"Little Angel" chorus oscillator architecture name

[knutolai]

This is probably a easy one. What is the name of the oscillator architecture used in the "Little Angel" chorus?
Thread link: http://www.diystompboxes.com/smfforum/index.php?topic=86297.0

I can't seem to find it being used anywhere else than in the Little Angel and the Dimension P dual chorus.
Some resources would be grand!

[charmonder]

What in particular about it? I may be wrong but it looks very similar to the LFO on 4ms pedals (if you take out the extra waveshaping of tremulus lune for example http://www.commonsound.com/kits/doku.php?id=commonsound:tremulus )

[knutolai]

Well spotted! I hate the tremulus lune schematic. It could have been drawn much less confusing.
I mean I'm looking for the 'official' naming of that particular component-arrangement.  Would be nice to have a read about it as I'm totally in the dark on how it works.

[teemuk]

It looks a lot like relaxation oscillator.

[duck_arse]

I think that is a comparator/integrator oscillator, but done with a single opamp instead of a dual. there is a thread hereabouts in which rg does a deconstruction of the dual version.

[Seljer]

It's also the LFO in the MXR Phase 90/45


The opamp is set up as a comparator, so it's output jumps between the top rail and the bottom rail voltage. Refencing the Little Angel schematic: the 10uF cap is charged/discharged through the speed potentiometer (the upper speed being limited by the 4k7 resistor). The bottom 220k resistor adds positive feedback to the reference and shifts it up or down (adding hysteresis). The small 10nF cap is in the position it would be for an integrator but it's purpose is more for making the thing work during the period the output jumps so it doesn't get stuck in one state, not for the actual timing.

A cycle of the LFO would be:

• output jumps from low to high (from 0 to 9V) due to end of previous cycle
• + input gets pulled slightly up due to lower 220k and voltage on 10uF capacitor starts charging through speed pot
• theres barely any current flowing through the top 220k so the voltage on the - input is roughly the same as the voltage on the capacitor
• when the negative input voltage becomes higher than the positive input, the output goes to its low state
• + input gets pulled slightly down, capacitor begins discharging
• when - input is lower than + input voltage output jumps high
• and repeat

The LFO shape is "roughly" triangular. You're basically getting segments of an exponential curve (or if you want, you can take the signal directly from the output for a choppy square LFO). The capacitor is acting like as an integrator, in the way that voltage on the capacitor is related to the integral of the current, but if you're charging it with a constant voltage source, as the voltage rises the current charging it falls.
If you use another opamp to build an integrator you can then make a proper ramp/triangle oscillator (visibile both in the EHX Small Clone and Boss CE2, but both ciruits then do some extra filtering and such to the LFO before it goes to the actual circuit)

In the case of the tremulus lune they used a second opamp to buffer the signal so it could drive the LED because the capacitor doesn't take well to things that draw large amoutns of current.