted to do some experimenting with various types of oscillators. I have seen two different configurations of Armstrong oscillators. One type seems to have the tickler coil as the input to the amplifier and the other seems to have the tank circuit as the input to the amplifier. My design is using the tank circuit as the input to the amplifier, and the amplifier feeds the tickler coil.

Reply to
Bob Monsen

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