Offset and oscilloscopes?

[danial stocks]

>>As I turn up the offset the line gets thicker
>>and and fuzzier still while the peaks of the waveform come together and
>>transpose themselves when the offset is turned all the way up. Could
>>someone explain whats going on here?
>
>That's funny, one channel of my Tek 465 does the same thing *all by
>itself* in in XY mode. I assumed it was oscillating.
>
>>     I can't take a picture of the wave but a made a little diagram to
>>describe what is happening
>
>Looks familiar, only there's no offsetting going on.
>
>
>--
>Terry Bowman, KA4HJH
>"The Mac Doctor"

looks like high frequency spurious oscillations to me.. if you make the 
scope sweep really fast, the 'fuzz' on the tri wave may well be really high 
freq oscillations.. this means something is wrong... one thing to note is 
that most oscilloscopes have the probe ground = chassis/ mains ground.. same 
applies to many mains powered signal generators.. I've noticed on some 
circuits that spurious oscillations can be caused by connecting the osc 
probe ground to the wrong point on the circuit.. for instance.. most of my 
gear is designed to run on plug packs.. so I use a op amp supply splitter to 
make a +/- supply - 9v plugpack turns into +/-4.5v inside the circuit.. but 
while internal circuit functions are referred to the synthetic ground, the 
probe ground must go to V- ie plugpack (-) terminal - as do the input/output 
audio sockets.. putting the probe gnd onto the internal synthetic ground 
causes the supply splitter op amp to go into oscillations due to the 
excessive capacitive loading. I've also noticed occasional spurii on audio 
power amps wired up to scopes and grounded sig gens.. poss a ground loop 
causing instability...
maybe it might be worth listening to the filter with the scope attached and 
not attached to see whether the measuring itself is causing the problem..
Cheers,
Dan
________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com