Variable Frequency Osc circuit? Whats a good one?

MrAl said:

Hi BeeBop,

Wow, i havent seen one of those things in ages now, thanks for posting.

Just a tiny problem however...
The two output diodes are connected such that neither one of them
will ever conduct, so there has got to be a problem with the connection
of those two. Perhaps you can find the error and correct this schematic?
Thanks.

Click to expand...

They're zeners, and they are in series. When the voltage across the reverse biased one reaches 5.1V it begins to conduct. The forward biased one then acts like a normal diode. Same thing for the other direction.

Anaximander said:

I assumed the diodes were in parellel with each other, because that's how the wien bridge circuits I've built have worked.

Click to expand...

Yes, I recall trying some like that, and some with a light bulb, but this one seemed the simplest one which worked.

When I get some time later today, I'll breadboard this one and have a play with it...take some measurements....

Edit> Oh wait, I see the problem! They are marked as small signal! I think I did this one when I first started using EAGLE, and couldn't find a zener in the library.

BeeBop said:

They're zeners, and they are in series. When the voltage across the reverse biased one reaches 5.1V it begins to conduct. The forward biased one then acts like a normal diode. Same thing for the other direction.

Click to expand...

No.
Your zener diodes are high current ones and are tested at 49mA which will not occur in this circuit. At a couple of volts with a fraction of one mA they will begin conducting and cause distortion.
Low current zener diodes like the 1N4689 (tested at 50uA and with a very sharp breakdown voltage) should have been used.

audioguru said:

No.
Your zener diodes are high current ones and are tested at 49mA which will not occur in this circuit. At a couple of volts with a fraction of one mA they will begin conducting and cause distortion.
Low current zener diodes like the 1N4689 (tested at 50uA and with a very sharp breakdown voltage) should have been used.

Click to expand...

Thanks for that... I'm not sure if the one I put in the drawing was the one I used, but the advice you just gave, I will keep.

audioguru said:

My Wien Bridge oscillator uses the trimmer capacitors to make the capacitance of each half exactly the same. Including the capacitance of the amplifier's input and stray wiring capacitance. I can see the output level change a little (when I sweep the frequency faster than the AGC can control it) if I move a wire a small amount.

Click to expand...

Those trimmer capacitors do nothing to ballance the dual gang variable capacitor. If the capacitance varies by a couple of % from one section to another, then you'll have to adjust the trimmers everytime you adjust the gang. You could've used a sterio potentiometer, which might've given you better matching but it will generate noise and crackle when it's adjusted.

The components don't need to be ballanced anyway, it'll still work if they're different, it just makes the frequency harder to calculate.

Please post the schematic - we can't discuss your circuit, if we can't see it.

BeeBop said:

They're zeners, and they are in series. When the voltage across the reverse biased one reaches 5.1V it begins to conduct. The forward biased one then acts like a normal diode. Same thing for the other direction.

Edit> Oh wait, I see the problem! They are marked as small signal! I think I did this one when I first started using EAGLE, and couldn't find a zener in the library.

Click to expand...

Hi again Bee,

Yes, they are zeners *TODAY*, but two days ago they were drawn as
small signal diodes
Thanks for updating the schematic!

MrAl said:

Hi again Bee,

Yes, they are zeners *TODAY*, but two days ago they were drawn as
small signal diodes
Thanks for updating the schematic!

Click to expand...

Yes, and what I learned this time, was not to be in such haste.

That schematic was drawn up >5years ago, as far as I remember ... it was a quick and dirty post. The good thing is that my very old project file is updated. I wonder how many others, from when I first started using cad, have small screw ups...

Anyway, thanks for noticing.