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Rule of thumb: Signal bandwidth from clock frequency

17 Jan 2014  | Eric Bogatin

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This rule of thumb relates the bandwidth of a signal when all you know is the clock frequency of the signal.



Where:

BW = the bandwidth of the signal, in GHz

Fclock = clock frequency in GHz


Of course, the bandwidth of a signal is about the rise time of the signal, not the clock frequency. We can have exactly the same clock frequency of different signals, but with very different rise times, and each signal will have a very different bandwidth. The figure shows three different signals, each with exactly the same clock frequency but very different rise times and very different bandwidths.


Figure: Three different signals with the same clock frequency, but very different bandwidths.


Clearly, just knowing the clock frequency of a signal is not enough to know the highest sine wave frequency in the signal. We have to know the rise time of the signal.

But, "sometimes an OK answer NOW! is better than a good answer late." Sometimes, we just don't have all the information we need to get a good answer and rather than wait around until we can get all the information we need, we have to make do with what we have.

In order to estimate the bandwidth of a signal, we have to know what its rise time is. If we only know the clock frequency, we have to "guess" the rise time. If we want to sound like we have put some more thought into it, we can say, "estimate" or if we want to sound like we really know what we are doing, we can say, "extrapolate" the rise time.

The question is, what per cent of the period is the rise time? It may be 1% the period if we have a leading edge ASIC driving a legacy bus. It may be 25% if we are driving the TX as fast as its little heart will go.

We always use as much information as we can about the specific signal to "estimate" its rise time. If we don't know anything then a good starting place is to assume the rise time is

RT = 7% x Period

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