Looking to modify this circuit for 30Hz operation

I was given the parts to build this sine wave generator. As far as I
can tell it's a Wein Bridge with the two LEDs serving the function
originally reserved for the incandescent lamp.

http://www.stephenhobley.com/sinewave.gif

Unfortunately it's fixed frequency, the 5K variable only trims one
"side" of the wave, not selecting the fundamental as I'd hoped. It
looks liket the 68K resistors and 0.047uF caps determine the period of
oscillation (one pair for each half).

Does anyone know the mathmatical relationship between the values of R5/
R6 and C1/C2 and the frequency of oscillation? I'd like to get this
working in the 30Hz range.

Thanks,
Steve

 

[John Popelish]

stephen@stephenhobley.com wrote:

I was given the parts to build this sine wave generator. As far as I
can tell it's a Wein Bridge with the two LEDs serving the function
originally reserved for the incandescent lamp.

http://www.stephenhobley.com/sinewave.gif

Unfortunately it's fixed frequency, the 5K variable only trims one
"side" of the wave, not selecting the fundamental as I'd hoped. It
looks liket the 68K resistors and 0.047uF caps determine the period of
oscillation (one pair for each half).

Does anyone know the mathmatical relationship between the values of R5/
R6 and C1/C2 and the frequency of oscillation? I'd like to get this
working in the 30Hz range.

To change the frequency, you have to (I think) change the
values of R5 and 6, simultaneously. Double both of these
and the frequency should fall to about half.

If you want to make a variable oscillator, you need a ganged
pair of pots that track each other well.

--
Regards,

John Popelish

 

[John Popelish]

stephen@stephenhobley.com wrote:

Any idea of the approx values to get this near to 30Hz?
I could just make it up with sockets and keep plugging away, but I
wondered if the math was relatively simple.

If I recall, the basic formula is f=1/(2*pi*R*C) for each of
the RC pairs in the positive feedback loop, so with C being
4.7nF and f being 30 Hz, R would be 1.13 meg. It might be
better to use a larger pair of capacitors so that the
resistors could be proportionately smaller.

Any RC pairs that have an R*C product of 1/(2*pi*30)=0.00531
seconds would produce 30 Hz. For instance
1.13meg*4.7nF=0.00531 but so does 53.1k*100nF.

100nF=0.1uF.


--
Regards,

John Popelish

 

On Jul 18, 10:24 am, John Popelish <jpopel...@rica.net> wrote:

step...@stephenhobley.com wrote:
I was given the parts to build this sine wave generator. As far as I
can tell it's a Wein Bridge with the two LEDs serving the function
originally reserved for the incandescent lamp.

http://www.stephenhobley.com/sinewave.gif

Unfortunately it's fixed frequency, the 5K variable only trims one
"side" of the wave, not selecting the fundamental as I'd hoped. It
looks liket the 68K resistors and 0.047uF caps determine the period of
oscillation (one pair for each half).

Does anyone know the mathmatical relationship between the values of R5/
R6 and C1/C2 and the frequency of oscillation? I'd like to get this
working in the 30Hz range.

To change the frequency, you have to (I think) change the
values of R5 and 6, simultaneously.  Double both of these
and the frequency should fall to about half.

If you want to make a variable oscillator, you need a ganged
pair of pots that track each other well.

--
Regards,

John Popelish

Thanks John,

Any idea of the approx values to get this near to 30Hz?
I could just make it up with sockets and keep plugging away, but I
wondered if the math was relatively simple.

 

That's great thanks - I ordered some caps close to your value, and a
ganged 200K dual pot - I should be able to get pretty close now.