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BrownOut

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At a meeting today, an age-old engineer was ranting about an old bus technology we have deployed. He said that because it uses Low Power Schottky logic for drivers at each node, that makes the bus niosey, and the more nodes, the more noise. He said the noise is generated because of the Schottky logic. I think this is wrong. I think he's confused with the lower noise margin due to the switching levels ( only .3V margin for sensing a 'low' and .7V margin for sensing a 'high ) The noise issue is because of the narrow noise margins, and not that Schottky logic genertes more noise. Any thoughts?
 

MikeMl

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My experience is that TTL (be-it Shottky or not) has very asymmetrical drive, excessive sink, crappy source, and that is what causes reflections, ground current bounce, etc.
 
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carbonzit

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So just curious: what, in your opinion, is the best interface system? Or does it always depend on the application? (I'm asking out of genuine ignorance here.)
 

BrownOut

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MikeMI said:
My experience is that TTL (be-it Shottky or not) has very asymmetrical drive, excessive sink, crappy source, and that is what causes reflections, ground current bounce, etc.
Thanks Mike. That makes sense. He claims the niose comes from the Schottky diodes in the logic. Maybe he heard of Schottky noise somewhere, and thinks it comes from Schottky doides :confused: (He's a systems engineer, not an EE) Also, he says the source resistors are wrong because they are 25 ohm and the line impeadance is 50 ohm. But 25 ohms would be correct, since it's a diffenertial driver.

carbonzit said:
So just curious: what, in your opinion, is the best interface system? Or does it always depend on the application? (I'm asking out of genuine ignorance here.)
Many modern systems define the physical drivers. For example, our DDR3 defines SSTL-1.5V interfaces. There are numerous factors that decide. I"m sure that doesn't answer your question though :)
 
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crutschow

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Schottky logic can be noisier since it has faster edge rates (rise and fall times). The harmonics and high peak currents from this can create more noise in the system.

Carbonzit, a commonly used and good interface system for high speed digital signals is some variation of LVDS (Low Voltage Differential Signaling) sent over differential wire pairs. It generates little external noise, generates negligible ground currents, and is also resistant to external noise. Differential connections are used in sending Ethernet and HDMI signals for example.
 

carbonzit

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Carbonzit, a commonly used and good interface system for high speed digital signals is some variation of LVDS (Low Voltage Differential Signaling) sent over differential wire pairs. It generates little external noise, generates negligible ground currents, and is also resistant to external noise. Differential connections are used in sending Ethernet and HDMI signals for example.
I hadn't even thought of differential interfaces, since I assumed all digital interfaces were single-ended (but then I completely forgot about differential SCSI, which I've actually worked with).

Differential good. Common-mode rejection and all that.
 
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jrz126

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Maybe he heard of Schottky noise somewhere, and thinks it comes from Schottky doides :confused: (He's a systems engineer, not an EE)
just another case of the systems engineer blaming the hardware :p
 

MikeMl

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... He claims the niose comes from the Schottky diodes in the logic. ...
TTL was noisy, and Schottky TTL is faster than TTL, so it got noisier when used to drive long unterminated busses.
 
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