strange modular experience

[Haible_Juergen#Tel2743]

Hi!

I produced some strange sounds on my modular last
weekend - and one thing I just don't understand.

I did the following:

Clocked a sequencer with a VCO output. (Audio rate)
Now I modulated the same VCO with the sequencer output.
(expo FM).
I set the first two steps of the sequencer to a perfect 5th,
the 3rd step to zero, and so on.
Now I experimented with the number of steps involved.
Fist, I tried 2 steps. As both steps had the same CV, the
result was quite predictable: Just a constant tone from
the VCO a fifth above the initial pitch.
Ok, now I switched to 3 steps.

The intention (or what I would have expected) was:

Now I have exponential FM in a feedback configuration. The
Modulator would be at 1/3 frequency of the output signal,
with a waveshape of a 66% with pulse. I didn't expect anything
special, besides an interesting new waveform.

What I discovered instead:

Yes, there was an interesting new timbre. Nice Overtones, rather
clean - *but* only in the upper octaves !!!
There was a very distinct point on the keyboard (iI think it was F1),
where there was a sharp cut in the result. Everything above that
point was just fine - everything below was - well - "interesting", but
not at all harmonic anymore.

What has happened?

I just don't know. perhaps someone has seen something similar, or
has some good explanation for that? I know what I have done is
not just FM with fixed carrier / modulator frequency relation, but with
a fixed *output*/modulator frequency relation. It's basically a
feedback oparator structure with a frequency divider (:3) and waveshaper
(66% pulse) in the feedback loop.
Is there any theory for such a configuration? Any experiences of
a similar thing?
I cannot explain this strange "cutoff" point from harmonic to disharmonic.

Any idea is highly welcome!

JH.