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

To: Bilow who wrote (65336)	2/4/2001 5:27:09 PM
From: John Walliker	Read Replies (2) \| Respond to of 93625

Carl, Since DDR is using a signal swing that is only half the amplitude of SDRAM, and is better terminated, don't you agree that DDR266 has simpler radio frequency problems than PC133 SDRAM? If all other things were equal, that might be true. However, they are not. One big difference is that DDR systems tend to distribute multiple clock signals, so as to improve the timing accuracy of the clocks. This gives more scope for radiated emissions. Secondly, the voltage swing is not the only consideration. The signal rise and fall times are an important factor in determining the emissions. These need to be shorter with DDR because there is less than half the time available for each bit to be transmitted than for an equivalent SDRAM. (Less than half for equal device setup times, that is.) You had previously stated "it will be much harder to obtain CE approval for DDR designs because of radio frequency emissions". By this you were implying that DDR would have difficulty in these areas, as compared to RDRAM. But it is clear that DDR has less problems than SDRAM, and the SDRAM problems were obviously solved by industry. Yes, I was implying that. DDR has many more simultaneously switching signals than Rambus. DDR does not have as well terminated a bus as Rambus, allowing resonances which selectively enhance emissions at some frequencies. No it is not clear that DDR has less (or fewer) problems than SDRAM. Nor is it even clear that these problems have been satisfactorily solved for SDRAM. There have been several prosecutions in the UK of suppliers who were shipping PCs which were falsely claimed to be CE compliant. I have measured the emissions from SDRAM-based PCs of very reputable brands which barely meet the emission requirements. They only did so by the use of spread spectrum clocking which spreads the interference out over a wide frequency range. I have never claimed that it is impossible to meet the emissions requirements, nor that these systems will be harmful in any way to their users. Most problems can be solved, at a cost. John