Wavetek 288 Amplitude & Offset 2X The displayed value - Page 1
Forum section: electronics / repair

author: montydtm
author_rank: Newbie
timestamp: August 01, 2022, 03:25:51 am
content: I have a Wavetek 288 Military version function generator that I've owned for several years that has the strangest problem that is not corrected by either auto or manual calibration. No Matter what the waveform the amplitude is exactly 2X what the display shows and the same goes for the DC-offset.For example, if I set the waveform to DC and offset to 1V the generator(and multi-meter) will output exactly 2V. If I set a waveform - irregardless of shape to 1VPP I will get 2VPP. Changing from VPP to VP or others are consistently the same - exactly 2X what the display shows. So to summarize, no matter what the waverform or offset, the actual values shown on my scope & multi-meter show 2X what is on the Wavetek displayI've checked that the line-voltage pc board is installed correctly, manually calibrated and verified(and adjusted R208) to 7.071v per the manual and all calibration tests pass w/o issue. The autocalibrate feature also passes w/o issue.I've lived with this for some time and thought it might have something to-do with the military version. The waveforms all look great, minimal distortion and really love this generator so hope someone out there has a clue regarding this very strange problem. Because the output is exactly 2X whats on the display I'm thinking there is a simple explanation for this...

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 01, 2022, 11:08:12 am
content: There is indeed a simple explanation as this question has been asked and answered many times. Most function generators display an amplitude assuming the load is 50 ohms. Since you are using a multi-meter with a high input impedance (tens of megohms), you will measure double on everything. Similarly, if you hook the generator up to an oscilloscope (typically 1 Mohm), you will see double. Terminate your output into 50 ohms and your readings will be as shown on the generator's display.

author: montydtm
author_rank: Newbie
timestamp: August 04, 2022, 08:40:01 am
content: Appreciated, strange that my Wavetek 145 does not have this issue but it's a lot less sophisticated and not designed primarily for radio-work. My oscilloscope will also have a high-Z input so guess I'll just have to live with it as I''ve been doing but at least the mystery is solved, thanks much.

author: alm
author_rank: Super Contributor
timestamp: August 04, 2022, 08:49:26 am
content: For best performance (amplitude flatness over frequency, square wave fidelity) you'll want to make sure the signal from the generator is terminated into 50 Ohms. Particularly at higher frequencies or with longer cables.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2022, 08:54:07 am
content: Some generators have an option to switch between 50 Ohms and high Z mode - just to change the displayed amplitude to 2x the value in high Z mode.Chances are the military wanted the more common display for a terminated output.

author: montydtm
author_rank: Newbie
timestamp: August 04, 2022, 03:03:34 pm
content: This model can switch up-to 600ohms and has an unbalanced output, so no hi-Z option however I certainly can create a dummy-load for testing and have a large-one for antenna work. Regarding fidelity, I suspect that if I'm not modulating carriers and just generating output waveform this won't be an issue.

author: alm
author_rank: Super Contributor
timestamp: August 04, 2022, 04:39:12 pm
content: The wavelength of a 20MHz signal is about 15m, so as long as your cables are an order of magnitude shorter you should indeed be fine.