# Help with Scope & Function Generator Snafu

• posted

I am trying to do a simple experiment involving an ocilliscope and a function generator in the same circuit. My plane is to have the function generator set to produce a 10V (no load) sine wave an vary the freq from 100-2700 Hz.

A .1 uf capactictor and a 1K resistor are connected in series across the output leads of the function generator. I want to connect the ocilliscope leads across the resistor to measure the voltage drop across the resistor. I calculate that the voltage drops should range between .6V and 8.6V using the above frequencies.

Scope: Tektronix D10 5103 Function Generator: MCP SG1639A

The problem that I am having is that I think that the scope and function generator share a common ground because as soon as I connect the positive leads of the scope and function generator together (without connecting the ground leads together) I get a wavefrom on the scope that represent the 10V sine wave. If I connect both leads of the scope probe across the resistor I get the same 10V sine wave signal not the .6V AC signal. Why are the gound test leads of the scope anf freq generator common. I can't see any switch on the function generator to control this.

Is my assumption about the common ground correct? Do I need to connect the scope or function generator to an isolation transformer?

Also, is it safe to short the output leads of the function generator together.

Any help would be greatly appreciated. Thanks

• posted

We just ran across a similar problem when trying to use a function generator and a scope to measure low-amplitude waveforms. Both devices were AC-powered. The grounds on some scopes and function generators are tied to the common chassis ground and are common through the AC-lines. Oddly enough, we tried using an isolation transformer trying to solve the problem and it did not help. We traced it down to the isolation transformer itself. The ground (third wire) was common between input and output on the isolation transformer. I'm not sure if this is always the case but it was true of two different models we checked.

Our ultimate solution was to go to either a totally isolated (battery-operated) scope or function generator. We bought battery-operated scopes and the problems went away. I'm putting together a battery-operated function generator as a solution when the scope has to be plugged in, also.

Good luck.

Dave

• posted

If your devices are sharing common ground (quite common) then you could isolate one of them, or you could connect the resistor to the grounded side of the generator and just use your probe on the other side.

Tom

• posted

isolate one of them, or

If you cannot connect the resistor to ground, then most oscilloscopes have a 'subtract' mode. Connect a pair of leads (one to each side of the resistor) and then subtract channel B from channel A to get the voltage across the resistor.

• posted

A: Most lab instruments have unbalanced inputs/outputs, with one side grounded.

B: Function generator -> cap -> resistor -> ground. Connect the scope to the junction of the cap and resistor, and you're seeing the voltage across the resistor.

What's the problem?

--
Tim Wescott
Wescott Design Services
• posted

You need a scope with floating commons for the probes.

I got a hand held that does just that even if its running off the wellwart.! It was cheap but it works!