Agilent 33220A f(x) generator..."Vpp" actually "Vp"?

[TimNJ]

Hi guys,

I was working on one of my circuit analysis labs and I was just boggled by the equipment and readings I was getting. I was supposed to be using the 33220A function generator to make an 8V peak-to-peak sine wave. On screen, I used the "Ampl" button to dial in 8V. Then I went back to make sure DC bias was 0V, and from there it showed Hi-level to be +4V and Low-level to be -4V.

But then once I measured the signal on a scope, it said that the signal was roughly double in amplitude at ~16V. ( I tested with other function generators and I got exactly double the amplitude.)

What am I not understanding? It says "Vpp" very clearly, but it's working as if it said "Vp".

Thanks!

[Odysseus]

Press utility, output setup, and change it from 50 ohms to Hi-Z.

Otherwise, it expects a 50 ohm load (in combination with it's 50 ohm output) to divide the output in half.

[electronic_eel]

How did you connect the generator and the scope? Is the input of your scope set to 50 Ohm or 1Meg?

What output mode did you set on the generator? 50 Ohm or High-Z?

Most probably your generator is set to 50 Ohm and the scope is at High-Z.

[alex.forencich]

Did you set the scope input to 50 ohms?  Generally these genrators wil have a 50 ohm output impedance and they will output voltages that are double what you expect when the output is open-circuit.  The 1M scope input is aproximately open circuit.  Many generators provide a way to switch to 'high impedance' mode.  This just changes the readout so that the voltage is accurate for an unterminated output. 

[TimNJ]

Well that makes sense! I measured it first by probing the circuit using a scope probe (~10M). I did another test with a piece of RG-58 (50R). However, I think the HP 54645D scopes have 10M input impedances. So the impedance the generator sees is still very high compared to the 50R that it expects.

It's expecting its 50R output impedance to be in series with a 50R input impedance, but it's instead a 50R output impedance in series with a Hi-Z. Makes sense.

Thanks everyone!

PS Alex & Electronic eel, I didn't know you could set a scope to change it's input impedance. Is that a feature of modern DSOs? I have an old analog Tektronix and that's not possible as far as I know.

[electronic_eel]

I didn't know you could set a scope to change it's input impedance. Is that a feature of modern DSOs? I have an old analog Tektronix and that's not possible as far as I know.

It's not really about DSO vs. analog, more about features and market-positioning. The better scopes have a switchable input impedance, the more basic ones don't. The advantages of using a 50 Ohm system get stronger the higher the frequency. So you usually won't find it in scopes < 100 MHz.

[alex.forencich]

I didn't know you could set a scope to change it's input impedance. Is that a feature of modern DSOs? I have an old analog Tektronix and that's not possible as far as I know.

It's not really about DSO vs. analog, more about features and market-positioning. The better scopes have a switchable input impedance, the more basic ones don't. The advantages of using a 50 Ohm system get stronger the higher the frequency. So you usually won't find it in scopes < 100 MHz.

And the really high end scopes (>> 1 GHz) only support 50 ohm inputs.  Generally you use 50 ohm inputs when you can either come out of a circuit directly at 50 ohms or when you use an active probe that drives 50 ohms all the way to the scope.