RE: [SI-LIST] : Frequency or time domain for component characterization

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From: mbenson@wavecrestcorp.com
Date: Tue Feb 29 2000 - 03:31:41 PST


>Anyone have experience in correlating frequency domain measurements
>with baseband pulse performance?

>regards,

>scott

Any pulse or square wave will have a large number of harmonics that will
range in frequency from the fundamental to more than five times the
fundamental. I.e. if you are looking at a square clock at 500MHz you MAY
need to be concerned with the frequency response up to 5 X 500 MHz = 2500MHz
, since the square wave needs a minimum of two odd harmonics to look really
square. Think of the components and traces you send this squarewave through
as a network. This network will have some frequency response. If the network
has some unwanted and unexpected attenuation at a certain frequency (due to
capacitor resonance maybe) this may be a problem. It is a problem when the
attenuation is at a frequency that might destroy an important frequency
component (say the third harmonic of our square wave). Basically, if your
frequency domain measurements show that the frequency response is flat and
has no group delay or phase shifts through the highest frequency of concern
you will be fine. If you see a non flat response you MAY be in trouble, and
if you see phase shifts and group delays you MAY be in trouble. Keep in mind
also that when using a vector network analyzer one needs to be mindful of
the termination impedances for the DUT and the analyzer. Also keep in mind
that more complex waveforms will have a more complicated frequency spectrum
(use a spectrum analyzer to see all of the components of the waveform).

Hope this helps a little

Mike B

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