[time-nuts] 10 MHz reference switching

[Bob Camp]

Hi

Assuming you are switching between an external reference and the internal OCXO:

The "bleed through" of the OCXO is going to show up as a discrete spur close to the external reference carrier. Unless you have a very unusual architecture, it will be inside the loop for everything you are doing. Attenuating it to a "suitable" level may be quite difficult. 

Bob


On Mar 8, 2010, at 6:34 PM, life speed wrote:

> Hello Time Nuts,
> 
> I'm back again.  I designed bandwidth-limited (30 MHz) ADA4899-1 opamp circuits to buffer the 10 MHz reference, with theoretical noise of less than 3 nV/rtHz.  Oscillator noise is about -155 dBc/rtHz from 1 KHz to 10 KHz, and -160 dBc/rtHz from 10 KHz to 100 KHz.  Unfortunately the rest of the design is holding up PCB manufacture, so results on these won't be available for awhile.
> 
> Which brings me to the other design-stopping considerations:  Is anybody aware of low-noise analog switches that can be used to reconfigure the 10 MHz reference paths without significantly degrading the phase noise?  As always, I believe 1/F noise would be the concern.  Non-reflective is great, but I can accomplish that by using more switches and 50 ohm resistors if need be.  Isolation is a key consideration as well.  Mechanical relays are out of the question.
> 
> An alternative to switching is phase-locking the OCXO to an external reference.  I have thoroughly looked into this possibility, and it is likely my preferred approach.  But I need to understand how the switching approach compares, hence my question.  I do not need help with phase-locking; this feature is readily available from OCXO vendors who have dedicated years of research to optimizing their products.
> 
> Any switch experience out there?
> 
> 
> 
> 
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