Albert wrote on Sun, 03 October 2004 11:49 |
Another question: Would you use a BNC t-bar and terminator on gear that is already internally terminated? Would that yield better results, or would it damage the gear?
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It wouldn't damage the gear, but eventually it would load down the source wordclock so that the system would not function or would not function well.
1) I suggest you start with an oscilloscope on the wordclock generator. Measure its output, which should be 4 v p-p if possible. Any lower and some following devices will not like it, but most following devices are comfortable with as low as 1 v p-p---
TERMINATED!!! Hey, there's no standard! It's a wild wild west.
2) Then, put a terminator on it in parallel with the scope input. The level should drop by 1/2. If not, then the source is not truly 75 ohms. This is not a big big deal with wordclock frequencies, which are not so high generally as to be subject to reflection problems. I'm sure Dan can explain the compromises there, I don't know. It would cause some limit as to the length of the cable and how many devices you can attach.
3) Using this level as your goal (2 v p-p terminated), run the signal into each of your wordclock devices and use the scope to see if the device terminates the line. Chances are it does and you'll have to go inside the device and remove the 75 ohm resistor that's there, so you can then use the BNC-T.
If the wordclock generator is truly 75 ohm, 4 v p-p (unterminated), 2 v p-p terminated, then I don't see a problem using low loss coax for even 20-40 feet and having 3 to 6 high impedance (unterminated) dropoffs in between. Scope the end of the chain. If it's still a clean square wave at approximately 2 v p-p into the terminator at the end of the chain, then I'd say it's a win-win!
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Also, would you use a BNC t-bar on gear that has a WC in and out, or would you pass through the gear, using it's own circuitry? And if you did use the BNC t-bar to bypass the gears internal routing, would you then use a terminator on the units WC output or rely on its own termination?
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The problem is that most word clock equipment has internally terminated inputs and will have to be modified (real real pity, since Dan's approach is absolutely valid). If you remove the 75 ohm resistor, then you will be able to use a BNC T, as this effectively bypasses the gear's internal routing. All you are concerned with at that point is the impedance of the gear's loading. Usually it's a small coil or transformer loaded by a 75 ohm resistor. Remove the resistor and (hopefully) it becomes high impedance, that is, negligible load on the rest of the circuit.
By the way, if the equipment to be slaved is NOT a converter, but simply a digital processor or DAT machine or whatever, then a wordclock distribution amplifier will do no harm, as the interface jitter is irrelevant. I use a Lucid Clk-X to feed wordclock to my Digital Timepiece, which is NOT feeding converters, to my DAT machine, to my TC Electronic System 6000, whose converters I am not using, etc. etc.
You could take the Clk-X box, remove the input terminator, and feed using Dan's BNC-T method through the input of the Clk-X, and feed (modified unterminated) critical converters like the Digidesign, and so on, through the BNC T system, and use the Clk-X wordclock distribution box to feed the rest of things.
My Tascam DAT machine has a Wordclock through jack and some kind of automatic termination. How does that automatic termination sensing circuit work? I don't know!!!! I don't trust it, either, but I suppose I could sniff it out with a scope...