--- In TekScopes@..., "Philip" <ndpmcintosh@...> wrote:
I have been studying and working with an FG 502 function generator. I recently went through the adjustment procedure and was able to do everything that did not require a distortion analyzer or spectrum analyzer. Actually I did those adjustments too but eyeballed the sine shape and used my best judgement as what gave the best appearance.
I am not convinced it is operating properly though. I uploaded three picture and perhaps someone who has experience with this unit can tell me if this is as good as it gets or if there is an indication of a fault or two somewhere.
The closeup of the sine shows (barely) that it appears to be composed of a number of signals closely bunched together.
The low frequency square shows that there are a number of tops stacked upon each other. This does not appear at high frequency.
Finally, the closeup of the high frequency square shows what it looks like at high frequency.
I also am unable to adjust the 20V power supply below 21.9 V. I replaced one cracked resistor on the front end of the power supply and am continuing to troubleshoot that problem.
Thanks for any insights.
Phil...
I don't know the magnitudes of the waveform amplitudes (noise and signal pk-pk), but it looks like a classic ground loop problem to me: TM500 mainframe grounded through ground pin in power cable, as the scope is too. Ground through the BNC cable makes one huge! ground loop. Any dynamic magnetic field passing through the loop induces a current into what is essentially a shorted transformer secondary turn. Current is only limited by impedance of the conductors, and could reach a few dozen amps!. This induces a voltage across the impedances, with the largest possibly being the BNC cable ground. Ground at the front panel of the FG502 does not equal ground at the vertical input connector of the scope. Resulting potential difference is superimposed on the displayed waveform.
If you have a differential amplifier, try using it, without plugging the BNC cable into to it. Connect the + input to the BNC cable connector center pin, and the ¨C input to the BNC ground shell. If you don't have a diff amp, try quasi differential using two scope channels, added with one inverted. Make sure both as set to the same V/div.
- Steve
