HP 33120A Function Generator Amplitude Offset Error - Page 1
Forum section: electronics / repair

author: Shay
author_rank: Regular Contributor
timestamp: September 17, 2023, 02:49:53 pm
content: Hello,My HP 33120A has an issue. The output amplitude is limited to 2.8V AC RMS, on the sine wave option, and the output amplitude is always at a known offset:Setting the output voltage to 100mV RMS, the output is 80mV RMSSET 200mV RMS = OUT 160mV RMSSET 300mV RMS = OUT 240mV RMSand so on, up to 2.8V RMS maximum. 20mV*range offset.The AMP_OUT pin at the highest set voltage corresponds to the output voltage (2.8V AC at both), which means it's something before the attenuation.I'd like to get any insight into fixing this function generator.Thanks!

author: Swake
author_rank: Frequent Contributor
timestamp: September 17, 2023, 04:35:40 pm
content: How did you measure that?Input impedance / output impedance matching?Frequency ?No other options active such as FM/AM modulation?What does the waveform looks like on a scope?Selftest ok?Specs says: Amplitude (into 50 ohms): 50 mVpp - 10 VppI don't know if that is valid for your specific selection but limited to 2.8V does not seem right indeed.

author: Swake
author_rank: Frequent Contributor
timestamp: September 17, 2023, 04:43:36 pm
content: BTW, how did it happen? All of a sudden or was the generator confronted to some 'signal' on the input?

author: Swake
author_rank: Frequent Contributor
timestamp: September 17, 2023, 04:47:21 pm
content: Might help:Quote

author: Shay
author_rank: Regular Contributor
timestamp: September 17, 2023, 06:18:20 pm
content: Quote from: Swake on September 17, 2023, 04:35:40 pm1. Measured using a scope and also a DMM2. Tried 50 ohm termination and also HIGH-Z.3. Frequency does not change much the ampltiude.4. No other options active5. Waveform is very clear and crisp, no jitter or anything unusual. The function generator otherwise works great.6. Self test is OK.I am not sure how did it happen, the gen used to work great with the correct amplitude. All of a sudden it started doing this issue.

author: Swake
author_rank: Frequent Contributor
timestamp: September 17, 2023, 07:57:02 pm
content: QuoteThat is not normal. In high impedance mode you should measure twice the set voltage.QuoteI've never used this generator, but that voltage should remain stable.The output limit of 2.8Vrms sounds like a setting somewhere. Try resetting everything to factory defaults.

author: Swake
author_rank: Frequent Contributor
timestamp: September 17, 2023, 08:09:24 pm
content: Let me write that in other words: The gen has an output Z of 50 Ohm, your scope must be set to an input impedance of 50 Ohm too else the voltages on the scope and the set voltage in the gen will not match.Your DMM most likely has an input Z of about 10 MOhm. Therefor it will never show the same voltage as set on the gen unless you have explicitly configured the gen to do so. With standard settings it will show about twice the set voltage.

author: Shay
author_rank: Regular Contributor
timestamp: September 17, 2023, 08:34:12 pm
content: Quote from: Swake on September 17, 2023, 08:09:24 pmYou are indeed correct, I wasn't clear enough - the 50 settings does increase the voltage by 2. High Z automtically cuts it off in half. This function works alright.I noticed the ampltiude error is on all settings except the square wave function.

author: macboy
author_rank: Super Contributor
timestamp: September 17, 2023, 09:45:22 pm
content: Quote from: Shay on September 17, 2023, 08:34:12 pmSo amplitude is correct on square wave but not other waveforms? I recall that square wave uses a different low pass reconstruction filter than used for other waveforms, to get fast edges with low overshoot. Maybe that hints at where the fault is.

author: Shay
author_rank: Regular Contributor
timestamp: September 18, 2023, 08:01:15 pm
content: Quote from: macboy on September 17, 2023, 09:45:22 pmSeems like this is related to the pre-attenuator. ill check this section and report back.