[sdiy] stupid scope

[jhaible at debitel.net]

> Scope probes both compensated correctly ???

I thought so. But honestly, that's for the upper range
of the freq response, hardly for < 150Hz, and subaudio
range, isn't it?

I don't even buy the AC coupled explanation. 
How would that
a) let pass 0.1Hz as good as 50Hz, and
b) create a +0.8dB *boost* below 150Hz ??

Something else must be wrong here.
Sometimes I also notice a (very low) 50Hz ripple
on the trace, even without anything connected. 
It's not big enough to be really annoying,
but it makes me think. 

JH.

> 
> 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->)
> 
> 




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