[time-nuts] FE-405B reprogramming

Skip Withrow skip.withrow at gmail.com
Thu Jun 9 14:37:53 EDT 2016

Hello time-nuts,

I'm a little late on this discussion, but I too was very intrigued by the
possibility of a 10MHz FE-405B.  I have tried it and it is possible
(maybe), there is still one big hurdle to get over.

1. There is a 15MHz crystal filter at the output of the 405.  This was
easily and cheaply replaced with a 10MHz unit.

2. There is a 15MHz (third overtone) crystal in the VCXO. This drives the
DDS and the output.  With some playing (new crystal and two component
deletions) this was changed to 10MHz.  BTW, this is not the 'good' crystal
that is ovenized and gives the FE-405 it's really good numbers.

3. The DDS is normally 15MHz in, 5MHz (+/-) out that is locked to the
'good' crystal.  So, it is reprogrammed to be 10MHz in, 5MHz (+/-) out.

It works, kinda.  The 15:5 ratio is enough to satisfy the Nyquist criteria
and a normal 405 is happy.  The 10:5 ratio creates some very interesting
outputs from the DDS, There is a very large low frequency component that
drives the PLL wacky.

I suppose there are several ways to get around this problem, but I was
looking for a way to change the unit with just minimal component changes.
Doesn't look like that can happen

I'm convinced now that the right way to do it is use a divider
(regenerative or otherwise) to get there from here.

Skip Withrow


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