Re: [time-nuts] FE-405B reprogramming

[Skip Withrow]

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.

Regards,
Skip Withrow


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Re: [time-nuts] FE-405B Reprogramming

[Logan Cummings]

Thanks Bob,

   I figured if it was that easy someone else would have already done
it. Went looking for a schematic and found Tom's page describing the
architecture at a high level and with ADEV plots providing evidence of said
architecture. Good block diagram from an FEI presentation at the bottom of
the page http://www.leapsecond.com/pages/fe405/

  Takeaway: no, can't be done - 5MHz DOCXO clocks a DDS which outputs
at 15MHz feeds into a PLL steering a standard 15MHz VCXO which then
provides the output directly.

  Very interesting architecture and gives me some food for thought, but
does not solve the 15 != 10 problem Nick was facing.

Cheers and thanks again!
-Logan

On Wed, Jun 8, 2016 at 3:06 PM, Bob Camp  wrote:

> Hi
>
> The 405 appears to have some “bandpass” elements in it. You can not tune
> it over a wide range.
>
> Bob
>
> > On Jun 8, 2016, at 3:08 PM, Logan Cummings 
> wrote:
> >
> > Hi All,
> >
> >Nick's request for a divide-by-three circuit to get his FE-405B to
> > output a 10MHz signal got me thinking - didn't want to threadjack.
> >
> >I had assumed that the FE-405B's DDS output and FE-5680 compatibility
> > made it a good candidate for output frequency reprogramming. Has anyone
> > determined that this is not possible? My biggest concern was that if the
> > internal crystal is also at 10MHz you'd get bad spurs trying to output
> the
> > DDS at the same frequency..
> >
> >Does anyone know if the FE-405B uses the original FE-5680 DDS circuit
> > or the more recent one that is only narrowly (EFC) tunable over the
> serial
> > interface? Would be great to have a stable reference at 1, 5, 10, etc.
> MHz.
> >
> >
> > Any info/references on the FE-XXX internals would be appreciated!
> >
> > Thanks,
> > -Logan
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Re: [time-nuts] FE-405B Reprogramming

[Bob Camp]

Hi

The 405 appears to have some “bandpass” elements in it. You can not tune it 
over a wide range.

Bob

> On Jun 8, 2016, at 3:08 PM, Logan Cummings  wrote:
> 
> Hi All,
> 
>Nick's request for a divide-by-three circuit to get his FE-405B to
> output a 10MHz signal got me thinking - didn't want to threadjack.
> 
>I had assumed that the FE-405B's DDS output and FE-5680 compatibility
> made it a good candidate for output frequency reprogramming. Has anyone
> determined that this is not possible? My biggest concern was that if the
> internal crystal is also at 10MHz you'd get bad spurs trying to output the
> DDS at the same frequency..
> 
>Does anyone know if the FE-405B uses the original FE-5680 DDS circuit
> or the more recent one that is only narrowly (EFC) tunable over the serial
> interface? Would be great to have a stable reference at 1, 5, 10, etc. MHz.
> 
> 
> Any info/references on the FE-XXX internals would be appreciated!
> 
> Thanks,
> -Logan
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[time-nuts] FE-405B Reprogramming

[Logan Cummings]

Hi All,

Nick's request for a divide-by-three circuit to get his FE-405B to
output a 10MHz signal got me thinking - didn't want to threadjack.

I had assumed that the FE-405B's DDS output and FE-5680 compatibility
made it a good candidate for output frequency reprogramming. Has anyone
determined that this is not possible? My biggest concern was that if the
internal crystal is also at 10MHz you'd get bad spurs trying to output the
DDS at the same frequency..

Does anyone know if the FE-405B uses the original FE-5680 DDS circuit
or the more recent one that is only narrowly (EFC) tunable over the serial
interface? Would be great to have a stable reference at 1, 5, 10, etc. MHz.


 Any info/references on the FE-XXX internals would be appreciated!

Thanks,
-Logan
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