[time-nuts] Re: 10 MHz Distribution Best Practices?

[Magnus Danielson]

Hi,

On 2023-03-24 03:28, John Miles via time-nuts wrote:
>> Has anyone run across any publications on best practices or examples
>> of 10MHz Lab wide distribution networks? I'm looking for a discussion
>> on how to physically locate oscillators/distribution amplifiers, cable
>> types and runs, RFI mitigation etc. I haven't come across any, and I'm
>> starting to build one at work. We've got a Cs oscillator and I'd like
>> to make sure we deliver that performance across our systems lab.
> A lot of things can be said in favor of low-noise distribution amps with
> good VSWR, good channel isolation, and good PSRR (not so fast, HP 5087A.)
> But over the years, I've learned that the three most important factors when
> it comes to piping 10 MHz signals around are:
>
> 1) Shield resistance
> 2) Shield resistance
> 3) Shield resistance
>
> I've spent a lot of time recommending double-shielded coax in the TimeLab
> manual and elsewhere, and I still stand by that advice, but what I've come
> to realize is that this is really just a proxy for low shield resistance.
> Good grades of single-shielded cable are basically as effective at HF as
> double-shielded cable.  To the extent your cable ground shield exhibits
> resistance, it's not a shield, it's a resistor.
You for sure want your cable tight as such, but for any low frequency 
variations, you might benefit from laying extra ground-wires in parallel 
with coax cables. The coax cable will tie grounds together, and the 
resistance in that will for sure limit how things are damped. A lab is 
just like a telecom setup. To further help, bring additional grounding 
wire to shunt the resistance of the coax shield is recommended.
> Avoiding ground loops is on the list too, but further down.  Never lift a
> ground to avoid a ground loop.  Use coax-to-coax baluns only when you can
> see a beneficial effect.
Figure out if the isolation needs to be at low or high frequency. A 
transformer achieves galvanic isolation and is still achieving some 
isolation all the way up at 50 or 60 Hz, but for RF it's just a cap that 
shorts quickly. If it is RF-power, then a common-mode choke that is the 
right solution, and some care should go into figuring out which 
frequency it will operate on.
>    Focus instead on providing a shared low-resistance
> common ground  to your entire network -- ideally not the ground all the way
> back at the service entrance -- and rely on low shield resistance on the RF
> side to do the rest.

Agree.

Look at ITU-T K.27 and read the Mesh-BN parts.

Cheers,
Magnus

>
> Every installation is different and your mileage will most certainly vary,
> but this is my take on it.
>
> -- john
>
>
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