Re: 50 ohm termination

On Thu, 20 Mar 2025 16:28:38 +1100
Bill Sloman <bill.sloman@ieee.org> wrote:

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:
>

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

>
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.
>

 
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.
 

Very fiddly, my hands shake too much for stuff this fine. I have a
driver and a mosfet:

BD2311NVX-LBE2 - driver
GAN190-650FBE  - mosfet

That driver is so tiny I'm wondering if the 10Mhz switching speeds are
worth it! Researching some strange field interactions so I needed
something that could switch a decent voltage (300V) at repetition rates
close to 10Mhz. Odd requirements but until I can narrow down parameters
I need to sweep up the frequency range as far as I can.

Thanks for all of your help.