[SI-LIST] Re: PureMode VNA vs 4port VNA
- From: "Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 5 Apr 2005 13:43:19 -0500
Jeff,
You're correct in that if you're going to attempt to
measure S-parameters of a non-linear device, you need
to provide the device with a constant power/voltage
level.
Most VNAs have options where you can
use directional couplers and power meters to
create a feedback loop to ensure a constant power
level to the device. I have not tried this with
our balun configuration, but I expect it is
possible in a pure-mode VNA as well.
One other alternative is to reduce the output power
down low enough that you're working in the "mostly linear"
region of the device. In general (not the case for mixers),
once the power is low enough, the non-linearities are
not quite as noticable.
Regards,
Pat
> -----Original Message-----
> From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
> Sent: Tuesday, April 05, 2005 1:08 PM
> To: Zabinski, Patrick J.; si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: PureMode VNA vs 4port VNA
>
>
> Hi Pat (and others),
> Maybe I'm having a temporary brain misfire here, but doesn't the
> argument of using a "pure-mode" differential VNA for
> non-linear systems
> imply that the excitation of the differential VNA has to be a voltage
> representative of the actual range under consideration?
>
> I.E., it seems not merely enough to be "pure-mode"; the voltage swing
> must also be modified/controlled. This would also seem to be true for
> measuring any non-linear system, including 2-port measurements, and
> TDR/TDT.
>
> If I'm not missing something, how do you solve characterizing and
> building measurement-based models with this added complexity (transfer
> as a function of frequency and voltage)?
>
> Jeff Loyer
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Zabinski, Patrick J.
> Sent: Tuesday, April 05, 2005 7:03 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: PureMode VNA vs 4port VNA
>
>
> Ray,
>
> In principle, the four-port VNAs (and two-port VNAs that
> have post-processing) will provide the same results at
> pure-mode VNAs under the condition that the device
> is linear. However, the assumption of linearity is
> not always the case (particuarly for any active circuits).
> For non-linear devices, pure-mode VNAs will produce
> different results than a four-port VNA. Note, however,
> S-parameters are defined to be linear relationships
> of voltages, so if you're measuring a non-linear device,
> you need to question the validity of our measurement.
>
> From a practical perspective, there are a few issues
> to consider:
>
> * Existance: I am not aware of a commercially-available
> pure-mode VNA, but you can readily create one with the
> use of baluns and hybrids; unless you're willing to
> invest in many microwave switches, the bummer is that
> each balun/hybrid is limited in bandwidth, so it takes
> a while to cover a large frequency span.
>
> * Mode-conversion Access: As part of four-port VNAs,
> getting differential-to-common mode conversion parameters
> is readily availble; in pure-mode VNAs, getting the
> different conversion parameters requires additional
> measurements/time/(equipment).
>
> * Calibration: calibrating four-port VNAs is accomplished
> with standard cal kits and is straight forward; pure-mode
> VNAs require custom cal kits, which I don't believe
> are as readily available.
>
> * Precision: In four-port VNAs, the conversion process
> is straight forward, but it's difficult to get down to
> very-low values of S-parameters given numerical roundoffs,
> where pure-mode VNAs should not have such a problem.
>
> In our labs, we most commonly use four-port VNAs (either
> as a single unit or a two-port unit with post-processing
> of data) for nearly all of our work. However, when
> characterizing active devices (e.g., LNAs, mixers, ...),
> we commonly bring out our collection of baluns and
> make our own version of pure-mode VNA (and try to stay
> small-signal enough to stay "mostly linear").
>
> Pat
>
>
>
> > Over the past year or so there has been a lot of discussion of
> > mixed-mode s-parameters, why they are useful, conversion of
> > single-ended
> > to mixed mode, etc., etc....
> >
> > As I understand it, mixed-mode s-parameters can be mesasured
> > with a true
> > pure-mode VNA, with a 4-port VNA, or with a TDR (the last
> 2 methods
> > requiring mathematical post-processing of the measured data
> to create
> > mixed-mode s-parameter data). This post processing can occur
> > either in
> > the instrument (with appropriate firmware) or after the fact in a
> > separate compute environment.
> >
> > Is it true that a true pure-mode VNA is still not available
> > commercially
> > from any of the test equipment vendors? I wonder if anyone
> > knowledgable
> > on the topic can comment in the practical differences a user
> > should be
> > aware of when utilizing a 4-port VNA to measure mixed mode
> > s-parameters
> > as opposed to the fabled pure-mode instrument.
> >
> > -Ray
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