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Technology Stocks : Qualcomm Moderated Thread - please read rules before posting -- Ignore unavailable to you. Want to Upgrade?

To: pheilman_ who wrote (27360)	10/2/2002 6:57:12 PM
From: RalphCramden	Respond to of 197244

That is certainly not true in general. A timing signal can certainly be aligned with the zero-crossing of the voltage waveform even with (especially with actually) a very narrow band sine wave. Consider 60 Hz AC. This signal has a bandwidth which is a tiny fraction of 1 Hz (it is as close to a CW signal as we can easily generate). With a 110VAC signal, and with the ability to measure zero volts +/- 1 microvolt, we can determine the zero crossing time to within ~1 microsecond. What limits the ability to get timing information from a narrow band signal is Signal to Noise Ratio (SNR). But with long integration times, SNR can be made quite high. To get ABSOLUTE time, we need to well-characterize the delays through the system, including the processor, to within the accuracy we wish to ultimately have. But this is true whether we use a broad band or a narrow band signal to get our timing reference. In the case where we want ABSOLUTE timing accuracy which is much less than 1/Bandwidth, we need to characterize the delays of our bandpass and lowpass filters in both the TX and RX systems to within that timing accuracy. This is the same as saying we need to characterize the phase delay through these filters to within a few degrees of phase, which is totally doable. High SNR is not a problem either. Getting high SNR is accomplished by averaging over many events. This is the analog equivalent of "Processing Gain." If we have an oscillator which is very stable, then we can spend minutes (if need be) estimating our best guess of exactly where 1 second lines up (1pps signal in timing references). While the jitter in the timing signal might be 1 millisecond if looked at instantaneously, if you have a 3 kHz bandwidth filled with your timing signal and you average for 3 seconds, the jitter in the net timing signal is beaten down by a factor of 100 to about 10 microseconds. Of course, we only really need relative timing. SO we just need to make sure the delay spread of all our base station receiving processors is scattered over only a few microseconds, our absolute knowledge of those delays can be worse. An overall time offset at all synchronized base stations is irrelevant to the operation of those synchronized base stations if they all have the same offset. To the moon, Ralph Message #27361 from pheilman_ at Oct 2, 2002 12:01 AM Ralph, The accuracy of a timing signal is related to its bandwidth. A voice width channel can only provide ms accuracy. That is why GPS is a wideband spread spectrum signal. Paul