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

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From: Scott McMorrow (scott@vasthorizons.com)
Date: Tue Feb 29 2000 - 10:41:12 PST


Craig,

Thank you.
It's time to read some papers again.

regards,

scott

C Deibele wrote:

> Scott McMorrow wrote:
> >
> > What are the issues if you would like to characterize a
> > circuit at baseband, when using a VNWA? Seems to me that
> > there are significant issues in dealing with the DC and low
> > frequency components.
> >
> > Anyone have experience in correlating frequency domain measurements
> > with baseband pulse performance?
> >
> > regards,
> >
> > scott
>
> Scott,
>
> You are certainly correct that there are significant issues for dealing
> with
> low frequency components when working with a VNWA and understanding
> baseband.
>
> Like I said, I did measure the time domain performance of a structure.
> And I
> did "verify" that I had a nice linear system. This is to say, I took
> the Fourier
> transform of my input signal In(t), I took the Fourier transform of my
> output signal
> Output(t), and came up with a transfer function which characterized my
> DUT. The two
> matched splendidly. Baseband as well as for frequencies up to 30 GHz.
> I think
> it is possible to investigate a bit into the literature and see
> countless papers
> which show this properly.
>
> It is noteworthy to point out that there are VNWA's that work down to
> relatively low
> frequencies -- I think one in particular is the HP 8751 -- it goes down
> to 5 Hz, but
> don't quote me on the model number and its appropriate lower frequency
> stopoff.
>
> When switching from one domain to another, one needs to keep several
> issues
> clearly in mind:
>
> a) The time domain measurement does not violate any upper frequency
> limit on
> cables. This can be a huge assumption with many and many cables one
> sees in
> many laboratories. Flexing a cable can add lots of dispersion....change
> attenuation, and if the cable is too big, then HOM's can be easily
> excited.
> These assumptions are critical in frequency domain measurements as well.
>
> b) some low frequency behavior of the DUT can be characterized. Like
> for a
> connector, it is probably safe to bet that it has a direct DC path.
> Most lower
> frequency measurements aren't the best suited for microwave measurement
> tools,
> but can be made easily with other tools.
>
> suffice it to say, there isn't any hard and fast answer. It depends on
> your DUT,
> your cables, your available equipment, etc.
>
> The best advice to say for VNWA's is to calibrate very carefully, choose
> your
> frequency points and change your frequency points to ensure you aren't
> missing some
> spurious resonance, and investigate your DUT below the lower frequency
> limit with some
> other measurement system.
>
> Craig
>
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--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

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