[sdiy] stupid scope

[harrybissell]

Scope probes both compensated correctly ???

My HP scope did this once... I worked for hours before
noticing it was lying to me. I had to disassemble it and
I guess I made a bad connection in the input amplifier 
better. Now it works OK.

H^) harry

"JH." wrote:
> 
> 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->)