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Technology Stocks : Rambus (RMBS) - Eagle or Penguin
RMBS 95.53+0.7%Nov 28 12:59 PM EST
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To: John Walliker who wrote (65338)2/5/2001 3:06:19 AM
From: Bilow  Read Replies (1) of 93625
 
Hi John Walliker; You wrote: "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."

While this is true, it is also the case that (1), the multiple clocks are much better terminated and have better defined loads than before, and (2) some of them show up as true/complement pairs, and consequently have somewhat less radiation than PC133.

Re: "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.)"

This is not the case. (1) SDRAM was saddled with relatively worse timing than the periods would indicate because of the time required for transients (ringing) to die out. (2) The impedance mismatches in SDRAM would tend to lead to shorter rise times over the range of possible designs in order to ensure that it works at worst case. (3) DDR is source synchronous, consequently the timing of the clocks is loosened considerably, as a percentage of the duty cycle, relative to SDRAM. Overall, DDR likely has worst case edge rates (in terms of max volts per nanosecond) considerably below SDRAM. You want to look it up?

-- Carl
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