dual lfo...

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dual lfo...

Postby swt » 08 Nov 2016, 13:16

hi guys. does anybody know of a good lfo design that puts out two in phase waves...one with half or one quarter the freq of the other?. i think there was a phaser or something like that using this to take a glance but can't remember which one.
the main purpose is to make a sort of rotary speaker modulation...univibe based...where i can split signal for upper register...with normal univibe values of caps...and lows in parallel of that with different caps...and maybe only two stages for a not so strong phase shifting...but i want the lows to have half or 1/4 the speed of the highs in phase with them...and the lfo will have to use only one speed pot for control...to get them always in phase.
is this possible? thanks a lot for your replies...
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Re: dual lfo...

Postby KTSEffects » 01 Jan 2017, 02:22

I'm new around here so forgive me if I don't get the etiquette just so.

What shapes are you looking for?
Have you any starting points or notions of how you want to achieve this?

At a glance my first approach would be to generate a square using your preferred type of configuration and then pass that through a frequency doubler, you can then use that to derive or trigger shapes at the full and half frequency as desired.

I hope that helps out a little,

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Re: dual lfo...

Postby mictester » 01 Jan 2017, 15:10

swt wrote:is this possible?

Of course it's possible. What waveshapes are you looking for? I presume that you want triangles, sine or hyperbolic if they're for phaser stages...

It's easiest to start with squarewaves - generate your LFO clock at (say) four times the higher frequency that you want, using a simple CMOS VCO like a 4046. Feed that to a 4024, 4020, or 4040, and pick off the outputs that you want. You'll have squarewaves that are harmonically related and synchronous. The problem that you'll have is the phase shifts when you convert the waveshapes to the ones that you want.......

Next approach (and one that I've used) - use the 8038 VCO chip. This will give good sine and triangular waveforms in addition to the squarewave - all will be in phase. Run your first 8038 at the frequency of the higher LFO, use the triangle or sine (whichever you prefer) and take the square output to a 4020 divider IC. Take the ÷4 squarewave to the reference input of a 4046 PLL IC. Run a second 8038 at the lower frequency that you want, taking its squarewave output to the other input to the 4046. Take the output from pin 13 of the 4046, lowpass filter it (with a simple RC filter) and use the derived voltage to tune the lower VCO - it will lock at one quarter of the upper VCO frequency, and will maintain a fixed phase relationship over a wide range of frequencies. If you voltage tune the upper frequency 8038, you could also add a simple ramp up / ramp down circuit for speed switching, and the lower frequency VCO will track the changes, maintaining the phase relationship!

Incidentally, I suggest using the upper frequency VCO as the controlling one because the frequency stability will be (slightly) better at higher frequency - you won't be as dependent on the quality of the timing capacitors!

Dual VCO.jpeg
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Re: dual lfo...

Postby swt » 07 Jan 2017, 13:05

Wow. Thanks a lot!! I'll try to aprox the idea in a schem to get corrections or other ideas.
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