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From: Toaster <toaster@dne3.net>
Newsgroups: sci.electronics.design
Subject: Re: 50 ohm termination
Date: Wed, 19 Mar 2025 21:55:32 -0400
Message-ID: <20250319215532.00003974@dne3.net>
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On Thu, 20 Mar 2025 01:41:29 +1100
Bill Sloman <bill.sloman@ieee.org> wrote:

> On 19/03/2025 9:23 pm, Toaster wrote:
> > On Tue, 18 Mar 2025 19:01:51 -0700
> > john larkin <jlArbor.com> wrote:
> >=20
> >> 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?
> >>
> >=20
> > digital
>=20
> 5V is a big swing for a modern digital system, but there are lots of=20
> fast switching transistors out there that can cope with a 5V swing.
>=20
> Discrete surface mount devices can be pretty compact, and there are
> some fast integrated circuit devices designed to drive them.
>=20
> Some of the ECL-to-TTL level shifters did generate a very fast full
> 0V to 5V swing. I got stuck with up-dating a very fast TTL-based
> timing circuit in the early 1990's, and used a bit of ECLinPS ECL to
> get rid of the usual TTL faults, and used 100k ECL-to-TTL converters
> to push out the TTL house-keeping signals.
>=20
> They were a lot better than the original TTL signals
>=20
> It involved adding -4.5V rail to drive the ECL, but with surface
> mount parts we could squeeze the additional stuff onto same sized
> printed circuit board that the original system had used.
>=20

fyi, im an amateur and may have made a mistake in my design, reason for
5V is so the logic chip in the other end gets around 2.5V after going
through the two 50 ohm resistors (voltage divider) and can trigger.

=46rom what everyone is saying I dont even need to do that and can get a
reliable termination just by using one 50 ohm resistor and avoid
dealing with the voltage divider side effect.

the hard part is coming up...soldering these tiny smd components...i
bought a microscope and a little platform to hold the boards. going to
try hot air soldering as i havent shelled out for a reflow oven yet.

the logic chips im using in my project are 74VHC series.