Re: 50 ohm termination

On 19/03/2025 9:23 pm, Toaster 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:
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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.

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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.
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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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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.

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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.
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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

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.
Discrete surface mount devices can be pretty compact, and there are some fast integrated circuit devices designed to drive them.
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.
They were a lot better than the original TTL signals
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.
--
Bill Sloman, Sydney