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Thread: HP33120A Incorrect Output Amplitude


Permlink Replies: 3 - Pages: 1 - Last Post: May 20, 2011 12:16 PM Last Post By: cldiller
maxy

Posts: 2
Registered: 05/05/11
HP33120A Incorrect Output Amplitude
Posted: May 5, 2011 11:55 AM
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Hi,
I am having problem with HP33120A function generator. The problem is the output amplitude. I set it to sine wave, 100mVpp, at 1kHz. I use 50 ohms coax cable and the oscilloscope to examine the output of the function generator. The oscilloscope display sine wave with amplitude 680mVpp at 1kHz. To be correct, the output should be less than 240mVpp sine wave at 1kHz. I try to set it to factory setting (100mVpp, sine wave at 1kHz) but it still give 680mVpp. Please help if anyone have encounter and solve this problem. Thanks in advance.
lhornburg

Posts: 477
Registered: 01/15/10
Re: HP33120A Incorrect Output Amplitude
Posted: May 5, 2011 3:25 PM   in response to: maxy in response to: maxy
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I would check the output termination setting on the 33120A and make sure it is set to 50ohms. If you go to menu, system, out term: you can check if you have it set for 50 ohms or Hi Z. Other than that I'm not sure what to do here, I would definately double check the impedance of your cabling. Maybe others have ran into this and have ideas?
maxy

Posts: 2
Registered: 05/05/11
Re: HP33120A Incorrect Output Amplitude
Posted: May 11, 2011 8:56 AM   in response to: maxy in response to: maxy
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Thank you for replying.
I setted to 50 ohms and also use a 50 ohms impedance coaxial cable. The result is still incorrect.
cldiller

Posts: 23
Registered: 08/05/09
Re: HP33120A Incorrect Output Amplitude
Posted: May 20, 2011 12:16 PM   in response to: maxy in response to: maxy
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If the 33120A is set for an output load of 50 Ohms, you are using 50 Ohm cabling and the oscilloscope has a 50 Ohm input impedance, then the oscilloscope should have the same reading as the function generator. In this scenario, I do not see how you can be outputting 100 mVpp and getting nearly 700 mVpp on the oscilloscope.

It sounds as though the oscillocope has a high input impedance (causing it to read twice the voltage of the 33120A when it is set for an output load of 50 Ohms) and then the oscilloscope is reading Vrms. (100 mVpp * 2) = 71 mVrms. If you are using a 10:1 probe setting on the oscilloscope, this will be displayed as 707 mVrms.

A few things to try:
  • Check the probe setting on the oscilloscope. The fact that you are getting a much higher reading makes me wonder if you have it set to 10:1 rather than 1:1

  • Set the output load on the function generator to match the input impedance of the oscilloscope. Most oscilloscopes are high impedance, although a few have an option for 50 Ohms. If the function generator is set to a 50 Ohm load but is terminated by a high impedance oscilloscope, the reading at the oscilloscope will be twice the displayed voltage.

  • Check to make sure both the function generator and the oscilloscope are set to mVpp.



I hope this helps!

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