Re: 50 ohm termination

On 20/03/2025 12:55 pm, Toaster wrote:

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:
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On Tue, 18 Mar 2025 18:59:44 -0400, Toaster <toaster@dne3.net>
wrote:
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On Tue, 18 Mar 2025 16:02:45 -0700
john larkin <jl@glen--canyon.com> wrote:
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On Tue, 18 Mar 2025 18:29:42 -0400, Toaster <toaster@dne3.net>
wrote:
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On Mon, 17 Mar 2025 19:17:13 -0700
john larkin <jlArbor.com> wrote:
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On Mon, 17 Mar 2025 19:50:17 -0400, Toaster <toaster@dne3.net>
wrote:
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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.
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LVDS line receivers are great at the receive end.
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>

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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.
>

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I had a really hard time finding a good line driver. I might look
into this chip.
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Thank you!

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Is your signal analog or digital?
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digital

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5V is a big swing for a modern digital system, but there are lots of
fast switching transistors out there that can cope with a 5V swing.
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Discrete surface mount devices can be pretty compact, and there are
some fast integrated circuit devices designed to drive them.
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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.
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They were a lot better than the original TTL signals
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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.
>

 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.
  From 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 haven't shelled out for a reflow oven yet.
 the logic chips im using in my project are 74VHC series.

I can sympathise with the difficulty of coming to terms with soldering smd chips. Around 1989 I got stuck with introducing smd parts to Cambridge Instruments in the UK, because the GaAs chips that I needed to use only came in surface mount packages. We bought a fairly expensive Groatmore hot-air reflow machine that would reflow individual packages.
When I wanted to use similar parts - Motorola ECLinPS devices - at Nijmegen University in the Netherlands, nearly ten years later, they just bought a much cheaper and smaller work station (but didn't let me use it). At Haffmans BV in the Netherlands around 2002 I just used a fine tipped soldering iron under a cheap binocular microscope.
It was fiddly work, but perfectly practical.
--
Bill Sloman, Sydney