Well even thoughe electricity flowing through a wire is somewhat slower
than the speed of light, exactly how much slower
is something I do not know about. What I do know is that the
approximation of it being "around" the speed of light has served me
very well over the years. One time on an an installation job at a
telephone switching office where all previoisly installed
processor complexes had been installed using 50 foot coax cables to get
the 12.352 mHzmaster clock to them all, this install
order called for the use of a 100 foot cable. The result was that the
newly installed processor complex was unable to synch
up with the rest of the switch. Nobody knew why. I calculated that
at the speed of light, the 12.352 mHz clock signal would be
late at the new processor by around half of a cycle. We talked this
over with network operations and they confirmed my calcualtions
and then instructed the crew to change out ALL the clock cables in the
entire exchange to 100 footers to match the new one.
There was lost of slack to stow, but it all came up just fine after
that. The install crew wanted to know how I managed to
know what the problem was. I said, "simple gentlemen, it's the speed
of light and our new intsall was getting the message rather late"
---- Original Message ----
From: "Adrian Godwin" <[email protected]>
Sent: 12/27/2020 5:42:02 PM
To: [email protected]
Subject: Re: [neonixie-l] Re: How close together do a controller and
crystal need to be?
https://en.wikipedia.org/wiki/Velocity_factor
On Sun, Dec 27, 2020 at 10:05 PM Paul Andrews <[email protected]>
wrote:
Hmm. My recollection from high school physics was that the
speed of propagation along a wave guide was around 90% the speed of light -
presumably limited by the dielectric. Signals along a plain old wire, on the
other hand, were more like 1/3 the speed of light. Now I'm going to have to
double-check that for the first time in over 40 years!
On Sunday, December 27, 2020 at 2:20:06 PM
UTC-5 Chuck wrote:
A handy way I use, to remember the
approximate speed of light, which is also the approximate
speed at which an electrical signal
travels in a wire is just to think of it
as 1 nanosecond per foot.
Approximately.
---- Original Message ----
From: "gregebert" <[email protected]>
Sent: 12/27/2020 12:32:41 AM
To: "neonixie-l"
<[email protected]>
Subject: Re: [neonixie-l] How close
together do a controller and crystal need to be?
I'm assuming you are routing the output
signal of an oscillator, not the crystal signals themselves.
The rise- & fall-times of the clock
signal will determine how long the trace can be without termination. Faster
edge-rates, say in the 2-3nsec range, will limit your trace to around 1 inch.
Signals propagate around 150psec/inch,
and if the rise/fall times are about 10x (or larger) longer than the
flight-time, then reflections should not have sufficient amplitude to cause
false clocking.
In the example above, 1 inch of trace
has a round-trip flight-time of 300psec. If the rise and fall delays are 3nsec
or larger, you can safely use 1 inch of trace without using termination
networks or controlled-impedance traces.
SPICE simulations are very helpful when
deciding how to design clock lines when you cant satisfy the above rule.
On Saturday, December 26, 2020
at 4:06:26 PM UTC-8 Bill van Dijk wrote:
As long
as there is not something very noisy on the other side of the board you’ll be
just fine.
Bill
From:
[email protected] [mailto:[email protected]] On Behalf Of Erick
Anderson
Sent:
Saturday, December 26, 2020 6:53 PM
To:
neonixie-l <[email protected]>
Subject: [neonixie-l] How close together do a controller and crystal need to be?
I
designed a board for the 6-digit All Spectrum controller, which uses the Dallas
TCXO chip. That's what goes in the DIP-14 socket in the picture. Right now
they're as close to each other as possible. I'm thinking about redesigning the
board to be a bit shorter, and moving the socket into the empty space at the
right of the board would help. This would make the clock signal trace much
longer, but is that actually a problem?
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