Rambus (RMBS) - Eagle or Penguin Message Board

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Technology Stocks : Rambus (RMBS) - Eagle or Penguin

RMBS 90.19

+2.8%

Nov 19 3:59 PM EST

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To: Dutch who wrote (23738)	6/26/1999 12:16:00 PM
From: John Walliker	Read Replies (1) of 93625

Dutch, Would this be the same type of spread spectrum technology that is used in digital telephones. Not really. This is a trick to enable circuits which emit large amounts of interference to meet the FCC and CE standards. It relies on the characteristics of the measuring receivers specified in the standards which have a bandwidth of 120kHz in the 30 - 1000 MHz measurement band. (This corresponds to a compromise between the bandwidths of FM broadcast and analog mobile radio systems.) By rapidly varying the clock frequency over a range of several MHz the unwanted signal only spends part of its time within the bandwidth of the measuring receiver. This therefore indicates a lower level, even though the total interference power is still the same. Higher frequencies are more easily radiated from electrical interconnections than lower ones. Rambus have done several things to minimise this problem. 1) The clock signal is the one that spends most time at a fixed frequency and therefore needs most attention. The use of a differential clock helps because interference from the two clock lines tends to cancel out. 2) The use of terminated transmission lines not only allows high data rates but avoids resonances which would otherwise greatly increase the emissions at some frequencies. 3) Motherboards and RIMMs must have ground planes which not only help to define the characteristic impedance of signal conductors but also provide a high degree of shielding. 4) The broad conductor around three edges of RIMMs reduces emissions by shielding the edge of the board. The random positioning of the vias (small dots visible on RIMM photographs which link different circuit board layers together) minimises resonances in the connection between this shield and the ground plane and further reduces emissions. 5) Rambus specify the allowable characteristics of spread spectrum clocks. This is necessary because the phase locked loops used to synchronise Rambus systems cannot cope with excessively sudden frequency changes. John

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