Need help with Wien-Bridge Oscillator

[(*steve*)]

Just for Fun, I decided to move my wien bridge oscillator from the breadboard to something a little more permanent.

I made a few small modifications:

1) a dual op-amp so that I can use one op-amp to split the power supply.
2) added a couple of DIP switches to switch in and out capacitors to vary the frequency (no, I don't have any dual gang pots on hand)

I did discover something very interesting.

There is a band of instability at certain points where a secondary oscillation can remain. In my case it was an approximately 1Hz variation in amplitude of the output signal.

It causes a modulation of the output that is only completely visible at a very slow scan rate on the CRO. It is both a thing of beauty and horror! I would imagine that the heating and cooling of the filament occurs at around 1Hz. The cure is to increase the overall gain a little.

I might try to get a picture of this from the CRO this afternoon.

 

[The Electrician]

I did discover something very interesting.

There is a band of instability at certain points where a secondary oscillation can remain. In my case it was an approximately 1Hz variation in amplitude of the output signal.

It causes a modulation of the output that is only completely visible at a very slow scan rate on the CRO. It is both a thing of beauty and horror! I would imagine that the heating and cooling of the filament occurs at around 1Hz. The cure is to increase the overall gain a little.

I might try to get a picture of this from the CRO this afternoon.

You might find the lead article in this issue of the HP Journal interesting and relevant:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1960-04.pdf

 

[(*steve*)]

Very interesting, thanks.

 

[(*steve*)]

So my problems may be caused by my op-amp being too good!

 

[The Electrician]

Fortunately that's easy to fix.

 

[(*steve*)]

And here is another site with a method of using a jfet for AGC.