[sdiy] stupid scope

[JH.]

So I'm building this quadrature VCO, using discrete OTAs and extra fast
opamps,
and test it with my scope in xy mode.

Perfect and smooth circle from 150Hz to 25kHz.
Below 150 hz, it becomes slightly elliptical.
At 5 Hz, the difference between the two axes is 10%.

I'm looking everywhere in the circuit.
Can't understand it.
I measure the two amplitudes at 150Hz with a good RMS voltmeter.
Exactly identical.
Measuring AC at 5Hz with the DVM doesn't work, of course.
But by then, I suspect my circuit to be ok, and the scope being
wrong, already.

Finally I do what I should have done right from the start: Swap
the two channels, and voila: The same elliptical shape as before.
I'm glad - the VCO seems ok then.

But what kind of misbehaviour of the scope is this ??
Both channels set to DC, of course. And running 0.1 Hz also
works (same elliptical shape), so it *is* DC coupled.
And both channels track so nicely above 150Hz.
Seems one channel has a slight "bass boost".
Stupid.

JH.

PS: QVCO circuit coming soon.
Uses discrete  linearised OTAs with 2 x MAT-02 + SSM2220 + 2SK170 + BC308A
each,
and AD8620 for the integrator
and 3 x AD633 + TL071 for amplitude stabilisation.
My most dopferising-proof circuit so far. (8->)