Oscillator troubles - Electronic Circuits Projects Diagrams Free

excetara2 · 13th March 2008, 05:01 AM

I am working on a beat frequency oscillator metal detector. The rest of the project is working properly. The mixer, filter, daq modules, and such.

I can't manage to get this oscillator working. It is a simple oscillator. The only difference on the other one to produce the frequency is I wind my own coil. Neither of them oscillate at all. Any ideas or something to try to change. Schematic of one oscillator attached.

Sorry, 220uH is the inductor if not visible.

**Nigel Goodwin** · 13th March 2008, 11:45 AM

An obvious ommission is the decoupling capacitor on the base of the transistor - it's not likely to work without that (I'm assuming you have a decoupling capacitor across the power rails?).

excetara2 · 13th March 2008, 08:30 PM

What size decoupling capacitor would be needed? No, I don't have anything across the rails.

**Nigel Goodwin** · 13th March 2008, 10:16 PM

Try something like 0.01uF, it's also vital in all circuits to have decoupling capacitors across the supply rails, try a 0.01uF and a 100uF in parallel (an electrolytic is VERY important on battery powered items).

excetara2 · 14th March 2008, 05:16 AM

Placed both in the circuit to no avail. Should I try another oscillator. I believe the desing is right. It worked in pspice.

eblc1388 · 14th March 2008, 08:31 AM

Please show us some of your simulation results, like op points and waveform.

Your design shows only flatline output when I simulated it in LTSpice. Adding capacitors to transistor base did not help neither.

on1aag · 14th March 2008, 02:19 PM

All my simulations seem to be working, maybe there's something
wrong with (me or) my simulator.

on1aag.

eblc1388 · 14th March 2008, 02:55 PM

Quote:

Originally Posted by on1aag

All my simulations seem to be working, maybe there's something
wrong with (me or) my simulator.

on1aag.

My inexperience in using simulator.

It turns out I just use the ".tran 0 20m" .op command the circuit does not seem to oscillate. See image. However, it oscillates normally after I added the extra parameters similar to yours.

Why would ".tran 0 20m 0 1u" make such a big difference?

excetara2 · 14th March 2008, 03:37 PM

I'll give that a try today and thanks for all the help so far.

sachin.kolkar · 15th March 2008, 04:45 AM

hi guys ,
please tell me which simulator you are using..........

eblc1388 · 15th March 2008, 07:54 AM

Quote:

Originally Posted by sachin.kolkar

hi guys ,
please tell me which simulator you are using..........

Its LTSpice by Linear Technology. Available free to anyone registered with Linear Technology or choose not to:

LTspice/SwitcherCAD III

Roff · 15th March 2008, 03:29 PM

You can usually get an oscillator to start immediately by adding the initial condition directive

Code:

.ic i(L1)=1u

or something similar, to your schematic.
You can also usually get it to start by switching your power supply on rapidly

Code:

PWL(0 0 1u 9)

instead of assigning it a DC value.
BTW, this circuit oscillates in simulation without the base bypass cap.

eblc1388 · 15th March 2008, 03:49 PM

Thanks Ron.

Tried the above simulation with each of your suggestions and the oscillator now starts up every time.

**Nigel Goodwin** · 15th March 2008, 03:50 PM

Quote:

Originally Posted by Roff

BTW, this circuit oscillates in simulation without the base bypass cap.

But would you ever build a common base circuit without one?

Roff · 15th March 2008, 03:58 PM

Quote:

Originally Posted by Nigel Goodwin

But would you ever build a common base circuit without one?

I probably wouldn't have even considered the possibility that it might work without one.

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13th March 2008, 11:45 AM	(permalink)
Nigel Goodwin	An obvious ommission is the decoupling capacitor on the base of the transistor - it's not likely to work without that (I'm assuming you have a decoupling capacitor across the power rails?). __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

13th March 2008, 08:30 PM	(permalink)
excetara2	What size decoupling capacitor would be needed? No, I don't have anything across the rails.

13th March 2008, 10:16 PM	(permalink)
Nigel Goodwin	Try something like 0.01uF, it's also vital in all circuits to have decoupling capacitors across the supply rails, try a 0.01uF and a 100uF in parallel (an electrolytic is VERY important on battery powered items). __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

14th March 2008, 05:16 AM	(permalink)
excetara2	Placed both in the circuit to no avail. Should I try another oscillator. I believe the desing is right. It worked in pspice.

14th March 2008, 08:31 AM	(permalink)
eblc1388	Please show us some of your simulation results, like op points and waveform. Your design shows only flatline output when I simulated it in LTSpice. Adding capacitors to transistor base did not help neither. __________________ L.Chung

14th March 2008, 03:37 PM	(permalink)
excetara2	I'll give that a try today and thanks for all the help so far.

15th March 2008, 04:45 AM	(permalink)
sachin.kolkar	hi guys , please tell me which simulator you are using.......... __________________ hm:

15th March 2008, 03:29 PM	(permalink)
Roff	You can usually get an oscillator to start immediately by adding the initial condition directive Code: .ic i(L1)=1u or something similar, to your schematic. You can also usually get it to start by switching your power supply on rapidly Code: PWL(0 0 1u 9) instead of assigning it a DC value. BTW, this circuit oscillates in simulation without the base bypass cap. __________________ Ron Last edited by Roff; 15th March 2008 at 03:31 PM.

15th March 2008, 03:49 PM	(permalink)
eblc1388	Thanks Ron. Tried the above simulation with each of your suggestions and the oscillator now starts up every time. __________________ L.Chung