Signal Generator is isolated?....and accuracy at low amplitude outputs?

[treez]

Hello,

I am considering this £100 sig gen........................

GFG-8015G sig gen
**broken link removed**


...but it doesnt say if its output is isolated?, or if it can produce sine wave voltages of peak just 15mV and do this accurately?

Do you know the answer to these?

 

[kripacharya]

according to the specs, it cannot. The attenuator specified is -10dB, and the output into 50ohm is 10v. It says nothing about any lower outputs.
you could add your own attenuator, but since the spec is vague about the exact output levels, this would be based on actual measurement once you actually buy and get the instrument.
Very strange SigGen.

 

[treez]

I suppose, to find out if the output is isolated, i must get out the ohmmeter, and probe between the earth in its plug, and the output common?

i hope its not one of those molded plugs......as i will never be able to get to the earth in the plug...and then i'll have to plug the blessed thing in, and then plug another open plug in, and then use the open plug's earth connection for the probing?

 

[sky_123]

I think it must have adjustable output, the spec just says '>10V', usually meaning that the adjustment will allow at least a
maximum of 10V.
If the attenuator is 20dB amplitude (guessing it is, since this is a low freq siggen), then you should be fine once you make
your adjustment, but it looks like a variable pot on the image, so you may want to have your own divider.
It probably is isolated from the mains, yes.

 

[treez]

It probably is isolated from the mains, yes.

thanks, though is the output common isolated from earth ground?

 

[sky_123]

I'd be very surprised if it isn't (even my 1980's [or maybe 1970's!] philip's low-freq siggen is isolated).

 

[treez]

silly question, i thought the output common's of generators and PSU's had to be hard wired to earth ground so as to stop them from floating up to high voltages?

 

[sky_123]

Hi,
No, I don't think this is normal at all. Here is an example PSU, you can see that the output is completely isolated.
The left red box shows the outputs, the right box shows the earth, and the dashed white line shows the isolation.

EDIT: Sorry I should point out that I double-checked the PSU in the image, and I noticed it actually does have AC coupling,
a capacitance to earth, of a low value (470pF in series with 22nF). This is to remove high frequency components according to **broken link removed**, due to interwinding capacitance.
However the output is still isolated for DC.

 

[Syncopator]

If I were you, treez, I would ask the company. They have a contact form on their site.

 

[kripacharya]

I think it must have adjustable output, the spec just says '>10V', usually meaning that the adjustment will allow at least a
maximum of 10V.
If the attenuator is 20dB amplitude (guessing it is, since this is a low freq siggen), then you should be fine once you make
your adjustment, but it looks like a variable pot on the image, so you may want to have your own divider.
It probably is isolated from the mains, yes.

I suppose it does mean this, though i'd write it as : a maximum of at least 10v
Sloppy spec sheet, but hey... its just 100 bucks ;-)

btw it turns out there is variable attenuator of 0 - 20dB, and another fixed attenuator of 20dB, totaling upto 40dB. It still won't give downto 15mV it seems.

silly question, i thought the output common's of generators and PSU's had to be hard wired to earth ground so as to stop them from floating up to high voltages?

i happened to go have a 2nd look, and guess what - the user manual is right there - complete discriptions and including the circuit diagrams!
Have a look Treez, you might get all your answers right there. Last page has details on the power wiring...

 

[treez]

yes i looked at the cct diadram , and you cant tell if secondary is dc connected to earth or not from that.
..it just shows a 2 pin plug coming in, and deosnt show a third earth wire coming in with the plug, so the cct diagram is not helpful in this case..................or rather , the cct diagram on pg 46 would make you conclude that earth ground never enters the product anywhere.....as it just shows the 2 pin plug and two wires.....................no earth symbol on the secondary side is reciprocated on the primary side...............so you would conclude there was no earth gnd connection to the secondary.

Anyway, i did contact the company...."goodwill" in China.........and they have got back

***************************************************************
Good Day!
Thank you for message.
Regarding to the GFG-8015G, the output of GFG-8015 is not isolated from Earth Ground.
All of our function generators are not isolated type.
*******************************************************************

So its not isolated.............earth ground is connected to the secondary by a wire, somewhere.

Why on earth would they want to do that?