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

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From: C Deibele (deibele@fnal.gov)
Date: Tue Feb 29 2000 - 07:20:07 PST


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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