[SOLVED] Basic function generator output voltage and Impedance question.

[David_]

Basic function generator output voltage and Impedance questions.

Hello,
There is something I'm not grasping regards to function generator outputs.

I have a rigol DG1120, i actulay can't remember the model name buts its not important.

In order to get a really precise output i need a matched load to the output impedance?

I do use it for varius tasks but i would like to tell it to give me for exampel 5,35Vpp and actualy at some point get a measureble(with high Impedance voltmeter) something like 5,35Vpp as a result and not 5,1Vpp and then increase the generator output untill the high Z voltmeter says 5,35Vpp and the generator some higher value.


I just thought to put a 100Ohm pot parallel with a oscilloscope probe accros the generator output and adjust untill both scope and generator shows the same value, whould the pot then show approximatly the output impedance as a pure resistance?
Or whould the probe capacitance impact the load impedance much?
When i write the questions I'm happen to think of a solution like calculating the probe load as the probe resistance paralleled with the probe capacitance and then add that to the pot value to get the impedance value that should match my generator output Impedance.

Is this valid?
I happen to disstrust that the 50Ohm label at the generator actualy means that the output Z is actualy 50Ohms and not more like 48,9Ohm or something. But what do i actually know? Not that much at all.

The Z load adjustment on the generator can be adjusted but it only increases the voltage, i really want to know the exact(or more precisly) output impedance value.

Allso i don't really understand how to estemate the maximum output current, I know ohms law and I=V/R or V/Z but is it not as well a question of how much power the internal load can dissipate? If i would underestimate the load applied can i fry the something in the generator?
Theoreticaly, i guess a modern generator whould shut down by a safe guard before that.
But it bugs me that the data sheat does not say max output current or wattage, it just says 50-10,000Ohms. And as i don't really grasp the whole basic consept that does not really tell me anything about what the generator Ocould handle if asked to deliver under varius surcumstances.

Edit// My scope does not have 50Ohms option but the generator has a High Z setting, what does that really mean for the generator output related to the 50ohms setting? Whatever it means the actuall output Impedance stays the same I'm sure.//

 

[kripacharya]

Ideally your Siggen should look like an ideal voltage source with a 50ohm in series at its output.

So yes, your experiment with the 100ohm pot should work. Adjustments for the scope probe - esp the cap - are more relevant at the higher frequencies only, where a typical 10pF probe will look like around 800ohm @20MHz

Notice that its specs say that you will get an output from 2mV to 10v with 50ohm load, and correspondingly a 4mV to 20v output with open circuit (i.e. Hi-Z)

 

[FvM]

High Z isn't but a factor when setting the output voltage, the output impedance is always 50 ohm.

As long as you don't work with high speed signals (which aren't generated by DG1000 series at all, I think) or impedance matched circuits, e.g. resistive combiners, I would work with "High Z" load impedance. It can be expected to give slightly lower magnitude error.

 

[David_]

Thanks for the replies.

With higher output voltage I worry that i will make some error of judgement and dump a lot of voltage on the internal load and combined with output current damage the generator.
The datasheet tells about safegards in case of overloading the generator, are they there only to prevent "catastrofic" errors or can I(preferible not) treat them as a more casual safeguard?

 

[kripacharya]

Imax would be 20v/ 50ohm = 400mA for a shorted output.

What does the specsheet say is the max allowed ? I don't have access....

 

[FvM]

The output voltage is 20 Vpp "high Z"/10 Vpp into 50 ohm. Means maximum output current with regular 50 ohm load is +/- 100 mA. The generator is also specified to have short circuit protection, so it will probably drive less than 200 mA into a shorted output.

 

[David_]

When the Z setting is adjusted between 50and 10,000ohms the max output voltage is decreased as the load goes down, i tok notes of which voltage it would produce at whitch load setting but i can't find them and have no possibillity to test again for a couple of days. But i remember that the numbers trough ohms law didn't make any sense to me, I will remeber when i sit down with it again but thers something that really confuses me about the whole thing, I'm sure its actually self explanetory but i just don't get it. I will return with some numbers shortly, hopfully tomorow.
Thanks for the help so far, great forum.

Edit// rigol DG1022 was the name.

 

[FvM]

Re-reading your initial post, it seems to me that you are simply misunderstanding a special instrument feature, "Set Output Load" in the Utility Menu. It's intended as a correction of set output amplitude for different load impedances, with respect to internal 50 output impedance. (User Guide p 2-53). It has nothing to do with maximum output voltage/current/whatsoever.

The function is essentially calculating a voltage divider ZL/(50 ohm + ZL). Usually people have a pocket calculator for similar operations, in lack of an "Utility Menu".

 

[schmitt trigger]

On an Agilent signal generator I used, that was precisely the purpose of the load impedance switch.

For instance, you command the generator to output 1 Vrms.
The generator will internally create the 1Vrms, which will go thru its internal 50 ohm series resistor, into the output connector and your load. If your load is high impedance, you will for practical purposes be applying 1 Vrms to your UUT.

However, if the UUT load is 50 ohms, then the applied level will be only half the generated value. To prevent you from applying an incorrect level to the UUT (in case you are not monitoring the output with a scope) then the "50 ohm" switch will halve the display value.

 

[FvM]

A 2:1 output scaling like "50 ohm/high Z load" can be found with many generators. The unusual (and apparently confusing) feature is to take account of arbitrary load impedances between 1 and 10kOhm. I don't remember to have seen it elsewhere, apart from the Z0 setting of network analyzers which has a different purpose.

 

[David_]

It's intended as a correction of set output amplitude for different load impedances, with respect to internal 50 output impedance.

The coin finaly fell down, thank you all for the replies.
Put toghether it all makes sense perfectly, could not be more pleased.

Cheers.