[time-nuts] The difficulty of low noise measurements

[jimlux]

On 1/11/20 9:57 AM, Poul-Henning Kamp wrote:
> At that level of the dungeon, you have a choice.
> 
> You can either bolt *everything*, including the chair you sit on
> and the pencil you write with, together with copperstraps to get a
> common potential.
> 
> Or you can arrange *all* your cables and other metalic connections
> (cabinets touching because the insulating legs were lost years ago?)
> to form a Directed Acyclic Graph[1].

Two theoretically isolated 2m tall, 1m deep racks separated by 1cm is a 
2nF capacitor.  At 10 MHz, that's a not so huge series 9 ohms in your DAG.


 > I have on my ever-growing TODO list to test if serial-BLE adapters
 > are any good.  Has anybody tried that yet ?

That is an interesting idea. I wonder if you could make a short distance 
fiber connection by just having your notional 10MHz source directly 
drive the diode, with maybe a series resistor (so you don't need a power 
supply).  You could almost certainly do this for an RS232 output. I 
wonder if there's a "no power required" approach for the receiving end.

or a low noise RS232-fiber adapter that steals power for the Rx side by 
using the Tx signals. No level shifters and charge pumps.