Help understanding sag1021i signal generator and output load

[richieHH]

Attached are two screenshots of my sds1104xe connected via ch4 to a sag1021i. In both the scope measures 5.6v pkpk. One is with the "output load" set to 50 ohm. The other set to Highz. What's confusing me (ignoring small inaccuracies) is the awg status top right left of the frequency counter. Why with 50 ohm "output load" is it saying 2.5v? The scope measures 5.6 pkpk still. I switch to HighZ and that same field now says 5v pkpk.

My understanding is that the signal generator is a fixed 50 ohm output impedance, therefore the "output load" is what's connected.  But with a highz output load, how come the awg status reading says 2.5v with the scope still reporting double? With a highz load I would expect most/all of the awg output to be presented to the scope via the output impedance/load voltage divider.

I hope this makes some sense, as I'm spinning around in circles here.

[bdunham7]

We don't have a FAQ section here, but if we did this question would be on it.

I think the easiest explanation goes like this:

The output section is a voltage source with a 50-ohm resistor in series, this is what a 50R source impedance means.

The HiZ and 50R settings do not affect the actual output in any way.  The Hi-Z setting indicates what the output of the voltage source is, the 50R just changes that so that it displays half that value.

If the output is connected to a high-impedance load, like an oscilloscope input, then that load will see almost the same exact voltage as the voltage source is putting out because there is no significant drop across the 50R output resistor.  If is connected to a 50R load, then the load will see half of the voltage at the voltage source because the other half is dropped across the 50R output resistor.  Thus the 50R 'mode' displays half the voltage at the source.

[richieHH]

Thanks for the reply and yes , I've seen this type of Q many times and watched a view videos.. but it hasn't helped. Something in this instance is tripping me up. I'll try and explain. Please don't read this as picky or antagonistic, I'm just trying to get to the root of my confusion 😀

###Firstly I made an error in my op. The awg reading on the scope shows half with 50 ohm load setting, not highz. Apologies.###

You say  , "The output section is a voltage source with a 50-ohm resistor in series, this is what a 50R source impedance means."

Yup. I understand that : "My understanding is that the signal generator is a fixed 50 ohm output impedance". (Note that source and input  impedance isn't a configurable thing in the awg or scope so that's cool.)

You say,  "The HiZ and 50R settings do not affect the actual output in any way.  The Hi-Z setting indicates what the output of the voltage source is, the 50R just changes that so that it displays half that value."

Here is the crux of my issue:

Why is the scope AWG reading showing half??  It says 2.5v pkpk when the awg is set for a 50 ohm *output load* (50 ohm output in series with 50 ohm load... Voltage divider.. yup) ***BUT*** (and the important part) the scope continues to show 5.6v pkpk? When it's highz the AWG reading top right shows double but the scope continues to show the same.

I have no problem understanding the voltage distribution if we put a 50ohm or Highz load on. I'm having an awful time reconciling that with the numbers shown on the scope.

I know this is going to be trivial .

Thanks for your time!

[Anthocyanina]

The 50 ohm or Hi Z settings are just for ease of use. if you set the generator for 1vpp into 50 ohm, it will output 2vpp into open circuit, and when you measure it with an oscilloscope which doesn't have a 50 ohm input, it will act pretty much as an open circuit, so the oscilloscope measurement will be 2vpp. If you set the generator for 2vpp into Hi Z, it will still behave like before, and your oscilloscope will read 2vpp. The only thing that changes when you set your generator for Hi Z or 50 ohm is what the generator displays on the settings, it will not change the actual output voltage. It behaves like this, again, for ease of use and readability, so that when you are connecting the generator output to a 50 ohm load, you don't have to divide by 2 in your head the setting in the generator to know what the voltage the load is seeing is.

to sum it up, if you connect a high impedance load to the generator, it will always see the voltage set into Hi Z or twice the voltage set into 50 ohm. If you connect a 50 ohm load to the generator, it will always see the voltage set into 50 ohm, or half the voltage set into Hi Z. The High Z or 50 ohm settings on the generator are only for display purposes.

[HKJ]

You say,  "The HiZ and 50R settings do not affect the actual output in any way.  The Hi-Z setting indicates what the output of the voltage source is, the 50R just changes that so that it displays half that value."

The impedance setting will affect the output, the generator will use it to calculate necessary voltage to output the specified voltage.
I.e.
Setting 1V & HiZ the generator will output 1V into the 50ohm output resistor
Setting 1V & 50ohm the generator will output 2V into the 50ohm output resistor, making the output 1V when a 50ohm load is applied (and 2V when unloaded).

[richieHH]

The 50 ohm or Hi Z settings are just for ease of use. if you set the generator for 1vpp into 50 ohm, it will output 2vpp into open circuit, and when you measure it with an oscilloscope which doesn't have a 50 ohm input, it will act pretty much as an open circuit, so the oscilloscope measurement will be 2vpp. If you set the generator for 2vpp into Hi Z, it will still behave like before, and your oscilloscope will read 2vpp. The only thing that changes when you set your generator for Hi Z or 50 ohm is what the generator displays on the settings, it will not change the actual output voltage. It behaves like this, again, for ease of use and readability, so that when you are connecting the generator output to a 50 ohm load, you don't have to divide by 2 in your head the setting in the generator to know what the voltage the load is seeing is.

to sum it up, if you connect a high impedance load to the generator, it will always see the voltage set into Hi Z or twice the voltage set into 50 ohm. If you connect a 50 ohm load to the generator, it will always see the voltage set into 50 ohm, or half the voltage set into Hi Z. The High Z or 50 ohm settings on the generator are only for display purposes.

Thank you. But in short, the output certainly does change. It seems the awg status on the scope is simply confusing me.. it reflects what the awg would be putting/showing into a 50 ohm load (when output load is set to 50ohms)... But of course my load is really highz (1 mega ohm oscilloscope input) meaning the scope measures effectively the doubled output!!!

I think I'm there.

Thanks all.

[Anthocyanina]

Thank you. But in short, the output certainly does change. It seems the awg status on the scope is simply confusing me.. it reflects what the awg would be putting/showing into a 50 ohm load (when output load is set to 50ohms)...

This is because your generator can only be controlled from the oscilloscope, so the settings for it will be only shown in the oscilloscope. But think of that bit at the top right as if it was separate from the oscilloscope. It is just the generator's settings display, which again, will display that value for convenience only, since if you set it to 50 ohm, it will "think" that you are indeed using a 50 ohm load, so it will display the set voltage as if it was connected to a 50 ohm load. Function generators will usually not have feedback from their output, so their displayed settings are showing a value "blindly" only considering the load setting you set, not the real world load you connected to it