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previous: BillnShell I was thinking, to get an idea of w... -- 8/24/2004 3:31 AM View Thread

Re: Finding source of hum via O-Scope and Sig. Generator

8/24/2004 7:48 AM
Re: Finding source of hum via O-Scope and Sig. Generator
It is good to be cautious, but these things are made to use. I use the 60Hz noise that permiates the environment. Can you think of anywhere you can go that touching the tip of your guitar cord would not make a loud hum? 60Hz fields are all over you all the time. If you touch the tip of your scope probe with a finger, amd the vertical gain is set to maybe a volt per division, you ought to see a waveform on the screen. No matter how distorted the roughly sine wave thing looks, it still is 60Hz.  
My scope usually sweeps at about 2ms. That speed gives me good view most of the time, otherwise whatever speed you like is fine. At that sweep speed, I get just a bit more than one full cycle on the screen. That is what 60Hz looks like. 120Hz will simply have twice as many bumps in the same space. 120Hz ripple will never look like a sine wave of a square wave or whatever else your generator puts out. It will look sorta sawtoothy, sorta.  
I also select that swep speed because 60 and 120Hz are usually what I am interested in, also I use 100Hz from my generator most of the time as it causes less ear stress than 1000Hz.  
You should use the 10x probe most of the time. That presents a higher impedance to the circuit under test and it reduces the voltage actually going into the scope. So with a 10x probe, a 1v/div setting on the scope really means 10v/dov in real life. My scope goes up to 10v/div. which means 100v/div. So 400VDC moves the trace up 4 divisions on the screen. I usually move the 0 position of the trace down to the bottom for this so my 400V is not off the top. More headroom so to speak.  
Older scopes almost always went to 10v, but meny newer scopes only go to 5v/div. In that case 400VDC becomes 8 divisions.  
Lets say we have 400VDC with 20V of ripple. If I set up for DC at 100v/div, that 20v of ripple is one fifth of a division tall, not very easy to see well. But if I flip the scope to AC, then it ignores the 400VDC and I can turn up the gain to say 10v/div, and now my ripple is two divisions tall. Remember we are using the 10x probe and the settings on the scope are actually 10v/div and 1v/div when I say 100V and 10V per div.  
Yes you cannot exceed the scope input limits, but that 10X probe only lets 1/10 of the voltage through. The scope is meant for this. Always make sure to connect the ground lead of the scope to the amplifier ground first.

BillnShell Thanx again Enzo, for the good info... -- 8/26/2004 3:45 PM