[time-nuts] 10 MHz 'failover' switch?

Bob kb8tq kb8tq at n1k.org
Thu Jul 26 18:21:20 EDT 2018


Regardless of how you do things, the input to input isolation needs to be better than you might 
think at first. With a PLL, a “spur” 60 db will come right through the loop if it’s close enough. In
general, the loop bandwidth will rarely be narrow enough to take care of the offending signal. You
have to do *something* to keep them isolated.


> On Jul 26, 2018, at 5:05 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> kb8tq at n1k.org said:
>> One interesting subtlety making something like this:
>> What if the two inputs aren’t quite on the same frequency?  Purely as an
>> example, say they are 1 Hz off from each other.  If you have 60 db of
>> isolation in your “switch” you get a 1 Hz offset spur that is 60 db down.
>> Even something much further  down is plenty to mess up the ADEV of the
>> output. 
> Has anybody built a microprocessor controlled PLL to handle this case?  If you 
> have the microprocessor and DAC, watching a second input and switching in 
> software doesn't seem like a big step.
> ---------
> Can I put two switches in series?  The is-it-working logic has to be look 
> before the first switch, but the first switch is on-off rather than A-B.  It 
> can be in a separate package placed to minimize feed-through.
> -- 
> These are my opinions.  I hate spam.
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