Dieter wrote:
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.
Uhm... are you saying that displays with BNCs are 50 Ohm?
We would hope so. What we do know is that the VGA standard is 75 ohm
and the standard impedance for BNC connectors is 50 ohms. Although
people kept putting BNCs on 75 ohm coax so now you can buy 75 ohm BNCs
but they don't have as high a frequency rating.
The monitor issue should be researched.
TM said that some high end monitors are 50 ohm, but all that I are 75 ohm.
http://bssc.sel.sony.com/BroadcastandBusiness/DisplayModel?m=0&p=8&sp=20074&id=67536
Perhaps the 75 ohm connections have taken over in video applications.
It also appears that we should use the HD-15 for the Hi Resolution since
I have found monitors that support 2048x1536 and don't have BNC
connections. Sony and Mitsubishi.
I hate to think what the VSWR of an HD-15 pin connector stuck in a 75
ohm coax line. Also they seem to use miniature coax.
And that the cables with HD-15 on one end and BNC on the other have matching
transformers hidden inside?
I'm thinking not.
That would be nice, but they probably just have BNC on the end of 75 ohm
cable. We can hope that they used 75 ohm BNC connectors.
--
JRT
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