[time-nuts] combining same frequency for redundancy

[Magnus Danielson]

On 06/04/2014 05:22 AM, Tony Greene wrote:
> About 50 years ago I ran into a Frequency Division Multiplexer that used a redundant master frequency system.  I beleive they combined the same frequency for the master oscillator, but it was something  like a special hybrid combiner.  I remember that they used a phase angle of 135 degrees - and then if they lost one signal, the power went down by 3 db on the main pilot.  Anybody no any details on this type of system ?
>
> What I am looking for is to have an input to a distribution amplifier where I can connect and disconnect two different inputs and keep the output systems up and running.

Two methods is being used as far as I know:

1) Phase microstepper to shift into phase. Quadrature phase 
multiplication is one approach to achieve it.

2) PLL and then alternate which is the reference source. As you loose 
the current reference, the oscillator in the PLL act in hold over until 
the PLL has locked to the other reference.

This is being used in essentially all the telecom SSU like redundant 
chassis.

Cheers,
Magnus