[time-nuts] Re: Input stage for time interval counter

[Magnus Danielson]

Hi,

On 2022-11-30 17:14, Hans-Georg Lehnard via time-nuts wrote:
> Hi all,
>
> I am currently working on a 2 channel time interval counter. It should
> measure frequencies from 5 to 20 MHz. And hopefully have a resolution
> about 1E-13 at 0,1s to measure HP10811 OCXO stability.
>
> For inputs should I use  UNBAL Rf transformers 1:4 (1:16) or drive the
> LTC9657 directly.
>
> Thanks for your suggestions

I think I would go without an transformer. The transformer only helps if 
you have low-frequency or DC differences you want to isolate from. For 
higher frequencies, it couples through capacitive effect straight 
through. If you want to isolate from RF frequencies bring the signal and 
"ground" through a ferrit choke to suppress the common mode, and be sure 
to have capacitive coupling to ground on the "inside" of that.

For most designs I've only see ferrit choke for common mode current 
reduction help. The coax shield should be bolted to the chassi and 
ground, but only sensed on the inside as reference, but not distributed 
further in on the board. RF transformer use is more relevant if you want 
to impedance convert or voltage convert, but isolation wise rarely worth 
the effort most of the times.

So, that is what my experience tell me. There are a couple of books from 
Henry Ott I would recommend you to consult, if I where to do a narrow 
pick from the ones in my bookshelf.

Good luck with your project!

Cheers,
Magnus