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From: Gerhard Hoffmann <dk4xp@arcor.de>
Newsgroups: sci.electronics.design
Subject: Re: 50 ohm termination
Date: Sun, 30 Mar 2025 03:59:47 +0200
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Am 30.03.25 um 00:53 schrieb Phil Hobbs:
> On 2025-03-19 10:41, john larkin wrote:> On Wed, 19 Mar 2025 06:23:25 
> -0400, Toaster <toaster@dne3.net> wrote:
>  >
>  >> On Tue, 18 Mar 2025 19:01:51 -0700
>  >> john larkin <jlArbor.com> wrote:
>  >>
>  >>> On Tue, 18 Mar 2025 18:59:44 -0400, Toaster <toaster@dne3.net> wrote:
>  >>>
>  >>>> On Tue, 18 Mar 2025 16:02:45 -0700
>  >>>> john larkin <jl@glen--canyon.com> wrote:
>  >>>>
>  >>>>> On Tue, 18 Mar 2025 18:29:42 -0400, Toaster <toaster@dne3.net>
>  >>>>> wrote:
>  >>>>>
>  >>>>>> On Mon, 17 Mar 2025 19:17:13 -0700
>  >>>>>> john larkin <jlArbor.com> wrote:
>  >>>>>>
>  >>>>>>> On Mon, 17 Mar 2025 19:50:17 -0400, Toaster <toaster@dne3.net>
>  >>>>>>> wrote:
>  >>>>>>>
>  >>>>>>>> Thank you for the advice. In my case I have a 10Mhz signal with
>  >>>>>>>> very sharp transitions (500ps, 5V) and wanted to make sure I
>  >>>>>>>> did things properly.
>  >>>>>>>
>  >>>>>>> Interesting. What's generating the 5v signal? Lots of AC and
>  >>>>>>> Tiny Logic chips are that fast, but might strain to drive 50
>  >>>>>>> ohms. We use several tiny triple buffers in parallel sometimes.
>  >>>>>>>
>  >>>>>>> Regular thick-film surface-mount resistors are fine as
>  >>>>>>> terminators at 500 ps.
>  >>>>>>>
>  >>>>>>> LVDS line receivers are great at the receive end.
>  >>>>>>>
>  >>>>>>>
>  >>>>>>
>  >>>>>> I used a THS3111CD. Split up my project into a timing and driver
>  >>>>>> board, so i have some 50 ohm BNC cables between and wanted to be
>  >>>>>> extra safe about reflections at these higher frequencies.
>  >>>>>
>  >>>>> Is the signal some analog thing, or a 10 MHz clock? The THS is an
>  >>>>> opamp, but they can make good cable drivers too, even for clocks.
>  >>>>>
>  >>>>> Lately I'm enamored of BUF602, a unity-gain 1 GHz beast.
>  >>>>>
>  >>>>
>  >>>> I had a really hard time finding a good line driver. I might look
>  >>>> into this chip.
>  >>>>
>  >>>> Thank you!
>  >>>
>  >>> Is your signal analog or digital?
>  >>>
>  >>
>  >> digital
>  >
>  > I use Tiny Logic triple buffers as line drivers, with all three
>  > sections in parallel, and then sometimes two or three chips.
>  >
>  > NL37WZ16US costs 10 cents.
>  >
>  > 
> https://www.dropbox.com/scl/fi/gw7wetgtovqc04as2gxol/NL37WZ16_5V_Pulse.JPG?rlkey=2eqbyhds8l1myrzfjsrqwn5b3&raw=1
>  >
>  > That US8 package is nasty to solder or probe.
>  >
> Just rereading this.  John,  the prop delay spreads in the datasheet are 
> all over the place--almost a factor of 2 from typical to max over 
> temperature.
> 
> I'd expect the three sections to match OK, but paralleling packages 
> seems quite a lot sportier.  How well does that work in production?
> 
> Cheers
> 
> Phil Hobbs

This here are 2 single TI 74LVC inverters, 100 Ohms on each output,
RG174 / RG188-like Coax into the 50 Ohm of an Agilent 2.4GHz scope.

< 
https://www.flickr.com/photos/137684711@N07/32245910240/in/album-72157662535945536/ 
      >    + 2 pics to the right.

Sorry for the blue on black trace; it is default for input 3 that
happened to be unused. Looks better if downloaded.
Having a GND/VCC pair for each output is friendlier to GND bounce
and temp rise is also smaller in comparison to multichannel gates.

Gerhard