Agilent 33120A outputs double output voltage - Page 1
Forum section: electronics / repair

author: JoeB83
author_rank: Regular Contributor
timestamp: March 26, 2021, 07:23:27 am
content: Hi all,I have an older function generator, an Agilent 33120A, that apart from the rotary encoder and putting out twice the stated output voltage, works great. The rotary encoder I can fix, it either needs replacement or cleaning. The output voltage is more confusing to me. I don’t have much experience with repair, I mostly make simple analog stuff so I don’t really know where to start. Maybe it’s just a setting issue, but going through the manual I don’t see anything that points to this issue. The output voltage is exactly double; if I tell it I want a 1kHz sine wave with 1Vp-p, I get a 1kHz sine wave with 2Vp-p. Same with square, triangle, and other waveforms.Any idea where to start? Thanks in advance.

author: alm
author_rank: Super Contributor
timestamp: March 26, 2021, 07:51:12 am
content: What is the load when you are measuring the voltage (50 Ohm or open circuit), and what is the output termination setting on the 33120A (in the sys menu). I think if you match those two, the output voltage will match the display.By default the function generator assumes a 50 Ohm load, and since the output impedance of the generator is also 50 Ohm, this would result in a 1:2 voltage divider relative to the open circuit voltage that you're measuring.

author: THDplusN_bad
author_rank: Regular Contributor
timestamp: March 26, 2021, 10:46:38 am
content: Good Day,almis correct. Just terminate the cable to the DUT/voltmeter with a 50 Ohm resistor and the measured levels shall all make sense...Cheers,THDplusN_bad

author: JoeB83
author_rank: Regular Contributor
timestamp: March 26, 2021, 11:50:06 am
content: Honestly the way it’s hooked up is just a BNC cable from the output going into a BNC on my scope, or to a BNC to banana adapter when hooked up to my meter. Maybe I should try a proper scope probe when hooking it up to my oscilloscope, and I’ll give what you suggested a try, thank you so much!

author: alm
author_rank: Super Contributor
timestamp: March 26, 2021, 12:29:03 pm
content: Just set the output termination to hi-Z, and the levels should be accurate for this kind of setup at low (audio) frequencies. For best fidelity at higher frequencies, or square waves, a 50 Ohm terminator will provide best results.

author: gamalot
author_rank: Super Contributor
timestamp: March 26, 2021, 12:36:01 pm
content: You need to set output termination toHigh impedanceif you are not terminating the output into a 50 ohm load.

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2021, 12:49:48 pm
content: Quote from: JoeB83 on March 26, 2021, 11:50:06 amUsing a scope probe instead of a BNC connection won't change a thing if the AWG doesn't have the correct output setting to match the load.Typically a 50 ohm output setting will halve the the displayed output when matched with a 50 ohm load yet with a HiZ setting into a HiZ load will have the correct output level and displayed output value.You should have 2 output settings, HiZ and 50 Ohm and you need to learn when to apply one or the other so to get an output level that matches the AWG display.....what the user sets.Think of this a little like scope input attenuation that needs match the probe in use for the correct values to show on its display.Hopefully these AWG's have some marker on the display that indicates which output mode you are in at just a glance.

author: JoeB83
author_rank: Regular Contributor
timestamp: March 27, 2021, 07:54:33 pm
content: Thanks all, I changed the output to high-Z and all is good again in the world. Thank you!

author: THDplusN_bad
author_rank: Regular Contributor
timestamp: March 27, 2021, 09:31:53 pm
content: Glad you have de-mystified this one!Greetings to the Rockies...Cheers,THDplusN_bad