LFO woes: part 2

[Brian Marshall]

here is my most succesful incarnation yet.  seems to get down to .5 hz (approx) withot much problem, but any lower than that and the triangle's edges start to look more like a relaxation wave.  at the very slowest they get really, and almost stop arround zero, and speed back up when charging.

Looking at it now, i think maybe changing the 22k resstor to a 33k may help??!?!?!  the design is prety amplitude stable.

[Brian Marshall]

o by the way... prety standard voltage divider.  2 10k resistors, and a 47 uf cap.

[Brian Marshall]

ANYONE?

[Nasse]

What opamp type u used? And power supply?

[Brian Marshall]

maybe you missed the previous thread.

9volts

tl072

[puretube]

help is on the way...

[puretube]

this is the kind of thing GEZ was propagating;

(maybe we`ll have to add 3 more resistors around the trimpot and at the non-inv. input of IC1,
but this pic should give you an idea, that it`s almost the same as yours,
but will work fine from 20Hz down to 5mHz (1 sway in 3min.)).

[Brian Marshall]

that is f***'ing genius, and now i feel so dumb for not thinking of something like that.... P2 limits the head room of the comparator, so that it wont "over charge" the integrator.  right?

I guess i was so frustrated i didnt really listen to gez earlier, or atleast didnt understand what he meant....  

not sure i need a 5 min sweep  :lol: 5 seconds should do it.

brian


oh, btw why a resistor on the noninverting input of the integrator?

Just to keep out noise from the voltage divider?

[puretube]

just had to draw it up, coz I noticed, you were "blocked" somehow - lol.

Wouldn`t wanna call it a "headroom", but rather a "threshold" matter...

Vb = Vref, or Vbias or Ub/2 or 4.5V ? do we agree?!?

Now: actually this circuit was intended for +/- 15V supply,
but basically, it should work on "halved 9V" also.

To reduce eventual offset problems, you may need to connect
the first opamp`s + input not directly, but thru a resistor of the
same value as R1 to Vb. (in the drawn case: 68k).

The other 2 add. resistors would be series-R`s on both sides of the
trimpot, to get a smoother setting:
1. make trimpot 1k;
2. 3k9 between lower lug an opamp-output;
3. 470ohms betw. upper lug and Vb.

B.T.W.: reducing R4 to 68ohms gives even longer min. rates,
but possibly oscillation can stop after some sweeps
(I`ll tell ya, it is p.i.t.a., to sit and count minutelong periods with a watch,
and find out after 20minutes, the thing has stopped....)

[Brian Marshall]

ok, cool.

I'm looking at the drawing, and it totally makes sense to me.

i'm still perplexed by the resistor at the the integrators + input.

when you say 'off set" i assume you mean that the voltage at the divider may not be that stable because of the schmidt trigger charging it.  in that case shouldnt the inverting input have a cap to gound as well... to absorb any noise voltage coming through the resistor.  in theory the opamp input has infinite impedance.... are we dealing with some type of real opamp property?

[puretube]

no, just the ordinary opamp-inherent offset-thing (as one can read e.g. @ Lancaster`s filtercookbook, both inputs ideally love to see the same impedances...

B.T.W.: you can take cheap 1/2 a LM324...

[gez]

PT, you're a generous man!  

[mattv]

PT, you're a generous man!

Indeed. Thank you Sir.

[RDV]

Hey, where do I plug into this thing! :wink:

Regards

RDV

[puretube]

ah, come on boys: just a textbook standard-thingy (none of my "secrets")

[Brian Marshall]

thanks a lot.  

it still looks prety simple too.

I'll throw it together tonight, or tomorrow.  see what happens:D

thanks again

bri

[puretube]

hey there: did it work out?

[Brian Marshall]

hey there: did it work out?

Funny, i ordered some better caps to use build it.

one thing that i noticed was that i had my scope on AC instead of DC which was affecting my readings.  That was the root of most of my problems i was having.... just plain stupidity.  lower the frequency the more messed up my triangle looked.

the wave was so slow that the AC reading made it inaccurate.

that said i got the caps a couple weeks later, and still have yet to put it together, and test it out.  

bot of my breadboards are being used for other things right now... but i have another one on order.... should be sitting by my door when i get home today.


Actually i am getting a bunch of stuff today.

[RedHouse]

Don Lancaster?, great author/mind.

Reminds me of years ago when I used to build bi-amp crossover boards to install into rack gear and home stereo's, I used his 4th order hi and low pass circuits from his "Active Filter Cookbook" he rocks and the circuits out perform most commercial stuff.

[puretube]

RedHouse: wrong thread?