Tek FG503 Repair Calibration - Still Problems?

[gmilliorn]

I recently got a Tek TM503 with several plug-ins, the FG503 function generator is the second I've looked at after recapping
the TM503 itself and a PS503A.  It seemed to go well, just needed a BNC replaced and some adjustment.  But when I came to the
sine distortion test step, since I don't have a distortion analyzer, I used Tek's recommendation: a 10kHz notch
filter and compare response with and w/o.


In doing so, I get about 11V pk-pk vs. 400mv or 600mV (depending on if the spike from the sine shaper
is part of this test).  Either way I get 3.5% or 5.3%, when it is supposed to be ~0.5%.


But then I started wondering, aren't I really just testing how good the notch filter is?  If I improved the
notch filter, the distortion result would be better, but I would not have changed anything in the unit itself?
Am I missing something obvious?

[Kleinstein]

The distortion test with the notch filter tests both the quality of the generator and filter. It already looks like the frequency is adjusted to get the best suppression, which is a good idea with a variable source.
The residual still looks a lot like the original frequency - still nearly a sine. The original frequency signal is not relevant, but only the other parts. So one should improve / adjust the filter a little more. With a variable frequency it is usually sufficient to trim just 1 resistor to get a much deeper notch.

Alternative one could use the FFT function of the scope to get an idea how much is in the harmonics and mow much is just leakage of the wanted signal. The notch than helps to suppress the main signal so that a more sensitive range can be used.

I would say he signal looks good, with the typical spike for this typ (square - triangle - sine converter).

[gmilliorn]

Thank you, an FFT is a good idea. 
That's going to be much more direct than fussing with the notch filter.

[gmilliorn]

The sine looks good on the scope, and the FFT shows no spurs pretty far out, so I think
I can leave it as-is - good enough for my random lab purposes.
 
Thanks

[gmilliorn]

The Tek wiki didn't have any pictures of the insides, here's what mine looks like
in case anyone ever wants to know.

[Kleinstein]

The FFT looks really good, especially if this is for the signal after the notch.
-50dB down for the harmonics is already close to what one may get from the scope alone. 

[Chappy425]

The Tek wiki didn't have any pictures of the insides, here's what mine looks like
in case anyone ever wants to know.

Perhaps you could email your photos to the administrator of the Tek Wiki site.  His address is available on the front page. If not, I will do it for you with your permission.

Sincerely,
Steve

[gmilliorn]

I had thought about that, but their photos are quite good - I guess they use a lightbox and
a decent camera.

I resized the ones above, so I'll send the fullsize ones there in case they want them.