On Fri, Mar 17, 2006 at 11:11:38AM -0500, howard parkin wrote:
> Here's the story with the 50ohm output impedance.
>
> The large majority of monitors are 75ohm, whether they are VGA
> or BNC. However this interface has been included to specifically
> drive a two types of monitor on the market that really are 50ohm.
> The D/A converters and amps on this video head are expensive.
> In fact they would add about $100 to the price of the board.
> Traversal will not be fitting them on the standard board that we sell.
>
> If somebody needs to drive 2048x1536, thay can use the
> 330Mpixel/s VGA on the other connector. If anybody needs to
> drive two 2048x1536 monitors or 2048x1536 at more than 75Hz
> frame rate, then Traversal may add the extra components or they
> can do it themselves. In this case 50 or 75 ohm resistors will be
> fitted as required and the D/A current set resistor modified
> accordingly to give the required voltage swing (usually 0.7Vp-p
> without syncs).
>
> To avoid any confusion, I will add a comment to the schematic
> to indicate that either 50ohm or 75ohm can be fitted.
Knock me over with a feather.
OK. I've got an idea how to route 50 and 75 ohm tracks to different
connectors, without having to use miniature coax to bypass the board.
Suppose you lay out the board with two locations for each backmatch
resistor, with a common pad for the upstream end driven by the op amp. Each
resistor would have a separate downstream pad, one driving the head end of
the 75 ohm track, and the other driving the 50 ohm track. The 75 ohm tracks
could go to the default connector, and the 50 ohm tracks could go to 50 ohm
BNCs. You'd probably have to change layers to get the 50 ohm tracks where
they're going, but that should have minimum effect if it's done right at the
backmatch resistors. Only one in-line resistor gets stuffed, so the unused
transmission lines don't load the signal.
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