Oscillator Quest

[Kipper4]

Ok so I'm looking at the shoot the moon oscillator.

What kind of oscillator is it please.
It looks like more than a Wien bridge and less than a PSO.
I'd like to find something with a nice speed range. Capable of driving an led (vactrol)
I'm not sure differant wave patterns are usefull for my intended application.
I think Sine wave would be all that's needed.
Maybe there's a way to amend the STM oscillator so it only does sine.
Perhaps someone can enlighten me.
Thanks

[anotherjim]

It's a Schmitt trigger Sqr/tri oscillator, but I don't know if it has a more accurate name than that. The integrator part  -  usually done with an other op-amp (the one with a feedback cap instead of resistor), is instead done with a passive RC network so its "triangle" wave is a curved shark-tooth shape - almost sawtooth. It's fed from the RC capacitor to a high impedance buffer amp so as not to load the timing capacitor or inject voltage back into it.
With a Vactrol control, the LFO shape works very well, I suppose due to the slow LDR rounding the response.

[nocentelli]

Ok so I'm looking at the shoot the moon....
I think Sine wave would be all that's needed.
Maybe there's a way to amend the STM oscillator so it only does sine.
Perhaps someone can enlighten me.
Thanks

I like the stm, it's a neat simplification of the tremulus lune LFO, I usually further simplify it by replacing the shape pot with a 470k from the vref on the first opamp non-inverting input to the
non inverting input of the second opamp buffer, and connect the 10uF-2k2-220k node directly to that point also: it hardwires the shape pot to the "triangle" output shape anotherjim describes, and works well for vibrato, chorus or tremolo stuff. I think I'd like a more swoopy, proper sinewave for modulating a delay, but I'm not sure it's that easy with just a dual opamp.

[Kipper4]

Thanks I got that now.
I googled Schmitt trigger and Integrator and now see both componant circuits.
I'm thinking of breaking the original and modded version (Leo's mods) to do some trials.
Much appreciated guys
Rich

[garcho]

do you need a real sine wave?
do you really need a sine wave?
real sine waves are fairly complicated to produce compared to square/triangle/ramp/saw. you can get by with faux sine waves, usually, but they're also a little more involved.

[Kipper4]

It's intended use is for a trem/vibe circuit.
So maybe not pure sine wave, also I'll be using the lfo to drive a vactrol led. Part of the reason I chose the lune/STM lfo.
Following my research and talks with Midwayfairer I'm assured that series vibe units is the way to go. Thanks JP.
I got excited when I discovered the Bubba oscillator and the potential for its use in a different but similar application.

Bubba can be seen here along with some others.


http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf


All thoughts and suggestions welcome ,so wade in guys, give me your thoughts
Rich

[anotherjim]

This is the "triangle" you get from an RC integrator. The shape is the natural exponential curve of a capacitor charging/discharging via a resistance.
An LDR won't respond fast enough to those sharp peaks and rounds it off. How much depends on the speed of the LDR and the LFO frequency. To see the effect, if you can scope it, rig the LDR up as one half of a resistive divider from V+ to 0v with, say a 100k for the other half. The waveform at the middle will show how the LDR followed the LED drive waveform. The LDR is probably doing just as well as any of the basic tri-sine converters do at rounding-off the peaks

[Kipper4]

Thanks Jim.
I get that and I will rig the scope up when I bread the lfo experiment.
Side note JP gave em an excellent tip about putting a cardboard box over the breadboard when working with vactrols.

I'm guessing that since vactrols , discrete or roll your own will react in a similar manner to whatever wave form is used because of the slow off time of an led in the vactrols used. Unless I'm mistaken about this.
The way I'm seeing it the PSO gives out a sine wave and works quite well for a trem/vibe set up with the concertina splitter. So I got to thinking why not use an op amp Lfo. One lfo driving and dual ldr set up.
I kinda want to make a circuit of my own rather than emulate escabedo.
Have two concertinas one set up with the 100nf cap the other without the cap. Which I found out gives a tremolo. By big booboo.
Maybe rather than summing them which I suspect would kinda lose the effect, do separate outs to get a differant effect to two amps but still in time with each other.
Or like I intimated with the bubba lfo. (Sine/cosine) out of phase with each other.
Also all these ideas are building up like a circuit Tourette's when I originally wanted a magna sound with more.
I hope that makes sense it made sense when I thought it.
So that's why I'm looking for the "best" wave form and lfo for that.

Simple right.........

[PRR]

> I discovered the Bubba oscillator

Op-amp sellers love plans like this: LOTS of op-amps.

To change frequency of Bubba you really have to vary *four* Rs (or four Cs).

And for wide range you want Reverse Audio taper pots.

The Fig.7 buffed faze shift plan is the same thing with only three places to vary (and no 90 deg outputs).

Note that SIN/COS is not 180 deg, but 90 deg. Take the head off a 4-cylinder engine. The pistons move 180 apart. Now take a head off a V-8. The pistons move 90 deg apart. A Straight-Six, 120 deg apart (the three RC oscillator). Now put all those heads back....

Since one opamp can easily give gain>26, there is little wrong with Fig.6. This IS the classic Fender tube LFO with a TI (or other) chip instead. If you trim the gain over say 50 and if your amplifier turns-down when overdriven (tubes do, opamps don't), you can get small freq shift by varying just one R.

Most of these schemes want to "wobble" when frequency is changed rapidly. Zener- or rail-limiting may wobble less than lamp control.

[anotherjim]

If it's a stereo effect you're after, as Paul says, 90deg or quadrature is the best approach. Simple 180deg opposite phase isn't too pleasant, especially heard on headphones. Not for nothing do recording engineers record rotary speakers in stereo with the mic's at 90deg.
Now I have had on the breadboard, a quadrature LFO that only needed a dual gang speed pot and sine output. It used CMOS inverters (CD4069UB) instead of op-amps. I haven't drawn it properly yet, but it was 2 of these...

Inverter a & b with R1/R2 are the schmitt trigger and c is the integrator. d has enough gain to soft clip the triangle into something very like a sine.
Tri & Sine look like this (noisy breadboard!)

For the quadrature output, that is repeated, only it's schmitt trigger is fed from the first triangle out of c using a much smaller R2 so the trigger thresholds are close to the zero crossing of the first triangle wave. This puts the extra triangle output 90deg out of phase. VR1 would be a dual gang pot.
I even had it working with 2 vactrols for the speed pots, with the idea of having a rotary speaker speed change effect and the quadrature wave tracked very well.
Don't try it with 4049UB inverters -  they're not symmetrical.
Op amps could be used instead. The quadrature would use a comparator detecting the first Triangle with reference at 1/2Vcc instead of schmitt trigger feeding its integrator. Faux sinewave wouldn't be so easy as the CMOS version.

[Kipper4]

Brilliant thanks guys.
Do tractor have four cylinders, V8s or straight sixes.

Of course armed with this info my Tourette's ticks are increasing......

[PRR]

> Do tractor have four cylinders, V8s or straight sixes.

And Ones, Twos, and Threes (I got a Three). Also at least one Straight Eight.

V-8s were quite rare, until someone noticed the Fordson 4-banger was about the size of a Ford flathead V-8 of twice the power, and spliced the flattie in. 1960s Ford and Chevy V-8s are now popular swaps. A car engine will NOT plow flat-out 12 hours a day for decades; these are now toys.

The Waterloo Boy/John Deere Jonny Popper 2-banger is famous in US agriculture. And the largest Deere 2-banger Diesel had a V4 gas/petrol engine to start it.

[TejfolvonDanone]

If you hook an integrator to a triangle output you get a pretty good sine like wave. (If i remember correctly with maximum 1% THD)

[anotherjim]

I think that's right, but only at one frequency, unless you make the second integrator track the first. Non-linear amplifier seems to be the most common approach, and they can be better than 1% distortion of the sine. For LFO, how good a sine does it need to be? Maybe "slow round the peaks and fast through the middle" is the important feature?

[Kipper4]

Thanks guys Noted.

[PRR]

> integrator to a triangle output you get a pretty good sine

I don't see 1%.

Triangle has 33% 3rd harmonic(?).

Integrator can reduce the 3rd to about 1/3rd of the 1st (fundamental).

That's like 11% 3rd?

Tracking doesn't change that; it makes *both* fundamental and harmonics grow weak with speed.

I am too sure that the "tips" will still stand out. (Too many wasted years tuning ARP VCOs to not give shrill "sines".)

A several-stage low-pass will probably work better, for narrow-range "trem" rates.

Over-driving CMOS into its soft-top zone is a cute trick. There are more complicated wave-shapers. The '8038 and kin VCO chips do this with fair success.

> Maybe "slow round the peaks and fast through the middle"

And smoooth... seems reasonable to me.

[TejfolvonDanone]

That's like 11% 3rd?

Looked up my uni notes and ran a simulation just to test it. Turns out i missed a 0 somewhere. 
That's what happens when i talk before i think.

[anotherjim]

I just tried it with a second integrator after the triangle on the CMOS scheme. It do look like a sine.
With fixed components, the amplitude falls too much. It only works over a narrow range. I expected that. With the same cap value and a dual gang pot so it's matching the oscillator, it tracks pretty well, but the amplitude is a little less than half of the triangle. With a cap value about 2/3rds of oscillator cap, amplitude increases, but still less than the triangle. As CMOS inverters are not perfectly linear, things might be better with op-amps.