[time-nuts] troubleshooting a FE-5680A (got two; only one works)
beale at bealecorner.com
Wed Nov 30 13:01:47 EST 2011
> [Peter Bell] ...check to make sure that the VXCO is actually sweeping through 10MHz - if not, you can try adjusting C217 to centralise the sweep range.
Thanks Pete, you are exactly right. As mentioned, the VCXO was sweeping from 9.999799 to 9.999994 MHz with no lock. I found that the C217 trimmer was nearly at minimum C already, but a tiny tweak to move it to absolute minimum C shifted up the center frequency 30 Hz, which (just) enabled a lock at 10.000000 MHz. Given the pre-existing C217 setting, I wonder if this unit was marginal even from the factory. I don't know what minimum value C217 has, but I could remove C217 completely which might center it more. Or, maybe there is a way to increase the VCO tuning range, no doubt at a cost in phase noise.
Asking for a replacement unit is another option, but I'm just too impatient for that, so I'm taking it as an opportunity to learn something. (in fact I did send him a note, but I'm not expecting anything.)
Thanks to Pete and the time-nuts list for the solution!
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