[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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