Frequency generator ground reference

[Evenekul]

I've seen many instances of checking output of a frequency generator directly with a scope. I have a wavetek 164 and an old analog tektronix scope. I will use short rg58 cables with a50ohm terminator on a T connector.

I checked the scope and the chassis, bnc lugs and ground points are direct short to mains ground.

The frequency generator has bnc grounds have a voltage differential with reference to chassis ground. The chassis is a direct short to ground.

Question, will this create problems? Is this normal? Or is this a product of the 50 ohm output on the generator that I don't understand yet?

Thanks!

Luke

[tautech]

Welcome to the forum.

Typically most FG's and AWG's have mains ground linked BNC outputs.
Though not common it could have an isolated output however there's no mention that it has here:
https://sites.fas.harvard.edu/~phys191r/Bench_Notes/D2/Wavetek164.pdf

A deeper dig into online info might provide better knowledge.

[Evenekul]

Thank you. I'll dig into that a bit deeper into the manual. The web seems to not want to cough up more info. I was surprised when none of the bnc's on the FG were linked to ground. Good thing this is a hobby with no deadlines :-)

[Cerebus]

How have you determined that the generator output grounds are at a different potential to the generator chassis, and did you determine it as AC or DC?

It's quite likely that the generator output is floating and what you're seeing, if tested with a high input impedance DVM, is a small common mode signal, often capacitively coupled between a transformer primary and secondary. What do you get if you take a current reading, AC coupled and DC coupled, from the output ground to chassis ground? (Obvious, but I'll say it for the sake of safety for the equipment, take the initial current reading current on a low (e.g. uA) setting so that you only shunt the two 'grounds' together via a high impedance.)

[David Hess]

There is no indication in the schematics that earth ground is tied to the output ground.  Instead, full galvanic isolation is provided by the power supply's transformer.  This is hardly unusual since it prevents ground loops and it is something to be expected in the best function generators.  Note that all of the signal grounds *are* tied together however.

The manual does not discuss this or give a specification for maximum output ground to earth ground voltage but typically it is like 50 volts for low voltage test instruments.

While the DC impedance is high, it is still shunted to earth ground through the power plug with 10s to 100s of picofarads of capacitance.

[Evenekul]

Yes, high impedance DVM was used. I'll work on this today.

What I'm hearing you guys say is that this is most likely normal and coupling the FG to my scope through a 50ohm feed-thru connector won't damage the FG's guts.

I've been meaning to better understand shunts anyway, so this is a welcome lesson for me without even having a device to experiment on. Neat.

Thanks to both of you for your patience with the noob.