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Technology Stocks : Rambus (RMBS) - Eagle or Penguin -- Ignore unavailable to you. Want to Upgrade?

To: grok who wrote (29576)	9/15/1999 11:12:00 AM
From: John Walliker	Read Replies (1) \| Respond to of 93625

KZNerd I believe the expectation for DDR and DDR-II is that they will use series termination at the source and pullup termination at the destination. So the two resistors are in series for power calculations and also much higher resistance than in the Rambus case. Yes, that is what I have assumed also (with the destination terminated at an intermediate voltage between logic states). As for the resistance value used, this must equal the characteristic impedance of the transmission line for the termination to be effective. For Rambus the characteristic impedance is about 28 ohms resistive. For (DDR) SDRAM the impedance will be complex because of the stubs leading to each memory module. This means that the termination requirements cannot be fully met with a pure resistance and that any reactive terminator will need to change in value as modules are added to the system. The characteristic impedance used by Rambus is relatively low, so perhaps it would have been better to use a higher value, maybe 50 ohms for (DDR) SDRAM. This depends on the width of the tracks used. Narrow tracks have a higher impedance and also receive and radiate interference more effectively as their impedance is closer to the impedance of free space. This means that the wide low impedance tracks used by Rambus are less susceptible to crosstalk and radiate less interference. John