[sdiy] Oscilloscope question

[Ian Fritz]

At 08:09 AM 2/21/2010, Justin Owen wrote:
>At 5 seconds I have a dot going across the screen as opposed to being able 
>to see a complete waveform or line/measurement.

Yes, that's how they work.  The dot is produced by a phosphor which dies 
out in a short time.  You used to be able to get special scopes with "long 
persistance" phosphors, but I doubt you'll find one nowadays.

The usual analog solution is to photograph the trace with a long exposure 
time.  Many old scopes were set up to use with a polaroid camera, so you 
could see the results fairly quickly.  Except that it usually took several 
tries to get the exposure right.

Your solution for today is to either get a digital storage scope, or to use 
a digital camera to photograph the scope trace.  For very slow traces you 
might need a camera with a "bulb" exposure setting.  It is hard to 
synchronize the exposure to the trace, but you can make it work if you are 
patient.

Ian




>Problem is, when measuring low frequencies (let's say 20Hz to LFO range) 
>to view an entire cycle I need the Time/CM setting above 0.5 seconds - 
>which makes it almost impossible to check the quality or shape or whatever 
>of a waveform.
>
>Right now I'm working on a kick drum oscillator - and for those big long 
>808 style Booooooms - that's a long old waveform - but it's almost 
>impossible for me to see it in its entirety.
>
>So - if I wanted to look at a scope that could display an entire cycle or 
>at least more info about lower frequency waveforms/signals but with the 
>refresh rate of higher frequencies - what sort of specs should I be looking at?
>
>Maybe I should mention at this point that I only paid a tenner for my 
>current scope. :)
>
>Thanks much.
>
>Justin
>
>
>
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