Timothy Miller wrote:
On 3/16/06, James Richard Tyrer <[EMAIL PROTECTED]> wrote:
Since no one commented on this, I repeat it:
And next question, is the monitor input 50 ohm or 75 ohm. RS-170 is 75
ohm. The schematic shows a 49.9 (standard 1% value but some suppliers
also sell actual 50s) ohm termination resistor. I think that this is an
error, it should be 75 ohm for R90, R100, & R1
I checked and monitors do have 75 ohm input impedance.
================
And I made a typo. :-|
it should be 75 ohm for R90, R100, & R110.
It was a good catch to notice that these resistors are incorrect for
most monitors. I think Howard put in 50's because a few of the really
high-res monitors he's familar with actually do have 50 Ohm impedance,
but we'll fix this.
Hmmmmm. Well I'm glad that I brought it up but this is something that
we need to consider further. What kind of connections is the hi
resolution head going to have?
If it is going to have BNC connectors, they it probably should have 50
ohm termination resistors.
But, if you are going to use a VGA "D" connector, the I would use 75 ohm.
This is a constant issue with many things -- 50 ohm vs 75 ohm.
If you are going to use BNC connectors and 50 ohm, you really need to
design the circuit board so the three traces also have 50 ohm impedance.
Otherwise with ps rise times the trace will cause ringing if the
propagation time for the trace is more than half the rise time.
--
JRT
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