Dieter wrote:
>>> If it is going to have BNC connectors, they it probably should have 50
>>> ohm termination resistors.
...snip...
>>> 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.
You've been able to buy 75R BNC connectors for as long as I can
remember, which goes back to the 1970's.
People tended not to bother, because the impedance discontinuity is
smeared out over the wavelength of the signal in the cable - which is
about 200mm at 1GHz, and proportionately longer at lower frequencies.
Back in the 1970's we thought in terms of regular TV signals, and
didn't expect to have to handle anything above 15MHz.
Minature 75R coax is available - Farnell stocks both RG179PE, which
is polyethylene "spaced" and cheap, and RG179BU, which uses a solid
PTFE/Teflon dielectric which is much more robust and easier to work
with but costs three times as much. Farnel also stock compatible 75R
crimp connectors from Amphenol at much the same sort of price as
comparable SMB parts (where they only stock 50R parts) and 75R Lemo
connectors which cost three times as much.
--
Bill Sloman, Nijmegen
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