[time-nuts] FE 5680A "new version" - Filtering the 10 MHz
Sylvain Munaut
246tnt at gmail.com
Tue Nov 15 22:26:49 UTC 2011
Hi,
> If the problem is only harmonics (cleaning up a square wave), a simple LC
> low pass filter with a cutoff midway between the fundamental and second
> harmonic might be a better choice because it minimizes the tempco-related
> phase shift that either a high Q filter, or one with a cutoff near the
> desired frequency, can exhibit.
Mmm, very interesting.
I was indeed a bit worried about introducing a temperature dependent
element, but didn't think that making a filter with a cutoff far from
the desired frequency might solve that issue.
The unit already has a LC filter but not really good enough I guess.
Has anyone measured it to know what its design parameters are ?
What I still don't get is how bad the signal looks at the RF connector inside.
* This is at the DB9 output (measured after a small wire from a DB9
female) : http://i.imgur.com/R33B6.png
There is some distortion near the 0 crossing and the fft shows
harmonics clearly.
* This is at the RF connector : http://i.imgur.com/Bg3SK.png
That's just nasty and indescriptible.
The signal _before_ the capacitor leading to that RF connector
doesn't look bad tough ...
(both measurements done with a 500 MHz scope and a 4 GHz FET probe,
with a ground blade to the nearest gnd each time).
Cheers,
Sylvain
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