[SI-LIST] Re: To measure impedance of a capacitor using VNA
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: Anto Davis <antokdavis@xxxxxxxxx>
- Date: Sun, 27 Jul 2014 21:35:31 -0400
Hi Anto,
I think the usual answer is in order here, too: it depends. A few
important things that matter in case you want to measure a component
after a cable and you do not want or can not do calibration at the end
of the cable: a) frequency range of interest, b) ratio of impedances of
your original DUT and impedance of DUT transformed through the cable.
In other words: what is the DUT you want to measure, up to how high
frequencies, and how long and what quality of cable do you want/need to
use? The more we want to deembed or back-calculate, the more cumulative
error we may have due to practical limitations.
Regards,
Istvan Novak
Oracle
On 7/25/2014 1:37 AM, Anto Davis wrote:
> Thanks for the replies,
>
> I am going through the book, Signal Integrity Characterization
> Techniques by Mike Resso and Eric Bogatin Ch.9,
> It is explaining the method of de-embedding for two port n/w.
>
> I am doing a one port measurement; the impedance of a single loop
> (inductance), using VNA.
> VNA is calibrated at the plane of VNA port, and using 50 ohm coaxial
> cable (which is not included in the calibration process), connected to
> the DUT (single loop).
>
> 1) Instead of using De-embedding, can I directly use the method
> explained in Section 9 of the paper "Inductance Calculations in a
> Complex Integrated Circuit Environment" by A. E. Ruehli?
> Or when we talk about one port measurement, does de-embedding and the
> method explained in that paper become same?
>
> 2) With De-embedding can I avoid buying the calibration kit, if the
> VNA is already calibrated at the connnection ports? (Say, I am ok with
> slight variations due to time)
> Basically, can I de-embed the coaxial cable connectors also with the
> fixtures?
>
> Anto
>
>
>
> On Thu, Mar 13, 2014 at 6:13 PM, Istvan Novak
> <istvan.novak@xxxxxxxxxxx <mailto:istvan.novak@xxxxxxxxxxx>> wrote:
>
> Hi Anto,
>
> 1) If you have a large-size DUT with low capacitance, it means
> that in addition to
> the impedance of the DUT we may also have a lot of stray
> capacitance from the
> large DUT body to ground, which distorts the results if we wanted
> to measure
> the DUT in the series connection scheme. Typical such scenario:
> measuring the
> power/ground plane capacitance of an unpopulated board at low
> frequencies.
>
> 2) Each instrument has finite resolution, repeatability, noise
> floor and accuracy.
> If we connect the DUT in shunt-through connection, the conversion
> formula
> giving you impedance from S21 becomes very sensitive to small data
> errors
> when the DUT impedance is large. There are low-frequency VNAs,
> such as the
> Agilent E5061B, which can work down to a few hertz quite
> accurately. And cable
> length actually matters much less as frequency goes down.
>
> Regards,
>
> Istvan Novak
> Oracle
>
>
> On 3/13/2014 1:51 AM, Anto Davis wrote:
>
> Dr. Istvan,
> 1) From your book, why series through connection for higher
> impedances? Why
> do you suggest size of DUT to choose between series through
> and shunt
> through? Shouldn't it be based on the impedance only?
>
> 2) VNA measurement for lower frequencies is a challenge; they
> extrapolate
> for lower frequencies?
> They rely on reflection coefficients for measuring impedance.
> So if I
> connect longer cable, will it give better results for lower
> frequencies?
> For eg, if have 1m (=0.5m+0.5m) total cable length and the DUT
> is very
> small, v = 2e8 m/s taking wavelength/4 for distinguishing
> between lumped
> equivalent and Transmission line, below 50MHz, will it be
> treated as a
> lumped equivalent? And in this case anything less than 50MHz
> will be
> extrapolated by VNA?
> Basically, is there a minimum length for connection cable for
> a minimum
> frequency?
>
> Thanks,
> Anto
>
>
> On Wed, Mar 12, 2014 at 6:23 PM, Istvan Novak
> <istvan.novak@xxxxxxxxxxx <mailto:istvan.novak@xxxxxxxxxxx>>wrote:
>
> Hi Anto,
>
> Fair question, but for most practical situations, you dont
> need to worry.
> Even with very simple instrumentation, like a VNA created
> around a little
> USB-powered oscilloscope with an arbitrary waveform
> analyzer), the
> Two-port shunt-through measurement will be reasonably good
> up to
> at least a kohm impedance magnitude. Professional network
> analyzers
> have better resolution and you get reasonable result at
> least up to ten
> kohms. For a 0.1uF capacitor this means you dont need to
> worry about
> this potential problem unless you want to measure the
> impedance at
> frequencies with single digits or below. If that was the
> case, you can
> use either traditional impedance bridges or just use the
> same two-port
> setup but switch to series-through connection.
>
> Best regards,
> Istvan Novak
> Oracle
>
>
>
> On 3/12/2014 7:12 AM, Anto Davis wrote:
>
> Hi,
> I am trying to get the impedance plot of a 0.1uF
> capacitor (using VNA),
> which has ESL and ESR associated with it. The
> frequeuncy response should
> be
> a V curve. From the book, Frequency Domain
> Characterization of PDN by
> Istvan Novak (ch-5) I realized that two port
> measurements are better
> (measuring Z12 or S12) when Z << 50 ohms.
> But for a single capacitor, how to get the V curve? If
> I connect it to
> measure as Z12, I can measure it correctly for
> frequencies where Z<<50
> ohms
> only?
>
> Thanks,
> Anto
>
>
>
>
>
>
>
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