Help with Scope & Function Generator Snafu

[jalbers@bsu.edu]

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

 

[Tim Wescott]

jalbers@bsu.edu wrote:

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

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
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Jamie]

jalbers@bsu.edu wrote:

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
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!

http://www.saelig.com/miva/merchant.mvc?Screen=PROD&Product_Code=PSSA017&Category_Code=PSSA

P.S.
They have a 60 mhz version that isn't much more..
http://www.saelig.com/miva/merchant.mvc?Screen=PROD&Product_Code=PSSA006&Category_Code=





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SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
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