[SI-LIST] Re: VNA and characterization of long cables
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: markfilipov81@xxxxxxxxx
- Date: Tue, 24 Apr 2012 18:56:25 -0400
Mark, Andy,
It depends on what we want to do with the measured data.
If we wanted to have measured data without any further
post-processing and avoid the undersampling of the phase
rotation, we need equidistant frequency points. For a long
cable measured up to high frequencies we then need both:
low-frequency data points AND many data points continuing
up to the upper end of the frequency range of interest. The
minimum number of points is determined by the total delay
of the cable, highest frequency we want to cover, and the
number of points we need in each 2*PI phase period.
We do not need many equidistant points (and we can use VNAs
with overlapping frequency ranges but with different frequency
steps) if we fit a trusted model over the measured data. Here
we need to assume that the group delay is relatively constant
or at least just a slowly varying function of frequency. Once
we have a continuous model fitted to the data, we can resample
it to get equidistant samples if that is what we actually want.
Regards,
Istvan Novak
Oracle
On 4/24/2012 2:15 PM, Mark Filipov wrote:
> That's great question Andy!
>
> Yes do we need to have a low starting frequency or do we need
> frequency spacing that is higher then 10MHz.
> Because on the VNA I'm using, I can use 16 000 points, and that VNA
> goes up to 26GHz, in that case it can surely measure cables longer
> then 10m!
>
> Best regards, Mark
>
>
> 2012/4/24, A. Ingraham<a.ingraham@xxxxxxxx>:
>>> ... Alternately
>>> you can take multiple network analyzers, each covering its own frequency
>>> range, and you can go down to a few Hz in frequency if you wish. ...
>> Is that sufficient? Do you only need a low starting frequency, or
>> doesn't the frequency spacing (even up at the higher end of the sweep)
>> need to be small enough to avoid phase ambiguity?
>>
>> Regards,
>> Andy
>>
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